High Energy Phenomena in Relativistic Outflows VII (HEPRO VII)

Aula Magna Enric Casassas (Facultat de Física, Universitat de Barcelona)

Aula Magna Enric Casassas

Facultat de Física, Universitat de Barcelona

C/ Martí i Franquès 1, 08028 Barcelona, Spain

The High Energy Phenomena in Relativistic Outflows (HEPRO) conference series are devoted to the discussion of the latest and more relevant observational, phenomenological, and theoretical developments in the field of high-energy astrophysics related to systems displaying relativistic winds and jets.  Some specific topics traditionally included in the HEPRO conferences include gamma-ray production in relativistic outflows, multi-wavelength jet/outflow emission, formation and propagation of relativistic jets, and particle acceleration in different relativistic contexts.

Previous editions of HEPRO were held in Dublin (2007), in Buenos Aires (2009), in Barcelona (2011), in Heidelberg (2013), in La Plata (2015), and in Moscow (2017). The next edition will take place in Barcelona on July 9th to 12th 2019, and it will focus on similar topics and with emphasis on the physics of high-energy processes in all kinds of relativistic outflows. 

The meeting will be hosted by the Institut de Ciències del Cosmos of the Universitat de Barcelona (ICCUB). The workshop will take place at the Faculty of Physics in the Aula Magna Enric Casassas, which can accommodate up to 190 attendants. There will be invited and contributed talks, posters, and plenty of time for discussion.




Invited Speakers

  • Lorenzo Amati (INAF)
  • Elena Amato (INAF)
  • Maxim Barkov (Purdue University)
  • Sergey Bogovalov (MEPHI)
  • Pol Bordas (Universitat de Barcelona/ICCUB)
  • Jo van den Brand (Nikhef)
  • Benoit Cerutti (Université Grenoble Alpes)
  • Luigi Costamante (ASI)
  • Evgeny Derishev (IAIP-RAS)
  • Jose L. Gómez (IAA-CSIC)
  • Sergey Komissarov (University of Leeds)
  • Amir Levinson (Tel Aviv University)
  • Emily Petroff (University of Amsterdam/API)
  • Maria Petropoulou (Princeton University)
  • Oliver Porth (API)
  • Frank Rieger (Universität Heidelberg)
  • Florencia Vieyro (IAR)
  • Roberta Zanin (MPIK)
  • Bing Zhang (UNLV)

      Scientific Organising Committee

     Local Organising Committee

  • Felix Aharonian (DIAS/MPIK)
  • Gennady Bisnovatyi-Kogan (IKI)
  • Ioannis Contopoulos (Academy of Athens)
  • Alice Harding (NASA)
  • Dmitry Khangulyan (Rikkyo University,)
  • Josep M. Paredes (Chair; Universitat de Barcelona/ICCUB)
  • Manel Perucho (Universitat de València) 
  • Elena Pian (INAF)
  • Tsvi Piran (HUJI)                
  • Nanda Rea (CSIC-IEEC)
  • Elisa Resconi (TUM)
  • Gustavo E. Romero (IAR/UNLP)
  • Samar Safi-Harb (University of Manitoba)
  • Megan C. Urry  (Yale University) 
  • Xiang-Yu Wang (Nanjing University) 
  • Pol Bordas 
  • Valentí Bosch-Ramon
  • Matteo Cerruti
  • Kazushi Iwasawa
  • Edgar Molina
  • Josep M. Paredes
  • Marc Ribó
  • Núria Torres-Albà




  • Alexey Gunya
  • Alexey Pozanenko
  • Alicja Wierzcholska
  • Amir Levinson
  • Andreas Zech
  • Andrzej Zdziarski
  • Asaf Pe'er
  • Asher Noel
  • Benito Marcote
  • Benoit Cerutti
  • Bidzina Kapanadze
  • Bing Zhang
  • Bruno Jiménez Fernández
  • Charalampos Sinnis
  • Constantinos Kalapotharakos
  • Dahai Yan
  • Damien Begue
  • Daniel del Ser
  • Daniel Kwabena Ansu
  • David Paneque
  • Davit Zargaryan
  • Elena Amato
  • Eleni Kefala
  • Emanuele Sobacchi
  • Emily Petroff
  • Evgeny Derishev
  • Felix Aharonian
  • Felix Mirabel
  • Frank Rieger
  • Gennady Bisnovatyi-Kogan
  • Giancarlo Mattia
  • Gustavo E. Romero
  • Hannes Thiersen
  • Hester Maria Schutte
  • Igor Kulikov
  • Ilia Kondratev
  • Ioulia Florou
  • Iria Ayan-Míguez
  • Ishika Palit
  • Josep Maria Paredes
  • Josep Martí
  • Joseph Affi
  • José L. Gómez
  • José Ortuño-Macías
  • Kazushi Iwasawa
  • Kenichi Nishikawa
  • Killian Long
  • Kwabena Berimah
  • Laenita Oberholzer
  • Lawuza Adam Ibrahim
  • Lente Dreyer
  • Lorenzo Amati
  • Luigi Costamante
  • Luigi Foschini
  • Luigi Pacciani
  • Manel Errando
  • Manel Perucho
  • Marc Ribó
  • Maria Glushikhina
  • Mariam Nikolashvili
  • Mark Eric Dieckmann
  • Markus Boettcher
  • Martin Lemoine
  • Matteo Balbo
  • Matteo Cerruti
  • Matthew Baring
  • Maxim Barkov
  • Michael Zacharias
  • Natalia Zywucka-Hejzner
  • Nick Kylafis
  • Nuria Álvarez Crespo
  • Núria Torres-Albà
  • Olga Toropina
  • Oliver Porth
  • Omar Kurtanidze
  • Oscar Blanch Bugas
  • Pedro Luis Luque-Escamilla
  • Pol Bordas
  • Roberta Zanin
  • Rui Xue
  • Sara TOMITA
  • Sergey Moiseenko
  • Shuta Tanaka
  • Silke Britzen
  • Silke Britzen
  • Silvano Massaglia
  • Stella Boula
  • Susumu Inoue
  • Taiki Ogihara
  • Tiffany Lewis
  • Valenti Bosch-Ramon
  • Vitalii Sliusar
  • Vyacheslav Krauz
  • Weihua LEI
  • Xiaoyi Xie
  • Xiying Zhang
  • Yakov Istomin
    • 08:30 09:15
      Registration Aula Magna Enric Casassas

      Aula Magna Enric Casassas

      Facultat de Física, Universitat de Barcelona

      C/ Martí i Franquès 1, 08028 Barcelona, Spain
    • 09:15 09:30
      Welcome Aula Magna Enric Casassas

      Aula Magna Enric Casassas

      Facultat de Física, Universitat de Barcelona

      C/ Martí i Franquès 1, 08028 Barcelona, Spain
    • 09:30 11:00
      Formation and propagation of relativistic outflows Aula Magna Enric Casassas

      Aula Magna Enric Casassas

      Facultat de Física, Universitat de Barcelona

      C/ Martí i Franquès 1, 08028 Barcelona, Spain
      Convener: Maxim Barkov (Purdue University)
      • 09:30

        Particle acceleration in relativistic jets to very high energies occurs at the expense of the dissipation of magnetic or kinetic energy. Therefore, understanding the processes that can trigger this dissipation is key to the characterization of the energy budgets and particle acceleration mechanisms at action in active galaxies. Instabilities and entrainment are two obvious candidates to trigger dissipation. On the one hand, supersonic, relativistic flows threaded by helical fields, as expected from the standard formation models of jets in supermassive black-holes, are unstable to a series of magnetohydrodynamical instabilities, such as the Kelvin-Helmholtz, current-driven, or possibly the pressure-driven instabilities. Furthermore, in the case of expanding jets, the Rayleigh-Taylor and centrifugal instabilities may also develop. With all these destabilizing processes at action, a natural question is how can some jets keep their collimated structure along hundreds of kiloparsecs. On the other hand, the interaction of the jet with stars and clouds of gas that cross the flow in their orbits around the galactic centers provides another scenario in which kinetic energy can be efficiently converted into internal energy and particles can be accelerated to non-thermal energies. In this talk, I will review the conditions under which these processes occur and their role both in jet evolution and propagation and energy dissipation.

        Speaker: Manel Perucho Pla (Universitat de València)
      • 10:00

        Plasma injection and magnetic field dissipation are two key issues in the theory of black hole jets. Pair production in either starved magnetospheric regions or dissipative boundary layers can supply the plasma required for black hole activation. These processes naturally produce rapidly
        varying VHE emission. In the first part of the talk I shall discuss pair creation in a starved BH magnetosphere and present recent GRPIC simulations of a spark gap. In the second part I will present results of recent force-free
        simulations of magnetic loops accretion into a Kerr black hole, and show that in such configurations rapid dissipation occurs in the current sheets of interacting loops, that can give rise to TeV emission and consequent pair creation.

