AGILE detection of variable gamma-ray activity from the BL Lac S during 2007 September - October observations F. D’Ammando 1,2, A. Chen 3, A. Bulgarelli 4, I. Donnarumma 1, A. Giuliani 3, F. Longo 5, L. Pacciani 1, G. Pucella 1, M. Tavani 1,2, S. Vercellone 3, V. Vittorini 1 on behalf of the AGILE Team (1) INAF / IASF-Roma (2) Dip. Fisica, Università degli Studi di Roma Tor Vergata (3) INAF / IASF-Milano (4) INAF / IASF-Bologna (5) Dip. Fisica, Università di Trieste and INFN with the collaboration of M. Villata and C. M. Raiteri, Osservatorio Astronomico di Torino

S was classified by Biermann (1981) as a BL Lac object, because of its featureless optical spectrum and high linear polarization The optical continuum is so featureless that every attempt to determine the spectroscopic redshift has failed; however, very recently through optical imaging of the underlaying galaxy was stimated a redshift of z = 0.31 ± 0.08 (Nilsson et al. 2008) The source belongs to the Intermediate BL Lac class in terms of its Spectral Energy Distribution. In fact, it shows evidence for a concave X-ray spectrum in the keV band (Tagliaferri et al. 2003, Foschini et al. 2007), a signature of the presence of both the steep tail of the synchrotron emission and the flat part of the Inverse Compton spectrum The variability of this blazar is strong in every band on both long and short intraday timescales The EGRET telescope onboard CGRO detected several times S with an integrated flux above 100 MeV varied between (13 ± 5) and (53 ± 13) x ph cm -2 s -1 AGILE detection of variable gamma-ray activity from the BL Lac S during 2007 September - October observations Filippo D’Ammando AGN 8 22 Maggio 2008

In September, the AGILE satellite was performing its Science Verification Phase and devoted three weeks to the observations of S between 2007 September 4 14:58 UT and September 23 11:50 UT S in mid-October shows increasing optical flux, reaching a peak of F R = 45.7 mJy on October 22.2 (Villata et al. 2008); at that time S , even if rather off-axis (~ 50° from the axis), was seen by AGILE-GRID to have a high gamma-ray flux. In particular, between 2007 October 22 12:33 UT and October 23 12:06 UT the source was detected at a significance level of 4.0-sigma with a flux of F E>100 MeV = (203 ± 75) x ph cm -2 s -1 After this flaring episode, AGILE repointed toward the source and observed S between 2007 October 24 9:47 and November 1 12:00 UT AGILE detection of variable gamma-ray activity from the BL Lac S during 2007 September - October observations Filippo D’Ammando AGN 8 22 Maggio 2008

A strong gamma-ray flare was detected between 7 and 12 September 2007 at a significance level of 9.6-sigma with an average gamma-ray flux of F (E>100 MeV) = (97 ± 15) x ph cm -2 s -1 AGILE detection of variable gamma-ray activity from the BL Lac S during 2007 September - October observations The peak level of the gamma-ray flux is F (E>100 MeV) = (193 ± 42) x ph cm -2 s -1, the highest flux ever detected from S

Filippo D’Ammando AGN 8 22 Maggio 2008 AGILE detection of variable gamma-ray activity from the BL Lac S during 2007 September - October observations Gamma-ray photon spectrum during two different levels of activity: the high activity state of mid-September and the low activity state of end October

AGILE detection of variable gamma-ray activity from the BL Lac S during 2007 September - October observations Filippo D’Ammando AGN 8 22 Maggio 2008 The Spectral Energy Distribution with AGILE and GASP - WEBT data of September 2007 can be fitted only with a model with two SSC components

Filippo D’Ammando AGN 8 22 Maggio 2008 Discrete Correlation Function (DCF) between the gamma-ray and the R-band light curves in October 2007 AGILE detection of variable gamma-ray activity from the BL Lac S during 2007 September - October observations DCF ~ 0.9 at a lag of -1 day suggest a possible delay of the gamma-ray flux variations with respect to the optical ones on the order of 1 day