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ATEL # 1260; M. Linares, N. Degenaar, R. Wijnands (Amsterdam)
on 6 Nov 2007; 19:28 UT
Password Certification: Manuel Linares (firstname.lastname@example.org)
Subjects: X-ray, Binaries, Neutron Stars, Transients
We observed the neutron star transient SAX J1810.8-2609 with Swift-XRT on November 3rd and 5th, in order to follow its way to quiescence. The source was not detected during the individual ~1.6 ksec and ~1.9 ksec observations. We place 95% upper limits on its 0.5-10 keV unabsorbed flux of [3.4-4.2]E-13 erg/cm2/s (November 3rd.) and [2.9-3.6]E-13 erg/cm2/s (November 5th.), where the range given corresponds to the uncertainty in the spectral model assumed (absorbing column density of 0.3E22 cm^-2 and a power law with index 2.5-1.5) and the prescription for low number statistics given by Gehrels (1986) has been applied. At a distance of 5kpc (Natalucci et al., 2000), this corresponds to a luminosity of ~1E33 erg/s.
This clearly indicates that the outburst that started in August 2007 (Atels #1175, #1185 and #1227) has finished. Furthermore, by adding both observations (total exposure ~3.5 ksec) a clustering of photons becomes apparent inside a 15 arcsec circle centered in the Chandra position (Jonker et al 2004). Assuming the spectral model above mentioned, this translates to an 0.5-10 keV unabsorbed flux of [1.3-1.7]E-13 erg/cm2/s, or a luminosity of ~4.5E32 erg/s.
Comparing our result with the RXTE-PCA flux measured on October 25th. (that corresponds to a 0.5-10 keV luminosity of ~7E35 erg/s assuming the same spectral model), we conclude that the source luminosity has decayed by three orders of magnitude in about ten days. However, given the low quiescent luminosity found by Jonker et al. (2004) in SAX J1810.8-2609 (~1E32 erg/s), we note that our measurement does not necessarily imply that the source has reached quiescence.
We thank the Swift team for scheduling the observations.