Swift/XRT detects a new outburst of the Galactic Center transient AX J1745.6-2901
ATel #13150; N. Degenaar (U. of Amsterdam), R. Wijnands (U. of Amsterdam), M. T. Reynolds (U. of Michigan), J. M. Miller (U. of Michigan), J. A. Kennea (PSU), on behalf of a larger collaboration
on 1 Oct 2019; 07:27 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Nathalie Degenaar (degenaar@uva.nl)
Subjects: X-ray, Neutron Star, Transient
Our daily Swift/XRT monitoring observations of the Galactic Center (Degenaar et al. 2015, JHEAp, 7, 137) show X-ray activity of a transient source located ~1.5' to the south-east of Sgr A*. This object is clearly detected in individual XRT images since September 17 (obs ID 00095329145), with a count rate of ~0.04-0.12 c/s.
Using the tool xrtcentroid on a combined image of the 13 observations taken between September 17 and 29 (obsID 00095329145-157), we determine that the position of the transient is RA = 17:45:36.16, DEC = -29:01:33.24 (J2000) with a 90% confidence error of 3.6". These coordinates are consistent with the Chandra localisation (ATel #1513) of the 8.4-hr eclipsing low-mass X-ray binary and thermonuclear X-ray burster AX J1745.6-2901. We thus conclude that we have likely detected a new outburst of this transient.
We extracted an average spectrum from the above mentioned observations. This spectrum can be described by an absorbed power-law model with an index of 2.4 +/- 0.4 and a hydrogen column density of (2.0+/-0.3)E+23 cm-2 (1-sigma errors). The inferred 2-10 keV unabsorbed flux for this fit is (3.7+/-0.9)E-11 erg cm-2 s-1, which translates into a luminosity of ~3E35 erg/s at a distance of 8 kpc.
AX J1745.6-2901 is very frequently active: our monitoring program previously detected outbursts from this system in 2006, 2007-2008, 2010 (e.g. Degenaar et al. 2015), 2013-2016 (e.g. ATels #5226, #9196), and two in 2017 (ATels #10323, #10900). The years-long outbursts that occurred in 2007-2008 and 2013-2016 reached a peak luminosity of several times E36 erg/s. All the other outbursts had a shorter duration (months) and a lower peak luminosity of a few times E35 erg/s. It seems that the current outburst belongs to the latter class.
Daily Swift/XRT monitoring of the Galactic Center is ongoing and the results of new observations are automatically posted on http://www.swift-sgra.com.
