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ATEL # 1055; M. Linares, R. Wijnands, M. van der Klis (University of Amsterdam)
on 14 Apr 2007; 5:52 UT
Password Certification: Manuel Linares (firstname.lastname@example.org)
Subjects: X-ray, Binaries, Neutron Stars, Pulsars, Transients
After the recent reports of activity of the accreting millisecond pulsar XTE J1751-305 (ATELs #1045, #1046 and #1051) we obtained another Swift/XRT (~4.3ksec) observation of the field starting on April 11th 2007 23:59. The source was not detected with a 2-10 keV upper limit of ~5E-14 erg/cm2/s, corresponding to a luminosity of 3.8E32 (d/8kpc)**2 erg/s. This shows that the source decayed further and might have reached quiescence again. We note, however, that this luminosity upper limit is somewhat higher than the ~0.2-2E32 (d/8kpc)**2 erg/s upper limit found by Wijnands et al (2005), so we cannot exclude that the source is still active at a low but higher than quiescent level.
Using an approximate average intensity of 5 mCrab and an outburst duration of 7 days we estimate a fluence of ~1E-4 erg/cm2 for this outburst, i.e. 10% of the estimate for the 2002 outburst (Markwardt et al. 2002). If dim and short episodes of activity like this outburst and the one of March 2005 (ATELs #446, #449) are common, they could have an appreciable effect on the time-averaged mass accretion rate. For instance, if two such small outbursts occur between two main outbursts this would increase the overall fluence and the time-averaged mass accretion rate by ~20%, and this increase could be higher if some outbursts are not detected. Frequent and sensitive observations can clarify this.
We thank the Swift team for a prompt scheduling of the observation.