SIMBAD references

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).