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ATEL # 1975; Melania Del Santo (INAF/IASF-Roma), Patrizia Romano (INAF/IASF-Palermo), Lara Sidoli (INAF/IASF-Milano)
on 17 Mar 2009; 19:46 UT
Password Certification: Melania Del Santo (email@example.com)
Subjects: X-ray, Binaries, Neutron Stars, Transients
On 2009 March 13th Swift/XRT observed the field of the Very Faint X-ray Transient (VFXT) SAX J1753.5-2349. In the ~2.2 ks (net exposure) photon counting mode observation, no sources are detected in the XRT error box for SAX J1753.5-2349. A 3-sigma upper limit can be measured at about 6E-3 counts/s. Adopting a spectral model with Nh=2.1E22 cm-2 and a photon index of 2.2, we obtain an unabsorbed flux as 6.0E-14 erg cm-2 s-1 (2-10 keV), corresponding to a luminosity level of about 5E32 erg s-1 (at 8 kpc).
SAX J1753.5-2349 is a neutron star X-ray binary discovered by BeppoSAX in 1996 during a type-I burst event without any persistent emission detected (in't Zand et al. 1996), i. e. at a luminosity level below the WFC sensitivity. Thereafter it was no longer detected until 2008 October 11th, when PCA/RXTE reported an outburst at 8 mCrab flux level (ATel #1799), corresponding to a peak luminosity of about 1E36 erg s-1 (d=8kpc).
The flux decreasing phase has been reported by the IBIS telescope on-board INTEGRAL in the middle of October (ATel #1810) and by Swift/XRT (2008 October 24th) when SAX J1753.5-2349 was about 3.5E35 erg s-1 (ATel #1809).
Due to the sensitivities, hard X-ray detectors (i. e. IBIS and BAT), cannot follow the complete evolution of outbursts of the Very Faint X-ray Transients (i. e. outburst luminosities of 1E34-1E36 erg s-1). However, from the end of October 2008 until March 3009 SAX J1753.5-2349 was no longer observed with Swift/XRT.
It should be noted that on 2008 February 12th, SAX J1753.5-2349 was quiescent (ATel #1809). This allow us to estimate the lower limit on the outburst duration as 5 months, while the upper limit is one year.
We would like to thank the Swift Team for performing these observations.