Gamma-ray Variability of Bright AGNs: Implications on Processes Powering the Emission and Jet-central-engine Connection

The X-ray/Ultraviolet/Optical emission from accretion-powered sources shows phenomenological similarities, characterized by a lognormal flux distribution, a linear relation between intrinsic variability and flux (rms-flux), and a (broken) power-law power spectral densities (PSDs). These features have normally been argued to be a result of the multiplicative combination of fluctuations in the accretion disk. Here, we present results from exploration of these properties in gamma-ray emission of four radio-loud Active Galactic Nuclei (AGNs): radio-galaxy NGC 1275 and three blazars - Mrk 421, PKS 1510-089, and B2 1520+31, using continuously sampled light curves between 2008 - 2015 from Fermi-LAT. All the sources, except Mrk 421, display flux spanning 2 orders of magnitude between the extremes with a lognormal profile describing the blazars flux histogram better compared to a Gaussian while none favored for the non-blazar source NGC 1275. The rms-flux relation, however, is linear for all, irrespective of histogram profile while the PSDs are typical of the accretion-powered source, exhibiting a red noise power-law spectrum of slope 1, though there is hint of breaks. The inferred results are consistent with the statistics of magnetic-reconnection powered minijets-in-a-jet model as well as with the statistics of X-ray emission from the whole Solar disk. The results thus suggest magnetic reconnection as the potential process powering the emission, consistent with the non-thermal spectrum observed from these sources. The broad similarity of properties with other accretion-powered sources also suggest that the magnetic reconnection may likely be an imprint of fluctuation in the accretion disk to the jet.