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Swift/XRT observations of the very-faint X-ray transient XMMU J174716.1-281048

ATEL # 1078; Nathalie Degenaar & Rudy Wijnands (University of Amsterdam)
on 19 May 2007; 8:00 UT
Password Certification: Rudy Wijnands (rudy@space.mit.edu)

Subjects: X-ray, Binaries, Neutron Stars, Transients

We observed the very-faint X-ray transient XMMU J174716.1-281048 during a total of 5.5 ksec Swift/XRT pointings, performed on May 13 and May 17 2007. We have clearly detected the source during our observations. By fitting the source spectrum with an absorbed powerlaw model (nH=9.0E22 cm-2, photon index=2.25), we find an unabsorbed flux of 7.4E-12 erg/cm2/s (2-10 keV). Del Santo et al. (2007; astro-ph/0704.2134) suggested a distance toward the source of approximately 3 to 4 kpc, based on the type-I X-ray burst detected by INTEGRAL (ATEL #970). Assuming a distance of 3.5 kpc, the derived flux translates to a luminosity of 1.1E34 erg/s. The XRT position of the source is found to be RA (J2000.0)=17 47 16.2, Dec J2000.0)=-28 10 46.81 (with an uncertainty of 3.8 arcsec), which is consistent with the XMM-Newton position of the source (ATEL #147).

The neutron star X-ray binary XMMU J174716.1-281048 was detected in outburst with XMM-Newton in 2003 and 2005 (ATEL #147 and Sidoli et al., 2006, A&A 456, 287). At these times the source displayed a luminosity of Lx~1E34 (d/3.5kpc) erg/s, with spectral parameters similar to the ones we find now. XMMU J174716.1-281048 was not detected during Chandra and XMM observations of the field performed in 2000 and 2001, which indicates that this source is a transient system with a quiescent emission at least two orders of magnitude lower. With such an anomalous low accretion rate during outburst, XMMU J174716.1-281048 can be classified as a very-faint X-ray transient. Based on the properties of the type-I X-ray burst detected from this source, Del Santo et al. (2007; astro-ph/0704.2134) argued that XMMU J174716.1-281048 is undergoing an accretion episode of several years and is therefore a quasi-persistent X-ray transient. Motivated by their paper, we proposed a short Swift/XRT observation to determine the current state of this system. The fact that we again detect XMMU J174716.1-281048 at similar fluxes as during the XMM-Newton observations strengthens the suggestion that the system is undergoing a prolonged accretion episode of many years. Studying this system at all wavelengths can be of great value for our understanding of binary evolution models.

It is a pleasure to thank the Swift team for making these observations possible.


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