Swift/XRT detects renewed activity of the Galactic center transient XMM J174457-2850.3

ATel #9551; N. Degenaar (Cambridge), R. Wijnands (UvA), M. T. Reynolds, J. M. Miller (Michigan), J. A. Kennea (PSU), N. Gehrels (GSFC), on behalf of a larger collaboration
on 26 Sep 2016; 21:47 UT
Credential Certification: Nathalie Degenaar (degenaar@ast.cam.ac.uk)

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Daily X-ray monitoring observations of the Galactic center with Swift (Degenaar et al. 2015, JHEAp 7, 137) reveal activity at a position consistent with the location of the transient neutron star low-mass X-ray binary (LMXB) XMM J174457-2850.3. This known thermonuclear X-ray burster is located ~14 arcmin NW of Sgr A*. The source is clearly detected at an XRT count rate of ~2E-2, 3E-2, and 9E-2 c/s during ~1 ks PC-mode observations performed on 2016 September 24, 25 and 26, respectively. The count rate at the source position is consistent with the local background (~3E-3 c/s) during a 1-ks pointing performed on September 23.

We extracted an average spectrum from the September 24 and 25 observations and fitted that together with the September 26 spectral data. Using an absorbed power-law model with the hydrogen column density fixed at 1.1E23 cm-2, we obtain photon indices of 1.6 +/- 0.5 for both spectra (comparable to that found for previous outbursts of this source; Degenaar et al. 2014, ApJ 792, 109). The inferred unabsorbed 2-10 keV fluxes are (1.0 +/- 0.2)E-11 and (3.3 +/- 0.5)E-11 erg/cm2/s for September 24-25 and 26, respectively. The corresponding luminosities are (4.1 +/- 1.0)E34 and (1.7 +/- 0.2)E35 erg/s for a distance of 6.5 kpc (inferred from thermonuclear burst analysis; Degenaar et al. 2014). Using these spectral parameters, the non-detection with Swift on September 23 implies a 2-10 keV luminosity of <5E33 erg/s at that time. This source exhibits occasional outbursts with a 2-10 keV peak luminosity of ~E35-E36 erg/s that last for a few weeks (Degenaar et al. 2014).

XMM J174457-2850.3 is currently the only active X-ray transient in the FOV of the Swift Galactic center monitoring observations (which cover ~20'x20' around Sgr A*), but it is the 6th active transient seen this year. Earlier on in the 2016 campaign, Swift observed activity of the neutron star LMXBs AX J1745.6-2901 (e.g. ATel #5222, #9196) and GRS 1741-2853 (ATel #8881, #9109), as well as the unclassified X-ray transients Swift J174540.7-290015 (e.g. ATel #8649, #8689, #8746, #9109, #9196, #9236), Swift J174540.2-290037 (ATel #9109, #9196, #9236), and Swift J174535.5-285921 (ATel #9236). All those other X-ray transients are currently not detected with the XRT. Daily X-ray observations of the Galactic center with Swift are ongoing.

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