New outburst of the Galactic center transient Swift J174535.5-285921 seen with Swift/XRT

ATel #17192; N. Degenaar (U. of Amsterdam), M. T. Reynolds (U. of Michigan/Ohio State U.), R. Wijnands (U. of Amsterdam), J. M. Miller (U. of Michigan), J. A. Kennea (PSU), on behalf of a larger collaboration
on 19 May 2025; 05:48 UT
Credential Certification: Nathalie Degenaar (degenaar@uva.nl)

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Our daily Swift/XRT monitoring observations of the Galactic center (Degenaar et al. 2015) reveal new activity of a transient X-ray source located ~1.4' to the north-east of Sgr A*. The source is clearly seen during a 0.8-ks observation taken in PC mode on 2025 May 13 (obsIDs 00098343019), where it is detected with a net XRT count rate of (9.8+/-1.4)E-2 c/s.

We utilized the online XRT data products tool to obtain a localization of the source, where the UVOT is used to derive the astrometry so that an "enhanced" position is obtained (Goad et al. 2007; Evans et al. 2009). This yielded R.A. = 17:45:35.58 and Dec. = -28:59:28.1 (J2000) with an uncertainty of 3.4" (90% confidence). These coordinates are consistent with the Swift and Chandra localisation of the unclassified very-faint X-ray transient (VFXT) Swift J174535.5-285921 (CXOGC J174535.4-285925; ATel #3525).

The X-ray spectrum extracted from observation 00098343019 can be modelled by an absorbed power-law with an index of 1.93+/-0.55 (1-sigma uncertainty) for a hydrogen column density fixed at 15E22 cm-2. The resulting unabsorbed 2-10 keV flux is (1.63 -1.06 +0.11)E-11 erg/s/cm2, which translates into a luminosity of 1.2E35 erg/s for an assumed distance of 8 kpc. Our monitoring campaign, which has been running since 2006, detected three previous outbursts from this VFXT: in 2011, 2016 and 2020 (ATels #3472, #9236 and #13839). During those outbursts, which lasted ~2 weeks, the source displayed 2-10 keV peak X-ray fluxes of ~E35 erg/s (for 8 kpc).

The Swift/XRT observations further detect ongoing activity of the bright transient X-ray binary MAXI J1744-294 (active since January 2025; ATels #16975, #17010), as well as the bursting, eclipsing neutron star X-ray binary AX J1745.6-2901, which has been active since 2023 June (ATel #16106). The VFXT Swift J174610-290018, which was reported to be in outburst in 2025 April (ATel #17132), is no longer detected in the Swift/XRT images.

Swift/XRT observations of the Galactic center continue to be performed daily and instant quick-look results posted at: http://www.swift-sgra.com.

References:
Degenaar et al. 2015, JHEA 7, 137
Evans et al. 2007, A&A 469, 379
Evans et al. 2009, MNRAS 397, 1177
Goad et al. 2007, A&A 476, 1401