        Speaker: Amir Levinson (Tel Aviv University)
      • 10:30
        MHD Accretion Disk Winds and the Blazar Sequence 15m

        Fermi telescope has detected a significant number of AGNs to allow statistical treatment of their properties. Among others, it confirmed the "Blazar Divide" in FSRQs and BL Lacs according to their gamma-ray spectral index and luminosity. We investigate this classification by proposing a model which reproduces in detail the broadband blazar spectra and their statistical properties based on the physical parameters of their MHD accretion disk winds. Τhis model describes the distribution of matter and magnetic fields in AGN at least over 5 decades in radius and it provides the vestiges of an account of the observed blazar classification in terms of a single parameter, their mass accretion rate.

        Speaker: Stella Boula (National and Kapodistrian University of Athens)
      • 10:45
        Magnetically arrested disks around black hole, and jet formation 15m

        Synchrotron origin of the radiation from extragalactic disks indicate to important role of the magnetic field in their formation and collimation. Jets from AGN show their close connection with supermassive black holes. Large magnetic fields around BH are formed during an accretion of matter with large-scale magnetic field. Approximate solutions describing such accretion lead to conclusion of formation of an accretion disk around BH supported by magnetic field against gravity, instead of a centrifugal force in the keplerian disk. The efficiency of this regime of accretion may reach 0.5Mc2. The properties of such disk had been studied it the middle of 70s, and much later is was named as “magnetically arrested disk”. Different models of jet origin from such disk had been studied in approximate way. 2-and 3 d numerical simulations had been performed during last years, confirming efficiency of jet formation, especially from the disks around rapidly rotating BH.

        Speaker: Gennady Bisnovatyi-Kogan
    • 11:00 11:30
      Coffee break 30m
    • 11:30 12:50
      Formation and propagation of relativistic outflows Aula Magna Enric Casassas

      Aula Magna Enric Casassas

      Facultat de Física, Universitat de Barcelona

      C/ Martí i Franquès 1, 08028 Barcelona, Spain
      Convener: Valenti Bosch-Ramon (Universitat de Barcelona/ICCUB)
      • 11:30

        The Event Horizon Telescope (EHT) has captured the first image of a black hole. The central compact radio source in the radio galaxy M87 has been resolved out into an asymmetric bright emission ring with a diameter of 42 uas, which is circular and encompasses a central depression in brightness with a flux ratio >10:1. The emission ring is recovered using different calibration and imaging schemes, and remains stable over four different observations carried out in different days. We compare our images to an extensive library of ray-traced GRMHD simulations showing that they are consistent in size and shape with the lensed photon orbit encircling a dark shadow caused by photon capture at the event horizon of a 6.5 billion solar masses black hole, as predicted by general relativity. Our EHT observations thus provide confirmation for the presence of supermassive black holes powering active galaxies, and present a new tool to explore gravity in its most extreme limit via repeated astronomical observations.

        Speaker: José Luís Gómez (Instituto de Astrofísica de Andalucía-CSIC)
      • 11:50

        Over the last two decades, general relativistic magnetohydrodynamic simulations (GRMHD) have drawn a robust theoretical picture of jet formation in low-luminosity sources. The observations by the Event Horizon Telescope now allow to test these theories at an unprecedented level. I will take this opportunity to review the state-of-the-art in 3D numerical source models. Nowadays, adaptive mesh refinement simulations resolve the black hole engine as well as the jet outflow over several decades. To demonstrate how results compare between different codes, I will also report on the first comprehensive GRMHD code comparison effort of black hole accretion in which nine groups have participated. As an example application, first conclusions drawn from the GRMHD simulation library which was applied to the 2017 EHT observations will be discussed. In particular the jet power derived from GRMHD simulations proved to be very constraining on the source parameters of M87.

        Speaker: Oliver Porth (University of Amsterdam, Anton Pannekoek Institute for Astronomy)
      • 12:20
        Probing the dynamics of AGN jets with advanced semi-analytical modelling 15m

        The jets launched by accreting super-massive black holes can be some of the brightest sources in the high-energy sky. However, despite being discovered decades ago, their physics and energetics are still poorly understood. The past decade has seen a dramatic improvement in the quality of available multi-wavelength data, particularly in the X-ray and $\gamma$-ray bands. However, the semi-analytical modelling of jets has advanced slowly, and simple one-zone models are still the preferred method of interpreting data, in particular for AGN jets. These models can roughly constrain the properties of jets but they can not unambiguously couple their emission to the launching regions and internal dynamics, which are usually probed with simulations. On the other hand, simulations are not easily comparable to observations because they cannot yet self-consistently predict spectra. I will discuss our group's ongoing efforts to develop a more advanced semi-analytical model which accounts for the dynamics of the whole jet, starting from a simplified parametrization of Relativistic Magnetohydrodynamics in which the magnetic flux is converted into bulk kinetic energy. We apply our model to state-of-the-art datasets, including recent observations taken during the Event Horizon Telescope multi-wavelength campaign, which combine combine multi-wavelength SEDs and direct VLBI imaging of the jet. Thanks to the quality of our data we are able to conduct a thorough exploration of parameter space. Compared to previous modelling efforts this approach produces some of the best SED fits for these sources to date and provides stronger constraints on jet internal properties physics.

        Speaker: Matteo Lucchini (Anton Pannekeok Institute for Astronomy)
      • 12:35
        Structure of a collisionless pair jet in a magnetized electron–proton plasma: flow-aligned magnetic field 15m

        Accreting black holes can eject jets that are composed of electrons, positrons and ions. Electromagnetic emissions by such jets reveal the presence of strong coherent magnetic fields, of hot plasma and relativistic leptons. Several mechanisms contribute to the magnetic field of the jet: magnetic field transport by the jet, the amplification of the ambient magnetic field by shocks and collisionless plasma instabilities. The latter two have been studied with particle-in-cell simulations. Collisionless shocks, which develop between plasma clouds with the same particle composition that collide at a relativistic speed, are mediated by magnetic fields that are driven by the filamentation (beam-Weibel) instability.
        We present results from a particle-in-cell simulation that models the interaction between a spatially localized pair cloud and a magnetized electron-proton plasma. The pair cloud has the temperature 400 keV and mean speed 0.9c. Its mean velocity vector is aligned with the background magnetic field that permeates the electron-proton plasma. A jet forms in time. Its outer cocoon consists of jet-accelerated ambient plasma and is separated by an electromagnetic piston from the jet material. This piston is the counterpart of a hydrodynamic contact discontinuity in collisionless plasma. Its coherent magnetic field enwraps the pair jet and its thickness is comparable to the thermal gyroradius of the relativistic jet particles. This piston constitutes a novel plasma structure that could contribute to the radio emissions of jets. A beam of electrons and positrons moves along
        the jet spine at its initial speed. Its electrons are slowed down and some positrons are accelerated as they cross the jet's head. The latter escape upstream along the magnetic field, which yields an excess of MeV positrons ahead of the jet. Some of the protons, which were located behind the electromagnetic piston at the time it formed, are accelerated to MeV energies.

        Speaker: Mark Eric Dieckmann (Linkoping University)
    • 12:50 14:30
      Lunch Break 1h 40m
    • 14:30 15:45
      Formation and propagation of relativistic outflows Aula Magna Enric Casassas

      Aula Magna Enric Casassas

      Facultat de Física, Universitat de Barcelona

      C/ Martí i Franquès 1, 08028 Barcelona, Spain
      Convener: Manel Perucho Pla (Universitat de València)
      • 14:30

        Extragalactic jets are launched from the innermost regions of galaxies, near the central supermassive black hole. As they propagate, they must cross the whole galaxy, and in this process they interact with a variety of obstacles; including gas clouds, populations of stars or even supernova remnants. The interaction between jets and penetrating obstacles has been studied as a possible method for jet mass-loading and deceleration, as well as of production of gamma-ray emission, through non-thermal particles accelerated in shocks. Interaction with individual objects, such as stars or gas clouds, can explain both rapid variability in blazars, and gamma-ray flares. Interaction with whole populations of obstacles, however, may lead to the production of persistent gamma-ray emission.

