Unprecedented temporal evolution of the broad-band emission of the BL Lac Mrk 501

TL;DR

This study presents a detailed multi-instrument observation of Mrk 501, revealing unprecedented temporal evolution in its broad-band emission and unique flare behaviors, providing new insights into blazar jet physics.

Contribution

First detailed multi-wavelength campaign capturing complex flare behaviors and polarization changes in Mrk 501, highlighting similarities across blazar subclasses.

Findings

Detected two VHE flares with distinct characteristics.

Observed a polarization angle rotation during a flare.

Identified a shift in the synchrotron bump during a flare.

Abstract

The TeV BL Lac object Markarian 501 has been the subject of a 4.5 month multi-instrument campaign conducted in 2009, which provided an excellent temporal and energy coverage from radio to very high energy gamma rays (>100 GeV, VHE). During the campaign, Mrk 501 was mostly in a comparably low state, but for two flares at VHE with very different characteristics. While the second flare seems to be correlated with a moderate increase in the X-ray flux, the first one is most likely accompanied by a shift of the synchrotron bump towards higher energies. Moreover, the first flare occurs during an abrupt change in the polarized optical flux, and was preceded by a rotation of the electric field vector position angle. This is the first time that such behavior is observed in a high-frequency-peaked BL Lac object, while similar events have been seen in the low-frequency peaked BL Lac object BL Lacertae and the flat spectrum radio quasar PKS 1510-089, hence suggesting that similar physical processes occur in the jets of different blazar subclasses.