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arXiv · 1709.03755

Rapid Gamma-ray variability of NGC 1275

V. Baghmanyan·S. Gasparyan·N. Sahakyan
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Abstract

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 $(2.21\pm0.26)\times10^{-6}\:{\rm photon\:cm^{-2}\:s^{-1}}$, achieving a flux of $(3.48\pm0.87)\times10^{-6}\:{\rm photon\:cm^{-2}\:s^{-1}}$ within 3 hours, which corresponds to an apparent isotropic $γ$-ray luminosity of $\simeq3.84\times10^{45}\:{\rm erg\:s^{-1}}$. The most rapid flare had e-folding time as short as $1.21\pm0.22$ 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 ($Γ_{\rm X}\simeq 1.75-1.77$) and the flux increased nearly $\sim3$ times as compared with the quiet state. The observed hour-scale variability suggests a very compact emission region ($R_γ\leq5.22\times10^{14}\:(δ/4)\:{\rm cm}$) 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.

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V. Baghmanyan, S. Gasparyan, N. Sahakyan. 2017-09-12. Rapid Gamma-ray variability of NGC 1275. https://doi.org/10.3847/1538-4357%2Faa8c7b

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