TITLE:   GCN CIRCULAR
NUMBER:  31991
SUBJECT: GRB 220427A: GRANDMA Early Optical Afterglow Detection
DATE:    22/05/02 19:34:07 GMT
FROM:    Alexander Kann at IAA-CSIC  <kann@iaa.es>

A. Klotz (CNRS-OMP-IRAP), M. Blazek (FZU), N. Christensen (OCA/Artemis), 
D. Datashvili (AbAO), P. A. Duverne (IJCLAB), F. Z. Guo (THU), N. 
Kochiashvili (AbAO), M. Lamoureux (UCLouvain), N. Leroy (IJCLAB), T. 
Sadibekova (AIM/CEA-UPS), P. Thierry (AGORA), A. Simon (Kyiv Univ), L. 
Eymar, S. Antier, M. Boer, A. de Ugarte Postigo (all CNRS-OCA-ARTEMIS), 
D. A. Kann (IAA-CSIC), A. Iskandar (XAO), A. Baransky (Kyiv Univ),
N. Sasaki (NEPA/UEA), W. Corradi (LNA), I. Tosta e Melo (INFN-LNS), J. 
P. Vignes (KNC), V. Godunova (IC ICAMER) report on behalf of GRANDMA 
collaboration:

The GRANDMA telescope network responded promptly to the alert of GRB 
220427A (A. D'Ai et al. GCN 31960, M. R. Goad et al. GCN 31962, A. P. 
Beardmore et al. GCN 31963, M. Stamatikos et al. GCN 31968).

The first observations with TAROT La Reunion started 2.35 min after the 
Swift/BAT trigger time, and were followed by observations with the Les 
Makes T60. Later observations were obtained by the CDK40 0.4m telescope 
that is part of the Kilonova-Cather citizen science programme. The light 
curve peaked around 4.5 min after the burst and then decayed steadily 
with a decay slope of alpha ~ 1.3, where F ~ t^-alpha.

The following table displays part of our photometry with magnitudes 
given in the AB system and calibrated with respect to field stars from 
the SkyMapper catalogue, using the MUPHOTEN pipeline (Duverne et al. 
2021).


T-T0 (hr)| MJD   | Obser.     |Exposure| Filter| Mag +/- err (AB)
_________________________________________________________________
0.04 |59696.878276| TRE       |     60s| Clear | 15.7 +/- 0.2
1.38 |59696.932836| Makes-T60 | 15x120s| Clear | 20.1 +/- 0.1
1.65 |59696.944294| Makes-T60 | 15x120s| Clear | 20.4 +/- 0.2
11.37|59697.3491  | KNC-CDK400| 10x300s| R     | > 21.0

T-T0 is the mid-time of observations related to the BAT trigger time.

These detections are consistent with the previous report of detections 
by the VLT (A. de Ugarte Postigo et al. GCN 31967).

GRANDMA is a worldwide telescope network (grandma.ijclab.in2p3.fr) 
devoted to the observation of transients in the context of 
multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518). 
Kilonova-Catcher (KNC) is the citizen science program of GRANDMA 
(http://kilonovacatcher.in2p3.fr/).