γ-Ray spectral evolution of NGC 1275 observed with Fermi large area telescope

J. Kataoka*, L. Stawarz, C. C. Cheung, G. Tosti, E. Cavazzuti, A. Celotti, S. Nishino, Y. Fukazawa, D. J. Thompson, W. F. McConville

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

54 Citations (Scopus)

Abstract

We report on a detailed investigation of the high-energy γ-ray emission from NGC 1275, a well-known radio galaxy hosted by a giant elliptical located at the center of the nearby Perseus cluster. With the increased photon statistics, the center of the γ-ray-emitting region is now measured to be separated by only 0.46 arcmin from the nucleus of NGC 1275, well within the 95% confidence error circle with radius ≃1.5 arcmin. Early Fermi Large Area Telescope (LAT) observations revealed a significant decade-timescale brightening of NGC 1275 at GeV photon energies, with a flux about 7 times higher than the one implied by the upper limit from previous EGRET observations. With the accumulation of one year of Fermi-LAT all-sky-survey exposure, we now detect flux and spectral variations of this source on month timescales, as reported in this paper. The average >100 MeV γ-ray spectrum of NGC 1275 shows a possible deviation from a simple power-law shape, indicating a spectral cutoff around an observed photon energy of εγ = 42.2 19.6 GeV, with an average flux of F γ = (2.31 0.13) × 10 -7 photons cm-2 s-1 and a power-law photon index, Γγ = 2.13 0.02. The largest γ-ray flaring event was observed in 2009 April-May and was accompanied by significant spectral variability above εγ ≳ 1-2 GeV. The γ-ray activity of NGC 1275 during this flare can be described by a hysteresis behavior in the flux versus photon index plane. The highest energy photon associated with the γ-ray source was detected at the very end of the observation, with the observed energy of εγ = 67.4 GeV and an angular separation of about 2.4 arcmin from the nucleus. In this paper we present the details of the Fermi-LAT data analysis, and briefly discuss the implications of the observed γ-ray spectral evolution of NGC 1275 in the context of γ-ray blazar sources in general.

Original languageEnglish
Pages (from-to)554-560
Number of pages7
JournalAstrophysical Journal
Volume715
Issue number1
DOIs
Publication statusPublished - 2010

Keywords

  • Galaxies: active
  • Galaxies: individual (NGC 1275)
  • Galaxies: jets
  • Gamma rays: general
  • Radiation mechanisms: non-thermal

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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