2010A&A...522A.109P

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.