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Swift/XRT detection of a transient source in the Galactic Center

ATel #753; J. A. Kennea (PSU), D. N. Burrows (PSU), S. Campana (OAB), O. Godet (U Leicester), J. Nousek (PSU) and N. Gehrels (GSFC)
on 28 Feb 2006; 18:59 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

Subjects: X-ray, Transient

Referred to by ATel #: 756, 892, 1005, 1006, 1058, 1513, 2690, 5332

We report the detection in late February 2006 of a variable X-ray source near Sgr A*, by the Swift X-Ray Telescope (XRT) in a series of short (~1ks per day) observations of the Galactic Center. We have analyzed data from four observations and detect a bright point source, Swift J174535.5-290135.6, 90 arcseconds from Sgr A*, at the following coordinates: RA = 17h 45m 35.5s, Dec = -29d 01m 35.6s (J2000) with an estimated uncertainty of 6 arcseconds radius (90% containment). We note that source location is 2.8 arcseconds from the Chandra discovered Galactic Center source, CXOGC J174535.6-290133 (Muno et al, 2004). This location is also 4.6 arcseconds from the position of an ASCA detected eclipsing X-ray burster with an ~8.4 hour period, AX J1745.6-2901 (Maeda et al., 1996, Kennea and Skinner, 1996).

This source was not detected during a 1025s observation starting on February 24th at 22:55UT, but is the brightest source within 10 arcmin of Sgr A* in a 900s observation 9.6 hours later with a flux of 2.3 x 10^-11 erg/s/cm^2 (0.2-10 keV). The 3 sigma upper limit on the flux in the initial observation is 2 x 10^-13 erg/s (0.2-10 keV). It has a power-law spectrum (photon index ~1.6) and its high absorption (1.4 x 10^23 cm^-2) suggesting it is located at the Galactic Center. Assuming a distance of 8.5 kpc, the X-ray luminosity of this source is approximately 3 x 10^35 erg/s (0.2-10 keV), corrected for absorption.

In 2 further observations starting on February 26th at 08:40UT and February 27 at 07:11UT the source remains on, fading gradually with a power-law slope of approximately -0.15. As we have no observations of this field prior to February 24th, we cannot determine if the non-detection of this source on that day was due to the source being in eclipse, or if the source was in a low flux state prior to the February 25th detection. If the source is AX J1746.7-2901, the eclipse hypotheses for the non-detection of the source is unlikely as Maeda et al. (1996) report the flux during eclipse is ~20% of the un-eclipsed level, which would have been detected in our observation.

We note that the spectral parameters for CXOGC J174535.6-290133 are consistent with Swift J174535.5-290135.6, however the reported absorbed flux is ~1000 times fainter than the Swift/XRT detected flux. AX J1745.6-2901 has been shown to vary in brightness over long time periods (e.g. Sakano et al., 2002), and we tentatively suggest that the Swift source may be AX J1745.6-2901 re-entering a high state.

Observations of this source are on-going.