Letter to the Editor

AGILE detection of variable γ-ray activity from the blazar S5 0716+714 in September–October 2007 ^*,**

¹ INAF/IASF–Milano, via E. Bassini 15, 20133 Milano, Italy e-mail: chen@iasf-milano.inaf.it
² CIFS–Torino, Viale Settimio Severo 63, 10133 Torino, Italy
³ INAF/IASF–Roma, via del Fosso del Cavaliere 100, 00133 Roma, Italy
⁴ Dip. di Fisica, Univ. “Tor Vergata”, via della Ricerca Scientifica 1, 00133 Roma, Italy
⁵ INAF, Osservatorio Astronomico di Torino, Italy
⁶ INAF/IASF–Bologna, via Gobetti 101, 40129 Bologna, Italy
⁷ Dip. di Fisica Generale dell'Universitá, via Pietro Giuria 1, 10125 Torino, Italy
⁸ Dip. di Fisica and INFN Trieste, via Valerio 2, 34127 Trieste, Italy
⁹ INFN–Pavia, via Bassi 6, 27100 Pavia, Italy
¹⁰ Armenzano Astronomical Observatory, Assisi, Italy
¹¹ ENEA, via Martiri di Monte Sole 4, 40129 Bologna, Italy
¹² Abastumani Astrophysical Observatory, Georgia
¹³ Astrophysikalisches Institut Potsdam, Germany
¹⁴ Landessternwarte Heidelberg-Königstuhl, Germany
¹⁵ Astronomical Institute, St.-Petersburg State University, Russia
¹⁶ Pulkovo Observatory, Russia
¹⁷ INFN–Roma “La Sapienza”, Piazzale A. Moro 2, 00185 Roma, Italy
¹⁸ CNR, Istituto Metodologie Inorganiche e dei Plasmi, Area Ricerca Montelibretti (Roma), Italy
¹⁹ INFN–Roma “Tor Vergata”, via della Ricerca Scientifica 1, 00133 Roma, Italy
²⁰ Tuorla Observatory, University of Turku, Finland
²¹ Dip. di Fisica, Univ. dell'Insubria, via Valleggio 11, 22100 Como, Italy
²² ENEA–Roma, via E. Fermi 45, 00044 Frascati (Roma), Italy
²³ ASI–ASDC, via G. Galilei, 00044 Frascati (Roma), Italy
²⁴ ASI, Viale Liegi 26, 00198 Roma, Italy

Received: 9 July 2008
Accepted: 18 August 2008

Abstract

Aims. We report the γ-ray activity from the intermediate BL Lac S5 0716+714 during observations acquired by the AGILE satellite in September and October 2007. These detections of activity were contemporaneous with a period of intense optical activity, which was monitored by GASP–WEBT. This simultaneous optical and γ-ray coverage allows us to study in detail the light curves, time lags, γ-ray photon spectrum, and Spectral Energy Distributions (SEDs) during different states of activity.

Methods. AGILE observed the source with its two co-aligned imagers, the Gamma-Ray Imaging Detector (GRID) and the hard X-ray imager (Super-AGILE), which are sensitive to the 30 MeV–50 GeV and 18–60 keV energy ranges, respectively. Observations were completed in two different periods, the first between 2007 September 4–23, and the second between 2007 October 24–November 1.

Results. Over the period 2007 September 7–12, AGILE detected γ-ray emission from the source at a significance level of 9.6-σ with an average flux (E > 100 MeV) of (97 ± 15) 10^-8 photons cm^-2 s^-1, which increased by a factor of at least four within three days. No emission was detected by Super-AGILE for the energy range 18–60 keV to a 3-σ upper limit of 10 mCrab in 335 ks. In October 2007, AGILE repointed toward S5 0716+714 following an intense optical flare, measuring an average flux of (47 ± 11) 10^-8 photons cm^-2 s^-1 at a significance level of 6.0-σ.

Conclusions. The γ-ray flux of S5 0716+714 detected by AGILE is the highest ever detected for this blazar and one of the most intense γ-ray fluxes detected from a BL Lac object. The SED of mid-September appears to be consistent with the synchrotron self-Compton (SSC) emission model, but only by including two SSC components of different variabilities.