Search for an extended VHE gamma-ray emission from Mrk 421 and Mrk 501 with the MAGIC Telescope

TL;DR

This study used the MAGIC telescope to search for extended gamma-ray emission around blazars Mrk 421 and Mrk 501, setting upper limits that inform constraints on extragalactic magnetic fields and the nature of cascade emissions.

Contribution

The paper introduces a new analysis method with improved angular resolution to detect or constrain extended gamma-ray emission around extragalactic sources.

Findings

Upper limits on extended emission are <5% and <4% of Crab Nebula flux for Mrk 421 and Mrk 501.

Constraints on extragalactic magnetic field strength are discussed, around a few times 10^{-15} G.

A new multidimensional decision tree method enhances the detection sensitivity for extended gamma-ray halos.

Abstract

Context: Part of the very high energy $\gamma$-ray radiation coming from extragalactic sources is absorbed through the pair production process on the extragalactic background light photons. Extragalactic magnetic fields alter the trajectories of these cascade pairs and, in turn, convert cosmic background photons to gamma-ray energies by inverse Compton scattering. These secondary photons can form an extended halo around bright VHE sources. Aims: We searched for an extended emission around the bright blazars Mrk 421 and Mrk 501 using the MAGIC telescope data. Methods: If extended emission is present, the angular distribution of reconstructed gamma-ray arrival directions around the source is broader than for a point-like source. In the analysis of a few tens of hours of observational data taken from Mrk 421 and Mrk 501 we used a newly developed method that provides better angular resolution. This method is based on the usage of multidimensional decision trees. Comparing the measured shapes of angular distributions with those expected from a point-like source one can detect or constrain possible extended emission around the source. We also studied the influence of different types of systematic errors on the shape of the distribution of reconstructed gamma-ray arrival directions for a point source. Results: We present upper limits for an extended emission calculated for both sources for various source extensions and emission profiles. We discuss possible constraints on the extragalactic magnetic fields strength. We obtain upper limits on the extended emission around the Mrk~421 (Mrk~501) on the level of < 5% (< 4%) of the Crab Nebula flux above the energy threshold of 300 GeV. Using these results we discuss possible constraints on the extragalactic magnetic fields strength around a few times $10^{-15}$~G.