Very High Energy outburst of Markarian 501 in May 2009

TL;DR

This paper reports on a major very high energy gamma-ray outburst of blazar Markarian 501 in May 2009, highlighting rapid flux variability, polarization changes, and multi-wavelength observations that provide insights into the flare's characteristics.

Contribution

It presents a detailed multi-wavelength observational analysis of a significant VHE flare in Markarian 501, including polarization and spectral variability, which advances understanding of blazar emission mechanisms.

Findings

VHE flux increased to 10 times the baseline during the flare.

Rapid flux variation with a factor of 4 increase in 25 minutes.

Polarization angle rotated by 15 degrees during the flare.

Abstract

The very high energy (VHE; E $>$ 100 GeV) blazar Markarian 501 was observed between April 17 and May 5 (MJD 54938--54956), 2009, as part of an extensive multi-wavelength campaign from radio to VHE. Strong VHE $\gamma$-ray activity was detected on May 1st with Whipple and VERITAS, when the flux (E $>$ 400 GeV) increased to 10 times the pre-flare baseline flux ($3.9{\times 10^{-11}}~{\rm ph~cm^{-2}~s^{-1}}$), reaching five times the flux of the Crab Nebula. This coincided with a decrease in the optical polarization and a rotation of the polarization angle by 15$^{\circ}$. This VHE flare showed a fast flux variation with an increase of a factor $\sim$4 in 25 minutes, and a falling time of $\sim$50 minutes. We present the observations of the quiescent state previous to the flare and of the high state after the flare, focusing on the flux and spectral variability from Whipple, VERITAS, Fermi-LAT, RXTE, and Swift combined with optical and radio data.