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ATEL # 1809; N. Degenaar and R. Wijnands (University of Amsterdam)
on 24 Oct 2008; 9:19 UT
Password Certification: Rudy Wijnands (firstname.lastname@example.org)
Subjects: X-ray, Binaries, Neutron Stars, Transients
Referred to by ATEL #: 1810
On 2008-10-23 Swift/XRT observed the field around the X-ray burster SAX J1753.5-2349, which was recently detected in outburst by both RXTE/PCA and Swift/BAT (Atel #1799). In the ~1 ksec photon counting mode observation, we detect one clear source located at a position of R.A. = 17:53:31.97 and Dec. = -23:49:12.32 (J2000) with an uncertainty of 3.7 arcsec. The Swift/XRT position lies just outside the reported RXTE/PCA error circle for SAX J1753.5-2349 (Atel #1799), but is consistent with the Swift/BAT (Atel #1799) and BeppoSAX (in 't Zand et al. 1998, Nucl. Phys. B 69) locations of the source. The refined position of SAX J1753.5-2349 is ~10 arcmin away from the location of IGR J17536-2339, which is well outside the estimated 4 arcmin Integral error circle for that source (Atel #790).
The Swift/XRT spectrum can be fit by an absorbed powerlaw model with a hydrogen column density of Nh~2.1 (+/-0.6) E22 cm-2 and a photon index of ~2.2 (+/-0.4). The resulting 2-10 keV absorbed and unabsorbed fluxes are ~3.6E-11 erg cm-2 s-1 and ~4.5E-11 erg cm-2 s-1 respectively. For a distance of 8 kpc the unabsorbed flux translates into a luminosity of ~3.4E35 erg s-1.
We note that this same field was observed with Swift/XRT on 2008-02-18. During that ~7.6 ksec exposure no source is detected within the XRT error circle for SAX J1753.5-2349. We can place a 95% confidence upper limit on the unabsorbed source flux of ~2.9E-14 erg cm-2 s-1, adopting a spectral model with Nh=2.1E22 cm-2 and a photon index of 2.2, and applying the prescription for small number statistics given by Gehrels (1986, ApJ 303). The corresponding upper limit on the X-ray luminosity is ~2E32 (d/8kpc) erg s-1.
We thank the Swift team for the quick scheduling of this observation.