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VVV near-infrared observations of the Swift J1753.7-2544 field

ATel #4802; A. F. Rojas (PUC, Santiago), N. Masetti (INAF/IASF, Bologna) & D. Minniti (PUC, Santiago)
on 9 Feb 2013; 21:00 UT
Credential Certification: Nicola Masetti (masetti@iasfbo.inaf.it)

Subjects: Infra-Red, X-ray, Transient

Referred to by ATel #: 4818, 4899, 4904

After the announcement of the arcsec-sized soft X-ray error box (ATel #4789) of the X-ray transient Swift J1753.7-2544 (ATel #4769), we searched the archival frames of the near-infrared (NIR) VVV survey (vvvsurvey.org; Minniti et al. 2010, New Astron., 15, 433) of the Galactic Bulge and inner disk, which is being obtained with the 4.1m VISTA telescope at Cerro Paranal (Chile), to look for further NIR objects within the Swift/XRT error circle besides the bright one reported in ATel #4789.

Initial JHKs images were acquired on 9 April 2010 between 07:26 and 07:32 UT. Seven further Ks images were secured between 21 August 2011 and 12 September 2012. Exposure times were 6 s in the J band and 4 s in both H and Ks; the image scale is 0.34 arcsec/pixel for all images.

In the VVV frames we find 2 more sources at the edge of the Swift/XRT X-ray error circle (ATel #4789). Their coordinates and magnitudes are:

 
 #   RA(J2000)    Dec(J2000)     J(2010)      H(2010)      Ks(2010)  
  
 1   17 53 39.72  -25 45 12.3  18.48+-0.12  16.40+-0.08  15.60+-0.10  
 2   17 53 39.96  -25 45 10.9  18.37+-0.10  16.92+-0.09  16.06+-0.14 

Coordinate errors are +-0.1 arcsec at 1 sigma confidence level; magnitude uncertainties are at 1 sigma confidence level as well.
None of these sources shows significant variability among the Ks epochs.

The brighter source mentioned in ATel #4789 also does not show significant variability in the Ks band and its magnitudes (J=14.08+-0.07; H=13.61+-0.06; Ks=13.44+-0.05) are consistent with the 2MASS ones reported in ATel #4789 (note that the 2MASS Ks-band magnitude value for this object is actually an upper limit).

We do not detect any other NIR source within the XRT error circle down to J ~ 20.0; H ~ 19.0; Ks ~ 18.5.

At present we cannot tell which of the above sources (if any) is the quiescent NIR counterpart of Swift J1753.7-2544. Comparison with NIR imaging acquired during the present outburst or availability of an X-ray position with a subarcsecond error, and of course optical and/or NIR spectroscopy on the brighter object in the Swift/XRT error circle, are therefore needed to pinpoint the actual counterpart.