[vsnet-grb-info 18690] GRB 161104A: Gemini South optical observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  20137
SUBJECT: GRB 161104A: Gemini South optical observations
DATE:    16/11/05 22:12:41 GMT
FROM:    Eleonora Troja at GSFC  <eleonora.troja at nasa.gov>

E. Troja (UMD/GSFC), A. Watson (UNAM), S. Covino (INAF), W. H. Lee
(UNAM), N. Butler (ASU), J. Becerra-Gonzalez (UMD/GSFC), A. Lien
(UMBC), T. Sakamoto (AGU) and N. Gehrels (NASA/GSFC) report:

We observed the field of the short GRB 161104A (Mingo et al., GCN 20123)
with the Gemini Multi-Object Spectrograph (GMOS) on the 8m Gemini South
telescope. We acquired 6x120s exposures in the r-band beginning at
02:53 UT on 05 November 2016 (~17 hours after the Swift trigger) and
under good observing conditions.

The three sources reported by Nicuesa Guelbenzu et al. (GCN 20132) are
detected with similar magnitudes, showing no significant variability
between our observations and the GROND exposures taken 1 hour later.
Within the enhanced XRT position (Goad et al., GCN 20124) we detect an
additional faint, point-like source not reported by Nicuesa Guelbenzu
et al. (GCN 20132). The position of this source is RA, Dec = 05:11:34.5,
-51:27:33.90 (J2000), accurate to about 0.5" in each coordinate. We
measure a preliminary magnitude of r~24.6 (AB).

The lack of detection in the subsequent GROND observations suggests that
the source significantly faded, however such rapid decay rate is unusual
for a GRB optical afterglow.

We thank the Gemini staff and observers for rapidly scheduling and executing
these observations.