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Swift-XRT detects X-ray burst from Circinus X-1

ATel #2651; M. Linares (MIT), P. Soleri (Groningen), D. Altamirano, M. Armas-Padilla, Y. Cavecchi, N. Degenaar, M. Kalamkar, R. Kaur, M. van der Klis, A. Patruno, A. Watts, R. Wijnands, Y. Yang (Amsterdam), P. Casella (Southampton), N. Rea (CSIC-IEEC), D. Chakrabarty, J. Homan (MIT)
on 28 May 2010; 22:38 UT
Credential Certification: Manuel Linares (linares@mit.edu)

Subjects: X-ray, Binary, Neutron Star

Referred to by ATel #: 2653, 2671, 2674, 2699

Following the recent re-brightening (ATel #2608) and RXTE-PCA detection of X-ray bursts from the peculiar X-ray binary Cir X-1 between May 15 and 25 (ATel #2643), we obtained a series of Swift-XRT observations of the field (see also ATel #2650).

Swift-XRT detected an X-ray burst on 2010-05-28 at 12:08:19 UTC, which lasted about 60 seconds and had a FRED profile, typical of type I X-ray bursts. The total peak count rate (full XRT energy band; including persistent emission) was about 14 c/s.

Previous bursts from this source could only be localized to within ~0.75 deg (Tennant et al. 1986; EXOSAT-ME) and ~1 deg (ATel #2643; RXTE-PCA) of Cir X-1. Thanks to Swift-XRT we are able to reduce this uncertainty to a position error box that is about 23 arcminutes long and a few arcseconds wide (XRT operating in WT mode), consistent with the Chandra position of Cir X-1 (Iaria et al. 2008). This further strengthens the identification of Cir X-1 as a neutron star.

We thank N. Gehrels and the Swift team for promptly scheduling the observations, and J. Gelbord for comments on the observing strategy.