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Swift and INTEGRAL observations of SAX J1747.0-2853

ATEL # 1951; S. Campana (INAF-OAB, Italy) and J. Chenevez (National Space Institute, Denmark), E. Kuulkers (ESA/ESAC, Spain; on behalf of the INTEGRAL Galactic bulge monitoring team)
on 5 Mar 2009; 11:46 UT
Password Certification: Sergio Campana (campana@merate.mi.astro.it)

Subjects: X-ray, Neutron Stars, Transients

On Feb 25, 2009 an X-ray burst from SAX J1747.0-2853 was detected by the INTEGRAL, but no persistent emission was seen (Chenevez et al. 2009, ATEL #1944). RXTE PCA scans of the Galactic center region detected the source at 15 +/- 7 mCrab (2-10 keV) on Feb. 26, 2009 (Markwardt et al., 2009 ATel #1945), even if the field is crowded and the source might be fainter. We observed SAX J1747.0-2853 with Swift XRT on Mar. 3, 2009 between 09:54 and 22:58 UT for 4.6 ks. The source is well detected with a 0.3-10 keV count rate of (2.2+/-0.1)E-1 c/s. The light curve is consistent with being constant. We extracted photons from a 20 pixel circular region, collecting about 900 photons. The source spectrum is well fitted with a power-law or black body model. However, the power-law model requires a very steep spectrum (Gamma=3.0+/-0.5) and a very high column density [N_H=(1.1+/-0.2) E23 cm^-2, errors are 90% c.l.]. The black body model, on the contrary, provides the same good fit (reduced chi^2 of 1.2 for 26 degrees of freedom) but with a smaller column density [N_H=(6.4+/-1.2) E22 cm^-2]. The black body temperature is 1.1+/-0.1 keV and the equivalent radius is 1.1+/-0.3 km (for a source distance of ~8 kpc, based on radius expansion Type I bursts). The 2-10 absorbed (unabsorbed) flux is 2.1(3.4) E-11 erg/cm^2/s. At 8 kpc this corresponds to 2E35 erg/s. Simultaneous observations with INTEGRAL between 11:42 and 15:24 UT confirm the faintness of the source, providing only upper limits: 8.E-11 erg/cm2/s (3-10 keV), 2.E10-11 erg/cm^2/s (10-25 keV) and 1.E-10 erg/cm^2/s (15-40 keV) adopting a Crab like spectrum. Further observations and monitoring will assess if the source, after the bright Type I burst, is going to start a new outburst, remains in this quasi-persistent state or turns down to quiescence. We thank the Swift team for the quick scheduling of this observation.


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