Emitting electrons and source activity in Markarian 501

TL;DR

This study models the spectral energy distribution of Mrk 501 across different activity states using a one-zone SSC model, revealing complex variability patterns and differences from similar sources like Mrk 421.

Contribution

It provides a detailed analysis of how SSC model parameters vary with source activity and compares these variations with those in Mrk 421, highlighting differences in jet properties.

Findings

The Compton component varies between Thomson and Klein-Nishina regimes.

Electron spectra are softer in quiescent states and harder during flares.

Jet energy density remains particle dominated across all states.

Abstract

We study the variation of the broad-band spectral energy distribution (SED) of the BL Lac object Mrk 501 as a function of source activity, from quiescent to flaring. Through chi-square-minimization we model eight simultaneous SED datasets with a one-zone Synchrotron-Self-Compton (SSC) model, and examine how model parameters vary with source activity. The emerging variability pattern of Mrk 501 is complex, with the Compton component arising from gamma-electron scatterings that sometimes are (mostly) Thomson and sometimes (mostly) extreme Klein-Nishina. This can be seen from the variation of the Compton to synchrotron peak distance according to source state. The underlying electron spectra are faint/soft in quiescent states and bright/hard in flaring states. A comparison with Mrk 421 suggests that the typical values of the SSC parameters are different in the two sources: however, in both jets the energy density is particle dominated in all states.