        Speaker: Núria Torres-Albà (Universitat de Barcelona (ICCUB))
      • 15:00
        Laboratory simulation of jet propagation in ambient medium with Plasma Focus facilities 15m

        Plasma focus (PF) facilities have proved to be an effective tool in the modeling of outflows from compact astrophysical objects [1]. The specific feature of the experimental scheme with PF is the possibility to study the propagation of plasma flows over long distances, while we are able to change the parameters of the ambient medium. The talk provides a brief overview of the latest experiments on the PF-3 (NRC "Kurchatov Institute", Moscow) and PF-1000 (IPPLM, Warsaw) facilities. The main emphasis is made on the simulation of the bow shock wave. The effect of pre-ionization of background gas by the radiation of the plasma flow is shown. The effects of radiation cooling and magnetic fields on the collimation and stability of the plasma flow are analyzed. The results of experiments with an external poloidal field are presented. The increase in both the poloidal and toroidal components of the captured magnetic field is shown. This effect may be due to the rotation of the plasma flow.

        The reported study was funded by RFBR according to the research project № 18-29-21006

        [1]. V.I. Krauz, V.S. Beskin and E.P. Velikhov. IJMP D, 27, (2018) 1844009

        Speaker: Vyacheslav Krauz (NRC Kurchatov Institute)
      • 15:15
        Relativistic Jets from black hole accretion disc 15m

        Astrophysical jets from AGNs and microquasars are often relativistic and collimated. We study magnetic and radiative driving of jets to address these issues. The plasma is described by relativistic equation of state which depends on the composition. We show that the matter content may not affect the streamline of magnetically driven jets, but the poloidal velocity and temperature distribution strongly depend on composition of the jet. We also discuss the salient features of radiatively driven jets. Although consensus in the community precludes radiation driving to be effective acceleration mechanism, we show that is certainly not the case. For black holes surrounded by luminous discs, jets may be accelerated up to few Lorentz factors for baryon dominated jets. Interestingly, the terminal Lorentz factor may reach to a value of few tens for lepton dominated jets. We also show internal shock driven by radiation is also possible for jets. Moreover, consideration of temperature dependent scattering cross-section can produce relativistic jets which stats with very low speed and quite moderate temperatures which are expected in the inner region of the accretion discs. Although we have studied magnetic driving and radiative driving separately, but it is apparent that both the processes should be incorporated in order to solve collimation and acceleration enigma of astrophysical jets.

        Speaker: Indranil Chattopadhyay (Aryabhatta Research Institute of Observational Sciences (ARIES))
      • 15:30
        Reconnection and Associated Flares in Global Relativistic Jets Containing Helical Magnetic Fields with PIC Simulations 15m

        In the study of relativistic jets one of the key open questions is their interaction with the environment on the microscopic level. Here, we study the evolution of both electron–proton and electron–positron relativistic jets containing helical magnetic fields, focusing on their interaction with an ambient plasma. We have performed simulations of “global” jets containing helical magnetic fields in order to examine how helical magnetic fields affect kinetic instabilities such as the Weibel instability, the kinetic Kelvin-Helmholtz instability (kKHI) and the Mushroom instability (MI) using a large jet radius. In our simulation study these kinetic instabilities are excited and new types of jet evolution takes place. In the electron-proton jet simulation a recollimation-like instability occurs near the center of jet. In the electron-positron jet simulation mixed kinetic instabilities grow and the jet electrons are accelerated. The evolution of electron-proton jets shows that helical magnetic fields are untangled and electrons are accelerated in turbulent magnetic field further. We also investigate mechanisms of flares possibly due to rapid acceleration.

        Speaker: Kenichi Nishikawa
    • 15:45 16:15
      Coffee break 30m
    • 16:15 17:15
      Particle acceleration and Radiation processes Aula Magna Enric Casassas

      Aula Magna Enric Casassas

      Facultat de Física, Universitat de Barcelona

      C/ Martí i Franquès 1, 08028 Barcelona, Spain
      Convener: Prof. Frank Rieger
      • 16:15

        The present paper proposes a generalized description of Fermi-type acceleration processes, by following the momentum of the particle through a continuous sequence of accelerated frames, defined in such a way that the electric field vanishes at each point along the particle trajectory. This unified description of Fermi acceleration applies equally well in sub- and ultra-relativistic settings, in Cartesian or non-Cartesian geometries, flat or non-flat space- time.

        Speaker: Martin Lemoine (Institut d'Astrophysique de Paris)
      • 16:45
        Pitch-angle Diffusion and Bohm-type Approximations in Diffusive Shock Acceleration 15m

        The problem of accelerating cosmic rays is one of fundamental importance, particularly given the uncertainty in the conditions inside the acceleration sites. In this talk, I describe our work in which we examine diffusive shock acceleration in arbitrary turbulent magnetic fields, constructing a new model that is capable of bridging the gap between the very weak (δB/B_0 ≪ 1) and the strong turbulence regimes. To describe the diffusion we provide a quantitative analytical description of the “Bohm exponent” in each regime.
        I show that our results converge to the well known quasi-linear theory in the weak turbulence regime. In the strong regime, we quantify the limitations of the Bohm-type models. Furthermore, our results account for the anomalous diffusive behavior which has been noted previously. Finally, I will discuss the implications of our model in the study of possible acceleration sites in different astronomical objects.

        Speaker: Asaf Pe'er (Bar Ilan University)
      • 17:00
        Acceleration of Relativistic Outflows with Tangled Magnetic Field 15m

        Extracting the rotational energy by magnetic braking is the most promising mechanism driving relativistic outflows of active galactic nuclei, gamma-ray bursts, and pulsar winds, i.e., they must be Poynting dominated flow at their base.
        On the other hand, the observed spectral and dynamical signatures of these relativistic objects infer that the magnetization $\sigma$ (the ratio of Poynting to particle energy fluxes) of the outflows is much less than unity at a dissipation point.
        Many studies intended to accelerate the Poynting dominated outflows by reducing the magnetization have been done, called $\sigma$-problem.

        Recently, we extended the one-dimensional spherically symmetric steady outflow model by Kennel & Coroniti (1984) by formulating the effects of the turbulent magnetic field in order to resolve the $\sigma$-problem of the Crab Nebula (Tanaka et al. 2018).
        Here, we will show the results for applications of our model to the relativistic outflows.
        Interestingly, even cylindrical jets (e.g., Fanaroff-Riley type II) can be accelerated by the effects of the magnetic turbulence.
        Although the magnetic turbulence does not reduce the flow magnetization, the radiative cooling of the relativistic flow accompanied with the dissipation of the turbulent magnetic field leads to further acceleration and reduction of the magnetization.

        [1] Kennel C. F., & Coroniti F. V., 1984, ApJ, 283, 694
        [2] Tanaka S. J., Toma, K. & Tominaga, N., 2018, MNRAS, 478, 4622

        Speaker: Shuta Tanaka (Aoyama Gakuin University)
    • 17:15 18:00
      Poster session Aula Magna Enric Casassas

      Aula Magna Enric Casassas

      Facultat de Física, Universitat de Barcelona

      C/ Martí i Franquès 1, 08028 Barcelona, Spain
    • 18:00 20:00
      HEPRO VII Welcome Reception Atri Solar (Facultat de Física)

      Atri Solar

      Facultat de Física

      besides meeting room in Aula Magna Enric Casassas

      Reception cocktail

    • 09:30 11:00
      Particle acceleration and Radiation processes Aula Magna Enric Casassas

      Aula Magna Enric Casassas

      Facultat de Física, Universitat de Barcelona

      C/ Martí i Franquès 1, 08028 Barcelona, Spain
      Convener: Dr Eugeny Derishev
      • 09:30

        Relativistic outflows are ubiquitous in high-energy cosmic phenomena. Whether their reservoir of free energy at launch is in the form of bulk kinetic, magnetic or gravitational, it is ultimately channeled into energetic particles and non-thermal radiation. A key question is to understand how this transfer operates efficiently and under which conditions. In this talk, I will review some of the recent developments in the modeling of three particle acceleration processes, namely relativistic diffusive shock acceleration, relativistic magnetic reconnection and collisionless turbulence. These results will be discussed in the context of pulsars and pulsar wind nebulae, black hole magnetospheres and relativistic jets.

        Speaker: Benoît Cerutti (CNRS & Université Grenoble Alpes)
      • 10:00

        In the broad-band spectra of blazars there are two broad and widely separated maxima. According to the existing paradigm, the origin of the lower-frequency maximum is explained by the synchrotron radiation of accelerated electrons, and the higher-frequency maximum is produced via inverse Compton radiation from the same population electrons. This paradigm makes it necessary to adopt such physical parameters in the radiating region (magnetic field energy density, electron acceleration rate, etc.), which differ by orders of magnitude from their natural values ​​dictated by dimensional considerations and comparison with other sources of synchrotron radiation. We analyse the extreme regimes of particle acceleration and radiation, discuss physical limits to the parameters of blazars' emitting zones, and show that in many cases model requirements cannot fit into these fundamental limits.

