TITLE:   GCN CIRCULAR
NUMBER:  33014
SUBJECT: GRB 221202A: VLT/FORS2 Afterglow Discovery
DATE:    22/12/03 17:28:59 GMT
FROM:    Andrea Rossi at INAF  <andrea.rossi@inaf.it>

A. Rossi (INAF), G. Pugliese (API, Amsterdam university), D. A. Kann 
(Goethe Univ.), B. Schneider (CEA Paris-Saclay), and V. D'Elia (INAF/OAR 
and ASI/SSDC) report on behalf of the Stargate collaboration:

We performed photometric observations of the afterglow of GRB 221202A 
detected by the Swift Burst Alert Telescope (BAT) at 20:06:10.56 UT 
(Caputo et al., GCN 33005; De Wet et al., GCN 33007; Beardmore et al., 
GCN 33008;  Strausbaugh et al., GCN 33009; Evans et al., GCN 33010) with 
the FORS2 instrument at the ESO VLT-UT1 in the R_Special, I_Bessel and 
z_Gunn filters (in the following RIz). The observation started at 
06:50:29 UT (about 10.73 hours after the BAT detection), and consisted 
of 500 s in R, 800 s in I, and 1320 s in z.

We detect a source in the XRT error circle in all filters at position 
RA, Dec. (J2000) = 05:08:19.94,-59:49:58.6, astrometrized against Gaia 
DR3 field stars. We measure the following photometry, consistent with 
the upper limits of MeerLICHT (S. de Wet et al., GCN 33007) and LCOGT 
(R. Strausbaugh et al., GCN 33009):

R = 23.10 +/- 0.05 mag,
I = 22.68 +/- 0.06 mag,
z = 22.52 +/- 0.07 mag.

Magnitudes are calibrated against SkyMapper field stars and are in the 
AB system. For the R and I bands we use the Lupton formulas to convert 
the standard stars to the BVRI photometric system, then convert the 
measured magnitudes back to the AB system.

A source at the same position is visible in r'-band images downloaded 
and stacked from the Legacy Imaging Survey, with r' ~ 24.3 mag (AB). 
Therefore, we conclude that the detected source is dominated by the 
afterglow of GRB 221202A and slightly contaminated by the GRB host 
galaxy. We derive an X-ray to optical slope beta_OX ~ 0.8, therefore the 
afterglow is not dark, just faint.

We acknowledge the support provided by T. C. Emy and the Paranal staff, 
in particular Ana Escorza, Ricardo Arrue, Marcela Espinoza, and Michael 
Abdul-Masih.