Spectral characteristics of Mrk 501 during the 2012 and 2014 flaring states

TL;DR

This paper analyzes the spectral and flux variability of Mrk 501 during 2012 and 2014 flaring states using H.E.S.S. observations, revealing spectral hardness changes, high-energy behavior, and implications for jet physics and fundamental physics.

Contribution

It provides detailed spectral and variability analysis of Mrk 501 during flaring states, extending energy coverage above 20 TeV and exploring implications for EBL, LIV, and jet physics.

Findings

Spectral index of 2.15 during 2014 flare

Flux-dependent spectral hardening observed

Detection of variability on four-minute timescales

Abstract

Observations at Very High Energies (VHE, E > 100 GeV) of the BL Lac object Mrk 501 taken with the High Energy Stereoscopic System (H.E.S.S.) in four distinct periods between 2004 and 2014 are presented, with focus on the 2012 and 2014 flaring states. The source is detected with high significance above $\sim$ 2 TeV in $\sim$ 13.1 h livetime. The observations comprise low flux states and strong flaring events, which in 2014 show a flux level comparable to the 1997 historical maximum. Such high flux states enable spectral variability and flux variability studies down to a timescale of four minutes in the 2-20 TeV energy range. During the 2014 flare, the source is clearly detected in each of these bins. The intrinsic spectrum is well described by a power law of index $\Gamma=2.15\pm0.06$ and does not show curvature in this energy range. Flux dependent spectral analyses show a clear harder-when-brighter behaviour. The high flux levels and the high sensitivity of H.E.S.S. allow studies in the unprecedented combination of short timescales and an energy coverage that extends significantly above 20 TeV. The high energies allow us to probe the effect of EBL absorption at low redshifts, jet physics and LIV. The multiwavelength context of these VHE observations is presented as well.