A two-zone photohadronic scenario for EHBL-like behavior of Mrk 501

TL;DR

This paper introduces a two-zone photohadronic model to explain extreme high-energy flaring events in Mrk 501, addressing limitations of the standard model and suggesting these events are transient phenomena.

Contribution

The paper proposes a novel two-zone photohadronic scenario with a new high-energy spectral index to explain extreme flaring behavior in Mrk 501.

Findings

Standard model inadequate for extreme flares

Two-zone model fits observed spectra

Extreme events are transient and possibly repeat

Abstract

Major outbursts have been observed from the well-known high-energy peaked blazar Markarian 501 since its discovery in 1996. Especially, two episodes of very high energy gamma-ray flaring events during May-July 2005 and June 2012 are of special significance, when the source exhibited extreme HBL-like behavior. The successful standard photohadronic model seems inadequate to explain these extraneous behaviors. We propose a two-zone photohadronic scenario to overcome this problem. In this picture, the low energy regime (zone-1) of the spectrum follows the standard photohadronic interpretation, while the high energy regime (zone-2) of the spectrum is new, with a spectral index $\delta_2\geq 3.1$, which is solely due to the extreme nature of the flaring event. We also estimate the bulk Lorentz factor corresponding to these extreme flaring events. By analyzing many flaring events before and after these extreme events we argue that the extreme HBL-like events are transient and may repeat in future.