Temporary EHBL-like behavior of Markarian 501 during July 2014 VHE flaring

TL;DR

This paper analyzes the July 2014 VHE gamma-ray flaring of Markarian 501, explaining the observed peak-like feature with a two-zone photohadronic model, highlighting its temporary EHBL-like behavior.

Contribution

It applies a two-zone photohadronic model to explain the transient VHE gamma-ray features of Markarian 501 during a specific flare event.

Findings

The peak-like feature at 3.18 TeV is explained by the model as a cutoff energy.

The spectrum shows a slow increase below 3.18 TeV and a faster fall above it.

The observed feature is mild and not as prominent as previously claimed.

Abstract

Markarian 501, a BL Lac object well-known as a high energy gamma-ray source, has exhibited several epochs of very high energy (VHE) gamma-ray flaring events when its synchrotron peak frequency shifted above $10^{17}$ Hz, a signature of extreme behavior. From July 16 to July 31, 2014 such flaring events were observed for 15 days by various telescopes. On July 19 (MJD 56857.98), the X-ray outburst from the source was at its highest and on the same day an intriguing narrow peak-like feature around 3 TeV was observed by the MAGIC telescopes, a feature inconsistent with standard interpretations. Using the well-known two-zone photohadronic model, we study these VHE gamma-ray spectra on a day-by-day basis and offer explanation. Our two-zone photohadronic scenario shows that, on MJD 56857.98, the peak-like feature appears at a cutoff energy of $E^c_{\gamma}=3.18$ TeV. Below this energy the VHE spectrum increases slowly and is in high emission state. However, for $E^c_{\gamma}\, > 3.18$ TeV, the spectrum falls faster, resulting in a mild peak-like feature, not prominent enough as claimed by the MAGIC collaboration.