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Swift Observation of the Microquasar Cygnus X-3

ATel #1830; G. Stratta (ASDC), F. D'Ammando, Y. Evangelista, M. Tavani (INAF/IASF Roma), N. Gehrels (NASA/GSFC), J. A. Kennea (PSU), G. Pucella, V. Vittorini, E. Costa, M. Feroci, I. Donnarumma, L. Pacciani, E. Del Monte, F. Lazzarotto, P. Soffitta, I. Lapshov, M. Rapisarda, S. Sabatini, G. Piano, A. Argan, A. Trois, G. De Paris (INAF/IASF Roma), M. Marisaldi, A. Bulgarelli, F. Gianotti, M. Trifoglio, G. Di Cocco, C. Labanti, F. Fuschino, M. Galli (INAF/IASF Bologna), A. Chen, P. Caraveo, A. Giuliani, S. Mereghetti, A. Pellizzoni, S. Vercellone, F. Perotti, M. Fiorini, A. Zambra (INAF/IASF Milano), G. Barbiellini, F. Longo, E. Vallazza (INFN Trieste), P. Picozza, A. Morselli (INFN Roma-2), M. Prest (Universita` dell'Insubria), P. Lipari, D. Zanello (INFN Roma-1), P. Cattaneo (INFN Pavia), and S. Cutini, C. Pittori, F. Verrecchia, B. Preger, P. Santolamazza, S. Colafrancesco, P. Giommi (ASDC), L. Salotti (ASI)
on 6 Nov 2008; 09:46 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Marco Feroci (marco.feroci@iasf-roma.inaf.it)

Subjects: X-ray, Request for Observations, Binary

Referred to by ATel #: 1839

SWIFT observed the Galactic microquasar Cygnus X-3 following the AGILE detection of the gamma-ray source AGL J2030+4043, whose position is consistent with that of Cygnus X-3 within the AGILE-GRID error box, as reported by Tavani et al. (ATel #1827). Very recently radio observations with the AMI Large Array showed an interesting low flux density of the source, which could precede a major radio flare (Pooley 2008, ATel #1828). SWIFT-XRT started the observation of Cygnus X-3 on 2008-11-04 at 04:44:28 UT. Data were taken in Windowed Timing mode for a total exposure of 5.9 ksec spanning a total of three orbits. During the first orbit (about 3 ksec) the 0.3-10 keV light curve shows an average count rate of 40-60 counts/s. During the second orbit, the count rate initially started at a value near 100 counts/s showing an increase by an additional 40% within a time scale of order of 500 sec. The third orbit shows an average count rate near 90 counts/s. The XRT spectrum in the 2.5-10 keV range during the first orbit can be fitted by an absorbed power law model plus an emission line (pcfabs*(power law + gaussian). Best fit parameters are N_H = (15 +/- 1)E22 cm^-2 with covering factor of 0.93, photon index = 4.0 +/- 0.1, E_line = 6.66 +/- 0.03 keV (sigma = 0.1 keV fixed). The average 2.5-10 keV flux is 3.6E-9 erg cm^-2 s^-1. During the second orbit, data can be fitted by the same model from 2.5 keV up to 9 keV, showing an excess at energies above 9 keV. The emission line is no more visible. Best fit parameters are N_H = (11 +/- 1)E22 cm^-2 and photon index = 3.6 +/- 0.1. The average 2.5-9 keV flux is 9.8E-9 erg cm^-2 s^-1. Compared to the previous SWIFT-XRT observations of Cygnus X-3, in which the 2-10 keV flux ranged between 0.78 and 20.2 E-9 (Moore et al. 2008, ATel #1655), the flux now observed is high with a rapid increase of the activity of the source during the second orbit. Further multifrequency observations of Cygnus X-3 are encouraged