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Available from: Felix Spanier, Jul 01, 2015
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    ABSTRACT: This paper presents a detailed analysis of the temporal and spectral variability of the low-energy peaked BL Lac object S5 0716+714 with a long (~74 ks)X-ray observation performed by XMM-Newton on 2007 September 24-25. The source experiences recurrent flares on timescales of hours. The soft X-ray variations, up to a factor of ~4, are much stronger than the hard X-ray variations. With higher energy, the variability amplitude increases in the soft X-rays but decreases in the hard X-rays. The hard X-ray variability amplitude, however, is effectively large. For the first time, we detect a soft lag of ~1000s between the soft and hard X-ray variations. The soft lags might become larger with larger energy differences. The overall X-ray spectra exhibit a softer-when-brighter trend, whereas the soft X-ray spectra appear to show a harder-when-brighter trend. The concave X-ray spectra of the source can be interpreted as the sum of the high-energy tail of the synchrotron emission, dominating in the soft X rays, and the low-energy end of the inverse Compton (IC) emission, contributing more in the hard X-rays. The synchrotron spectra are steep (\Gamma~2.6), while the IC spectra are flat (\Gamma~1.2). The synchrotron spectra appear to harden with larger synchrotron fluxes, while the IC spectra seem to soften with larger IC fluxes. When the source brightens, the synchrotron fluxes increase but the IC fluxes decrease. The synchrotron tail exhibits larger flux variations but smaller spectral changes than the IC component does. The crossing energies between the two components and the trough energies of spectral energy distributions (SEDs) increase when the source brightens. The X-ray spectral variability demonstrates that the synchrotron and IC SED peaks of S5 0716+714 shift to higher energies when it brightens.
    The Astrophysical Journal 03/2010; 713(1):180. DOI:10.1088/0004-637X/713/1/180 · 6.28 Impact Factor
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    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.
    The Astrophysical Journal 04/2010; 715(1):554. DOI:10.1088/0004-637X/715/1/554 · 6.28 Impact Factor
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    ABSTRACT: Our goal is to research the upper limits on the extragalactic background light (EBL). The upper limits on the extragalactic background light (EBL), using the Fermi and very high energy (VHE) spectra recently observed in TeV blazars, are presented. We use an assumption that the VHE intrinsic photon index cannot be harder than the Fermi index measured by the Fermi-LAT. Totally, these upper limits on the EBL are compatible with ones given by most of EBL models. However, the models of high EBL density are denied by TeV blazars.
    Astronomy and Astrophysics 06/2010; 522. DOI:10.1051/0004-6361/201014724 · 4.48 Impact Factor