The near-infrared counterpart to XTE J1856+053
M. A. P. Torres (CfA), D. Steeghs (CfA/Warwick), P. G. Jonker (SRON/CfA), N. Morrell, M. Roth (LCO),  F. Kerber (ESO, Garching)
We report the detection of the near-infrared counterpart to the X-ray transient and black hole candidate XTE J1856+053 (Marshall et al. 1996, IAUC # 6504 ; Barret et al. 1996, IAUC # 6519 ). This finding is based on follow-up observations of the currently ongoing outburst (Levine et al. 2007; ATel #1024) with the 6.5m Magellan Baade telescope at Las Campanas observatory.

Following the improved X-ray localisation of XTE J1856+053 (Steeghs et al. 2007; ATel #1026), we acquired a series of 15s Ks-band images on 2007 Mar 11 9:47-10:03 UT (MJD 54170.4) and again on 2007 May 8 8:12-8:25 (MJD 54228.3) using the PANIC camera and totaling 450s on source for both nights. The frames were acquired with a seeing of 0.5 arcsec on Mar 11 and 0.8 arcsec on May 8. Additional 3s exposures of the field were obtained on May 8 for an absolute flux calibration of the images. The astrometric and photometric scale were defined using 2MASS sources in the field of view achieving a positional RMS of 0.1 arcsec and a zero-point error of 0.03 mag.

Several near-infrared sources are visible inside the Swift error circle for XTE J1856+053, but only one point-like source with position R.A(J2000)=18:56:42.92, Dec(J2000)=+05:18:34.3 shows significant variability. PSF-fitting photometry yields Ks=16.43 +/- 0.05 on Mar 11 and Ks=17.8 +/- 0.1 on May 8. Note that the Mar 11 data were obtained close to the outburst peak whereas the May 8 images were acquired when the X-ray flux has declined by more than a factor of 10 compared to the outburst peak (see public RXTE ASM light curve).

Both he positional coincidence with the Swift X-ray position and the photometric variability support this source as the near-infrared counterpart to XTE J1856+053.
infra-red,x-ray,binary,black hole,transient