        We also propose another paradigm in which both maxima in blazars' spectra are explained by synchrotron radiation of different populations of particles. The higher-frequency maximum is associated with the emission of primary accelerated particles directly in the acceleration zone, and the lower-frequency maximum with emission of secondary electron-positron pairs, which are produced outside of the acceleration zone. Then the required physical parameters in emitting zones of blazars turn out to be close to their natural values, and the resulting spectra qualitatively agree with the observed ones.

        Speaker: Eugeny Derishev (Institute of Applied Physics RAS)
      • 10:30
        Weibel Mediated Shocks Propagating into the Inhomogeneous Media 15m

        The Weibel instability occurs in collisionless plasma with the temperature anisotropy. It is thought to be important for particle acceleration and generation of magnetic fields in relativistic shocks. Observations of afterglows of gamma ray bursts (GRBs) suggest that magnetic fields are amplified in the large downstream regions of relativistic shocks. However, the magnetic field produced by the Weibel instability decays rapidly, which cannot explain the observed properties of afterglows of GRBs. The nonlinear evolution of the Weibel instability has been studied in uniform plasmas so far. In reality, there must be density fluctuations in interstellar or circumstellar medium. We performed two-dimensional particle-in-cell simulations of relativistic shocks propagating to the inhomogeneous electron-positron plasmas. We found that, for the no-uniform case, the downstream magnetic field keeps a higher amplitude and larger length-scale than that for the uniform case. Furthermore, there is a much larger temperature anisotropy in the far downstream region than that for the uniform case. The downstream temperature anisotropy results from the downstream sound waves excited by an interaction of incident waves with shock waves. The observed temperature anisotropy is sufficiently large to generate the magnetic field required by the GRB observations. We could expect that the upstream inhomogeneity is crucial role for the generation of magnetic field and particle acceleration in relativistic collisionless shocks.

        Speaker: Sara Tomita (Aoyama Gakuin University)
      • 10:45
        Relativistic Jet of Markarian 421: Observational Evidences of Particle Acceleration Mechanisms 15m

        Markarian 421 is one the most extreme blazars characterized by complex, unpredictable timing/spectral variability, exclusively strong X-ray flares in some epochs, very broad, nonthermal spectral energy distribution (SED) extending over 19 orders of the frequency and showing a typical two-"hump" structure. The lower-energy component, ranging from the radio to X-rays, is widely accepted to be a synchrotron radiation emitted by ultra-relativistic electrons/positrons/protons, while the origin of higher-energy emission (the MeV-TeV range) still remains controversial (synchrotron self-Compton, external Compton, hadronic etc.). All these mechanisms need the presence of highly-relativistic particles in the jet, to be initially accelerated via the Blandford-Znajek mechanism and magneto-hydrodynamic processes in the vicinity of the central super-massive black hole. However, particles loose the energy, sufficient for emitting the KeV photons, very quickly and the source can maintain its flaring state on the daily-weekly timescales if some additional acceleration mechanisms are continuously at work. According to different studies and simulations, the particles can gain a tremendous energy due to the propagation of relativistic shocks through the jet: by means of first-order Fermi mechanism at the shock front, or they undergo an efficient stochastic (second-order Fermi) acceleration close to the shock, in the turbulent jet medium. Our intense X-ray spectral study of Mrk 421 has revealed the dominance of these processes in different epochs: while the spectral curvature and photon index show a positive cross-correlation during some flares (expected in the framework of energy-dependent acceleration probability scenario: a particular case of first-order Fermi mechanism), other epochs clearly demonstrate the observables of the stochastic (second-order Fermi) acceleration (low spectral curvature; anti-correlation between the spectral curvature and the position of the synchrotron SED peak). During several X-ray flares, the source also showed hard power-law spectra, expected in the case of relativistic magnetic reconnection.

        Speaker: Bidzina Kapanadze (Ilia State University)
    • 11:00 11:30
      Coffee break 30m
    • 11:30 12:45
      Relativistic outflows from extragalactic sources Aula Magna Enric Casassas

      Aula Magna Enric Casassas

      Facultat de Física, Universitat de Barcelona

      C/ Martí i Franquès 1, 08028 Barcelona, Spain
      Convener: Kazushi Iwasawa (Universitat de Barcelona / ICCUB)
      • 11:30

        Relativistic jets in AGN emit radiation at all wavelengths,
        from radio to TeV gamma-rays. The main source of their power seems the rotation of the black hole. Their radiation mechanisms however remains elusive. I will review the current status and some recent observations which are challenging
        common views, in particular about variability, location of the gamma-ray emitting regions, particle acceleration and main emission mechanisms among different classes of sources.

        Speaker: Luigi Costamante
      • 12:00
        The extreme character of our closest VHE blazars, Mrk421 and Mrk501 15m

        Owing to their brightness and proximity (z=0.03), Mrk 421 and Mrk 501 are among the blazars that can be studied with the greatest level of detail; and hence a sort of astrophysical laboratories to study the blazar's phenomena. In the conference I will show that these two objects can change their personality from one season to the next, sometimes showing a remarkably extreme character in the broadband SED, as well as in the variability and Doppler factors that are measured or inferred from the data. I will report detailed observational and theoretical results related to the 2-week long highest X-ray activity in Mrk501 observed with Swift-XRT since its launch 14 years ago, which suggests the presence of narrow spectral components at TeV energies. And I will also show an unprecedented correlation study of VHE gamma-rays and X-rays for Mrk421, revealing a large degree of complexity in these relations when quantified on different temporal and energy bins, and supporting the presence of multiple components in the X-ray and VHE gamma-ray emission. These multi-instrument observations on Mrk421 and Mrk501 have yielded thought-provoking results, and demonstrate the importance of performing a continuous monitoring over multi-year timescales to fully characterise the dynamics of blazars.

        Speaker: David Paneque (Max Planck Institute for Physics)
      • 12:15
        5.5 years multiwavelength variability of Mrk 421: evidence of leptonic emission from radio to TeV 15m

        Mrk 421 is a high-synchrotron-peaked blazar featuring bright and persistent GeV and TeV emission. We use the longest and densest ongoing unbiased observing campaign obtained at TeV and GeV energies during 5.5 years with the FACT telescope and the Fermi LAT detector. The contemporaneous multi-wavelength observations were used to characterize the variability of the source and to constrain the underlying physical mechanisms. We study and correlate light curves obtained by nine different instruments from radio to gamma rays and found two significant results. The TeV and X-ray light curves are very well correlated with sub-day lag. The GeV light curve varies independently and accurately leads the variations observed at long wavelengths, in particular in the radio band. We find that the observations match the predictions of leptonic models and suggest that the physical conditions vary along the conical jet, where the emitting region is moving outwards and is becoming first transparent to GeV photons and later to radio.

        Speaker: Vitalii Sliusar (University of Geneva, Department of Astronomy)
      • 12:30
        CTA 102 -- year over year receiving you 15m

        The FSRQ CTA 102 (z=1.032) has been tremendously active over the last few years. During its peak activity lasting several months in late 2016 and early 2017, the gamma-ray and optical fluxes rose by up to a factor 100 above the quiescence level. We have interpreted the peak activity as the ablation of a gas cloud by the relativistic jet, which can nicely account for the months-long lightcurve in 2016 and 2017. The peak activity was in the middle of a 2-year-long high-state, which was characterized by increased fluxes and increased rms variability compared to the previous low-states, and which was flanked by two bright flares. In this presentation, we put the cloud-ablation scenario into the broader context of the 2-year-long high-state.

        Speaker: Michael Zacharias (Institut für theoretische Physik IV, Ruhr-Universität Bochum, Germany)
    • 12:45 14:30
      Lunch Break 1h 45m
    • 14:30 15:45
      Relativistic outflows from extragalactic sources Aula Magna Enric Casassas

      Aula Magna Enric Casassas

      Facultat de Física, Universitat de Barcelona

      C/ Martí i Franquès 1, 08028 Barcelona, Spain
      Convener: Dr Luigi Costamante
      • 14:30

        We compare the energy requirements of different scenarios that allow addressing ultrafast gamma-ray variability recently reported by the MAGIC collaboration from two extragalactic sources, IC 310 and NGC 1275. Currently, the following three models are accepted as a feasible explanation for minute-scale variability: (i) external cloud in the jet, (ii) relativistic blob propagating through the jet material, and (iii) production of high-energy gamma rays in the magnetosphere gaps. Our
        analysis shows that the first two scenarios are not constrained by the flare luminosity. On the other hand, there is a robust upper limit on the luminosity of flares generated in the black hole (BH) magnetosphere (MSph). The maximum luminosity of magnetosphere flares depends weakly on the mass of the central BH and is determined by the accretion disk magnetization, viewing angle, and the pair multiplicity. For the most favorable values of these parameters, the luminosity for 5-minute flares are limited by 2*10^43 erg/s, which excludes a BH MSph origin of the flare detected with MAGIC from IC 310, and NGC 1275. In the scopes of scenarios (i) and (ii), the jet power, which is required to explain the flares detected from these sources, exceeds the jet power estimated based on the radio data. To resolve this discrepancy in the framework of the scenario (ii), it is sufficient to assume that the relativistic blobs are not distributed isotropically in the jet reference frame. A realization of scenario (i) demands that the jet power during the flare exceeds by a large factor, 100, the power of the radio jet relevant on a timescale of 10^8 years.

