AGILE detection of extreme γ-ray activity from the blazar PKS 1510-089 during March 2009

Multifrequency analysis^⋆

¹ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, via U. La Malfa 153, 90146 Palermo 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/IASF–Roma, via Fosso del Cavaliere 100, 00133 Roma, Italy
⁴ INAF, Oss. Astronomico di Torino, viaOsservatorio 20, 10025 Pino Torinese, Torino, Italy
⁵ ENEA–Frascati, via E. Fermi 45, 00044 Frascati, Roma, Italy
⁶ NASA / Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁷ INAF, Oss. Astronomico Brera, via Bianchi 46, 23807 Merate ( LC), Italy
⁸ INAF/TNG Fundación Galileo Galilei, Rambia José Ana Fernández Pérez 7, 38712, Brena Baja, Tenerife, Spain
⁹ INAF-Istituto di Radioastronomia, via P. Gobetti 101, 40129 Bologna, Italy
¹⁰ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
¹¹ INAF Osservatorio Astronomico di Trieste, via G. Tiepolo 11, 34143, Italy
¹² Dip. di Fisica and INFN Trieste, via Valerio 2, 34127 Trieste, Italy
¹³ INFN–Pavia, via Bassi 6, 27100 Pavia, Italy
¹⁴ Dip. di Fisica Nucleare e Teorica, Univ. di Pavia, via Bassi 6, 27100 Pavia, Italy
¹⁵ INAF/IASF–Bologna, via Gobetti 101, 40129 Bologna, Italy
¹⁶ INAF/IASF–Milano, via E. Bassini 15, 20133 Milano, Italy
¹⁷ Dip. di Fisica Generale dell’Universitá, via P. Giuria 1, 10125 Torino, Italy
¹⁸ ENEA–Bologna, via dei Martiri di Monte Sole 4, 40129 Bologna, Italy
¹⁹ INFN–Roma “La Sapienza”, Piazzale A. Moro 2, 00185 Roma, Italy
²⁰ INFN–Roma “Tor Vergata”, via della Ricerca Scientifica 1, 00133 Roma, Italy
²¹ INAF–Oss. Astronomico di Cagliari, loc. Poggio dei Pini, strada 54, 09012 Capoterra ( CA), Italy
²² Dip. di Fisica, Univ. dell’Insubria, via Valleggio 11, 22100 Como, Italy
²³ Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
²⁴ Instituto de Astrofísica de Andalucía, CSIC, Apartado 3004, 18080 Granada, Spain
²⁵ Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA
²⁶ Pulkovo Observatory, Russian Academy of Sciences, 196140, St.-Petersburg, Russia
²⁷ MPIfR, 53121 Bonn, Germany
²⁸ Instituto de Astronomia, Universidad Nacional Autonoma de Mexico, D. F. Mexico, Mexico
²⁹ Tuorla Observatory, Department of Physics and Astronomy, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
³⁰ Astron. Inst., St.-Petersburg State Univ., 198504 St.-Petersburg, Russia
³¹ INAF – Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
³² Institute of Astronomy, National Central University, Taiwan
³³ Lulin Observatory, Institute of Astronomy, National Central University, Taiwan
³⁴ INAF, Osservatorio Astronomico di Roma, via di Frascati 33, 00040, Monte Porzio Catone, Italy
³⁵ INAF, Osservatorio Astronomico di Collurania, via Mentore Maggini, 64100 Teramo, Italy
³⁶ Agrupació Astronòmica de Sabadell, Spain
³⁷ Harvard-Smithsonian Center for Astrophysics, Cambridge, Garden st. 60, MA 02138, USA
³⁸ School Of Cosmic Physics, Dublin Institute For Advanced Studies, Ireland
³⁹ ZAH, Landessternwarte Heidelberg, Königstuhl, 69117 Heidelberg, Germany
⁴⁰ Instituto de Astronomia, Universidad Nacional Autonoma de Mexico, 2280 Ensenada, B.C. Mexico, Mexico
⁴¹ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kymälä, Finland
⁴² Department of Physics, Purdue University, 525 Northwestern Ave., West Lafayette, IN 47907, USA
⁴³ Abastumani Observatory, 383762 Abastumani, Georgia
⁴⁴ Isaac Newton Institute of Chile, St.-Petersburg Branch, Russia
⁴⁵ School Of Cosmic Physics, Dublin Institute For Advanced Studies, Ireland
⁴⁶ Department of Physics and Astronomy, University of Southampton S017 1BJ, UK
⁴⁷ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, 21550 Piikkiö, Finland
⁴⁸ ASI–ASDC, via G. Galilei, 00044 Frascati, Roma, Italy
⁴⁹ ASI, Viale Liegi 26, 00198 Roma, Italy

Received: 11 November 2010
Accepted: 15 March 2011

Abstract

We report on the extreme γ-ray activity from the flat spectrum radio quasar (FSRQ) PKS 1510−089 observed by the AGILE satellite in March 2009. In the same period a radio-to-optical monitoring of the source was provided by the GASP–WEBT and REM facilities. In the radio band we made use also of multi-epoch 15-GHz Very Long Baseline Array data from the MOJAVE Program to get information on the parsec-scale structure. Moreover, several Swift target of opportunity observations were triggered, adding important information on the source behaviour from optical/UV to hard X-rays. We paid particular attention to the calibration of the Swift/UVOT data to make it suitable to the blazars spectra. Simultaneous observations from radio to γ rays allowed us to study in detail the correlation among the emission variability at differentfrequencies and to investigate the mechanisms at work during this high activity state of the source.

In the period 9–30 March 2009, AGILE detected γ-ray emission from PKS 1510−089 at a significance level of 21.5-σ with an average flux over the entire period of (311 ± 21) × 10^-8 photons cm^-2 s^-1 for photon energies above 100 MeV, and a peak level of (702 ± 131) × 10^-8 photons cm^-2 s^-1 on daily integration. The activity detected in γ rays occurred during a period of increasing activity from near-infrared to UV, as monitored by GASP–WEBT, REM and Swift/UVOT. A flaring episode on 26–27 March 2009 was detected from near-IR to UV, suggesting that a single mechanism is responsible for the flux enhancement observed at the end of March. By contrast, Swift/XRT observations seem to show no clear correlation of the X-ray fluxes with the optical and γ-ray ones. However, the X-ray observations show a harder photon index (Γ_x ≃ 1.3–1.6) with respect to most FSRQs and a hint of harder-when-brighter behaviour, indicating the possible presence of a second emission component at soft X-ray energies. Moreover, the broad band spectrum from radio-to-UV confirmed the evidence of thermal features in the optical/UV spectrum of PKS 1510−089 also during high γ-ray state. On the other hand, during 25–26 March 2009 a flat spectrum in the optical/UV energy band was observed, suggesting an important contribution of the synchrotron emission in this part of the spectrum during the brightest γ-ray flare, therefore a significant shift of the synchrotron peak.