AGILE detection of intense γ-ray activity from the blazar PKS 0537–441 in October 2008
INAF/IASF–Roma, via del Fosso del Cavaliere 100,
2 Dip. di Fisica, Univ. “Tor Vergata”, via della Ricerca Scientifica 1, 00133 Roma, Italy
3 INAF/IASF–Palermo, via U. La Malfa 153, 90146 Palermo, Italy
4 Dip. di Fisica, Univ. dell’Insubria, via Valleggio 11, 22100 Como, Italy
5 INAF – Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34143 Trieste, Italy
6 Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126, Pisa, Italy
7 Dip. di Fisica, Univ. di Perugia, via B. Bonfigli, 06126 Perugia, Italy
8 INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
9 ASI–ASDC, via G. Galilei, 00044 Frascati (Roma), Italy
10 NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
11 INAF/IASF–Bologna, via Gobetti 101, 40129 Bologna, Italy
12 INAF/IASF–Milano, via E. Bassini 15, 20133 Milano, Italy
13 Dip. di Fisica and INFN Trieste, via Valerio 2, 34127 Trieste, Italy
14 INFN–Pavia, via Bassi 6, 27100 Pavia, Italy
15 Dip. di Fisica Nucleare e Teorica, Univ. di Pavia, via Bassi 6, 27100 Pavia, Italy
16 ENEA, via Martiri di Monte Sole 4, 40129 Bologna, Italy
17 INFN–Roma “La Sapienza”, Piazzale A. Moro 2, 00185 Roma, Italy
18 INFN–Roma “Tor Vergata”, via della Ricerca Scientifica 1, 00133 Roma, Italy
19 INAF–OA Cagliari, loc. Poggio dei Pini, strada 54, 09012 Capoterra, Italy
20 ENEA–Frascati, via E. Fermi 45, 00044 Frascati (Roma), Italy
21 ASI, via le Liegi 26, 00198 Roma, Italy
Received: 7 May 2010
Accepted: 26 July 2010
Context. 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.
Aims. 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.
Methods. 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 30 MeV − 30 GeV and 18 − 60 keV 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.
Results. 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) × 10-8 photons cm-2 s-1 for energies higher than 100 MeV. 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.
Conclusions. 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.
Key words: gamma rays: general / BL Lacertae objects: individual: PKS 0537 − 441 / radiation mechanisms: non-thermal
© ESO, 2010