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Chandra position and possible IR counterpart for the very-faint X-ray transient XMMU J174716.1-281048

ATel #1136; N. Degenaar (UvA), M. Krauss (MIT), D. Maitra, R. Wijnands (UvA), P. Jonker (SRON/CfA), A. Juett (U. Virginia), C. Markwardt (CRESST/U. Md./GSFC), C. Bailyn (Yale) and D. Chakrabarty (MIT)
on 10 Jul 2007; 16:03 UT
Credential Certification: Rudy Wijnands (rudy@space.mit.edu)

Subjects: Infra-Red, Optical, X-ray, Binary, Neutron Star, Transient

Referred to by ATel #: 1174, 1496, 2050

As part of an existing Chandra program for the accurate localization of neutron-star X-ray transients, we observed the very-faint X-ray transient XMMU J174716.1-281048 (Del Santo et al. 2007, astro-ph/0704.2134) with the ACIS-S detector aboard Chandra, since this source was found to be still actively accreting in May 2007 (ATel #1078). During our ~1 ksec observation, performed on July 6, 2007 (04:54-05:28 UT), we clearly detected the source at a position of R.A. = 17:47:16.16 and Dec. = -28:10:48.0, with an uncertainty of 0.5 arcseconds (J2000, 90% confidence). This refined position is consistent with previous reported coordinates based on XMM-Newton (ATel #147) and Swift (ATel #1078) observations. The ACIS-S source spectrum could be fitted with a simple absorbed powerlaw model with a hydrogen column density of Nh~9E22 cm-2 and a powerlaw component of ~2.2. The inferred unabsorbed flux is ~3.0E-12 erg/cm2/s (2-10 keV), which translates into an X-ray luminosity of ~4.4E33 erg/s for a distance of 3.5 kpc (Del Santo et al. 2007, astro-ph/0704.2134). The displayed X-ray flux and spectral shape are similar to what was seen during our Swift observations of May 13 and May 17 (ATel #1078).

We obtained V, I and H-band images of the field of XMMU J174716.1-281048 using the SMARTS 1.3m telescope and ANDICAM instrument at CTIO on May 27.298 UT. While both USNO-B1.0 and 2MASS point source catalogs do not have any sources within the 0.5 arcsec Chandra error circle, a faint source coincident with the Chandra position was detected at a S/N-ratio of 4.3 in our H-band image, suggesting that this source could be the IR counterpart of XMMU J174716.1-281048. Comparing with the brightnesses of the nearby stars in the 2MASS point source catalog, we estimated the H-band brightness of the tentative counterpart of XMMU J174716.1-281048 to be 15.3 +/- 0.1 magnitude when the frame was taken. Given the number density of sources in the field, the probability of finding at least one background star that is at least as bright as the IR candidate in the Chandra error circle is 1.55%. The source was not detected in I or V bands, most likely due to the high Nh in the direction of the source.