        Speaker: Maxim Barkov (Purdue University)
      • 15:00
        Unraveling the complex nature of the very high-energy gamma-ray blazar PKS 2155-304 15m

        PKS 2155-304 is a blazar located in the Southern Hemisphere, monitored with the High Energy Stereoscopic System (H.E.S.S.) at very high energy (VHE) gamma rays every year since 2004. Thanks to the large data set collected in the VHE range and simultaneous coverage in optical, ultraviolet, X-ray and high energy gamma-ray ranges, this object is an excellent laboratory to study spectral and temporal variability in blazars. However, despite many years of dense monitoring, the nature of the variability observed in PKS 2155-304 remains puzzling.

        In this presentation, I will discuss the complex spectral and temporal variability observed in PKS 2155-304. I will focus on the VHE gamma-ray data collected with H.E.S.S. starting in 2013, and which is complemented with multi-wavelength observations from Fermi-LAT, Swift-XRT, Swift-UVOT, SMARTS, the Steward Observatory, and the ATOM telescope. I will also discuss the complexity of the temporal evolution of the broadband emission with a special focus on the presence of different kinds of flaring activity, including orphan outbursts. The theoretical implications are manifold, ranging from acceleration and emission processes to different locations of the emission regions. I will also demonstrate the importance of dense, multi-wavelength monitoring that is needed to fully characterize the behaviour of PKS 2155-304.

        Speaker: Alicja Wierzcholska (Institute of Nuclear Physics PAN)
      • 15:15
        Optical variability modelling of newly identified blazars and blazar candidates behind Magellanic Clouds 15m

        We present results of optical variability study of 44 newly identified blazars and blazar candidates behind the Magellanic Clouds (Żywucka et al. 2018). The sample contains 27 flat spectrum radio quasars (FSRQs) and 17 BL Lacertae objects (BL Lacs), but only nine of them are recognized as blazars, while the classification of 35 objects is still uncertain. All objects possess high photometric accuracy and infrequently sampled optical light curves (LCs) from the long-term ($\sim$2 decades) monitoring conducted by the Optical Gravitational Lensing Experiment in V and I filters. The LCs were modelled with the Continuous-time Auto-Regressive Moving Average (CARMA) process using the publicly available Markov Chain Monte Carlo sampler described by Kelly et al. (2014) and with the Lomb-Scargle (LS) periodogram.

        The CARMA models allow to investigate variability features of irregularly sampled LCs, especially their power spectral density (PSD), to determine variability-based classification of astrophysical objects and to detect quasi-periodic oscillations (QPOs). We found that some of the examined objects require high-order fits, implying a deviation from the simple single-Lorentzian PSD.

        The power law PSD is indicative of a self-affine stochastic process characterised by the Hurst exponent $H$, underlying the observed variability. An estimation of the $H$ values was performed with a wavelet lifting transform (Knight, Nason & Nunes 2017). We find that most objects have $H\leq 0.5$, indicating short-term memory, but four BL Lacs and two FSRQs have $H>0.5$, implying long-term memory.

        The power law PSDs of blazars are thought to be the result of synchrotron, synchrotron self-Compton and external Compton emission. The higher-order CARMA fits suggest there are additional variations present in blazar jets and/or accretion discs that affect both the overall shape of the PSD, and can give rise to QPOs. The non-power law features are also visible in some of the LS periodograms, and signs of flattening of the PSD at low frequencies observed in some of the CARMA fits hints at the blazar nature of the objects.

        Speaker: Natalia Zywucka-Hejzner (Centre of Space Research, North-West University, Potchefstroom, South Africa)
      • 15:30
        Are BL Lac jets weakly magnetised? 15m

        The Spectral Energy Distribution (SED) of BL Lacs is usually modelled assuming that the momentum distribution of the non-thermal particles is isotropic. The modelling of the SED typically suggests the presence of strongly sub-equipartition magnetic fields in the emission region, which contradicts the paradigm of dynamically important magnetic fields in AGN jets. I will argue that the non-thermal electrons responsible for producing the observed radiation are instead primarily accelerated in the direction of the background magnetic field. The key point is that gyroresonant pitch angle scattering, which might isotropize the electron momentum distribution, can be effective only out to some electron energy that is typically smaller than the spectral break due to efficient cooling. I will present a simple phenomenological model that takes this effect into account. Using the new model, the physical properties of the emission region that are inferred from the observed SED change dramatically. In particular, allowing for an anisotropic electron distribution removes the need for strongly sub-equipartition magnetic fields in the emission region.

        Speaker: Emanuele Sobacchi (Ben Gurion University of the Negev)
    • 15:45 16:15
      Coffee break 30m
    • 16:15 17:30
      Relativistic outflows from extragalactic sources Aula Magna Enric Casassas

      Aula Magna Enric Casassas

      Facultat de Física, Universitat de Barcelona

      C/ Martí i Franquès 1, 08028 Barcelona, Spain
      Convener: Prof. Markus Böttcher (NWU Potchefstroom)
      • 16:15

        AGN are believed to be potential sites of ultra-high energy (UHE) cosmic ray acceleration. I will highlight observational findings as well as requirements on source energetics, and then discuss the relevance of different acceleration sites and mechanisms, such as black hole gap or large-scale jet shear acceleration.

        Speaker: Frank Rieger (ITA Univ. & MPIK Heidelberg)
      • 16:45
        Gamma-rays from large-scale outflows in starburst galaxies 15m

        The combined effects of supernova explosions and stellar winds produce a hot bubble in the central regions of starburst galaxies. As the bubble expands, it can outbreak into the galactic halo driving a superwind that transports hot gas and fields to the intergalactic space. I present estimates of cosmic ray generation and gamma-ray emission in this large-scale wind.

        Speaker: Gustavo E. Romero (Instituto Argentino de Radioastronomía (IAR))
      • 17:00
        On the interaction of pulsar winds and clumpy stellar winds 15m


        Speakers: Eleni Kefala, Valenti Bosch-Ramon (ICCUB / UB)
      • 17:15
        Spectral ageing in powerful radio galaxies 15m

        Relativistic radio jets create large-scale lobes -- the extragalactic footprints of accretion onto supermassive black holes, central to radio-loud active galaxies. In these dynamic and energetic structures, the phenomenon of spectral ageing occurs -- the progressive steepening of the radio spectrum in the lobes due to radiative losses. Recent advances in computing capabilities have enabled this physical process to become a tool that can be used to calculate the age of radio galaxies -- an important parameter used to determine their jet power (the time averaged kinetic feedback into the environment). In building the jet power function for all radio galaxies, accurate sources ages are required. Spectral ages, however, are almost always underestimated relative to the dynamical ages
        of radio galaxies. In this talk I will present a detailed investigation of spectral and dynamical ages of two powerful cluster-centre radio galaxies, using broad-band VLA data at multiple frequencies, and deep X-ray observations with Chandra and XMM-Newton of the shocked medium being driven by the central radio source. We find that the use of broad-band radio data can give a close agreement between spectral and dynamical ages, which is rarely found. These observational results are repeated when analytically modelling the radio sources and their X-ray environments. These findings will be key to building future tools to determine jet powers of all radio-loud AGN, which will be observed in the dawn of deep radio surveys such as the SKA, and will lead to information on the total power output of radio galaxies over all cosmic time.

        Speaker: Vijay Mahatma
    • 09:30 10:30
      Relativistic outflows from extragalactic sources Aula Magna Enric Casassas

      Aula Magna Enric Casassas

      Facultat de Física, Universitat de Barcelona

      C/ Martí i Franquès 1, 08028 Barcelona, Spain
      Convener: David Paneque (Max Planck Institute for Physics)
      • 09:30

        I will review several open questions in GRB physics as well as how latest observational data shed light into them. The topics include jet composition, energy dissipation mechanism, radiation mechanism, progenitors, and central engine. The exciting aspects of gravitational wave - GRB associations will be discussed, with GW170817/GRB 170817A as an example.

