TITLE:   GCN CIRCULAR
NUMBER:  22820
SUBJECT: GRB 180613A: Afterglow confirmation and added photometry
DATE:    18/06/21 14:47:01 GMT
FROM:    Christina Thoene at IAA-CSIC  <christina.thoene@gmail.com>

C.C. Thoene (HETH/IAA-CSIC), A. de Ugarte Postigo (HETH/IAA-CSIC,
DARK/NBI), N.R. Tanvir (Univ. of Leicester),  D.A. Kann, L. Izzo (both
HETH/IAA-CSIC), J.P.U. Fynbo (DAWN/NBI) and D.B. Malesani (DAWN/NBI,
DARK/NBI) report on behalf of the Stargate consortium:

We re-observed the field of GRB 180613A (Tohuvavohu et al., GCN 22769) with
HAWK-I/VLT on 14 June 2018 at 23:37 UT (32.02 hr after the burst). The
observation consisted of a combined exposure of 36 min in the J-band. The
seeing of this observation was worse than during the first epoch (Thoene et
al., GCN 22775) with an average value of 0.8", but the exposure time was
significantly longer, allowing us to reach a deeper limit. In this second
epoch we do not detect any more the source found by Thoene et al. (GCN
22775) down to a 3-sigma limit of J(Vega) > 24.2 mag. This implies that the
source has faded, confirming that this source was indeed the afterglow of
GRB 180613A.

We also revise and provide the full photometry of our first HAWK-I epoch
(GCN 22775, Thoene et al.) and obtain the following values:
J(Vega)=23.24+/-0.26 or J(AB)=24.15+/-0.26
H(Vega)=22.23+/-0.22 or H(AB)=23.62+/-0.22
Ks(Vega)=21.04+/-0.27 or Ks(AB)=22.89+/-0.27

These values result in an IR spectral slope of 2.02 +/- 0.64, which would
indicate a significant amount of extinction, explaining the faintness of
the afterglow. Due to the lack of further data, we are not able to draw any
definite conclusions in this matter.

We acknowledge the continued excellent support from the ESO staff, in
particular Frederic Vogt, Ivan Aranda and Eleonora Sani.