Gamma-ray Spectral Evolution of NGC1275 Observed with Fermi-LAT

Abstract

We report on a detailed investigation of the high-energy gamma-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 gamma-ray emitting region is now measured to be separated by only 0.46' from the nucleus of NGC1275, well within the 95% confidence error circle with radius ~1.5'. Early Fermi-LAT observations revealed a significant decade-timescale brightening of NGC1275 at GeV photon energies, with a flux about seven 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 gamma-ray spectrum of NGC1275 shows a possible deviation from a simple power-law shape, indicating a spectral cut-off around an observed photon energy of E = 42.2+-19.6 GeV, with an average flux of F = (2.31+-0.13) X 10^{-7} ph/cm^2/s and a power-law photon index, Gamma = 2.13+-0.02. The largest gamma-ray flaring event was observed in April--May 2009 and was accompanied by significant spectral variability above E > 1-2 GeV. The gamma-ray activity of NGC1275 during this flare can be described by a hysteresis behavior in the flux versus photon index plane. The highest energy photon associated with the gamma-ray source was detected at the very end of the observation, with the observed energy of E = 67.4GeV and an angular separation of about 2.4' from the nucleus. In this paper we present the details of the Fermi-LAT data analysis, and briefly discuss the implications of the observed gamma-ray spectral evolution of NGC1275 in the context of gamma-ray blazar sources in general.

20 pages, 6 figures, accepted for publication in the ApJ

References in corpus (14)

• Fermi Large Area Telescope First Source Catalog
• Fermi Large Area Telescope Bright Gamma-ray Source List
• The First Catalog of Active Galactic Nuclei Detected by the Fermi Large Area Telescope
• Bright AGN Source List from the First Three Months of the Fermi Large Area Telescope All-Sky Survey
• Radio Imaging of the Very-High-Energy Gamma-Ray Emission Region in the Central Engine of a Radio Galaxy
• Fermi Discovery of Gamma-Ray Emission from NGC 1275
• Fermi Large Area Telescope Gamma-Ray Detection of the Radio Galaxy M87
• MAGIC Gamma-ray Telescope Observation of the Perseus Cluster of Galaxies: implications for cosmic rays, dark matter, and NGC1275
• Discovery of very high energy gamma-ray emission from the low-frequency peaked BL Lacertae object BL Lacertae
• Galaxy Clusters in the Swift/BAT era: Hard X-rays in the ICM
• Multiwavelength observations of a TeV-Flare from W Comae
• Discovery of Very High Energy gamma-rays from the blazar S5 0716+714
• Discovery of Variability in the Very High Energy Gamma-Ray Emission of 1ES 1218+304 with VERITAS
• VERITAS Upper Limit on the VHE Emission from the Radio Galaxy NGC 1275

Cited by in corpus (44)

• The Second Catalog of Active Galactic Nuclei Detected by the Fermi Large Area Telescope
• The Third Catalog of Active Galactic Nuclei Detected by the Fermi Large Area Telescope
• Blazars as Ultra-High-Energy Cosmic-Ray Sources: Implications for TeV Gamma-Ray Observations
• The First Fermi-LAT Catalog of Sources Above 10 GeV
• Fermi Large Area Telescope Observations of Misaligned AGN
• Contribution of the Gamma-ray Loud Radio Galaxies Core Emissions to the Cosmic MeV and GeV Gamma-Ray Background Radiation
• Limb-Brightened Jet of 3C 84 Revealed by the 43-GHz Very-Long-Baseline-Array Observation
• A Statistical Approach to Recognizing Source Classes for Unassociated Sources in the First Fermi-LAT Catalog
• Detection of very-high energy γ-ray emission from NGC 1275 by the MAGIC telescopes
• Broad Line Radio Galaxies Observed with Fermi-LAT: The Origin of the GeV Gamma-Ray Emission
• Locating the gamma-ray emission region of the flat spectrum radio quasar PKS 1510-089
• The Extragalactic Gamma-ray Sky in the Fermi era
• High energy gamma-ray properties of the FR I radio galaxy NGC 1275
• On the spine-layer scenario for the very high-energy emission of NGC 1275
• Contemporaneous observations of the radio galaxy NGC 1275 from radio to very high energy gamma-rays
• Discovery of a new subparsec counterjet in NGC 1275: the inclination angle and the environment
• The gamma-ray emission region in the FRII Radio Galaxy 3C 111
• The Fermi All-sky Variability Analysis: A list of flaring gamma-ray sources and the search for transients in our Galaxy
• First detection in gamma-rays of a young radio galaxy: Fermi-LAT observations of the Compact Symmetric Object PKS 1718-649
• The connection between the radio jet and the gamma-ray emission in the radio galaxy 3C 120
• X-ray properties of the Youngest Radio Sources and their Environments
• A Non-thermal Study of the Brightest Cluster Galaxy NGC 1275 - The Gamma-Radio Connection Over Four Decades
• Fermi-LAT Observation of Non-Blazar AGNs
• Rapid Gamma-ray variability of NGC 1275
• Non-thermal Emissions from Cool Cores Heated by Cosmic-Rays in Galaxy Clusters
• VLBI and Single Dish Monitoring of 3C84 in the Period of 2009-2011
• Suzaku Observations of gamma-Ray Bright Radio Galaxies: Origin of the X-ray Emission and Broad-Band Modelin
• The origins of the gamma-ray flux variations of NGC 1275 based on 8 years of Fermi-LAT observations
• A Stacked Analysis of Brightest Cluster Galaxies Observed with the Fermi Large Area Telescope
• Implications for the structure of the relativistic jet from multiwavelength observations of NGC 6251
• Discovery of gamma-ray emission from the Broad Line Radio Galaxy Pictor A
• Unveiling the nature of the gamma-ray emitting AGN PKS 0521-36
• GENJI Programme: Gamma-ray Emitting Notable AGN Monitoring by Japanese VLBI
• Simultaneous optical monitoring of BL Lacertae object S5 0716+714 with high temporal resolution
• Fermi-LAT and Suzaku Observations of the Radio Galaxy Centaurus B
• Gamma-Rays from Radio Galaxies: Fermi-Lat Observations
• Radio Galaxies with the Cherenkov Telescope Array
• Origin of the UV to X-ray emission of radio galaxy NGC 1275 explored by analyzing its variability
• Earth Occultation Imaging of the Low Energy Gamma-Ray Sky with GBM
• Multiwavelength monitoring of NGC 1275 over a decade: Evidence of a shift in synchrotron peak frequency and long-term multi-band flux increase
• A search for $\gamma$-ray emission from a sample of local universe low-frequency selected radio galaxies
• Two blobs in a jet model for the gamma-ray emission in radio galaxies
• Very High Energy gamma-ray emission from RBS 0970
• Very high energy $\gamma$-ray emission from RBS 0679