        Speaker: Bing Zhang (University of Nevada, Las Vegas)
      • 10:00

        The Transient High-Energy Sky and Early Universe Surveyor (THESEUS) is a space mission concept currently under Phase A study by ESA as candidate M5 mission, aiming at exploiting Gamma-Ray Bursts for investigating the early Universe and at providing a substantial advancement of multi-messenger and time-domain astrophysics. Through an unprecedented combination of X-/gamma-rays monitors, an
        on-board IR telescope and automated fast slewing apabilities, THESEUS will be a wonderful machine for the detection, characterization and redshift measurement of any kind of GRBs and many classes of X-ray transients. In addition to the full exploitaiton of high-redshift GRBs for cosmology (pop-III stars, cosmic re-ionization, SFR and metallicity evolution up to the "cosmic dawn"), THESEUS will allow the identification and study of the electromagnetic counterparts to sources of
        gravitational waves which will be routinely detected in the late '20s / early '30s by next generation facilities like aLIGO/aVirgo, LISA, KAGRA, and Einstein Telescope (ET), as well as of most classes of transient sources, thus providing an ideal sinergy with the large e.m. facilities of the near future like LSST, ELT, TMT, SKA, CTA, ATHENA.

        Speaker: Lorenzo Amati (INAF - OAS Bologna)
    • 10:30 11:00
      Relativistic outflows from galactic sources Aula Magna Enric Casassas

      Aula Magna Enric Casassas

      Facultat de Física, Universitat de Barcelona

      C/ Martí i Franquès 1, 08028 Barcelona, Spain
      Convener: David Paneque (Max Planck Institute for Physics)
      • 10:30
        Black Hole High Mass X-ray Binary Microquasars at Cosmic Dawn 15m

        Theoretical models and observations suggest that primordial Stellar Black Holes
        (Pop-III-BHs) were prolifically formed in HMXBs, which are powerful relativistic jet sources of synchrotron radiation called Microquasars (MQs).

        Large populations of BH-HMXB-MQs at cosmic dawn produce a smooth synchrotron cos-
        mic radio background (CRB) that could account for the excess amplitude of atomic hydrogen absorption at z∼17, recently reported by EDGES.

        BH-HMXB-MQs at cosmic dawn precede supernovae, neutron stars and dust. BH-HMXB-
        MQs promptly inject into the IGM hard X-rays and relativistic jets, which overtake the slowly expanding HII regions ionized by progenitor Pop-III stars, heating and partially ionizing the IGM over larger distance scales.

        BH-HMXBs are channels for the formation of Binary-Black-Holes (BBHs). The large masses of BBHs detected by gravitational waves, relative to the masses of BHs detected by X-rays, and the high rates of BBH-mergers, are consistent with high formation rates of BH-HMXBs and BBHs in the early universe.

        Recent references on this subject:
        Mirabel, I.F. 2017, New Astron. Revs. 78, 1
        Mirabel, I.F. 2019, Invited review at IAUS 346: arXiv#1902.00511.pdf

        Speaker: Felix Mirabel (IAFE-CONICET-UBA-Argentina)
      • 10:45
        Gamma-ray emission from Cyg X-1 and Cyg X-3 15m

        In two recent papers (Zdziarski et al. 2017, 2018), we presented measurements by the Fermi/LAT and theoretical interpretation of the gamma-ray spectra from two Galactic microquasars, Cyg X-1 and Cyg X-3. In both sources, orbital modulation (due to Compton anisotropy of scattered stellar photons from the donor) has been measured, which has allowed us to estimate of the location of the bulk of the gamma-ray emission. The theoretical interpretation of the broad-band spectra (from radio to gamma-rays) is based on extended-jet and accretion models, developed earlier. We have also measured cross-correlations between the gamma-rays and both radio and X-ray emission, which put constraints on the respective emission sites and, in the case of Cyg X-3, implies the presence of extended periods of advection of magnetic fields through the accretion flow. The obtained set of results allows us to make predictions for the spectra and orbital modulation of those sources when observed by the CTA.

        Speaker: Andrzej Zdziarski
    • 11:00 11:30
      Coffee break 30m
    • 11:30 12:45
      Relativistic outflows from galactic sources Aula Magna Enric Casassas

      Aula Magna Enric Casassas

      Facultat de Física, Universitat de Barcelona

      C/ Martí i Franquès 1, 08028 Barcelona, Spain
      Convener: Marc Ribó (Universitat de Barcelona / ICCUB / IEEC-UB)
      • 11:30

        Relativistic jets are ubiquitous phenomena present in a variety of galactic sources. These jets can carry a significant fraction of the system's energy reservoir up to distances of a few tens of parsecs. Particle acceleration along the jets or at the interaction sites with the surrounding medium leads to the production of copious non-thermal emission, which is observed in a broad energy range, from radio to very-high-energy gamma-rays. While powerful galactic jets are typically associated to accretion processes in BH/NS X-ray binaries, jet-like features have recently been imaged also from isolated systems, most notably from young pulsars moving at high-speeds through the interstellar medium. In this talk I will review recent findings related to some of the most extreme examples of galactic jets, discussing the implications these findings have in our understanding of jet formation and propagation mechanisms.

        Speaker: Pol Bordas
      • 12:00
        GRS 1758-258 as a winged microquasar 15m

        The large-scale morphology of the prototypical microquasar GRS 1758-258 resembles the Z-shape of the subclass of extragalactic radio sources known as winged radio galaxies. This new quasar-microquasar analogy allows us to study the nature of the origin of this peculiar morphology, which seems to be related to the hydrodynamical back flow. Some other implications could be stated assuming that physical processes at work are similar in Galactic and extragalactic cases

        Speaker: Josep Martí (Universidad de Jaén)
      • 12:15
        Lorentz factors of compact jets in black hole X-ray binaries 15m

        Compact, continuously launched jets in black hole X-ray binaries (BHXBs) produce radio to optical–infrared (OIR) synchrotron emission. These jets are launched in the hard X-ray state, and are quenched in the soft state. They are not spatially resolved except in a few cases using VLBI radio observations. One of the basic properties of these jets is the bulk Lorentz factor, which defines how fast and how relativistic these jets are. The bulk Lorentz factor is notoriously difficult to measure, with to date only weak constraints for a few BHXBs. Here, we adopt simple models to constrain the Lorentz factor of the compact jets in several BHXBs using the amplitude of the jet fade and recovery at infrared (IR) wavelengths over state transitions. We investigate why some BHXBs have prominent IR excesses and some do not, quantified by the amplitude of the IR quenching or recovery over the transition from/to the hard state. Using the amplitude of the IR fade/recovery, known orbital parameters and simple analytical models, we constrain for the first time the Lorentz factor of compact jets in several BHXBs. We also find that the very high amplitude IR fade/recovery seen repeatedly in GX 339–4 requires a much lower inclination angle than previously expected. Our results are strongly supportive of the IR excess being produced by synchrotron emission in a relativistic outflow, and demonstrates how useful OIR monitoring of BHXB is for studying jet properties.

      • 12:30
        Inclination dependence of the time-lag – photon-index correlation in BHXRBs and its explanation with a simple jet model 15m

        It is widely accepted that the power-law X-ray spectra of black-hole X-ray binaries (BHXRBs) are produced by inverse Compton scattering in a corona, which is typically taken to be the hot inner flow around the black hole or the base of the jet. However, if a photon finds itself in the hot inner flow or the base of the jet, nothing prevents it from traveling higher up in the body of the jet. In other words, Comptonization in the hot inner flow and the base of the jet is, by necessity, followed by Comptonization in the body of the jet, because the jet is fed by the hot inner flow and lies above and below the hot inner flow. Over the years, we have demonstrated with a simple jet model that Comptonization in the body of a jet can explain not only the spectrum from radio to hard X-rays, but also the evolution of the photon index and the time (phase) lags as functions of Fourier frequency, the correlation between time lag and photon index (in Cyg X-1, in GX 339-4, and in the class of BHXRBs as a whole), and the correlation between time lag and cut-off energy in GX 339-4. Here we demonstrate observationally that the time-lag – photon-index correlation is different for systems with different inclinations. We also demonstrate theoretically, with Comptonization in the jet, that the same jet model reproduces nicely the inclination dependence of the correlation. Photons escaping the jet at different viewing angles obey different time-lag – photon-index correlations. The time lag is due to the random walk of the photons in the body of the jet.

        Speaker: Nick Kylafis (University of Crete and Institute of Astrophysics, FORTH)
    • 12:45 14:30
      Lunch Break 1h 45m
    • 14:30 15:45
      Relativistic outflows from galactic sources Aula Magna Enric Casassas

      Aula Magna Enric Casassas

      Facultat de Física, Universitat de Barcelona

      C/ Martí i Franquès 1, 08028 Barcelona, Spain
      Convener: Dr Roberta Zanin
      • 14:30

        Pulsar Wind Nebulae are highly intriguing astrophysical objects in many respects. They are the brightest and closest class of relativistic sources, and hence the ultimate laboratory for the physics of relativistic plasmas: several processes observed (or inferred to occur) in other classes of relativistic sources can here be studied with unique detail, like the acceleration and collimation of relativistic outlfows, or the acceleration of particles at relativistic shocks.

