Swift Observation of SAX J1747.0-2853

ATel #641; A. K.H. Kong (MIT), R. Wijnands (U. Amsterdam), J. Homan (MIT)
on 26 Oct 2005; 07:55 UT
Credential Certification: Albert Kong (akong@cfa.harvard.edu)

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Following the recent report of renewed activity of the neutron star X-ray transient SAX J1747.0-2853 (ATel #637, #638), we have obtained a Swift TOO observation of the field on 2005 October 25. The 1500-sec X-ray Telescope (XRT) observation was obtained in Photon Counting (imaging) mode and detected about 1000 counts from a single point source near the center of the field. The bright source is located at R.A.=17:47:03.0, Decl.=-28:52:56 with an uncertainty of 6 arcsec radius. It is 5.9 arcsec from the Chandra position of SAX J1747.0-2853 (Wijnands et al. 2002) and we therefore conclude that the Swift source is consistent with SAX J1747.0-2853.

We extracted the X-ray spectrum of SAX J1747.0-2853 and it can be fit by an absorbed power-law model with N_H=(7.2+/-1.6)E22 cm^-2 and a photon index of 1.6+/-0.4 (reduced chi square = 0.91 for 46 degrees of freedom). The N_H is consistent with previous X-ray observations. The absorbed 2-10 keV flux was 1E-10 erg/cm^-2/s. For a distance of 8 kpc, this corresponds to 7.7E35 erg/s. This is clearly fainter than the Chandra observation taken on 2005 October 18 (ATel #637) and is consistent with another Chandra observation on 2005 October 21. This suggests that the outburst may start to decline. We note that the source is significantly harder than previous XMM-Newton observations taken at similar flux level (Werner et al. 2004). We did not detect any X-ray bursts from the source.

SAX J17470.0-2853 is the only source in the central 10 arcmin of the XRT. We found another bright source near the edge of the detector. The source is located at R.A.=17:46:21.6, Decl.=-28:43:45. Since it is more than 10 arcmin from the aim point and part of the source is outside the detector, the positional uncertainty can be up to 10 arcsec. This source is about 7 arcsec from 1XMM J174621.1-284343 in the XMM-Newton Serendipitous Source Catalog. The absorbed 2-10 keV flux of the source is 1.5E-11 erg/cm^-2/s (assuming a column density of 7E22 cm^-2 and a photon index of 2) which is roughly consistent with the flux level detected with XMM-Newton.