Chandra imaging of IGR J16493-4348

ATel #654; L. Kuiper (SRON), P. Jonker (SRON/CfA), W. Hermsen (SRON/Amsterdam), K. O'Brien (ESO)
on 9 Nov 2005; 12:06 UT
Credential Certification: Lucien Kuiper (L.M.Kuiper@sron.nl)

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We observed the environment of IGR J16493-4348 with the Chandra HRC-I instrument on 2005 Oct 21.1965 for 4.1 ks in order to shed light on its very unlikely association with the normal and non-energetic radio pulsar PSR J1649-4349 (see e.g.ATEL #232 and Bird et al. 2004, ApJ 607, L33). Within the 2 arcminute error circle of IGR J16493-4348 (see ATEL #457) we detect one single point source at R.A. = 16 49 26.92, Decl.= -43 49 8.96 (equinox 2000; estimated uncertainty 0.6 arcsec in each coordinate) at an angular distance of 1.2 arcminute from PSR J1649-4348. We associate this source with IGR J16493-4348. No X-ray photons were detected within 5 arcsec from PSR J1649-4349 in this 4.1 ks HRC-I observation. The time averaged HRC-I count rate over the observation is 0.255(8) c/s. A lightcurve showed strong evidence for variability. We found no evidence for a coherent timing signal in the 1 - 500 s range. No X-ray spectral information can be derived from this observation. In the 2MASS source catalogue we found one source, 2MASS J16492695-4349090, coincident with the Chandra position with the following J,H and Ks magnitudes: 14.59(5), 12.86(6) and 11.94(4). We obtained ourselves 42 Ks-band images of the field of IGR J16493-4348 with the NTT using SOFI on May 18, 2005. The exposure time of each of the images was 3 seconds. The seeing was 1.5 arcseconds. We calibrated the resultant image using the 2MASS magnitudes of several relatively isolated stars. We find a Ks band magnitude of 12.0(1), consistent with the magnitude quoted in the 2MASS source catalogue showing that this source is not likely to be highly variable in the Ks band. No significant source can be found at the position of the Chandra source in the optical DSS maps, indicating strong absorption in the direction of the source. We suspect this source to be a heavily absorbed X-ray binary in line with the RXTE PCA measurements reported in ATEL #465 by Markwardt et al. Note, however, that the RXTE PCA spectral measurements may be unreliable, because these are possibly polluted by the presence of another X-ray source, 1RXS J164913.6-435527, within the PCA field of view, detected significantly (22.4 sigma) in the HRC-I image. Spectroscopic observations at X-rays, optical/IR bands are required to determine the true nature of the source.