Swift/XRT observations of the X-ray transients KS1741-293 and XTE J1719-291

ATel #1541; N. Degenaar and R. Wijnands (University of Amsterdam)
on 20 May 2008; 15:15 UT
Credential Certification: Rudy Wijnands (rudy@space.mit.edu)

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After our recent report of activity of the neutron star X-ray transient KS1741-293 (ATEL #1531) we obtained a ~1.6 ksec Swift/XRT observation of the field on May 18th 2008. We clearly detect a source at a position consistent with that of the proposed Chandra counterpart of KS1741-293 (ATEL #1531). The source spectrum could be fit with an absorbed power-law model using a hydrogen column density Nh~2.0(+/-0.6)E23 cm-2 and a photon index of ~2.3(+/-0.7). The inferred unabsorbed 2-10 keV flux is ~1.4E-10 erg/cm2/s, which translates into an X-ray luminosity of ~1.1E36 erg/s for an assumed distance of 8 kpc. The source intensity has thus increased by almost a factor of 10 compared to the Chandra detection of the source on May 10th 2008 (ATEL #1531).

We also acquired three new observations of the recently discovered very faint X-ray transient XTE J1719-291 (ATEL #1442), to further monitor the X-ray activity of this source (ATELs #1451, #1467). During the first additional (~1.7 ksec) observation, which was carried out on April 16th 2008, the source displayed an X-ray luminosity of ~2E35 erg/s in the 2-10 keV band (corrected for absorption and assuming a source distance of 8 kpc). Two weeks later, on April 30th 2008 (obtained with a ~2.0 ksec exposure), the source luminosity had decreased over a factor of 10, down to ~9E33 erg/s. During our last ~1.2 ksec Swift/XRT pointing on May 14th 2008, the source is not detected. We can place a 95% confidence upper limit on its 2-10 keV unabsorbed flux of ~5.2E-14 erg/cm2/s (adopting an absorbed power-law spectrum with Nh=0.6E22 cm-2 and index Gamma=3.0), where the prescription for low number statistics given by Gehrels (ApJ 303, 1986) has been applied. Assuming a source distance of 8 kpc, the corresponding upper limit on the X-ray luminosity is ~4E32 erg/s. This indicates that XTE J1719-291 is indeed a transient X-ray source that has likely returned to quiescence after an X-ray outburst that lasted almost two months and showed large X-ray variability (ATELs #1442, #1451, #1467).

We thank the Swift team for scheduling these observations.