Locating the gamma-ray emission region of the flat spectrum radio quasar PKS 1510-089

Abstract

I present a study of the high-energy gamma-ray properties of the flat spectrum radio quasar, PKS 1510-089, based on 3.75 years of observations with the Large Area Telescope (LAT) detector on-board the Fermi gamma-ray Space Telescope. Throughout the observing period, the 0.1 GeV < E < 300 GeV gamma-ray flux was highly variable, undergoing several flaring events where the daily flux exceeded 10^{-5} photons cm^{-2} s^{-1} on 3 separate occasions. The increased photon statistics of these large flares allowed the observations to be re-analysed in 6 and 3 hour intervals, revealing flux doubling timescales as small as 1.3\pm0.12 hours during the flare rise time, and flux halving timescales of 1.21\pm0.15 hours during the flare decay. These are the smallest variability timescales measured to date at MeV-GeV energies for the Flat Spectrum Radio Quasar class of Active Galactic Nuclei. The >10^{-5} photons cm^{-2} s^{-1} flare events were also studied in more detail in an attempt to uncover evidence for the location of PKS 1510-089's gamma-ray emission region. In particular, two approaches were used: (i) searching for an energy dependence to the cooling timescales, and (ii) searching for evidence of a spectral cut-off. Furthermore, an analysis of the highest photon events within the 3.75 year data set finds PKS 1510-089 to be a source of >20 GeV gamma-rays at the 13.5σconfidence level; a observational property which is difficult to explain in the traditional view that gamma-ray emission from Active Galactic Nuclei originates from the base of the relativistic jet. The combined results of these two approaches, along with the confirmation of >20 GeV photon flux from PKS 1510-089, suggest the presence of multiple gamma-ray emission regions being located in both the broad line region and molecular torus region of PKS 1510-089.