TITLE: GCN CIRCULAR NUMBER: 37319 SUBJECT: SVOM GRB 240821A: optical afterglow discovery DATE: 24/08/28 15:57:55 GMT FROM: Daniele B. Malesani at IMAPP / Radboud University d.malesani@astro.ru.nl
J. Quirola-Vasquez (Radboud Univ.), D. B. Malesani (DAWN/NBI and Radboud Univ.), A. J. Levan (Radboud Univ. & Warwick Univ.), P. G. Jonker (Radboud Univ), F. E. Bauer (PUC), J. van Dalen (Radboud Univ.), M. E. Ravasio (Radboud Univ.), D. Mata Sanchez (IAC), M. A. P. Torres (IAC), A. Martin-Carrillo (UCD), S. D. Vergani (GEPI / Obs. de Paris) report on behalf of a larger collaboration:
We observed the location of the candidate X-ray afterglow Turpin et al., GCN 37230; Bernardini et al., GCN 37249; Turpin et al., GCN 37316) discovered by Einstein Probe within the SVOM/ECLAIR localization of GRB 240821A (Cangemi et al., GCN 37220; He et al., GCN 37226; Murphy et al., GCN 37232; Dalessi & Meegan, GCN 37239). Observations were carried out in the z band using the GMOS-N instrument on the Gemini North telescope, on two epochs: 2024 August 22 (0.73 days after the GRB) and 2024 August 28 (6.58 days after the GRB). The first epoch was affected by the bright, nearby Moon.
Several sources are visible consistent with the X-ray afterglow error region (10" radius). Carrying out image subtraction between the two epochs, a clear transient is detected at the following coordinates (J2000):
RA = 23:37:04.90 Dec = -10:11:22.2
An (unresolved) archival counterpart is well detected at these coordinates in the Legacy Survey, with a magnitude z ~ 22 AB. The optical transient is slightly offset (~0.5") from its nucleus, ruling out AGN variability. The Legacy Survey catalog (Zhou et al. 2021, MNRAS, 501, 3309) provides a photometric redshift 0.488 +- 0.074.
Given the joint fading of the optical and X-ray flux, we consider this target to be the likely optical afterglow + host of GRB 240821A.
We thank the EP / SVOM teams for privately sharing the location of the X-ray source prior to GCN dissemination.
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