Very High Energy Gamma-Ray Observations of a Strong Flaring Activity in M87 in 2008 February

TL;DR

This paper reports the detection of a rapid, day-scale gamma-ray flare from M87 in 2008, with flux variability suggesting a very compact emission region or high Doppler boosting, providing insights into the source's energetic processes.

Contribution

First observation of rapid VHE gamma-ray variability in M87, constraining the size and location of the emission region during a flare.

Findings

Flux varied on a 1-day timescale with a significance of 5.6σ.

The energy spectrum follows a power law with photon index 2.30.

Flux at lower energies remained constant during the flare.

Abstract

M87 is the only known non blazar radio galaxy to emit very high energy (VHE) gamma-rays. During a monitoring program of M87, a rapid flare in VHE gamma-rays was detected by the MAGIC telescope in early 2008. The flux was found to be variable above 350 GeV on a timescale as short as 1 day at a significance level of $5.6\sigma$. The highest measured flux reached 15% of the Crab Nebula flux. We observed several substantial changes of the flux level during the 13 day observing period. The flux at lower energies (150 -- 350 GeV), instead, is compatible with being constant. The energy spectrum can be described by a power law with a photon index of $2.30 \pm 0.11_\mathrm{stat} \pm 0.20_\mathrm{syst}$. The observed day-scale flux variability at VHE prefers the M87 core as source of the emission and implies that either the emission region is very compact (just a few Schwarzschild radii) or the Doppler factor of the emitting blob is rather large in the case of a non expanding emission region.