Continued Swift/XRT observations of the new Galactic center transients SWIFT J174540.2-290037 and SWIFT J174540.7-290015

ATel #9196; N. Degenaar (IoA Cambridge), M. T. Reynolds (U. of Michigan), R. Wijnands (U. of Amsterdam), J. M. Miller (U. of Michigan), J. A. Kennea (PSU), G. Ponti (MPE), D. Haggard (McGill), and N. Gehrels (GSFC), on behalf of a larger collaboration
on 28 Jun 2016; 11:08 UT
Credential Certification: Nathalie Degenaar (degenaar@ast.cam.ac.uk)

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Daily monitoring observations of the Galactic center performed with the Swift/XRT show continued activity of the transient X-ray source SWIFT J174540.2-290037, which was first detected on 2016 May 28 and is located ~10" to the south of Sgr A* (ATel #9109). Following its initial detection, the XRT (PC mode) count rate of this source gradually rose to ~0.8 c/s around mid June. The XRT count rate has been lower, ~0.3-0.4 c/s, for the past few days.

A spectrum extracted from a 1.0-ks observation obtained in WT mode on June 22 suggests that near the peak intensity, the source spectrum could be described by an absorbed power-law model with N_H = (1.6 +/- 0.2)E23 cm-2 and a photon index of 1.9 +/- 0.2 (1-sigma errors). The implied unabsorbed 2-10 keV flux is (3.8 +/- 0.3)E-10 erg/cm2/s. For a distance of 8 kpc this translates into a 2-10 keV luminosity of (2.9 +/- 0.2)E36 erg/s. An averaged spectrum extracted from the two most recent PC-mode observations, obtained on June 25 and 27, suggest that the spectrum can currently be described with N_H = (1.7 +/- 0.5)E23 cm-2 and a photon index of 1.5 +/- 0.5, yielding an unabsorbed 2-10 keV flux of (1.2 +/- 0.2)E-10 erg/cm2/s (1-sigma errors). The current 2-10 keV luminosity is (9.2 +/- 0.9)E35 erg/s for a distance of 8 kpc.

Due the brightening and increased number of observations, we can now obtain an improved position for SWIFT J174540.2-290037. For this purpose we utilize the online XRT data products tool (Evans et al. 2007, 2009), using all PC-mode observations obtained between May 28 and June 27 (amounting to 22.5 ks of exposure time). This yields a standard XRT position of R.A. = 17:45:40.60 and Dec. = -29:00:36.4 (J2000) with an uncertainty of 3.5" (90% confidence), and an UVOT-enhanced position of R.A. = 17:45:40.38 and Dec. = -29:00:42.8 (J2000) with an uncertainty of 1.9" (90% confidence; see Goad et al. 2007; Evans et al. 2009). The improved localization is ~10" from the Chandra position of the 7.9-hr eclipsing transient LMXB and black hole candidate CXOGCJ174540.0-290031 (e.g., Muno et al. 2005), which makes an association unlikely.

The nearby X-ray transient SWIFT J174540.7-290015 that was discovered earlier this year and is located ~16" to the north of Sgr A* (ATel #8649), continues to be active to date. We extracted an averaged spectrum from the latest two PC-mode observations (June 25 and 27), which suggest that the spectral data can currently be described with an absorbed power-law with N_H = (7.0 +/- 0.5)E22 cm-2 and a photon index of 2.4 +/- 0.9 (1-sigma errors). The unabsorbed 2-10 keV flux for this fit is (2.1 +/- 0.6)E-11 erg/cm2/s, which corresponds to a luminosity of (1.6 +/- 0.5)E35 erg/s for a distance of 8 kpc. Up till now, there are no notable features in the XRT light curves such as bursts or eclipses for either of the two unclassified Swift X-ray transients.

The neutron star LMXB AX J1745.6-2901, seen active since 2013 July 18 (ATel #5222), faded into the XRT background around 2016 June 3. Summing all PC-mode observations obtained between June 3 and 23 yields a 2-10 keV flux upper limit of ~1.2E-12 erg/cm2/s, assuming an absorbed power-law spectrum with N_H=2E23 cm-2 and an index of 2.6 (e.g., Degenaar et al. 2014). This upper limit is relatively high due to contamination of SWIFT J174540.2-290037, which is ~1' away. This flux translates into a luminosity upper limit of ~9E33 erg/s at 8 kpc and suggests that AX J1745.6-2901 has likely transitioned to quiescence after an outburst that lasted for almost 3 yr.

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

References:
Degenaar et al. 2014, IAUS 303, 315
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. 2005, ApJ 633, 228