Renewed activity of the galactic center X-ray transient XMM J174457-2850.3 detected with Swift/XRT

ATel #4305; N. Degenaar (U. Michigan), R. Wijnands (U. Amsterdam), J. A. Kennea (PSU), J. M. Miller (U. Michigan) and N. Gehrels (GSFC)
on 12 Aug 2012; 00:17 UT
Credential Certification: Nathalie Degenaar (degenaar@umich.edu)

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Regular monitoring observations of the Galactic center with Swift's X-ray Telescope (XRT) have revealed renewed activity of the X-ray transient XMM J174457-2850.3 (Sakano et al. 2005). The source is first detected during a ~1.1 ks photon counting (PC) mode observation performed on 2012 August 5, at a count rate of ~1E-2 counts s-1. In the subsequent PC mode observation obtained on August 8 (~1.0 ks), its intensity has increased to ~0.1 counts s-1. The source is located near the edge of the CCD during these two observations, which prohibits a detailed spectral analysis. For a simple absorbed powerlaw model with N_H=7.5E22 cm-2 and an index of 1.5 (Degenaar & Wijnands 2010), the XRT count rates of August 5 and 8 translate into a 2-10 keV luminosity of ~1E34 and 1E35 erg s-1, respectively (assuming a distance of 8 kpc).

XMM J174457-2850.3 is an unclassified X-ray transient that has frequently been active in the past years (for an overview, see Degenaar et al. 2012). It typically reaches a peak 2-10 keV luminosity of ~1E36 erg s-1 during outbursts, while its quiescent luminosity is ~1E32 erg s-1. Between 2012 June 27 and August 2, the source intensity was consistent with the XRT background (~2E-3 counts s-1). Using the spectral parameters mentioned above, we estimate a corresponding upper limit on the 2-10 keV luminosity of ~3E33 erg s-1. This suggests that the source was in quiescence during this epoch, and therefore that the new outburst started between 2012 August 2 and 5.

We note that Swift's Burst Alert Telescope (BAT) triggered on XMM J174457-2850.3 on 2012 August 11 (GCN #13619), which is six days after the first activity detected with the XRT. The reported flux seen during the first seconds of the automated follow-up XRT observations would translate into a luminosity of ~3E37 erg/s (for an assumed distance of 8 kpc). This is a factor >10 brighter than the peak intensity that has ever been seen for this source during outburst. Alternatively, the BAT may have detected a thermonuclear X-ray burst, which would then classify XMM J174457-2850.3 as a neutron star low-mass X-ray binary.

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

References:
Sakano et al. 2005, MNRAS 357, 1211
Degenaar & Wijnands 2010, A&A 524, 69
Degenaar et al. 2012, A&A in press, arXiv:1204.6043