Improved IR (ISAAC/VLT) and radio (ATCA) astrometry of the likely counterpart to XTE J1720-318
ATel #117; Kieran O'Brien (ESO, Chile), Fraser Clarke (ESO, Chile), Rob Fender (Amsterdam), Catherine Brocksopp (MSSL), Danny Steeghs (CfA), Rob Hynes (Texas) and Phil Charles (Southampton)
on 23 Jan 2003; 14:40 UT
Credential Certification: Kieran O'Brien (kobrien@eso.org)
Subjects: Radio, Infra-Red, X-ray, Black Hole, Transient
We report VLT observations (Proposal 270.D-5025)
of the field of the newly discovered X-ray transient, XTE J1720-318 (ATEL
#113), together with an improved radio position.
We obtained a series of 60 Ks band images with ISAAC on VLT/UT1
on 21st Jan 2003 between UT 09:27 and UT 09:57. The combined image has a
limiting magnitude of approx 20.2 and an image quality (FWHM) of 0.5". The
proposed IR counterpart of Kato et al. (VSNET alert) is clearly detected,
along with several fainter nearby sources (<2" sep.). The source has a Ks
magnitude = 15.31 ± 0.03, consistent with the previously reported
observation. By cross-correlating our image with the 2MASS catalogue
(positional accuracy < 0.2"), the position in our IR image is found to be RA
= 17:19:58.994, Dec = -31:45:01.25, with an uncertainty of 0.05"
relative to 2MASS.
By combining four observations of the radio counterpart to XTE J1720-318 at 8.64 GHz with the Australia Telescope Compact Array (ATCA) we arrive at the following refined radio position: RA = 17:19:58.985, Dec = -31:45:01.109 (J2000) with a total positional uncertainty of 0.25". This improved radio position (relative to IAUC # 8054 ) remains consistent with the aforementioned infrared counterpart, strengthening the case for an association. (see image at http://www.sc.eso.org/~fclarke/research/XTE1720/XTE1720.html )
We have also extracted Ks band lightcurves for this source and two other
nearby candidates, but none show significant variability at the level of
+- 0.1 mag. We strongly encourage further monitoring and spectroscopic
observations of this relatively bright IR counterpart.
ISAAC/VLT image of proposed IR counterpart to XTE J1720-318
