Rapid <i>γ</i> -Ray Variability in the Lensed Quasar PKS 1830−211 Observed by Fermi-LAT

Abstract We report a detailed analysis of the γ -ray light curve from the source PKS 1830–211, where five γ -ray flares with peak fluxes larger than 10 −5 ph cm −2 s −1 were obtained. The observed shortest variability timescale of 0.83 ± 0.33 hr suggests a very compact emission region of R ≤ 10 15 cm and a Doppler factor of ∼40, implying that the observed emission is most likely produced in the subparsec-scale jet. The majority of short-timescale flares show an asymmetric temporal profile, implying that the flares may be attributed to the fast injection of accelerated particles or escape from the emission region. Moreover, the γ -ray spectra show an obvious spectral break with cutoff energies between 3.83 and 5.57 GeV, which may be caused by an intrinsic break in the energy distribution of radiating particles. The obtained γ -ray power spectral densities imply that the seed photons could originate from the dusty torus with a total photon luminosity L ph ∼ 1.2 × 10 50 erg s −1 for the magnetic island in comoving frame. According to the intermittency of turbulent cascade, we get the scale of the reconnection layers of ∼0.2 × 10 17 cm and the survival time of a sheet of τ cs ∼ 2.4 × 10 4 s, which is less than the shock acceleration timescale of t acc ≈ 6.0 × 10 4 s. These results hint that the ultrafast γ -ray variability from PKS 1830−211 may be due to the magnetic reconnection mechanism.