VVV near-infrared observations of the Swift J174540.7-290015 field

ATel #8737; N. Masetti (INAF/IASF, Bologna), R. K. Saito (Univ. Federal de Sergipe), A. F. Rojas & D. Minniti (UNAB, Santiago)
on 26 Feb 2016; 13:23 UT
Credential Certification: Nicola Masetti (masetti@iasfbo.inaf.it)

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Following the outburst of the Galactic Center X-ray transient Swift J174540.7-290015 (ATel #8649, #8684) and its arcsec-sized soft X-ray localization (ATel #8649), we searched the archival frames of the near-infrared (NIR) VVV survey (vvvsurvey.org; Minniti et al. 2010, New Astron., 15, 433) covering the Galactic Bulge and inner arms, and obtained with the 4.1m VISTA telescope at Cerro Paranal (Chile); this search was made in order to look for a possible quiescent NIR counterpart to the aforementioned X-ray transient.

JHKs images were acquired on 15 August 2010 between 03:23 and 03:32 UT under an average seeing of 0.9 arcsec. Further Z- and Y-band frames were secured on 24 August 2011 between 02:46 and 02:51 UT with a seeing of 1.0 arcsec. Exposure times were 24 s in the J band, 8 s in both H and Ks, and 20 s in both Z and Y bands; the image scale is 0.34 arcsec/pixel for all frames.

Within the Swift/XRT error circle we found one single NIR source, with coordinates (J2000) RA = 17:45:40.67, Dec = -29:00:15.3 (coordinate errors are +-0.1 arcsec at 1 sigma confidence level). This position locates the VVV source at 1.1 arcsec from the Swift/XRT error circle centre. Magnitudes, and 3-sigma lower limits thereof, for this object are the following:

 
Z > 20.1 
Y > 19.4 
J = 16.47+-0.03 
H = 12.59+-0.01 
Ks = 9.34+-0.01 

(magnitude uncertainties are at 1 sigma confidence). We remark that the VVV magnitudes are based on aperture photometry and should be used with caution in this field due to its crowdedness.

Because of the unavailability of positional information for the three sources described in ATel #8689, we cannot make any variability statement concerning the VVV source. Comparison with NIR imaging acquired during the present outburst or availability of an X-ray position with a subarcsecond error are therefore needed to confirm or disprove the VVV source as the actual counterpart of Swift J174540.7-290015.