High energy gamma-ray properties of the FR I radio galaxy NGC 1275

Abstract

We report on our study of the high-energy $\gamma -$ray emission from the FR I radio galaxy NGC 1275, based on two years of observations with the Fermi-LAT detector. Previous Fermi studies of NGC 1275 had found evidence for spectral and flux variability on monthly timescales during the first year of Fermi-LAT observations. This variability is also seen in the larger two year data set, during which we observe a large $\gamma -$ray flare (June-August 2010). The increased photon statistics from this large flare have allowed the discovery of flux variability from NGC 1275 on the timescales of days. The largest flux variation observed during this flare being a factor of $\sim 3$ from one day to the next and a resultant $e$-folding risetime of $1.51\pm 0.2$ days. The two year averaged $E>$100 MeV $\gamma -$ray spectrum is adequately described by a power-law spectrum, with a photon index, $\Gamma$, of $2.09\pm 0.02$, and a resultant integrated flux of $F_{\gamma }=(2.2\pm 0.1)\times 10^{-7}$ ph cm${}^{-2}$s${}^{-1}$. While no hysteresis was observed in the photon index$-$flux (${\Gamma }_{\gamma }$ vs F${}_{\gamma }$) parameter space, there was obvious `harder-when-brighter' behaviour observed during the large $\gamma -$ray flare. Furthermore, during this large flare, NGC 1275 appeared to migrate from the FR I radio galaxy to the BL Lac object region of the photon index$-$luminosity (${\Gamma }_{\gamma }$ vs L${}_{\gamma }$) paramater space. In this paper we present details of our Fermi-LAT analysis of NGC 1275, including a brief discussion on its implications for $\gamma -$ray blazar sources.

Accepted for publication in MNRAS: 7 pages, 5 figures, 1 table

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