Chandra Localization of the Galactic Center X-ray Transient Swift J174535.5-285921

ATel #3525; Deepto Chakrabarty (MIT), Manuel Linares (MIT), Peter G. Jonker (SRON), Craig B. Markwardt (NASA/GSFC)
on 2 Aug 2011; 18:45 UT
Credential Certification: Deepto Chakrabarty (deepto@space.mit.edu)

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As part of an ongoing Chandra program for precise localization of X-ray transients in low-mass X-ray binaries, we obtained a short Chandra/ACIS-I observation of the X-ray transient Swift J174535.5-285921 during its 2011 July discovery outburst (ATEL #3472, #3476, #3481, #3508). The exposure time was 966 s, using a (1/8)-subarray of the I3 chip with 0.5 s readout time. Our observation was made on 2011 July 21, 06:35 TT. This was at least one week after the transient is inferred to have returned to X-ray quiescence based on Swift observations, but it is also the same date that Swift observed marginal evidence for residual enhanced emission from the source (ATEL #3508).

Our Chandra observation weakly detected the source at an intensity of 0.023(5) count/s. The best-fit X-ray source position is:

RA(J2000) = 17h 45m 35.746s
Dec(J2000) = -28d 59' 29.06"

with an error radius of 0.6 arcsec (90% confidence). This position is inconsistent with the reported Swift XRT-UVOT enhanced position (3.3 arcsec error radius; ATEL #3472), lying 8.2 arcsec away. However, our Chandra position is consistent with the unenhanced Swift XRT position (3.8 arcsec error radius; ATEL #3472), lying 3.5 arcsec away. It is also consistent with the previously reported faint Chandra point source CXOUGC J174535.6-285928 (source 1680 in Table 2 of Muno et al. 2009, ApJS, 181, 110), lying 0.6 arcsec away. Given the number density of sources from the Muno et al. catalog in this field, the chance coincidence probability is 2.2 percent. We note that recent infrared counterpart searches during the X-ray outburst were evidently analyzing the wrong source position (ATEL #3476, #3481).

Assuming the average X-ray spectrum measured by Swift (an absorbed power-law spectrum with photon index 1.4 and hydrogen column density 5.7E22 cm^(-2)), the absorbed (unabsorbed) 2-10 keV flux in our Chandra observation was 8.2E-13 (1.1E-12) erg/cm^2/s, corresponding to a luminosity of 8.0E33 erg/s for a distance of 8 kpc. This is roughly consistent with the July 21 luminosity inferred from the marginal Swift detection (ATEL #3508). It is an order of magnitude fainter than the mean outburst luminosity reported by Swift (ATEL #3508), but nearly three orders of magnitude brighter than the quiescent luminosity for CXOUGC J174535.6-285928 reported by Muno et al. (2009).