AGILE detection of a rapid γ-ray flare from the blazar PKS 1510-089 during the GASP-WEBT monitoring^*

¹ INAF/IASF – Roma, via Fosso del Cavaliere 100, 00133 Roma, Italy e-mail: filippo.dammando@iasf-roma.inaf.it
² Dip. di Fisica, Univ. di Roma “Tor Vergata”, via della Ricerca Scientifica 1, 00133 Roma, Italy
³ INAF, Oss. Astronomico di Torino, via Osservatorio 20, 10025 Pino Torinese (Torino), Italy
⁴ CIFS – Torino, viale Settimio Severo 3, 10133 Torino, Italy
⁵ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, via U. La Malfa 153, 90146 Palermo, Italy
⁶ Dip. di Fisica and INFN Trieste, via Valerio 2, 34127 Trieste, Italy
⁷ INFN – Pavia, via Bassi 6, 27100 Pavia, Italy
⁸ Dip. di Fisica Nucleare e Teorica, Univ. di Pavia, via Bassi 6, 27100 Pavia, Italy
⁹ INAF/IASF – Bologna, via Gobetti 101, 40129 Bologna, Italy
¹⁰ INAF/IASF – Milano, via E. Bassini 15, 20133 Milano, Italy
¹¹ Dip. di Fisica Generale dell'Universitá, via P. Giuria 1, 10125 Torino, Italy
¹² ENEA – Bologna, via dei Martiri di Monte Sole 4, 40129 Bologna, Italy
¹³ INFN – Roma “La Sapienza”, Piazzale A. Moro 2, 00185 Roma, Italy
¹⁴ INFN – Roma “Tor Vergata”, via della Ricerca Scientifica 1, 00133 Roma, Italy
¹⁵ INAF – Oss. Astronomico di Cagliari, loc. Poggio dei Pini, strada 54, 09012 Capoterra (CA), Italy
¹⁶ Dip. di Fisica, Univ. dell'Insubria, via Valleggio 11, 22100 Como, Italy
¹⁷ ENEA – Roma, via E. Fermi 45, 00044 Frascati (Roma), Italy
¹⁸ Instituto de Astrofísica de Andalucía, CSIC, Apartado 3004, 18080 Granada, Spain
¹⁹ Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA
²⁰ Pulkovo Observatory, Russian Academy of Sciences, 196140 St.-Petersburg, Russia
²¹ MPIfR, 53121 Bonn, Germany
²² Instituto de Astronomia, Universidad Nacional Autonoma de Mexico, Mexico, D. F. Mexico
²³ Tuorla Observatory, Department of Physics and Astronomy, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
²⁴ Astron. Inst., St.-Petersburg State Univ., 198504 St.-Petersburg, Russia
²⁵ INAF – Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
²⁶ Institute of Astronomy, National Central University, Taiwan
²⁷ Lulin Observatory, Institute of Astronomy, National Central University, Taiwan
²⁸ INAF, Osservatorio Astronomico di Roma, via di Frascati 33, 00040, Monte Porzio Catone, Italy
²⁹ INAF, Osservatorio Astronomico di Collurania, via Mentore Maggini, 64100 Teramo, Italy
³⁰ Harvard-Smithsonian Center for Astrophysics, Cambridge, Garden st. 60, MA 02138, USA
³¹ Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
³² ZAH, Landessternwarte Heidelberg, Königstuhl, 69117 Heidelberg, Germany
³³ Instituto de Astronomia, Universidad Nacional Autonoma de Mexico, 2280 Ensenada, B.C. Mexico
³⁴ Abastumani Observatory, 383762 Abastumani, Georgia
³⁵ INAF – IRA, contrada Renna Bassa, 96017 Noto (SR), Italy
³⁶ Department of Physics and Astronomy, University of Southampton S17 1BJ, UK
³⁷ INAOE, Apdo. Postal 51 y 216, 72000 Tonantzintla, Puebla, Mexico
³⁸ Lowell Observatory, Flagstaff, AZ 86001, USA
³⁹ ASI-ASDC, via G. Galilei, 00044 Frascati (Roma), Italy
⁴⁰ ASI, viale Liegi 26, 00198 Roma, Italy

Received: 23 May 2009
Accepted: 15 September 2009

Abstract

Aims. We report the detection by the AGILE satellite of a rapid γ-ray flare from the source 1AGL J1511-0908, associated with the powerful γ-ray quasar PKS 1510-089, during a pointing centered on the Galactic center region from 1 March to 30 March 2008. This source has been continuosly monitored in the radio-to-optical bands by the GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope (WEBT). Moreover, the γ-ray flaring episode triggered three ToO observations by the satellite in three consecutive days, starting from 20 March 2008. The quasi-simultaneous radio-to-optical, UV, X-ray and γ-ray coverage allows us to make a detailed study of the multifrequency time evolution, the spectral energy distribution of this source, and its theoretical interpretation based on the synchrotron and inverse Compton (IC) emission mechanisms.

Methods. During the radio-to-optical monitoring provided by the GASP-WEBT, AGILE observed the source with its two co-aligned imagers, the gamma-ray imaging detector (GRID) and the hard X-ray imager (SuperAGILE), which are sensitive in the 30 MeV–30 GeV and 18–60 keV energy bands, respectively.

Results. In the period 1–16 March 2008, AGILE detected γ-ray emission from PKS 1510-089 at a significance level of 6.2-σ with an average flux over the entire period of  photons cm^-2 s^-1 for photon energies above 100 MeV. After a predefined satellite re-pointing, between 17 and 21 March 2008, AGILE detected the source at a significance level of 7.3-σ, with an average flux ( MeV) of  photons cm^-2 s^-1 and a peak level of  photons cm^-2 s^-1 with daily integration. During the observing period January-April 2008, the source also showed an intense and variable optical activity, with several flaring episodes and a significant increase in the flux was observed at millimetric frequencies. Moreover, in the X-ray band, the /XRT observations seem to show a harder-when-brighter behavior of the source spectrum.

Conclusions. The flat spectrum radio quasar PKS 1510-089 showed strong activity between January and April 2008, with episodes of rapid variability from radio to γ-ray energy bands, in particular with a rapid γ-ray flaring episode. The spectral energy distribution of mid-March 2008 is modeled with a homogeneous one-zone synchrotron self Compton (SSC) emission plus contributions from inverse Compton scattering of external photons from both the accretion disk and the broad line region. Indeed, some features in the optical-UV spectrum seem to indicate Seyfert-like components such as the little and the big blue bumps.