Detection of very-high energyγ-ray emission from NGC 1275 by the MAGIC telescopes

Abstract

We report on the detection of very-high energy (VHE, E > 100 GeV) γ-ray emission from NGC 1275, the central radio galaxy of the Perseus cluster of galaxies. The source has been detected by the MAGIC telescopes with a statistical significance of 6.6σ above 100 GeV in 46 h of stereo observations carried out between August 2010 and February 2011. The measured differential energy spectrum between 70 GeV and 500 GeV can be described by a power law with a steep spectral index of Γ = −4.1 ± 0.7 stat ± 0.3 syst , and the average flux above 100 GeV is F γ = (1.3 ± 0.2 stat ± 0.3 syst ) × 10 −11 cm −2 s −1 . These results, combined with the power-law spectrum measured in the first two years of observations by the Fermi-LAT above 100 MeV, with a spectral index of Γ −2.1, strongly suggest the presence of a break or cut-off around tens of GeV in the NGC 1275 spectrum. The light curve of the source above 100 GeV does not show hints of variability on a month time scale. Finally, we report on the nondetection in the present data of the radio galaxy IC 310, previously discovered by the Fermi-LAT and MAGIC. The derived flux upper limit F U.L. γ (>300 GeV) = 1.2 × 10 −12 cm −2 s −1 is a factor ∼3 lower than the mean flux measured by MAGIC between October 2009 and February 2010, thus confirming the year time-scale variability of the source at VHE.

NGC 3894: a young radio galaxy seen by Fermi-LAT

“…Different is the case of 3C 84, which has two-sided compact (parsec scale) jets and a distance similar to NGC 3894, but it presents a γ-ray luminosity more than 200 times higher (2 × 10 44 erg s −1 ). Unlike NGC 3894, for which no observations with Cherenkov telescopes have been performed, M 87 and 3C 84 are also detected at very high energy (VHE, E > 100 GeV) (Aharonian et al 2006;Aleksić et al 2012). In particular, at VHE, M 87 displayed strong variability on timescales as short as one day, but no unique signature of the region responsible for the VHE flares has been identified (Abramowski et al 2012).…”

Radio Galaxies at VHE Energies

“…NGC 1275 (3C 84), the central Perseus cluster RG at a distance of ∼ 70 Mpc, has been detected at VHE energies above 100 GeV by MAGIC, initially (based on data between 2009-2011) at moderate flux levels (∼ 3% of the Crab Nebula) and with a very steep VHE spectrum (photon index of Γ γ ∼ −4.1 if characterized by a power law) extending up to ∼ 650 GeV [18,19]. When HE (Fermi-LAT) and VHE data are combined, the average ("quiescent") γ-ray spectrum appears compatible with either a log-parabola or a power-law with a sub-exponential cut-off, suggestive of a common physical origin and of a peak or cut-off around several GeV.…”

Numerical models of neutrino and gamma-ray emission from magnetic reconnection in the core of radio-galaxies

“…3 The spatial and temporal scales are defined as R g = GM BH /c 2 and R g /c, respectively, where G is the gravitational constant, c the speed of light and M BH is the mass of the super massive BH of Cen A. 4 A more appropriate model for the electron temperature should possibly consider its dependence on the plasma magnetisation, see e.g., [35], [36], [33]. 5 The temperature of thermal protons is obtained assuming a γ-law equation of state p = (γ − 1)u, where p is the gas pressure and u the gas internal energy and thus, T p ∝ u/ρ.…”