2017/09/12 by V. Baghmanyan, S. Gasparyan, N. Sahakyan · 1 citation
Physics and Astronomy · #astro-ph.HE
paper · pdf · doi:10.3847/1538-4357/aa8c7b
Accepted for publication in ApJ
arxiv created 2017/09/12 · arxiv updated 2017/10/24
We report on a detailed analysis of the γ-ray light curve of NGC 1275 using the Fermi large area telescope data accumulated in 2008-2017. Major γ-ray flares were observed in October 2015 and December 2016/January 2017 when the source reached a daily peak flux of (2.21±0.26)×10-6 \rm photon cm-2 s-1, achieving a flux of (3.48±0.87)×10-6 \rm photon cm-2 s-1 within 3 hours, which corresponds to an apparent isotropic γ-ray luminosity of ≃3.84×1045 \rm erg s-1. The most rapid flare had e-folding time as short as 1.21±0.22 hours which had never been previously observed for any radio galaxy in γ-ray band. Also γ-ray spectral changes were observed during these flares: in the flux versus photon index plane the spectral evolution follows correspondingly a counter clockwise and a clockwise loop inferred from the light curve generated by an adaptive binning method. On December 30, 2016 and January 01, 2017 the X-ray photon index softened (Γ\rm X≃ 1.75-1.77) and the flux increased nearly ∼3 times as compared with the quiet state. The observed hour-scale variability suggests a very compact emission region (Rγ≤5.22×1014 (δ/4) \rm cm) implying that the observed emission is most likely produced in the subparsec-scale jet if the entire jet width is responsible for the emission. During the active periods the γ-ray photon index hardened, shifting the peak of the high energy spectral component to >\rm GeV, making it difficult to explain the observed X-ray and γ-ray data in the standard one-zone synchrotron self-Compton model.