Rapid Gamma-ray variability of NGC 1275

Abstract

We report on a detailed analysis of the $\gamma$-ray light curve of NGC 1275 using the Fermi large area telescope data accumulated in 2008-2017. Major $\gamma$-ray flares were observed in October 2015 and December 2016/January 2017 when the source reached a daily peak flux of $(2.21\pm 0.26)\times 10^{-6}\mathrm{photon}\mathrm{c}{\mathrm{m}}^{-2}{\mathrm{s}}^{-1}$, achieving a flux of $(3.48\pm 0.87)\times 10^{-6}\mathrm{photon}\mathrm{c}{\mathrm{m}}^{-2}{\mathrm{s}}^{-1}$ within 3 hours, which corresponds to an apparent isotropic $\gamma$-ray luminosity of $\simeq 3.84\times 10^{45}\mathrm{erg}{\mathrm{s}}^{-1}$. The most rapid flare had e-folding time as short as $1.21\pm 0.22$ hours which had never been previously observed for any radio galaxy in $\gamma$-ray band. Also $\gamma$-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 (${\Gamma }_{\mathrm{X}}\simeq 1.75-1.77$) and the flux increased nearly $\sim 3$ times as compared with the quiet state. The observed hour-scale variability suggests a very compact emission region ($R_{\gamma }\le 5.22\times 10^{14}(\delta /4)\mathrm{cm}$) 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 $\gamma$-ray photon index hardened, shifting the peak of the high energy spectral component to $>\mathrm{GeV}$, making it difficult to explain the observed X-ray and $\gamma$-ray data in the standard one-zone synchrotron self-Compton model.

Accepted for publication in ApJ

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