Pre-outburst optical/NIR observations of the field around the accreting millisecond X-ray pulsar IGR J17498-2921

ATel #3634; M. van den Berg (Utrecht, CfA), J. Grindlay, P. Zhao, J. Hong, M. Servillat (CfA)
on 7 Sep 2011; 12:04 UT
Credential Certification: Maureen van den Berg (maureen@head.cfa.harvard.edu)

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We report on pre-outburst optical (VRIHalpha) and near-infrared (NIR; JHKs) imaging observations of the field containing the newly discovered transient accreting milli-second X-ray pulsar IGR J17498-2921 (ATel #3551). The observations were done with the Mosaic (optical) and ISPI (NIR) cameras on the CTIO-4m Blanco telescope as part of our X-ray and optical/NIR Bulge Latitude Survey.

The optical images were taken on 2008 May 10. The astrometric calibration has an uncertainty of ~0.1" in each coordinate. The R (1200 sec total exposure time), I (540 sec), and Halpha (4500 sec) images show a source at RA(J2000)=17h 49m 55.34s, Dec(J2000)=-29d 19m 19.7s. At an offset of 0.16", this is consistent with the Chandra position. The Mosaic source lies 0.79" from the proposed optical counterpart found in Sloan i' images from the 2m Faulkes Telescope South taken during the ongoing outburst (ATel #3622). Given the error on the Faulkes position (0.3"), and the absence of other sources in the Mosaic and Faulkes images within 1.8" of the Chandra position, we consider our Mosaic source and the proposed optical counterpart to be the same. We note that the finding charts of ATel #3622 suggest that the proposed counterpart is closer to the Chandra position than 0.79", so we suspect that there may be a mistake in the reported position in ATel #3622. The magnitudes of our Mosaic source are R=23.2(2), I=21.06(5), and Halpha-R=0.4(3). The values in brackets give the internal errors of the PSF-photometry routine, while photometric calibration errors are on the order of 0.1 mag (Zhao et al. 2005, ApJS 161, 429). We have used the transformation equations in Jordi et al. 2006, A&A, 460, 339 to convert our I magnitude to the Sloan i' band, which gives i'=21.9(2). This is consistent with the magnitude of i'=22.5(7) in ATel #3622. As the true counterpart is expected to show a large difference between the outburst and pre-outburst optical magnitudes, it is likely that the proposed counterpart is a chance alignment. The distance estimate for the X-ray pulsar (7.6 kpc; ATel #3568) and column density derived from the X-ray spectrum (3e22 cm-2; ATel #3555) also suggest that the optical source is an unassociated foreground source.

At an offset of 0.16", the astrometric match found in pre-outburst NIR images reported in ATel #3562 is consistent with a source in our ISPI images from 2008 July 31 found at RA(J2000)=17h 49m 55.34s, Dec(J2000)=-29d 19m 19.6s (error 0.13" in each coordinate). With J=17.3(1), H=15.7(1), and Ks=15.0(1) the ISPI magnitudes are consistent with the values from ATel #3562.

Mosaic and ISPI finding charts