Astronomy and Astrophysics, volume 522, A109-109 (2010/11-1)
AGILE detection of intense γ-ray activity from the blazar PKS 0537-441 in October 2008.
PUCELLA G., D'AMMANDO F., ROMANO P., TREVES A., PIAN E., VERCELLONE S., VITTORINI V., PIANO G., IMPIOMBATO D., FUGAZZA D., VERRECCHIA F., KRIMM H.A., DONNARUMMA I., TAVANI M., BULGARELLI A., CHEN A.W., GIULIANI A., LONGO F., PACCIANI L., ARGAN A., BARBIELLINI G., BOFFELLI F., CARAVEO P., CATTANEO P.W., COCCO V., COSTA E., DEL MONTE E., DE PARIS G., DI COCCO G., EVANGELISTA Y., FEROCI M., FIORINI M., FROYSLAND T., FUSCHINO F., GALLI M., GIANOTTI F., LABANTI C., LAPSHOV I., LAZZAROTTO F., LIPARI P., MARISALDI M., MEREGHETTI S., MORELLI E., MORSELLI A., PELLIZZONI A., PEROTTI F., PICOZZA P., PILIA M., PREST M., RAPISARDA M., RAPPOLDI A., SABATINI S., SOFFITTA P., STRIANI E., TRIFOGLIO M., TROIS A., VALLAZZA E., ZAMBRA A., ZANELLO D., PERRI M., PITTORI C., SANTOLAMAZZA P., GIOMMI P., ANTONELLI L.A., COLAFRANCESCO S. and SALOTTI L.
Abstract (from CDS):
We report the detection by the AGILE satellite of intense γ-ray activity from the source 1AGL J0538-4424, associated with the low-energy-peaked BL Lac PKS 0537-441, during a target of opportunity (ToO) observation performed on 2008 October 10-17, triggered by a Fermi-LAT alert, together with REM and Swift observations. The quasi-simultaneous near-infrared, optical, UV, X-ray, and γ-ray coverage allowed us to investigate the behaviour of the source in different energy bands and study the spectral energy distribution and a theoretical model that can describe the γ-ray state observed in mid-October. AGILE observed the source with its two co-aligned imagers: the Gamma-Ray Imaging Detector (GRID) and the hard X-ray imager (SuperAGILE), sensitive in the 30MeV-30GeV and 18-60keV ranges, respectively. During the AGILE observation, the source was monitored simultaneously in the UV and X-ray bands by the Swift satellite through 6 ToO observations carried out between 2008 October 8 and 17. Moreover, the source was observed in the near-infrared and optical bands by the REM telescope on 2008 October 7, 8, and 9. During 2008 October 10-17, AGILE-GRID detected γ-ray emission from PKS 0537-441 at a significance level of 5.3-σ with an average flux of (42±11)x10–8photons/cm2/s for energies higher than 100MeV. A significant increase in the γ-ray activity was detected between the first and the second halves of the observing period. REM and Swift/XRT detected the source in near-infrared/optical and X-rays during a relatively low and intermediate activity state, respectively, with no signs of evident variability in the different observations. However, Swift/UVOT detected an increase between the first and the second parts of the observing period, smaller than in the γ-rays. The average γ-ray flux of PKS 0537-441 detected by AGILE is close to the average flux observed for this source by the EGRET and Fermi-LAT instruments, with an increase of a factor 3 throughout the observation period up to a flux level slightly lower than the highest flux observed by Fermi-LAT during the first 11 months of operation. The spectral energy distribution of PKS 0537-441 in mid-October 2008 seems to require two synchrotron self-Compton components to be modelled, to account for both the near-infrared/optical bump and the X-ray data, together with the information on the γ-ray flux level observed by AGILE. An alternative model based on the external Compton radiation, which requires an accretion disk with a relatively high luminosity, is also proposed.