TITLE: GCN CIRCULAR NUMBER: 19971 SUBJECT: GRB 161001A: X-shooter spectroscopy, candidate host galaxy and redshift DATE: 16/10/01 12:57:15 GMT FROM: Daniele Malesani at Dark Cosmology Centre, Niels Bohr Inst T. Kruehler (MPE Garching), D. Xu (NAOC/CAS), J. Bolmer (MPE Garching and ESO Santiago), K. Wiersema (Univ. Leicester), D. Malesani (DARK/NBI and DTU Space), R. Sanchez-Ramirez (IAA-CSIC), J. P. U. Fynbo (DARK/NBI), report on behalf of a larger collaboration: We observed the field of GRB 1601001A (Page et al., GCN 19967) with the ESO Very Large Telescope Unit 2 (Kueyen) equipped with the X-shooter spectrograph. Close to, but outside, the enhanced XRT position (Osborne et al., GCN 19969), in our acquisition image we detect a bright, pointlike source at coordinates RA = 04:47:40.71, Dec = -57:15:36.8. This source is faintly visible in the archival DSS data and is pointlike in our images (0.75" seeing). A spectrum was secured of this object, for a total of 4x600 s, starting on Oct 1.299 UT (6.1 hr after the GRB). The continuum is consistent with a stellar spectrum, and we conclude that this target is not associated with GRB 161001A. Our slit, at a position angle of 319.6 deg (CCW from north), does however cover a portion of the XRT error circle. Within the wings of the the PSF of the DSS object, approximately 0.8 arcsec SE, we detect a number of emission lines which are consistent with [O II], Hbeta, [O III] and Halpha at a common redshift of z = 0.891, revealing the presence of a background object. While we cannot accurately determine its position from the slit information alone, that is consistent with being at the edge of the XRT error circle. Our images also hint a faint extension of the DSS object at a position consistent with that of the emission lines, which could represent the continuum emission of the object. As such, this is a candidate host galaxy of GRB 161001A. We caution that we have no way to confirm the association between the emission line object and the GRB. No variability can be inferred from our data, and as we lack an estimate of the continuum flux, we can not presently quantify the chance association probability. A finding chart showing the region surrounding the XRT position is shown at this URL: http://www.astro.ku.dk/~malesani/GRB/161001A/GRB161001A_slit.png We acknowledge excellent support from the ESO observing staff at Paranal, in particular Zahed Wahhaj, Jonathan Smoker, and Claudia Cid.