Chandra Localization of IGR J17503-2636

ATel #11990; Deepto Chakrabarty (MIT), Peter G. Jonker (SRON), Craig B. Markwardt (NASA/GSFC)
on 26 Aug 2018; 00:30 UT
Credential Certification: Deepto Chakrabarty (deepto@space.mit.edu)

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As part of an ongoing Chandra X-ray Observatory program for the precise localization of X-ray transients in low-mass X-ray binaries, we obtained a short Chandra/HRC-S observation of IGR J17503-2636, following reports of its 2018 August outburst (ATEL #11952). Our observation was made in TIMING mode on 2018 August 23, 23:31 TT (MJD 58353.98), which was 12 days after the outburst was first detected by INTEGRAL. The exposure livetime was 1016.6 s.

The X-ray source had faded considerably by the time of our observation, yielding only a weak detection. We observed 11 counts within the 2.2 arcsec Swift error circle (ATEL #11952), compared to an expected background of 1 count. All 11 counts fell within a 1.8 arcsec radius of their mean position, and 6 counts fell within 0.5 arcsec radius of this position. Given the background count rate of 0.068 count/arcsec^2, the chance probability of detecting at least 11 counts within a 2.2 arcsec radius is 1.2e-8, corresponding to an equivalent Gaussian significance of 5.7 sigma. The chance probability of detecting at least 6 counts within a 0.5 arcsec radius is 3.0e-11, corresponding to an equivalent Gaussian significance of 6.6 sigma. We associate the 6 closely clustered counts with the core of the Chandra point-spread function (PSF). The mean sky position for these 6 counts is

RA(J2000) = 17h 50m 17.99s
Dec(J2000) = -26d 36' 16.7"

with an uncertainty radius of 1.1 arcsec (from a statistical uncertainty of 0.9 arcsec and a systematic uncertainty of 0.6 arcsec). This is consistent with the Swift X-ray error circle (ATEL #11952, 2.2 arcsec radius), lying 1.0 arcsec from its center.

The arrival times of the 11 events showed evidence for variability or flaring. In particular, we observed 2 events in the PSF core within a 3-second interval with a spatial separation of 0.16 arcsec, part of a cluster of 5 events (from the entire sample) observed within a 1-minute interval. Associating all 11 counts with the source region, the background-subtracted mean count rate was 0.010(3) count/s. The spectral shape in the HRC-S band is not well determined but is highly absorbed (ATEL #11952). Assuming an absorbed power-law X-ray spectrum with photon index gamma=2.0 and absorption column N_H = 1e23 cm^(-2), our count rate corresponds to an absorbed X-ray flux of 2.9e-12 erg/cm^2/s in the 0.2-10 keV band.