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Further Chandra observations of SAX J1747.0-2853 and the region around Sgr A*

ATel #638; R. Wijnands (U. Amsterdam), M. Rupen (NRAO), D. Steeghs (CfA), Q. D. Wang (U. Massachusetts), E. Cackett (U. St Andrews), E. Kuulkers (ISOC, ESA/ESAC), R. Fender, R. Cornelisse, T. Maccarone (U. Southampton), J. Grindlay (CfA), J. Miller-Jones, M. van der Klis (U. Amsterdam), J. Homan (MIT), J. in 't Zand (SRON-Utrecht), C. B. Markwardt (U. Maryland, NASA/GSFC)
on 22 Oct 2005; 19:45 UT
Credential Certification: Rudy Wijnands (rudy@space.mit.edu)

Subjects: X-ray, Binary, Neutron Star, Transient

Referred to by ATel #: 641, 642, 892, 1531

On 20-21 October 2005, the remaining six Chandra/HRC-I observations (as mentioned in ATEL #637) were performed as part of the XMM-Newton/Chandra monitoring campaign of the 1.7 square degree region around Sgr A* (Wijnands et al. 2005, A&A submitted, astro-ph/0508648). The neutron star X-ray transient SAX J1747.0-2853 was in the field-of-view of one of these observations and we detected the source at a 2-10 keV luminosity (at a distance of 8 kpc) of ~8E35 erg/s (obtained using PIMMS and assuming an absorbed power-law model with a column density of 7E22 cm^-2 and a photon index of 2; uncorrected for vignetting effects). This luminosity is a factor of 2 lower than reported two days ago in ATEL #637. This decrease in luminosity cannot be a vignetting effect since during the current observation the source was located only ~8.7 arcminutes away from the aim-point compared to ~15 arcminutes on 18 October 2005. Again no X-ray bursts were seen.

Aside from SAX J1747.0-2853 we did not detect any other transient active in the target region with 2-10 keV luminosities above 1E34 erg/s. However, we did detect activity for two very faint X-ray transients discovered by Muno et al. (2005, ApJ, 622, L113). Both CXOGC J174541.0-290014 and CXOGC J174535.5-290124 were detected at a 2-10 keV flux of ~3E-13 erg/cm2/s (assuming a column density of 6e22 cm^-2 and a photon index of 1.8) resulting in an 2-10 keV luminosity of ~2E33 erg/s for a distance of 8 kpc. This is in the range of X-ray luminosities reported by Muno et al. (2005) but at least an order of magnitude higher than their quiescent X-ray luminosities. Therefore, our observations strongly indicate that these two sources are currently active. Neither source was detected during our June 2005 observations. However, their current fluxes are similar to the detection limits during the June observations.

Follow-up observations at infrared and radio observations are encouraged for all three sources to search for counterparts at these wavelengths.