TITLE: GCN CIRCULAR NUMBER: 39406 SUBJECT: GRB 250221A: REM optical/NIR afterglow detection DATE: 25/02/21 13:50:48 GMT FROM: Andrea Melandri at INAF-OAR andrea.melandri@inaf.it
A. Melandri (INAF-OAR), R. Brivio, M. Ferro, P. D’Avanzo, S. Covino, D. Fugazza (INAF-OAB) report on behalf of the REM team:
We observed the field of GRB 250221A detected by Swift (Caputo et al., GCN 39396; Beardmore et al., GCN 39404) with the REM 60cm robotic telescope located at the ESO observatory of La Silla (Chile). The observations were carried in the g, r, i, z, J, H, K bands, starting on 2025 February 21 at 03:35:39 UT (i.e. ~60 s after the Swift trigger), and lasting for about 2 hours.
From preliminary photometry we detect the counterpart in the optical and NIR images at the position of the optical afterglow (Watson et al., GCN 39397) with the following early-time magnitudes:
r = 16.5 +/- 0.2 (AB; calibrated against the Pan-STARRS catalogue) at a mid-time of t - t0 = 70 s after the trigger.
H = 14.8 +/- 0.3 (Vega; calibrated against the 2MASS catalogue) at a mid-time t - t0 = 87 s after the trigger.
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