Variable Very High Energy γ‐Ray Emission from Markarian 501

TLDR

The observed variability can be explained by a synchrotron self‑Compton model in which fresh high‑energy electrons boost the γ‑ray flux. The blazar Mrk 501 was monitored above 0.10 TeV with the MAGIC Telescope from May to July 2005, allowing night‑by‑night flux and spectrum determination. During the campaign the flux varied by an order of magnitude, with intranight doubling times as short as 2 min—the fastest seen for this source—and a 20‑min flare on July 9 showed a 4‑min lag between low‑ and high‑energy peaks, while spectral hardening and peaks at 0.43 TeV (June 30) and 0.25 TeV (July 9) correlated with luminosity and were absent on other nights.

Abstract

The blazar Mrk 501 was observed at energies above 0.10 TeV with the MAGIC Telescope from 2005 May through July. The high sensitivity of the instrument enabled the determination of the flux and spectrum of the source on a night-by-night basis. Throughout our observational campaign, the flux from Mrk 501 was found to vary by an order of magnitude. Intranight flux variability with flux-doubling times down to 2 minutes was observed during the two most active nights, namely, June 30 and July 9. These are the fastest flux variations ever observed in Mrk 501. The ~20 minute long flare of July 9 showed an indication of a 4 ± 1 minute time delay between the peaks of F(<0.25 TeV) and F(>1.2 TeV), which may indicate a progressive acceleration of electrons in the emitting plasma blob. The flux variability was quantified for several energy ranges and found to increase with the energy of the γ-ray photons. The spectra hardened significantly with increasing flux, and during the two most active nights, a spectral peak was clearly detected at 0.43 ± 0.06 and 0.25 ± 0.07 TeV, respectively, for June 30 and July 9. There is no evidence of such a spectral feature for the other nights at energies down to 0.10 TeV, thus suggesting that the spectral peak is correlated with the source luminosity. These observed characteristics could be accommodated in a synchrotron self-Compton framework in which the increase in γ-ray flux is produced by a freshly injected (high energy) electron population.