Swift/XRT detects new outbursts of the galactic center X-ray transients GRS 1741-2853 and XMM J174457-2850.3

ATel #2770; N. Degenaar, R. Wijnands (UvA), J. A. Kennea (PSU) and N. Gehrels (GSFC)
on 3 Aug 2010; 08:02 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Nathalie Degenaar (degenaar@uva.nl)

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Monitoring observations of the Galactic center with the X-ray Telescope (XRT) aboard Swift have revealed renewed activity of the two known transient X-ray sources GRS 1741-2853 and XMM J174457-2850.3.

The neutron star X-ray binary GRS 1741-2853 (Sunyaev 1990, IAU Circ. 5104), located ~10 arcmin from Sgr A*, is first seen active during a 1.2 ks photon counting (PC) mode observation obtained on 2010 July 18. During this pointing the source is weakly detected at a count rate of ~0.01 c s-1, but the intensity increases up to ~0.5 c s-1 in subsequent observations carried out on 2010 July 20, 25, 27, 30 and August 1. GRS 1741-2853 is not detected in any previous Swift/XRT observations of this region performed in 2010, which indicates that the source is undergoing a new outburst. Between July 18 and August 1 the source is detected at an average count rate of 0.16 c s-1 and the mean X-ray spectrum in this period can be described by an absorbed powerlaw model with index 2.6 +/- 0.2, for a fixed hydrogen column density of 1.4E23 cm-2 (taken from Degenaar & Wijnands 2010, arXiv:1007.0249). The resulting absorbed (unabsorbed) flux in the 2-10 keV energy range is 2.3E-11 (6.4E-11) erg cm-2 s-1. This translates into a luminosity of 4.0E35 erg s-1 for a distance of 7.2 kpc (which was inferred from type-I X-ray burst analysis; Trap et al. 2009, A&A 504, 501).

The unclassified X-ray transient XMM J174457-2850.3 (Sakano et al. 2005, MNRAS 357, 1211) is located ~14 arcmin from Sgr A* and is detected at count rates of ~0.02, 0.09 and 0.05 c s-1 during PC mode observations obtained on 2010 July 27, 30 and August 1, respectively. The source is not detected during any of the other Swift/XRT monitoring observations carried out in 2010. The mean source spectrum extracted from the XRT data of 2010 July 27-August 1 can be fit with an absorbed powerlaw model with index 2.0 +/- 0.7, for a fixed hydrogen column density of 7.5E22 cm-2 (from Degenaar & Wijnands 2010). The resulting absorbed (unabsorbed) flux in the 2-10 keV passband is 1.0E-11 (1.8E-11) erg cm-2 s-1, which corresponds to a luminosity of 1.4E35 erg s-1 for an assumed distance of 8 kpc.

Finally, we report that the Swift monitoring observations of the Galactic center detect continued activity of the neutron star X-ray transient AX J1745.6-2901, which started a new outburst between 2010 June 12 and 15 (ATel #2690). This source has remained active since that time, displaying an average absorbed (unabsorbed) flux of 8.9E-12 (4.4E-11) erg cm-2 s-1, as deduced by fitting the mean X-ray spectrum with a powerlaw model yielding an index of 3.1 +/- 0.3 for a fixed hydrogen column density of 2.4E23 cm-2 (from Degenaar & Wijnands 2010). The inferred luminosity is 3.4E35 erg s-1 for an assumed distance of 8 kpc.

Monitoring observations of the Galactic center with Swift/XRT are continuing.