Deep observation of the NGC 1275 region with MAGIC: search of diffuse γ-ray emission from cosmic rays in the Perseus cluster

Clusters of galaxies are expected to be reservoirs of cosmic rays (CRs) that should produce diffuse gamma-ray emission due to their hadronic interactions with the intra-cluster medium. The nearby Perseus cool-core cluster, identified as the most promising target to search for such an emission, has been observed with the MAGIC telescopes at very-high energies (VHE, E greater than or similar to 100GeV) for a total of 253 h from 2009 to 2014. The active nuclei of NGC1275, the central dominant galaxy of the cluster, and IC310, lying at about 0.6 degrees from the centre, have been detected as point-like VHE gamma-ray emitters during the first phase of this campaign. We report an updated measurement of the NGC1275 spectrum, which is described well by a power law with a photon index Gamma = 3.6 +/- 0.2(stat) +/- 0.2(syst) between 90GeV and 1200 GeV. We do not detect any diffuse gamma-ray emission from the cluster and so set stringent constraints on its CR population. To bracket the uncertainties over the CR spatial and spectral distributions, we adopt different spatial templates and power-law spectral indexes alpha. For alpha = 2.2, the CR-to-thermal pressure within the cluster virial radius is constrained to be less than or similar to 1-2%, except if CRs can propagate out of the cluster core, generating a flatter radial distribution and releasing the CR-to-thermal pressure constraint to less than or similar to 20%. Assuming that the observed radio mini-halo of Perseus is generated by secondary electrons from CR hadronic interactions, we can derive lower limits on the central magnetic field, B-0, that depend on the CR distribution. For alpha = 2.2, B-0 greater than or similar to 5-8 mu G, which is below the similar to 25 mu G inferred from Faraday rotation measurements, whereas for alpha less than or similar to 2.1, the hadronic interpretation of the diffuse radio emission contrasts with our gamma-ray flux upper limits independently of the magnetic field strength.