On the infrared counterpart to IGR J17177-3656

ATel #3241; M. A.P. Torres (SRON/CfA), P. G. Jonker (SRON/CfA/RU), D. Steeghs (Warwick/CfA), J. E. Ybarra, K. Romita and E. A. Lada (University of Florida)
on 28 Mar 2011; 18:09 UT
Credential Certification: Manuel Torres (mtorres@cfa.harvard.edu)

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We report infrared follow-up observations of the field containing the currently active X-ray transient IGR J17177-3656 (Frankowski et al. 2011, ATel #3223).

We observed the field covering IGR J17177-3656 using the NEWFIRM widefield infrared imager mounted on the 4m Blanco telescope at CTIO. The data consist of multiple 10s Ks-band images obtained on 2011 Mar 20 starting at 08:58 UT totaling 10 min on source. Sky conditions were good with an image quality of 1.1 arcsec. The projected pixel scale was of 0.4 arcsec/pixel. Additional 2s exposures of the field were obtained the same night to allow for an astrometric and absolute flux calibration of the frames. The astrometric and photometric scales were defined using 2MASS objects in the field achieving a positional RMS < 0.1 arcsec and a photometric zero-point error < 0.1 mag.

We searched for counterpart candidates consistent with the 0.6 arcsec Chandra X-ray position for IGR J17177-3656 (Paizis et al. 2011, ATel #3236). No obvious point-like sources were found within the reported Chandra error circle.

One arcsec from the Chandra position is 2MASS J17174269-3656039 (J=15.171,K=12.879, see also ATel #3226, #3236). Given the large Hydrogen column to IGR J17177-3656 (ATel #3236), we would expect an E(J-K) of 5.1 mag. The colour of the 2MASS source therefore rules it out as a counterpart. Furthermore, PSF photometry of our images reveals a Ks= 15.8 +/- 0.2 source blended with 2MASS J17174269-3656039. The location for this faint source is R.A (J2000)=17:17:42.66, Dec (J2000)=-36:56:05.35 which lies 1.0 arcsec from the X-ray position.

The above faint source is also unlikely to be the counterpart given its significant separation from the X-ray position. Thus it is plausible that IGR J17177-3656 was not brighter than Ks = 17.5 (upper limit derived from our images) at the time of our observations.