        I will review the current status of our theoretical understanding of Pulsar Wind Nebulae in light of the most recent 2D and 3D MHD modeling of these sources. I will discuss how these studies are taking us to the point when we can reliably use multi-wavelength observations of these nebulae as a diagnostics of the hidden physics of the pulsar wind and of the mechanism(s) through which particles are accelerated at the highly relativistic shock that terminates the wind. Finally I will briefly discuss the role of Pulsar Wind Nebulae as sources of cosmic ray leptons.

        Speaker: Elena Amato (INAF - Osservatorio Astrofisico di Arcetri)
      • 15:00
        The Pulsar Sequence 15m

        A decade of Fermi-LAT operations has provided a wealth of observational data that shifted the study of gamma-ray pulsars from discovery to astronomy. Moreover, recent observations from ground-based imaging atmospheric Cherenkov telescopes has revealed multi-TeV pulsed emission. The consensus from the latest theoretical modeling is that the high-energy pulsar emission is produced in the equatorial current sheet outside the light cylinder. I will discuss how the observational data along with theoretical considerations constrain the various emission processes (curvature, synchrotron, inverse Compton). I will show that the entire Fermi pulsar population (young and millisecond pulsars) lies on a Fundamental Plane that connects the total gamma-ray luminosity, the cut-off energy, the spin-down power, and the stellar surface magnetic field, which is consistent with curvature radiation emission. Nonetheless, synchrotron radiation can reproduce the lower energy (up to infrared) segment of the pulsar spectrum, while its photons can serve as the seeds that produce emission up to multi-TeV radiation in IC interactions with the high-energy curvature emitting electrons. Finally, I will present our innovative kinetic PIC models of global pulsar magnetospheres with magnetic-field-line dependent particle injection. I will show not only how our simulations validate the above description but also how the particle population, which is injected near the separatrix that separates the open from the closed field lines, regulates the Fermi-LAT gamma-ray emission. Our simulations, more accurately than ever before, reproduce the observed Fermi gamma-ray phenomenology of the millisecond and young pulsars for the entire range of spin-down powers.

        Speaker: Constantinos Kalapotharakos (NASA GSFC / UMCP)
      • 15:15
        TeV and X-ray emission from the 50-year period binary PSR J2032+4127/MT91 213 during periastron passage 15m

        We report on the observations of the pulsar/Be binary system PSR J2032+4127/MT91 213 which was recently discovered at TeV and X-rays energies with MAGIC, VERITAS and Swift-XRT. This is the second gamma-ray binary in which the nature of the compact object is known. The system was detected at TeV energies during the periastron passage, which took place in November 2017. This observation was a once-in-a-lifetime opportunity due to the 50-year orbital period of the pulsar around the Be star. Our observations covered from 18 months prior to periastron passage to one month after. This allowed to study not only the pulsar/Be interaction but also the extended source TeV J2032+4130, most likely a PWN potentially associated with the pulsar. The observations revealed a cut-off in the TeV spectrum of the binary and high flux variability on a daily basis. The X-ray observations also revealed strong variability. Significant revision of the existing models is required to explain the detected TeV emission.

        Speaker: Oscar Blanch Bigas (IFAE)
      • 15:30
        Observations of the accreting X-Ray pulsar GX 301-2 with the X-Calibur Hard X-ray polarimetry mission 15m

        Measurements of polarization of the X-ray emission in accreting neutron stars probe the particle acceleration region and can distinguish between fan-beam and pencil-beam approximations of the geometry of the accretion column.
        In Dec 29th 2018, NASA’s hard X-ray polarimetry mission X-Calibur was launched on a 2.5-day stratospheric balloon flight from McMurdo station in Antarctica. During the campaign, X-Calibur observed the high-mass X-ray binary GX 301-2 in a rare flaring state at orbital phase ~0.35, close to apastron, resulting in a clear detection of GX 301-2 in the hard X-ray band. The ability of X-Calibur to measure the azimuthal scattering angle of the incoming X-rays resulted in a measurement of a 22% +/- 14% polarization fraction of the hard X-ray emission form GX 301-2 during the main peak of its 685s-long pulse profile. The measured polarization agrees better with fan-beam emission predictions (~20%) than pencil beam scenarios (~0%).
        We will report on the results of the X-Calibur observations of GX 301-2 including a 15-60 keV light curve, hard X-ray energy spectrum, phase-resolved analysis, and constraints on linear polarization. We will also present contemporaneous observations of GX 301-2 in the soft X-ray band with the Neil Gehrels Swift observatory and NICER X-ray telescope that show significant variability of the shape of the pulse profile during the flaring event. We will conclude with a discussion of how neutron star, accretion, and fundamental physics can be constrained with future deeper polarimetric observations of accreting X-ray pulsars.

        Speaker: Manel Errando (Washington University in St Louis)
    • 15:45 16:15
      Coffee break 30m
    • 16:15 17:00
      Relativistic outflows from galactic sources Aula Magna Enric Casassas

      Aula Magna Enric Casassas

      Facultat de Física, Universitat de Barcelona

      C/ Martí i Franquès 1, 08028 Barcelona, Spain
      Convener: Josep Martí (Universidad de Jaén)
      • 16:15

        Flaring activity above 100 MeV was serendipitously detected from the Crab nebula by Agile and Fermi-LAT in 2010 . Since then, a tens of flaring events have been observed by Fermi-LAT showing different spectral and flux behaviours within each other. In the attempt of identifying the exact site of this enhanced emission observations by high-resolution lower-frequency instruments were triggered, although all unsuccessful so far. In this talk I will review the results of the observed Crab flares obtained by Fermi-LAT, and the subsequent multi-wavelength campaigns. These results will be discussed in the light of the state-of-the-art theoretical models. I will conclude by discussing the capabilities of future instruments to constrain the origin of these still enigmatic phenomena.

        Speaker: Roberta Zanin (CTAO)
      • 16:45
        Gamma-rays and positrons from colliding wind binaries 15m

        Fermi-LAT and HESS detection of $\eta$ Carinae confirm that diffuse shock acceleration occurs in the complex geometry of the wind collision zone. Hydrodynamic simulations provide a convincing match with the observations if a few percent of the wind mechanical energy dissipated in the shock goes into particle acceleration. The intrinsic $\pi_0$ decay spectrum is a complex convolution of the maximum energy, luminosity, particle drift and obscuration. Accelerated particles cool down mainly via inverse-Compton, synchrotron radiation, and photo-pion production. If the final spectra of accelerated electrons from primary side and secondary side overlap significantly and have similar luminosities, the final $\gamma$-ray spectra will not present the observed two bumps. High-energy $\gamma$-rays interact also with the pool of anisotropic UV photons emitted by both luminous stars, creating $e^\pm$ pairs and strongly modifying the observed spectrum. Quick variations of the optical depth are expected along the orbit, due to changes in shape, position, and gas density of the shocked region. Various CTA simulations confirm that flux variabilities down to few days timescale could be detected above 30 GeV. These variations will disentangle the intrinsic particle spectral cut-off from that related to $\gamma$-$\gamma$ opacity and determine the flux of relativistic protons and positrons injected in the interstellar medium, the geometry of the colliding wind region and the magnetic field configuration, as well as the geometrical orientation of the binary system. CTA will also enlighten the nature of the high-energy component, the mechanisms and the percentage of kinetic energy channelled into particle acceleration.

        Speaker: Matteo Balbo (Université de Genève)
    • 17:00 17:30
      Multiwavelength and multi-messenger aspects Aula Magna Enric Casassas

      Aula Magna Enric Casassas

      Facultat de Física, Universitat de Barcelona

      C/ Martí i Franquès 1, 08028 Barcelona, Spain
      • 17:00
        Synchrotron maser from weakly magnetised neutron stars as the emission mechanism of fast radio bursts 15m

        The origin of Fast Radio Bursts (FRBs) is still mysterious. All FRBs to date show extremely high brightness temperatures, requiring a coherent emission mechanism. Using constraints derived from the physics of one of these mechanisms, the synchrotron maser, as well as observations, we show that accretion induced explosions of neutron stars with surface magnetic fields of $B_*$<$10^{11}$ G are favoured as FRB progenitors.

