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MAXI J0556-332: Swift/XRT localization

ATel #3103; J. A. Kennea (PSU), P. A. Evans (U Leicester), H. Krimm (CRESST/GSFC/USRA), P. Romano, V. Mangano (INAF-IASFPA), P. Curran (CEA-Saclay) and K. Yamaoka (AGU)
on 12 Jan 2011; 04:56 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

Subjects: X-ray, Transient

Referred to by ATel #: 3104, 3110, 3112, 3116, 3119, 3327, 3349, 4524

Starting 20:35 UT on January 11th, 2011, Swift performed a series of target of opportunity observations of the MAXI error circle of the newly discovered transient MAXI J0556-332 (Matsumura et al., ATEL #3102). Utilizing 0.5ks of Photon Counting (PC) mode data, and correcting for the astrometric errors utilizing Swift/UVOT data using the method described by Evans et al. (2009, MNRAS, 2009, 397, 1177), we find a bright uncataloged X-ray source at the following location: RA, Dec (J2000), = 89.19300, -33.17451, which is equivalent to:

RA(J2000) = 05h 56m 46.32s,
Dec(J2000) = -33d 10m 28.2s,

with an estimated uncertainty of 1.7 arc-seconds radius (90% confidence). This lies inside the MAXI error circle, 7 arc-minutes from position reported by Matsumura et al. (ATEL #3102). A catalog search of this position reveals no previously known X-ray source at this location, however near the edge of the XRT error circle is a B=19.4 star, which we tentatively suggest to be the optical counterpart.

During the Swift observations MAXI J0556-332 is bright, with a flux of ~2 x 10-9 erg/s/cm2 (0.3-10 keV), and shows some evidence for brightening during the observing period of 20:35UT to 22:23UT. The PC mode spectrum can be fit by an absorbed power-law model with N_H = 9.8+/-0.6 x 1020 cm-2, and a photon index of 1.87 +/- 0.02.