Letter to the Editor

AGILE and Swift simultaneous observations of the blazar S50716+714 during the bright flare of October 2007

¹ ASI Science Data Center, ASDC c/o ESRIN, via G. Galilei snc 00044 Frascati, Italy e-mail: paolo.giommi@asi.it
² Dipartimento di Fisica, Università Sapienza, P.le Aldo Moro 2, 00185 Roma, Italy
³ INAF – IASF Bologna, via Gobetti 101, 40129 Bologna, Italy
⁴ INAF, IASF – Milano via E. Bassini 15, 20133 Milano, Italy
⁵ INAF, IASF – Roma, via Fosso del Cavaliere 100, 00133 Roma, Italy
⁶ INFN Trieste, via Padriciano 99, 34012 Trieste, Italy

Received: 13 May 2008
Accepted: 11 June 2008

Abstract

We present the results of a series of optical, UV, X-ray and γ-ray observations of the BL Lac object S50716+714 carried out by the Swift and AGILE satellites in late 2007 when the blazar was flaring close to its historical maximum at optical frequencies. We have found that the optical through soft X-ray emission, likely due to synchrotron radiation, was highly variable and displayed a different behavior in the optical UV and soft X-ray bands. The 4-10 keV flux, most probably dominated by the inverse Compton component, instead remained constant. The counting statistics in the relatively short AGILE GRID observation was low and consistent with a constant γ-ray flux at a level similar to the maximum observed by EGRET. An estimate of the γ-ray spectral slope gives a value of the photon index that is close to 2, suggesting that the peak of the inverse Compton component in the Spectral Energy Distribution (SED) is within the AGILE energy band. The different variability behavior observed in different parts of the SED exclude interpretations predicting highly correlated flux variability like changes in the beaming factor or the magnetic field in simple SSC scenarios. The observed SED changes may instead be interpreted as due to the sum of two SSC components, one of which is constant while the other is variable and with a systematically higher synchrotron peak energy.