Speaker
Description
The North Polar Spur (NPS) is a giant structure that is clearly observed in both radio and X-ray all-sky maps. Although half a century has passed since its discovery, two competing ideas are still being actively debated to postulate its origin: one considers a local super-bubble near the solar system,and the other is based on a remnant of AGN and/or starburst outflow from the Galactic Center (GC) over 10 Myr ago. In this context, the recent discovery of gamma-ray Fermi bubbles, as well as even larger X-ray eROSITA bubbles, may suggest a possible connection between the NPS and these large structures. In this study, we analyzed broad-band radio observations covering a range between 22 MHz (VLA) to 70 GHz (Planck) for the first time, to provide a systematic analysis of thermal/non-thermal emissions associated with the NPS. We show that the radio emission of the NPS is composed of (1) synchrotron radiation, (2) free-free radiation, and (3) dust emission, but the synchrotron emission dominates over other emissions at high galactic latitudes. In most regions, the electron spectrum indicates a power-low relationship with its index, s, of $N(\gamma)\propto \gamma ^{-s}$ ($s\approx2.2-3.0$), moderated by a high-energy turnover cutoff around $\gamma \sim 10^{4}$ ($E\sim10$ GeV) ; this indicates that radio-emitting electrons are already cooled within the NPS. When assuming a typical magnetic field strength of $B \approx 5$ $\mu G$, the resultant cooling time is $\sim 10$ Myr, which provides additional support that the NPS is a structure within the GC. We estimated the non-thermal energy stored in the NPS to be $\sim 2.6\times10^{55}$ $[erg]$ in case of the GC. We also estimated that gamma-ray emission associated with the NPS, through inverse Comptonization of the CMB, peaks at approximately 100-1000 keV, with a flux of $\sim 10^{-9}$ $[erg/cm^{2}/s/str]$, which may be a good candidate for future detection by the Athena X-ray observatory.
