Speaker
Description
Multi-wavelength light curves in long-term campaigns show that, for
several blazars, the radio emission occurs with a significant delay with respect
to the $\gamma$-ray band, with timescales ranging from weeks to years. Such
observational evidence has long been a matter of debate and is usually
interpreted as a signature of the $\gamma$-ray emission originating upstream in the
jet, with the emitting region becoming radio transparent at larger scales.
We show, by means of self-consistent numerical modelling, that
the adiabatic expansion of a relativistic blob can explain these delays,
reproducing lags compatible with the observed timescales. We use the
JetSeT framework to reproduce the numerical modelling of the radiative and
accelerative processes, reproducing the temporal evolution of a single blob,
from the initial flaring activity and the subsequent expansion, following the
spectral evolution and the corresponding light curves, investigating the
relations among the observed parameters, rise time, delay, and decay time, and
we identify the link with physical parameters. We find that, when
adiabatic expansion is active, lags due to the shift of the synchrotron
frequency occur. The corresponding time lags have an offset equal to the
distance in time between the flaring onset and the beginning of the expansion,
whilst the rising and decaying timescales depend on the velocity of the
expansion and on the time required for the source to exhibit a synchrotron
self-absorption frequency below the relevant radio spectral window. We derive an
inter-band response function, embedding the parameters mentioned above, and we
investigate the effects of the competition between radiative and adiabatic
cooling timescales on the response. We apply the response function to long-term
radio and γ−ray light curves of Mrk 421, Mrk 501, and 3C 273, finding satisfactory
agreement on the long-term behaviour, and we use a Monte Carlo Markov Chain approach to
estimate some relevant physical parameters. We discuss applications of the presented analysis
to polarisation measurements and to jet collimation profile kinematics. The collimation
profiles observed in radio images agree with the prediction from our model.
