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AGILE detection of Cygnus X-3 above 400 MeV

ATel #3239; A. Bulgarelli (INAF/IASF Bologna, Italy), E. Striani (Univ. Tor Vergata), G. Piano, M. Tavani (INAF/IASF Roma), F. Gianotti, M. Trifoglio (INAF/IASF Bologna), M. McCollough, P. Savolainen (Smithsonian Astrophys. Observatory, USA), A. Argan, A. Trois, G. De Paris, V. Vittorini, E. Costa, I. Donnarumma, M. Feroci, L. Pacciani, E. Del Monte, F. Lazzarotto, P. Soffitta, Y. Evangelista, I. Lapshov, S. Sabatini, M. Cardillo (INAF-IASF-Rm), A. Chen, A. Giuliani, S. Mereghetti, F. Perotti (INAF-IASF-Milano), M. Marisaldi, G. Di Cocco, C. Labanti, F. Fuschino, M. Galli (INAF/IASF Bologna), G. Pucella, M. Rapisarda (ENEA-Roma), F. D'Ammando, S. Vercellone (INAF/IASF Palermo), A. Pellizzoni, M. Pilia (INAF/OA-Cagliari), G. Barbiellini, F. Longo (INFN and University of Trieste), P. Picozza, A. Morselli (INFN and University of Rome Tor Vergata), M. Prest (University of Insubria, Como), P. Lipari, D. Zanello (INFN and University of Rome La Sapienza), P. W. Cattaneo, A. Rappoldi (INFN Pavia), C. Pittori, F. Verrecchia, F. Lucarelli, P. Giommi, P. Santolamazza, (ASDC), L. Salotti, G. Valentini (ASI)
on 26 Mar 2011; 20:52 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Marco Tavani (tavani@iasf-roma.inaf.it)

Subjects: X-ray, Gamma Ray, Binary, Black Hole, Neutron Star, Transient

Referred to by ATel #: 3325

The AGILE satellite has been monitoring the Cygnus X-3 region in the spinning mode. The AGILE-GRID detector does not detect significant gamma-ray emission during the period of the Fermi-LAT detection on MJD 55642 (March 22, 2011, ATel #3233). Integrating during the period MJD 55641-55642 (a standard 2-day integration of the AGILE spinning mode) we obtain a 95% confidence level upper limit of 1.1 x 10^-6 ph cm^-2 s^-1 (E > 100 MeV). For E > 400 MeV, we obtain a 95% confidence level upper limit of 0.6 x 10^-6 ph cm^-2 s^-1.

However, integrating during the period MJD 55639-55640 the AGILE-GRID multi-source likelihood analysis finds enhanced gamma-ray emission above 400 MeV from a source positionally consistent with Cygnus X-3 at Galactic coordinates (l,b)=(80.23, 0.46) +/- 0.6 degrees (statistical plus systematic error), (R.A.,Dec.) J2000 =(308.66, 41.12) with a flux F = (0.9 +/- 0.3)x10^-6 ph cm^-2 s^-1 at 3.9 standard deviation significance level. For E > 100 MeV, we obtain a marginal detection at a 3 standard deviation significance level with a flux F= (1.4 +/- 0.8) x 10^-6 ph cm^-2 s^-1.

We note that this detection occurs a few days before the prominent hard-X ray flux rise shown by the Swift/BAT data near MJD 55643 after several weeks of low hard X-ray emission.

On MJD 55643 a Swift pointed observation was performed. The spectrum can be adequately fit by an absorbed partial covered disk blackbody with an absorption edge near 7 keV (n_H = 2.6x10^{22} cm^{-2}, n_H(covering) = 4.9x10^{22} cm^{-2}, f_covering = 0.79, T_bb = 1.9 keV). The measured 1-8 keV flux is 7.35x10^{-9} ergs/s/cm^2 (unabsorbed: 1.56x10^{-8} ergs/s/cm^2). The Swift/BAT spectra can be described by a steep (-3.5) power law.

Our detection at 400 MeV and lack of prominent emission in the 100-400 MeV energy band suggests a relatively hard gamma-ray spectrum of this remarkable event. The Cygnus X-3 spectral properties appear to be different from what detected by AGILE during the period preceding the strong radio flare and the hard X-ray spectral change of April 16-17, 2008, as reported in Tavani et al., Nature, 462, 620 (2009).