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Swift/XRT detects renewed activity of the galactic center X-ray transient AX J1745.6-2901

ATEL # 2690; N. Degenaar, R. Wijnands (UvA), J.A. Kennea (PSU) and N. Gehrels (GSFC)
on 22 Jun 2010; 15:21 UT
Distributed as an Instant Email Notice (Transients)
Password Certification: Rudy Wijnands (rudy@space.mit.edu)

Subjects: X-ray, Binaries, Neutron Stars, Transients
Referred to by ATEL #: 2770

Regular monitoring of the Galactic center with the Swift X-ray Telescope (XRT) has revealed activity of a transient X-ray source located ~1.5' SE from Sgr A*. The source is clearly detected during photon counting (PC) mode observations obtained on 2010 June 15 and 18, which had exposure times of 1.4 and 1.0 ks, respectively. However, this object is not detected in a 1.1 ks exposure performed on 2010 June 12, nor in any previous observations of this region carried out in 2009 and 2010. The XRT coordinates of the X-ray transient are R.A. = 266.3987 and Dec. = -29.0254, with an uncertainty of 3.6" (J2000). This coincides with the position of AX J1745.6-2901 (Degenaar & Wijnands 2009, A&A 495, 547). The Swift observations likely detect renewed activity from this galactic transient, which is an eclipsing X-ray burster with an ~8.4 hour period that was discovered by ASCA in 1993 (Maeda et al. 1996, PASJ 48, 417).

The XRT observations of 2010 June 15 and 18 suggest that the source intensity is rising. The spectra of both observations can be described by an absorbed powerlaw with index 3.4 +/- 1.2, for a fixed hydrogen column density of 2.3E23 cm-2 (Degenaar & Wijnands 2009). The resulting absorbed (unabsorbed) fluxes in the 2-10 keV energy band are 8.7E-12 (4.9E-11) and 1.4E-11 (8.0E-11) erg/cm2/s for June 15 and 18, respectively. The corresponding luminosities are 3.8E35 and 6.1E35 erg/s, for an assumed distance of 8 kpc. The 95% confidence upper limit on the unabsorbed 2-10 keV flux for the observation of June 12 is <7E-12 erg/cm2/s, which translates into a luminosity of <5E34 erg/s.

Swift monitoring observations of the Galactic center detected two previous outbursts from this source between 2006 February and June (Kennea et al. 2006, ATel #753; Degenaar & Wijnands 2009), and between 2007 February and 2008 September (Degenaar & Wijnands 2009; Degenaar & Wijnands in prep.). Prior to the Swift observations, no outbursts have been reported for this source other than the ASCA detections in 1993 and 1994 (Maeda et al. 1996).

Monitoring observations of the Galactic center, covering the region of this source, are continuing.


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