Title: γ-ray spectral evolution of NGC 1275 observed with FERMI large area telescope

Abstract

Here, 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 ε_γ = 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, Γ_γ = 2.13 ± 0.02. The largest γ-ray flaringmore » event was observed in 2009 April-May and was accompanied by significant spectral variability above ε_γ ≳ 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 ε_γ = 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.« less

Authors:

Kataoka, J. ^[1]; Stawarz, Ł. ^[2]; Cheung, C. C. ^[3]; Tosti, G. ^[4]; Cavazzuti, E. ^[5]; Celotti, A. ^[6]; Nishino, S. ^[7]; Fukazawa, Y. ^[7]; Thompson, D. J. ^[8]; McConville, W. F. ^[9]

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+ Show Author Affiliations

1. Waseda Univ., Shinjuku (Japan). Research Inst. for Science and Engineering
2. Inst. of Space and Astronautical Science, Sagamihara (Japan); Jagiellonian Univ., Krakow (Poland). Astronomical Observatory
3. Naval Research Lab. (NRL), Washington, DC (United States). Space Science Division
4. Istituto Nazionale di Fisica Nucleare (INFN), Perugia (Italy); Univ. di Perugia (Italy). Dipartimento di Fisica
5. Agenzia Spaziale Italiana (ASI), Rome (Italy). Science Data Center
6. International School for Advanced Studies (SISSA), Trieste (Italy)
7. Hiroshima Univ. (Japan). Dept. of Physical Sciences
8. NASA Goddard Space Flight Center (GSFC), Greenbelt, MD (United States)
9. NASA Goddard Space Flight Center (GSFC), Greenbelt, MD (United State; Univ. of Maryland, College Park, MD (United States). Dept. of Physics

Publication Date:

Thu Apr 29 00:00:00 EDT 2010

Research Org.:

SLAC National Accelerator Lab., Menlo Park, CA (United States)

Sponsoring Org.:

USDOE; National Aeronautics and Space Administration (NASA)

Contributing Org.:

The Fermi-LAT Collaboration

OSTI Identifier:

1357559

Grant/Contract Number:  

AC02-76SF00515

Resource Type:

Accepted Manuscript

Journal Name:

The Astrophysical Journal

Additional Journal Information:

Journal Volume: 715; Journal Issue: 1; Journal ID: ISSN 0004-637X

Publisher:

Institute of Physics (IOP)

Country of Publication:

United States

Language:

English

Subject:

79 ASTRONOMY AND ASTROPHYSICS; radiation mechanisms non-thermal; gamma rays general; galaxies jets; galaxies active; galaxies individual (NGC 1275)