GCN Circular 33014
Subject
GRB 221202A: VLT/FORS2 Afterglow Discovery
Date
2022-12-03T17:28:59Z (a year ago)
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.