MAXI J0158-744/XRF 111111A: Swift X-ray and U band detection

ATel #3758; J. A. Kennea, E. A. Hoversten (PSU), P. Curran (CEA-Saclay), P. A. Evans (U Leicester). H. A. Krimm (CRESST/GSFC/USRA), P. Romano, V. Mangano (INAF-IASFPA), K. Yamaoka (AGU) and H. Negoro (Nihon U.), M. Kimura (Osaka U.), H. Tomida (JAXA), T. Sootome, M. Serino, T. Mihara, S. Nakahira (RIKEN), M. Morii, N. Kawai (Tokyo Tech)
on 12 Nov 2011; 00:48 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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At 15:33:21UT on November 11th, 2011 Swift began a target of opportunity observation of the error circle of the new transient MAXI J0158-744/XRF 111111A (Kimura et al., ATEL #3756), approximately 12 hours after its initial detection by MAXI. We find an uncatalogued moderately bright (1.5 XRT c/s) X-ray source at the following location: RA/Dec (J2000) = 29.85647, -74.25795, which is equivalent to:

RA(J2000) = 01h 59m 25.55s
Dec(J2000) = -74d 15m 28.6s

with an estimated uncertainty of 2.4 arc-seconds radius (90% confidence). This lies 8.9 arc-minutes from the center of the refined MAXI error ellipse (Morii et al., GCN #12555), consistent with the ellipse when considering both statistical and systematic errors.

Within the XRT error circle lies a catalogued optical source, reported by Nishiyama et al, 2007 to be a candidate Herbig Ae/Be star in the Magellanic Bridge. This optical counterpart is detected by UVOT, which finds a position of: RA/Dec(J2000) = 29.85762, -74.25775, which is equivalent to:

RA (J2000) = 01h 59m 25.83s
Dec (J2000) = -74h 15m 27.9s

with an estimated uncertainty of 0.5 arcsec (radius, 90% confidence, statistical + systematic). UVOT measures a brightness of U = 13.06 +/- 0.02 (not including a 0.02 systematic error). In comparison the USNO-A2.0 catalog reports a brightness of B=15.4 for the optical counterpart, which lies inside the UVOT error circle. Since such a large U-B value is unlikely and suggests that the optical counterpart has brightened, implying that this is likely the counterpart of the X-ray transient MAXI J0158-744.

The X-ray source appears soft, with no emission above 2 keV. However, further longer observations will be needed to determine of the spectral parameters, due to pile-up issues and the relatively short exposure time. A search of archival images from the BAT transient monitor for the past month show no sign of emission in the 15-50 keV range with a an average 1-sigma limit of 0.001 ct/s/cm^2 (~5 mCrab).  This is consistent with the soft spectrum of the source.

Further observations of this target with Swift have been requested.