<i>FERMI</i>LARGE AREA TELESCOPE AND MULTI-WAVELENGTH OBSERVATIONS OF THE FLARING ACTIVITY OF PKS 1510-089 BETWEEN 2008 SEPTEMBER AND 2009 JUNE

Abstract

We report on the multi-wavelength observations of PKS 1510-089 (a flat\nspectrum radio quasar at z=0.361) during its high activity period between 2008\nSeptember and 2009 June. During this 11 months period, the source was\ncharacterized by a complex variability at optical, UV and gamma-ray bands, on\ntime scales down to 6-12 hours. The brightest gamma-ray isotropic luminosity,\nrecorded on 2009 March 26, was ~ 2x10^48erg s^-1. The spectrum in the Fermi-LAT\nenergy range shows a mild curvature well described by a log-parabolic law, and\ncan be understood as due to the Klein-Nishina effect. The gamma-ray flux has a\ncomplex correlation with the other wavelengths. There is no correlation at all\nwith the X-ray band, a weak correlation with the UV, and a significant\ncorrelation with the optical flux. The gamma-ray flux seems to lead the optical\none by about 13 days. From the UV photometry we estimated a black hole mass of\n~ 5.4x10^8 solar masses, and an accretion rate of ~ 0.5 solar masses/year.\nAlthough the power in the thermal and non-thermal outputs is smaller compared\nto the very luminous and distant flat spectrum radio quasars, PKS 1510-089\nexhibits a quite large Compton dominance and a prominent big blue bump (BBB) as\nobserved in the most powerful gamma-ray quasars. The BBB was still prominent\nduring the historical maximum optical state in 2009 May, but the optical/UV\nspectral index was softer than in the quiescent state. This seems to indicate\nthat the BBB was not completely dominated by the synchrotron emission during\nthe highest optical state. We model the broadband spectrum assuming a leptonic\nscenario in which the inverse Compton emission is dominated by the scattering\nof soft photons produced externally to the jet. The resulting model-dependent\njet energetic content is compatible with the accretion disk powering the jet,\nwith a total efficiency within the Kerr black hole limit.\n