Optical VLT/FORS Observations of a Fast X-ray Transient in the CDF-S

ATel #6603; Ezequiel Treister (U. de Concepcion, Chile), Franz Bauer (P. Universidad Catolica de Chile), Kevin Schawinski (ETH-Zurich, Switzerland)
on 21 Oct 2014; 19:58 UT
Credential Certification: Ezequiel Treister (etreiste@astro-udec.cl)

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On the night of Saturday, October 18, 2014 we observed the field of the fast X-ray transient reported on ATel #6541, 18 days after the X-ray transient was detected. Observations started at 05:37UT on Sunday, October 19, 2014 and finished at 06:30UT. Data were obtained using the FORS2 camera at the VLT/UT1 8m telescope in the R band, as part of DDT program 294.A-5005(A), PI: Franz Bauer. The ~7'x7' field of view covered by FORS2 was centered on the reported coordinates of the X-ray transient: RA=53.161661 and DEC=-27.859467. Observations were carried out in photometric conditions, seeing ~0.8" in the optical and airmass 1.02-1.0. Total exposure time was 2900 seconds. Based on existing HST observations of the field, the nearest counterpart is found at RA=53.161842, DEC=-27.859429 and corresponds to a R=27.38 (AB) galaxy with a photometric redshift of 0.31+-0.16 (Guo et al. 2013, ApJS, 207, 24 and Dahlen et al. 2013, ApJ, 775,93). No source is detected at this position in our VLT/FORS imaging, with an estimated magnitude limit of R=26.1 at 2 sigma. The nearest detected source is 1.8" away, at RA=53.16235, DEC=-27.859706 and a magnitude of R=25.79 which has however remained constant in optical flux according to existing imaging of the field. No variable source was found in the region of the X-ray transient. Following on telegrams ATel #6541 and ATel #6583, the transient continues to remain undetected in optical and radio observations, and thus its nature remains elusive. A further 1.5 hr observation in the optical R band is anticipated with Gemini-S/GMOS in the next couple of weeks as part of program GS-2014B-DD-4 (PI: Ezequiel Treister). We would like to acknowledge the ESO and Gemini staffs, in particular Nancy Levenson, Blair Conn, Rodolfo Angeloni, Rene Rutten, Christian Hummel and Linda Schmidtobreick, for their help in promptly accepting our DDT requests, preparing the observations and carrying them out.