        Speaker: Killian Long (University College Cork)
      • 17:15
        The Impact of Plasma Instabilities on the Constraints on the Intergalactic Magnetic Field 15m

        A relativistic electron–positron pair beam can be produced in the interaction between TeV photons from a blazar and extragalactic background light. The relativistic e ± pairs lose energy through inverse-Compton scattering (ICS) photons of the cosmic microwave background or plasma instabilities. The dominant energy-loss process is under debate. Based on the assumption that the dominant energy-loss process is ICS, the resulting cascade GeV radiation is usually used to constrain the intergalactic magnetic field (IGMF). Here, we include the energy-loss due to plasma oblique instability in the calculation of cascade gamma-ray flux, and investigate the impact of the plasma instability on the constraint of IGMF. Up-to-date GeV data and archival TeV data of the blazar 1ES 0229+200 are used. The results indicate that even if the oblique instability cooling is dominating ICS cooling, the cascade flux could still be used to constrain the IGMF.

        Speaker: Dahai Yan
    • 20:00 22:00
      Conference dinner 2h Font de Prades Restaurant

      Font de Prades Restaurant

      Font de Prades Restaurant: Poble Espanyol, Av. Francesc Ferrer i Guàrdia, 13-27, 08038 Barcelona

      The dinner will take place at the restaurant "Font de Prades", located in the Poble Espanyol open-air museum. The Poble Espanyol is easily reachable with public transport. Get to "Plaça d'Espanya" either by bus or using the green or red metro lines, metro stop "Espanya". From there, a 10 min walk up the Montjuïc hill will bring you to the Poble Espanyol entrance. Show your ticket and you will have access to the open-air museum. Follow indications to the restaurant, which is located at the northern-east corner of Poble Espanyol.

    • 09:30 11:00
      Multiwavelength and multi-messenger aspects Aula Magna Enric Casassas

      Aula Magna Enric Casassas

      Facultat de Física, Universitat de Barcelona

      C/ Martí i Franquès 1, 08028 Barcelona, Spain
      Convener: Gustavo E. Romero (Instituto Argentino de Radioastronomía (IAR))
      • 09:30

        Abstract: Abstract: Fast radio bursts (FRBs), bright millisecond duration radio transients, are quickly becoming a subject of intense interest in time-domain and high energy astrophysics. FRBs have the exciting potential to be used as cosmological probes of both matter and fundamental parameters, but such studies require large populations. Advances in FRB detection using current and next-generation radio telescopes will enable the growth of the population in the next few years from a few dozen to hundreds. Real-time discovery and follow-up, and new studies of the FRB population will provide us with some of the greatest insights in the coming years. I will discuss many observational aspects of the FRB population, including polarisation, searches for multi-wavelength emission, localisation, and repeating FRBs. I will also discuss how multi-wavelength and multi-messenger astrophysics with FRBs can be maximised with future facilities.

        Speaker: Emily Petroff (University of Amsterdam, Anton Pannekoek Institute )
      • 10:00
        Speaker: Jo van den Brand (Nikhef, Amsterdam University)
      • 10:30
        Resolving a decades-long transient: an orphan long gamma-ray burst or a young magnetar nebula? 15m

        The source FIRST J1419+3940 was recently discovered as a slow transient that has been fading for the last 30 years. The radio light-curve is consistent with an orphan long gamma-ray burst. However, our interest arises when comparing its host to the one where the Fast Radio Burst FRB 121102 is located: inside a low-metallicity star-forming region in a dwarf galaxy. Both sources show comparable compact radio emission and similar environments. We thus believe that FIRST J1419+3940 can be a potential host for FRBs, and could make a connection between long GRBs and FRBs. In this talk we present the results from observations conducted with the European VLBI Network (EVN) which allowed us to study the compactness of the source to unveil its origin. These results allowed us to discard several scenarios for the fading source, and to establish comparisons with the persistent counterpart of FRB 121102.

        Speaker: Benito Marcote (Joint Institute for VLBI ERIC (JIVE))
      • 10:45
        Magnetorotational supernovae and gravitational waves 15m

        We consider magnetorotational(MR) core-collapse supernova explosion mechanism.
        Numerical simulations show that the shape of the MR explosion depends on the initial configuration of the magnetic field. The explosion can develop preferably near equatorial plane (quadrupole-like field) or as a mildly collimated jet (dipole-like field). We have estimated the dimensionless amplitude of the gravitational wave with a frequency ~ 1300 Hz, radiated during the collapse of the rotating core of a pre-supernova with a mass of 1,2Msun. This estimate agrees well with many other calculations that have been done in 2D and 3D settings and which rely on more exact and sophisticated calculations of the gravitational wave amplitude.

        Speaker: Sergey Moiseenko
    • 11:00 11:30
      Coffee break 30m
    • 11:30 12:45
      Multiwavelength and multi-messenger aspects Aula Magna Enric Casassas

      Aula Magna Enric Casassas

      Facultat de Física, Universitat de Barcelona

      C/ Martí i Franquès 1, 08028 Barcelona, Spain
      Convener: Matteo Cerruti (Universitat de Barcelona / ICCUB)
      • 11:30

        Astrophysical neutrinos, the “ghost particles of the Cosmos”, are unique probes of the physical conditions in their sources, as they can escape from them and reach Earth almost unimpeded due to their extremely weak interactions with matter and radiation. High-energy neutrinos, which are mainly produced by inelastic collisions of relativistic protons or heavier nuclei with radiation or matter, can be used to trace the elusive sources of cosmic rays and to shed light on the underlying particle acceleration mechanisms. The recent discovery of a high-energy neutrino coincident with an electromagnetic flare from a blazar (TXS 0506+056) offers a unique opportunity to probe the physics of these powerful extragalactic sources using multiple messengers (i.e., photons and neutrinos). In this talk, I will review the lessons learned and the challenges created from the first high-energy neutrino source association to date.

        Speaker: Maria Petropoulou (Princeton University)
      • 12:00
        Photo-hadronically produced neutrinos from blazars 15m

        The tentative identification of the gamma-ray bright BL Lac object
        TXS 0506+05 with very-high-energy neutrinos detected by IceCube
        triggered a large number of works on the physics implications of
        neutrino production in blazar jets. Most of these works agree that
        GeV - TeV gamma-rays are unlikely to be produced by the same hadronic
        processes generating the IceCube neutrinos in the same emission region.
        In previous work, we had developed a method to derive general constraints
        on the physical conditions in the neutrino emission region and multi-wavelength
        electromagnetic signatures expected to go in tandem with such neutrino
        production. Our main conclusion is that, in order to produce IceCube
        neutrinos at the level of the 2014-15 neutrino flare from the direction
        of TXS 0506+056 (accompanied only by a slightly harder Fermi-LAT spectrum,
        but no flux enhancement), an intense UV - soft X-ray radiation field external
        to the jet is required as target photon field for photo-pion interactions.
        This result necessitates the proper treatment of anisotropies of the
        target photon field in the frame of the emission region. In this talk,
        we will briefly summarize previous work and present first results of
        our study of the effects of anisotropies on the resulting neutrino
        yield and electromagnetic signatures.

        Speaker: Markus Boettcher (North-West University)
      • 12:15
        Neutrinos from TXS 0506+056 15m

        We used archival data to perform a detailed analysis
        of the time evolution of the jet of TXS 0506+056.
        In this talk we will discuss the specifics of the jet
        kinematics and in particular its possible relation to
        neutrino emission.
        A paper describing the results has just been submitted.

        Speaker: Silke Britzen (Max-Planck-Institut für Radioastronomie, Bonn, Germany)
      • 12:30
        High-energy neutrino and gamma-ray emission from AGN-driven winds 15m

        Various observations are revealing the widespread occurrence of fast and powerful winds in active galactic nuclei (AGN) that are distinct from relativistic jets, likely launched from accretion disks. On sufficiently large scales, they are expected to interact with the gas of their host galaxies, leading to strong shocks that can accelerate nonthermal particles to high energies. Such winds have been suggested to be responsible for a large fraction of the observed extragalactic gamma-ray background (EGB) and the diffuse neutrino background, via the decay of neutral and charged pions produced in pp interactions between protons accelerated by the forward shock and the ambient gas. Taking into account processes such as adiabatic losses that were not properly included in earlier studies, we evaluate the production of gamma rays and neutrinos by AGN-driven winds in detail by modeling their hydrodynamic and thermal evolution. We find that they can only account for less than ~30% of the EGB flux, as otherwise the model would violate the independent upper limit derived from the diffuse isotropic gamma-ray background. We also identify ranges of parameters such as the spectral index for which AGN winds may still make an important contribution to the diffuse neutrino flux. Furthermore, we discuss the possibility of neutrino production via p-gamma interactions in the inner regions of the wind near the nucleus.

        Speaker: Susumu Inoue (RIKEN)
    • 12:45 13:15
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