Swift detection of an intermediately long X-ray burst from the very faint X-ray binary XMMU J174716.1-281048

We report on the Swift detection of a thermonuclear X-ray burst from the very-faint quasi-persistent neutron star X-ray binary XMMU J174716.1-281048, which triggered the satellite’s Burst Alert Telescope (BAT) on 2010 August 13. Analysis of the BAT spectrum yields an observed bolometric peak flux of ≃4.5 × 10−8 erg cm−2 s−1, from which we infer a source distance of ≲8.4 kpc. Follow-up observations with Swift’s X-ray Telescope (XRT) suggest that the event had a duration of ≃3 h and a total radiated energy of ≃9 × 1040 erg, which classify it as an intermediately long X-ray burst. This is only the second X-ray burst ever reported from this source. Inspection of Swift/XRT observations performed between 2007 and 2010 suggests that the 2-10 keV accretion luminosity of the system is ≃5 × 1034 erg s−1 for an assumed distance of 8.4 kpc. Despite being transient, XMMU J174716.1-281048 appears to have been continuously active since its discovery in 2003.