Search for gamma-ray emission from the Coma Cluster with six years of Fermi-LAT data

The Fermi-LAT Collaboration, Y. Rephaeli

Published 2015-07-31Version 1

We present results from gamma-ray observations of the Coma cluster incorporating 6 years of Fermi-LAT data and the newly released "Pass 8" event-level analysis. Our analysis of the region reveals low-significance residual structures within the virial radius of the cluster that are too faint for a detailed investigation with the current data. Using a likelihood approach that is free of assumptions on the spectral shape we derive upper limits on the gamma-ray flux that is expected from energetic particle interactions in the cluster. We also consider a benchmark spatial and spectral template motivated by models in which the observed radio halo is mostly emission by secondary electrons. In this case, the median expected and observed upper limits for the flux above 100 MeV are 1.7x10-9 ph/cm2/s and 4.2x10-9 ph/cm2/s respectively. These bounds are comparable to several recent predictions for the hadronic gamma-ray emission in such secondary models, although direct comparisons are sensitive to assumptions regarding the magnetic field strength and other factors. The minimal expected gamma-ray flux from radio and star-forming galaxies within the Coma cluster is roughly an order of magnitude below the median sensitivity of our analysis.