Swift/XRT observations of Cyg X-1 during state transition

ATel #2724; Evangelista, Y.; Campana, R.; Del Monte, E.; Sabatini, S. (INAF/IASF-Rm); Striani, E. (Univ. Roma Tor Vergata and INFN Roma); Campana, S. (INAF-OAB); Costa, E.; Donnarumma, I.; Feroci, M.; Lazzarotto, F.; Pacciani, L.; Soffitta, P.; Lapshov, I.; Tavani, M.; Vittorini, V.; Argan, A.; Trois, A.; Piano, G. (INAF/IASF-Rm); Rapisarda, M. (ENEA Frascati); Giuliani, A.; Chen, A.; Mereghetti, S.; Perotti, F.; Caraveo, P. (INAF/IASF Mi); Pellizzoni, A.; Pilia, M. (INAF/OA-Cagliari); D'Ammando, F.; Vercellone, S. (INAF/IASF Pa); Bulgarelli, A.; Gianotti, F.; Trifoglio, M.; Di Cocco, G.; Labanti, C.; Fuschino, F.; Marisaldi, M.; Galli, M. (INAF/IASF Bo); Barbiellini, G.; Longo, F.; Moretti, E. (INFN Ts); Picozza, P.; Morselli, A. (INFN and Univ. Roma Tor Vergata); Prest, M.; Vallazza, E. (Universita' dell'Insubria); Lipari, P.; Zanello, D. (INFN and Univ. Roma Sapienza); Cattaneo, P. W.; Rappoldi, A. (INFN Pv); Pittori, C.; Verrecchia, F.; Santolamazza, P.; Lucarelli, F.; Colafrancesco, S.; Giommi, P. (ASDC); Salotti, L. (ASI)
on 6 Jul 2010; 17:04 UT
Credential Certification: Ettore Del Monte (ettore.delmonte@iasf-roma.inaf.it)

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Following the MAXI/GSC (ATel #2711) and RXTE/ASM (ATel #2714) detection of a spectral softening of the BHC Cyg X-1, we requested and obtained a SWIFT/XRT ToO observation of the source.

The SWIFT/XRT observations were performed on 2010-07-02 between 14:06:17 UT and 18:27:40 UT in WT mode, for a total on-source time of about 6 ks distributed in three intervals of 2525 s, 2525 s and 885 s. We performed a spectral fit of the three observations using a model composed by an absorbed disc black body plus a power-law. Fit results are reported in the table below.

Begin Obs. [UT]	NH [1022cm-2]	kT [keV]	Photon Index	  Unabs. Flux 2-10 keV [10-8ergs cm-2s-1]Â
2010-07-02 14:58:39	0.63 +/- 0.02	0.47 +/- 0.01	2.21 +/- 0.07	3.60
2010-07-02 16:35:00	0.63 +/- 0.01	0.52 +/- 0.01	2.09 +/- 0.06	4.10
2010-07-02 18:11:21	0.64 +/- 0.02	0.51 +/- 0.01	2.20 +/- 0.10	3.53

Following Zdziarski et al., ApJ, 578, 357 (2002) and Malzac et al., Procs. ESA SP-604 (2006), the powerlaw photon indices from the fit show that the source has not completed the state transition at the epoch of the observations, but it was still in the intermediate spectral state (Belloni et al., ApJ, 472, L107, 1996).

We thank the Swift team for their prompt scheduling of these observations.