Swift/XRT captures a bright X-ray flare from the Galactic center
ATel #16642; N. Degenaar (U. of Amsterdam), M. T. Reynolds (Ohio State / U. of Michigan), J. M. Miller (U. of Michigan), R. Wijnands (U. of Amsterdam), J. A. Kennea (Penn State), on behalf of a larger collaboration
on 6 Jun 2024; 14:30 UT
Credential Certification: Nathalie Degenaar (degenaar@uva.nl)
Subjects: X-ray, Binary, Black Hole, Neutron Star, Transient
While carrying out its daily XRT monitoring program of the Galactic center (Degenaar et al. 2015 for a review), Swift detected a bright X-ray flare from a location consistent with Sgr A*. During the 1.0 ks exposure, which started on 2024 June 2 at 01:34:46 UT (obsID 00097302060), the average count rate at this position is 0.11 c/s with no pronounced rise or decay seen during the short observation. The preceding and subsequent observations, performed on June 1 and June 3, do not detect enhanced levels of X-ray emission compared to the long-term average at this position (~0.01 c/s; Degenaar et al. 2013).
The XRT spectrum of the X-ray flare can be fitted to a absorbed power-law model with a power-law index of 0.9+/-0.3 (1 sigma uncertainties) using a hydrogen absorption column density fixed at the value found in previous Swift studies of Sgr A* flares, NH=9.1E+22 cm-2 (Degenaar et al. 2013). The resulting 2-10 keV unabsorbed flux is (3.1+0.3-1.0)E-11 erg/cm2/s, which translates into a luminosity of ~2.7E35 erg/s at a distance of 8 kpc. These spectral properties, short duration of the activity, and positional coincidence with Sgr A*, together suggest that this event was a bright X-ray flare from the supermassive black hole.
Apart from the detection of this X-ray flare, we report that the transient neutron star low-mass X-ray binary AX J1745.6-2901, which started a new outburst in 2023 June (ATel #16106), remains active. The new X-ray transient Swift J174610-290018, which was discovered on 2024 February 22 (ATel #16481), faded beyond detectability between subsequent observations performed on March 11 and April 3 (obsIDs 00096991195 and 00097302002). The outburst thus had a duration of 3-6 weeks. A combined average spectrum obtained from 10 ks of XRT data can be characterized by an absorbed power-law model with NH=(19.3+5.3-4.6)E+22 cm-2 and an index of 2.7+0.8-0.7 (1 sigma uncertainties). The inferred unabsorbed 2-10 keV flux is (1.8+1.2-0.6)E-11 erg/cm2/s, which translates into a luminosity of ~1.6E35 erg/s for a fiducial distance of 8 kpc.
Updates of new observations are immediately posted at the Swift Sgr A* Monitoring Campaign Website: http://www.swift-sgra.com.
References: Degenaar et al. 2013, ApJ 769, 155; Degenaar et al. 2015, JHEA 7, 137
