Constraining cosmic rays and magnetic fields in the Perseus galaxy cluster with TeV observations by the MAGIC telescopes

Abstract

Galaxy clusters are being assembled today in the most energetic phase of hierarchical structure formation which manifests itself in powerful shocks that contribute to a substantial energy density of cosmic rays (CRs). Hence, clusters are expected to be luminous gamma-ray emitters since they also act as energy reservoirs for additional CR sources, such as active galactic nuclei and supernova-driven galactic winds. To detect the gamma-ray emission from CR interactions with the ambient cluster gas, we conducted the deepest to date observational campaign targeting a galaxy cluster at very high-energy gamma-rays and observed the Perseus cluster with the MAGIC Cherenkov telescopes for a total of ∼85 h of effective observing time. This campaign resulted in the detection of the central radio galaxy NGC 1275 at energies E > 100 GeV with a very steep energy spectrum. Here, we restrict our analysis to energies E > 630 GeV and detect no significant gamma-ray excess. This constrains the average CR-to-thermal pressure ratio to be 1-2%, depending on assumptions and the model for CR emission. Comparing these gamma-ray upper limits to models inferred from cosmological cluster simulations that include CRs constrains the maximum CR acceleration efficiency at structure formation shocks to be <50%. Alternatively, this may argue for non-negligible CR transport processes such as CR streaming and diffusion into the outer cluster regions. Finally, we derive lower limits on the magnetic field distribution assuming that the Perseus radio mini-halo is generated by secondary electrons/positrons that are created in hadronic CR interactions: assuming a spectrum of E −2.2 around TeV energies as implied by cluster simulations, we limit the central magnetic field to be >4-9 μG, depending on the rate of decline of the magnetic field strength toward larger radii. This range is well below field strengths inferred from Faraday rotation measurements in cool cores. Hence, the hadronic model remains a plausible explanation of the Perseus radio mini-halo.

CONSTRAINTS ON COSMIC RAYS, MAGNETIC FIELDS, AND DARK MATTER FROM GAMMA-RAY OBSERVATIONS OF THE COMA CLUSTER OF GALAXIES WITH VERITAS ANDFERMI

“…These upper limits are now starting to constrain the CR physics in self-consistent cosmological cluster simulations and cap the maximum CR acceleration efficiency at structure formation shocks to be <50%. Alternatively, this may argue for non-negligible CR transport processes such as CR streaming and diffusion into the outer cluster regions (Aleksić et al 2012). These are encouraging results in that we constrain the CR pressure (of a phase that is fully mixed with the ICM) to be at most a small fraction (<0.017) of the overall pressure.…”

“…Observations with EGRET (Sreekumar et al 1996;Reimer et al 2003) and the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope (Ackermann et al 2010) have provided upper limits on the gamma-ray fluxes (typically ∼10 −9 photons cm −2 s −1 for Fermi-LAT observations) for several galaxy clusters in the MeV to GeV band. Upper limits on the very high energy (VHE) gamma-ray flux from a small sample of clusters, including the Coma Cluster, have been provided by observations with ground-based imaging atmospheric Cerenkov telescopes (IACTs; Perkins et al 2006;Perkins 2008;Aharonian et al 2009b;Aleksić et al 2010Aleksić et al , 2012.…”

Gamma Rays From Star Formation in Clusters of Galaxies

“…, is approximately six orders of magnitude smaller than the upper limits reported in Aleksić et al (2012) and well below the sensitivity limits of current atmospheric Cerenkov telescopes. The gamma-ray emission associated with star formation in Perseus is therefore unlikely to be detected by ground-based instruments.…”

“…Recent studies of the Perseus Cluster by MAGIC did not yield a detection apart from the central AGN, NGC 1275, and a radio galaxy, IC 310 (Aleksić et al 2012). However, the gamma-ray spectrum of NGC 1275 as measured by MAGIC drops off above 630 GeV, which means that Perseus is a good potential candidate for detection in this energy range.…”

“…Atmospheric Cerenkov telescopes such as H.E.S.S. and MAGIC have also reported null results from observations of selected clusters of galaxies, such as Perseus (Aleksić et al 2012), Coma (Aharonian et al 2009a), and A0085 and A0496 (Aharonian et al 2009b). …”