Long-term multi-band photometric monitoring of Mrk 501^⋆

¹ Institute for Particle Physics and Astrophysics, ETH Zurich, Otto Stern Weg 5, 8093 Zurich, Switzerland
² Fakultät Physik, Technische Universität Dortmund, Otto Hahn Str. 4a, 44227 Dortmund, Germany
³ Department of Astronomy, University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland
e-mail: vitalii.sliusar@unige.ch, roland.walter@unige.ch
⁴ Institut für Theoretische Physik und Astrophysik, Universität Würzburg, Emil Fischer Str. 31, 97074 Würzburg, Germany
⁵ Physikalisches Institut III A, RWTH Aachen University, Otto Blumenthal Str., 52074 Aachen, Germany

Received: 27 July 2021
Accepted: 23 August 2021

Abstract

Aims. Radio-to-TeV observations of the bright nearby (z = 0.034) blazar Markarian 501 (Mrk 501), performed from December 2012 to April 2018, are used to study the emission mechanisms in its relativistic jet.

Methods. We examined the multi-wavelength variability and the correlations of the light curves obtained by eight different instruments, including the First G-APD Cherenkov Telescope (FACT), observing Mrk 501 in very high-energy (VHE) gamma-rays at TeV energies. We identified individual TeV and X-ray flares and found a sub-day lag between variability in these two bands.

Results. Simultaneous TeV and X-ray variations with almost zero lag are consistent with synchrotron self-Compton (SSC) emission, where TeV photons are produced through inverse Compton scattering. The characteristic time interval of 5−25 days between TeV flares is consistent with them being driven by Lense-Thirring precession.