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Swift/XRT monitoring observations detect an active X-ray transient near the Galactic center

ATel #5222; N. Degenaar (Michigan), R. Wijnands (UvA), M. T. Reynolds, J. M. Miller (Michigan), J. A. Kennea (PSU) and N. Gehrels (GSFC), on behalf of a larger collaboration
on 23 Jul 2013; 13:09 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Nathalie Degenaar (degenaar@umich.edu)

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

Referred to by ATel #: 5226, 5246, 5332, 8649, 8746, 9196, 9551

We report on ongoing daily X-ray monitoring observations of the Galactic center with the Swift/XRT (ATel #5006; see link below). In addition to continued activity of the magnetar SGR J1745-29 (ATels #5009,#5016, #5020, #5032, #5037, #5046, #5053; Kennea et al. 2013; Mori et al. 2013), a transient X-ray source located ~1.5 arcmin south-east of Sgr A* has become active. This object is first detected during a ~1.0 ks PC mode observation performed on 2013 July 18, at a net count rate of ~1E-2 counts/s. In the subsequent observation obtained on July 22 (~0.9 ks), the source has brightened to 8E-2 counts/s. No activity is seen at this position during previous Swift/XRT observations in 2013.

We combined the two XRT observations to extract an average X-ray spectrum. The spectral data can be described by an absorbed power-law model with NH~1E23 cm-2 and a photon index of ~1.9. The resulting absorbed (unabsorbed) 2-10 keV flux is 1.2E-11 (2.3E-11) erg/cm2/s. Assuming a distance of 8 kpc, the corresponding 2-10 keV luminosity is ~1.4E35 erg/s.

Using the online XRT data product tools (Evans et al. 2009), we determine a PSF-fitted position of R.A. = 17:45:36.42 and Dec. = -29:01:31.7 (J2000), with an uncertainty of 3.7" (90% confidence). These coordinates are ~10" from that of the transient neutron star low-mass X-ray binary AX J1745.6-2901 and ~13" from that of the unclassified very-faint X-ray transient CXOGC J174535.5-290124. Both are recurrent transients that have frequently been seen active with Swift/XRT in the past years (Degenaar & Wijnands 2009, 2010). AX J1745.6-2901 exhibits type-I X-ray bursts and displays X-ray eclipses that indicate a binary orbital period of ~8.4 hr (Maeda et al. 1996). Its outbursts typically last for several months and reach up to a 2-10 keV luminosity of a few times 1E35-1E36 erg/s (assuming a distance of 8 kpc). CXOGC J174535.5-290124 has been seen active for months at a time as well, but never brighter than ~1E35 erg/s. We tentatively associate the currently active transient with AX J1745.6-2901, but renewed activity from CXOGC J174535.5-290124 can not be excluded. Alternatively, Swift/XRT may have detected a new, previously unknown X-ray transient. Swift/XRT observations are ongoing and may allow us to distinguish between these different possibilities.

The Swift/XRT Monitoring Campaign Website can be found at: http://www.swift-sgra.com

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Degenaar & Wijnands 2010, A&A 524, 69
Evans et al. 2009, MNRAS 397, 1177
Kennea et al. 2013, ApJ 770, L24
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Mori et al. 2013, ApJ 770, L23