GCN Circular 46141
Subject
GRB 261029A: VST/OmegaCAM detection of the optical afterglow at 3 days
Event
Date
2026-11-04T01:24:41Z (a day ago)
From
Cheng Cheng at CAS South America Center for Astronomy (CASSACA) <chengcheng@nao.cas.cn>
Via
Web form
Cheng Cheng (CASSACA, NAOC), Liping Xin (NAOC), Jia-Sheng Huang (CASSACA, NAOC) report:
We observed the field of GRB 261029A (Fermi/GBM, GCN 46071; SVOM, GCN 46072)
with the ESO VLT Survey Telescope (VST) equipped with OmegaCAM, under CNTAC
proposal ID CN2026B-62 (PI: Jia-Sheng Huang) including VST project, ESO
programme 117.2B2A.001. Two 600-s exposures were obtained in the SDSS g and r
filters starting at 2026-11-01 01:59:53 UT, i.e. T0 + 72.55 hr, where
T0 = 2026-10-29 01:27:03.8 UT is the Fermi/GBM trigger time. The observations
were carried out at an airmass of 1.96-1.99, and the FWHM of point sources in
our images is ~0.8 arcsec.
The optical afterglow AT 2026ahaw (GOTO; GCN 46073; RA, Dec = 00:35:16.32,
+34:30:00.5, J2000) is detected in our r-band image and marginally in the g
band. Photometry calibrated using SDSS DR18 Petrosian magnitudes of stars in
the same CCD gives:
Filter T-T0 (hr) mag (AB) Flux (uJy)
r 72.63 23.25 +/- 0.09 1.82
g 72.82 24.91 +/- 0.21 0.39
The quoted uncertainties are 1-sigma statistical; an additional systematic
uncertainty of ~0.05 mag from the zero-point calibration should be added in
quadrature. The magnitudes are not corrected for Galactic extinction.
We note that the afterglow is located only about 3 arcsec from the edge of the
CCD on which it was detected, so the above magnitudes should be considered
preliminary. Our astrometric solution, derived from Gaia-based astrometry of
the same CCD, places the source within 0.2 arcsec of the coordinates given
above.
We thank the ESO Paranal staff and the VST team for the prompt execution of
these target-of-opportunity observations.