New Swift SMC transient = IGR J00515-7328?

ATel #3578; J. A. Kennea (PSU)
on 23 Aug 2011; 15:49 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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Sturm et al. (ATEL #3575) reported detection of a new transient in the SMC in recent (August 20-21st, 2011) observations of the SMC by Swift/XRT. We have examined these data, and archival data of this region and have determined that this transient has likely been seen previously in an observation of the SMC performed by Swift on June 21st, 2009, as a follow-up to identify the INTEGRAL detected target IGR J00515-7328. Coe et al (2010, MNRAS, 406, 2533) suggest that this Swift detected transient is the counterpart to IGR J00515-7328, and further suggest that this target is associated with ROSAT PSPC and HRI point sources.

Comparing the 2009 and 2011 data, we find a PSF fitted position for the 2009 transient of RA,Dec (J2000) = 12.9975, -73.4905, which is equivalent to

RA(J2000) = 00h 51m 59.41s
Dec(J2000) = -73d 29m 25.8s

with an estimated error of 3.9'' radius (90% confidence). In comparison the PSF fitted position for the 2011 transient (based on data taken on August 20th, 2011) is RA, Dec (J2000) = 12.9992, -73.4907, which is equivalent to:

RA(J2000) = 00h 51m 59.80s
Dec(J2000) = -73d 29m 26.5s

with an estimated error of 4.0'' radius (90% confidence). These two positions lie 1.9'' apart, making the positions of the 2009 and 2011 transients consistent within errors.

Comparing brightness, the source is seen in 2009 to have a flux of ~7 x 10^-12 erg/s/cm² (0.3-10 keV uncorrected for absorption), and in 2011, ~3.5 x 10^-12 erg/s/cm². Comparing the spectra of the sources in the two epochs, the 2011 spectrum appears to be harder, although low numbers of counts do not allow us to place strong limits on the differences. We therefore conclude that these transients are likely to be the same object, which has been previously identified as a counterpart of IGR J00515-7328.

This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester, and data analysis was performed utilizing the methods of Evans et al. (2009, MNRAS 397, 1177).