Discovery of VHE Gamma-ray Emission from the Starburst Galaxy M82

TL;DR

The paper reports the first detection of very high energy gamma-ray emission from the starburst galaxy M82, confirming theoretical predictions and linking star formation activity to cosmic-ray production.

Contribution

This work provides the first observational evidence of VHE gamma rays from M82, a starburst galaxy, validating models of cosmic-ray origins in such environments.

Findings

Detection of gamma rays from M82 at 5 sigma significance

Measured flux is 0.9% of the Crab Nebula flux above 700 GeV

Gamma-ray spectrum follows a power law with index 2.5

Abstract

The galaxy M82 has long been considered a promising target for VHE gamma-ray observations because of the compact starburst region in its core. Theoretical predictions have suggested it should be detectable by ground-based imaging Cherenkov telescopes like VERITAS and that a detection would have implications for the understanding of the origin of cosmic rays. M82 was observed with the VERITAS array during the 2007-2009 observing seasons. With an exposure of 137 hours, VERITAS was able to detect a gamma-ray signal at the 5 sigma level. This marks the discovery of gamma rays not only from M82 but also from the new source class of starburst galaxies. The observed flux from M82 is 3.7\pm 0.8(stat)\pm 0.7(syst)x10^-13 photons cm^-2 s^-1 above an energy threshold of 700 GeV, which corresponds to 0.9% of the Crab Nebula flux. The differential energy spectrum is a power law with a photon index Gamma=2.5\pm 0.6(stat)\pm 0.2(syst). Both the flux and the photon index is close to recent theoretical predictions. The VERITAS data places a strong correlation between the star-formation activity and the cosmic-ray production in M82.