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Chandra detects activity from the Galactic X-ray transients KS 1741-293, Swift J174535.5-290135.6 and CXOGC J174535.5-290124

ATel #1531; N. Degenaar, R. Wijnands (University of Amsterdam) and M. Muno (Caltech) on behalf of a larger collaboration
on 16 May 2008; 16:22 UT
Credential Certification: Rudy Wijnands (rudy@space.mit.edu)

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

Referred to by ATel #: 1541, 3632, 4840

On May 10-11 2008, a series of 7 Chandra/HRC-I observations was performed as part of our XMM-Newton/Chandra monitoring campaign of the region around Sgr A* (Wijnands et al. 2006, A&A, 449, 1117). Three X-ray transients were active during our observations.

We detect a very-faint X-ray transient with an HRC-I count rate of ~3E-2 count/s at a position of R.A. = 17h 44m 51.61s and Dec. = -29h 20m 42.2s (J2000, with an error radius of 0.6"). This source is also found active in XMM-Newton monitoring observations performed on September 6 2007. From that observation we extracted a source spectrum that could be fit with an absorbed powerlaw model with Nh~1.7(+/-0.2)E23 cm-2 and Gamma~1.6(+/-0.3). The corresponding unabsorbed 2-10 keV flux is ~2.0E-11 erg/cm2/s, which translates into an X-ray luminosity of ~1.5E35 (d/8 kpc) erg/s. With these spectral parameters, the current HRC-I count rate converts into an unabsorbed flux of 1.8E-11 erg/cm2/s in the 2-10 keV band, implying a similar luminosity of ~1.4E35 erg/s. In addition, we found the system active during a serendipitous Chandra/ACIS-S observation carried out on July 22, 2005. During that ~10 ksec exposure, the source is about a factor of 10 brighter, displaying a luminosity of ~1E36 erg/s. In several archival Chandra observations the source is detected at a quiescent luminosity of ~1E33 erg/s (named CXOGC J174451.6-292042, Muno et al. 2008, ApJ submitted).
CXOGC J174451.6-292042 lies within the region of the known neutron star transient KS 1741-293 (e.g., in 't Zand et al. 1991, Adv. Space Res. 11, 187), which has been detected by various X-ray instruments over the years (KVANT, BeppoSAX, ASCA, Integral). Recently, Marti et al. 2007 (A&A 462, 1065) identified KS 1741-293 with CXOGC J174451.0-292116, reported by Muno et al. 2006 (ApJS 165, 17). Since CXOGC J174451.6-292042 has been observed to undergo several X-ray outbursts that match both the level of activity and the spectral shape reported for KS 1741-293 (see Marti et al. 2007 for a detection history), we tentatively conclude that CXOGC J174451.6-292042 rather than CXOGC J174451.0-292116, for which no outburst has been seen in all archival Chandra nor in our XMM-monitoring data, is the Chandra counterpart of KS 1741-293.

We also detect continued activity of the neutron-star X-ray transient Swift J174535.5-290135.6/AX J1745.6-2901 (Atel #1513). The HRC-I source intensity was ~0.21 count/s. Assuming the previously reported absorbed power-law X-ray spectrum with photon index Gamma = 1.0 and absorption column Nh = 1.7E23 cm-2 (ATEL #1513), the unabsorbed X-ray flux is ~1.3E-10 erg/cm2/s in the 2-10 keV band. For a source distance of 8 kpc, this results in an X-ray luminosity of ~1.0E36 erg/s, which is consistent with the level of activity reported by Heinke et al. 2008 (Atel #1513).

A third transient is found active only 11.5" away from Swift J174535.5-290135.6, at a position that is coincident with that of the very-faint X-ray transient CXOGC J174535.5-290124 (Muno et al. 2005, ApJ, 622, L113), so likely we have detected renewed activity of this source. For an absorbed powerlaw spectrum with Nh=1.5E23 cm-2 and Gamma=2.0, the HRC-I count rate of ~1.4E-3 count/s translates into an unabsorbed flux of ~2.7E-13 erg/cm2/s or an X-ray luminosity of ~2E33 (d/8kpc) erg/s. CXOGCJ174535.5-290124 has been observed in outburst with Chandra and XMM-Newton on numerous occasions over the past years, displaying typical outburst luminosities of 10E33-10E34 erg/s (e.g, Muno 2005, ApJ 622, L113, Atel #638, Atel #892), i.e. similar to the current detection.