Swift/XRT detects renewed activity of the Galactic center X-ray transient Swift J174535.5-285921

ATel #9236; N. Degenaar (Cambridge), 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 11 Jul 2016; 19:53 UT
Credential Certification: Nathalie Degenaar (degenaar@ast.cam.ac.uk)

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Our daily Swift/XRT monitoring observations of the Galactic center (Degenaar et al. 2015) have revealed activity of a transient X-ray source located ~1.4' to the north-east of Sgr A*. An excess of photons was first seen during a 0.9-ks PC mode observation performed on 2016 July 7. The object remains to be detected in subsequent observations performed on July 8,9, and 10.

To obtain a position for the newly active X-ray source, we use the four PC-mode observations obtained between July 7 and 10 (amounting to 3.5 ks of exposure time) and utilize the online XRT data products tool (Evans et al. 2007, 2009). We obtain a "standard" position (J2000) of R.A. = 17:45:35.96 and Dec. = -28:59:27.2 with an uncertainty of 3.8" (90% confidence). When the UVOT is used to derive the astrometry (see Goad et al. 2007; Evans et al. 2009) an "enhanced" position (J2000) is obtained:

R.A. = 17:45:35.64
Dec. = -28:59:27.5

with an uncertainty of 2.2" (90% confidence). The XRT coordinates are fully consistent with the Chandra localization of the X-ray transient Swift J174535.5-285921 (ATel #3525). The positional coincidence makes it likely that we have detected renewed activity from this source. Swift J174535.5-285921 was detected in outburst only once before during our Swift/XRT Galactic center monitoring campaign: It exhibited a ~1-2 week active period in 2011 that peaked at a 2-10 keV luminosity of ~1E35 erg/s (Degenaar et al. 2015) for an assumed distance of 8 kpc. The source position is consistent with the dim X-ray source CXOUGC J174535.6-285928 (Muno et al. 2009), which has an estimated 2-10 keV luminosity of ~2E31 erg/s and may represent the quiescent X-ray counterpart of this transient (ATels #3472, #3525).

The source count rate is low and we therefore extracted an average spectrum from the four XRT/PC-mode observations of July 7-10. This can be described by an absorbed power-law model with a hydrogen column density of N_H=(1.5+/-0.9)E23 cm-2, and an index of 1.9+/-1.5. The inferred unabsorbed 2-10 keV flux is (8.8+/-3.3)E-12 erg/cm2/s, which translates into a luminosity of (6.7+/-2.5)E34 erg/s at a distance of 8 kpc. The count rate varied by a factor ~3 over the four different observations. A spectrum extracted from the one with the highest count rate (July 9) can be described with a power-law index of 2.8+/-0.9 for N_H=1.5E23 cm-2 fixed. This yields an unabsorbed 2-10 keV flux of (1.8+/-0.5)E-11 erg/cm2/s, or a luminosity of (1.4+/-0.4)E35 erg/s at 8 kpc.

Apart from Swift J174535.5-285921, we detect ongoing activity of two other X-ray transients: Swift J174540.7-290015 that is located ~16" to the north of Sgr A* and has been seen active since February of this year (e.g., ATels #8649, #8689, #8746, #9109, #9196), and Swift J174540.2-290037 that is located ~10" to the south of Sgr A* and became active in late May (ATel #9109, #9196). The 2-10 keV unabsorbed fluxes inferred from the XRT observations of July 7-10 are (4.0+/-0.3)E-11 erg/cm2/s for Swift J174540.7-290015, and (1.6+/-0.5)E-11 erg/cm2/s for Swift J174540.2-290037. The corresponding luminosity at a distance of 8 kpc is (3.1+/-0.2)E35 erg/s and (1.2+/-0.4)E35 erg/s, respectively.

Our daily Swift/XRT observations of the Galactic center are ongoing. The monitoring campaign webpage can be found 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
Muno et al. 2009, ApJS, 181, 110