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GCN Circular 19971

Subject
GRB 161001A: X-shooter spectroscopy, candidate host galaxy and redshift
Date
2016-10-01T12:57:15Z (7 years ago)
From
Daniele Malesani at Dark Cosmology Centre, Niels Bohr Inst <malesani@dark-cosmology.dk>
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.
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