We present the results of the monitoring of the GRB 970111 field that started
19 hours after the event. This observation represents the fastest ground-based
follow-up performed for GRB 970111 in all wavelengths. As soon as the detection
of the possible GRB 970111 X-ray afterglow was reported by Feroci et al. (1998)
we reanalyzed the optical data collected for the GRB 970111 field. Although we
detect small magnitude variability in some objects, no convincing optical
counterpart is found inside the WFC error box. Any change in brightness 19
hours after the GRB is less than 0.2 mag for objects with B < 21 and R < 20.8.
The bluest object found in the field is coincident with 1SAXJ1528.8+1937.
Spectroscopic observations revealed that this object is a Seyfert-1 galaxy with
redshift z=0.657, which we propose as the optical counterpart of the X-ray
source.
Further observations allowed to perform multicolour photometry for objects in
the GRB 970111 error box. The colour-colour diagrams do not show any object
with unusual colours. We applied a photometric classification method to the
objects inside the GRB error box, that can distinguish stars from galaxies and
estimate redshifts. We were able to estimate photometric redshifts in the range
0.2 < z < 1.4 for several galaxies in this field and we did not find any
conspicuous unusual object.
We note that GRB 970111 and GRB 980329 could belong to the same class of
GRBs, which may be related to nearby sources (z ~1) in which high intrinsic
absorption leads to faint optical afterglows.Comment: 10 pages with 11 encapsulated PostScript figures included. Uses
Astronomy & Astrophysics LaTeX macros. Accepted for publication in Astronomy
& Astrophysic