Optical Gemini-S/GMOS observations of a fast X-ray Transient in the CDF-S

ATel #6650; Ezequiel Treister (U. de Concepcion, Chile), Franz Bauer (P. Universidad Catolica, Chile), Kevin Schawinski (ETH-Zurich, Switzerland), Blair Conn (Gemini Observatory, Chile)
on 31 Oct 2014; 11:56 UT
Credential Certification: Ezequiel Treister (etreiste@astro-udec.cl)

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On the night of Monday, October 27, 2014 we observed the field of the fast X-ray transient reported in ATel #6541, 27 days after the X-ray transient was detected. Observations started at 07:36UT on Tuesday, October 28, 2014 and finished at 08:54UT. The observations were taken as part of DDT program GS-2014B-DD-4, PI: Ezequiel Treister. Data were obtained using the GMOS camera at the Gemini-South 8m telescope in the SDSS r band. The total integration time of the observations was 4500s divided in 5x900s dithered images. The new Hamamatsu CCD installed on the GMOS-S camera, which covers a ~5.5'x5.5' field of view was used. Images were centered on the reported coordinates of the X-ray transient: RA=53.161661 and DEC=-27.859467. Observations were carried out in clear conditions, seeing ~0.6" in the optical and airmass 1.3-1.1.

As reported in ATel # 6603, the nearest counterpart is found at RA=53.161842, DEC=-27.859429 and corresponds to an 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). Only very faint emission, <2sigma, is detected in our Gemini-S/GMOS imaging. This is consistent with a marginal detection of the galaxy previously described. No significant brightening is found at this position, in particular when compared to the VLT/FORS images described in ATel #6603, which reaches similar depths. Hence, we can safely conclude that no variable source was found in the region of the X-ray transient.

Thus, the nature of this X-ray transient still remains elusive. Twenty-eight days have passed since the detection of the X-ray transient and no variable optical counterpart has been observed at 1hr, 18 days and 28 days post-transient. If the transient is due to the shock breakout of a typical core-collapse supernova from a red supergiant, we should expect the optical emission to peak ~10-15 days after the X-ray detection. With limits to M_R=-15 to -16 at z=0.31, all but the faintest known SNe should have been detected. The observations still cannot exclude a SN1987-like Type II supernova from a blue supergiant progenitor, which would peak rather late with a maximum optical brightness ~100 rest-frame days after the explosion. Another possibility is that the transient is an X-ray flash from an off-axis GRB, for which the optical afterglow is exceptionally faint. Finally, the source could be at higher redshift, although as argued in ATel #6541, the source is more likely at z<=0.5. Further observations of this object in the next couple of months are required to further establish the nature of this very enigmatic source.

We would like to acknowledge the Gemini staff, in particular Nancy Levenson, Rodolfo Angeloni and Rene Rutten, for their help in promptly accepting our DDT request, preparing the observations and carrying them out.