Gamma-ray excess from a stacked sample of high- and intermediate-frequency peaked blazars observed with the MAGIC telescope

TL;DR

This study stacked observations of 21 blazars observed with MAGIC, revealing a significant gamma-ray excess at energies above 150 GeV, indicating a faint, persistent gamma-ray emission from these sources.

Contribution

First to perform a stacking analysis on non-detected blazars with MAGIC, uncovering evidence for a faint, steady gamma-ray signal across the sample.

Findings

Evidence for a 4.9 sigma gamma-ray excess in stacked data

The excess corresponds to a flux of about 1.5% of the Crab Nebula

The gamma-ray emission appears steady over the observation period

Abstract

Between 2004 and 2009 a sample of 28 X-ray selected high- and intermediate-frequency peaked blazars with a X-ray flux larger than 2 uJy at 1 keV in the redshift range from 0.018 to 0.361 was observed with the MAGIC telescope at energies above 100 GeV. Seven among them were detected and the results of these observations are discussed elsewhere. Here we concentrate on the remaining 21 blazars which were not detected during this observation campaign and present the 3 sigma (99.7 %) confidence upper limits on their flux. The individual flux upper limits lie between 1.6 % and 13.6 % of the integral flux from the Crab Nebula. Applying a stacking method to the sample of non-detections with a total of 394.1 hours exposure time, we find evidence for an excess with a cumulative significance of 4.9 standard deviations. It is not dominated by individual objects or flares, but increases linearly with the observation time as for a constant source with an integral flux level of ~1.5 % of that observed from the Crab Nebula above 150 GeV.