Journal Article

High-energy γ-ray properties of the Fanaroff–Riley type I radio galaxy NGC 1275

Anthony M. Brown and Jenni Adams

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 413, issue 4, pages 2785-2790
Published in print June 2011 | ISSN: 0035-8711
Published online May 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2011.18351.x
High-energy γ-ray properties of the Fanaroff–Riley type I radio galaxy NGC 1275

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We report on our study of the high-energy γ-ray emission from the Fanaroff–Riley type I (FR I) radio galaxy NGC 1275, based on 2 yr of observations with the Fermi Large Area Telescope (LAT) detector. Previous Fermi studies of NGC 1275 had found evidence for spectral and flux variability on monthly time-scales during the first year of Fermi-LAT observations. This variability is also seen in the larger 2-yr data set, during which we observe a large γ-ray flare (2010 June–August). The increased photon statistics from this large flare have allowed the discovery of flux variability from NGC 1275 on the time-scales of days. The largest flux variation observed during this flare being a factor of ∼3 from 1 day to the next and a resultant e-folding rise time of 1.51 ± 0.2 d. The 2-yr-averaged E > 100 MeV γ-ray spectrum is adequately described by a power-law spectrum, with a photon index, Γ, of 2.09 ± 0.02, and a resultant integrated flux of Fγ= (2.2 ± 0.1) × 10−7 photon cm−2 s−1. While no hysteresis was observed in the photon index–flux (Γγ versus Fγ) parameter space, there was obvious ‘harder-when-brighter’ behaviour observed during the large γ-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 (Γγ versus Lγ) paramater space. In this paper, we present details of our Fermi-LAT analysis of NGC 1275, including a brief discussion on its implications for γ-ray blazar sources.

Keywords: galaxies: active; galaxies: individual: NGC 1275; galaxies: jets; gamma-rays: galaxies

Journal Article.  4204 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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