TITLE: GCN CIRCULAR NUMBER: 39077 SUBJECT: GRB 250129A: Early afterglow detection by LCO. DATE: 25/01/29 09:12:17 GMT FROM: ankur ghosh at CAPP, University of Johannesburg ghosh.ankur1994@gmail.com

Ankur Ghosh, Soebur Razzaque (CAPP, University of Johannesburg), Alexander Moskvitin, Yulia Sotnikova (SAO RAS), Naveen Dukiya (ARIES), Rahul Gupta (NASA GSFC) on behalf of a larger collaboration.

We observed the field of the GRB 250129A triggered by Swift (Beardmore et al., GCN 39066) in V, r filters of the 0.4-m SCICAM QHY600 at the Las Cumbres Observatory Global Telescope (LCOGT) node located at Teide Observatory, Tenerife. The 0.4 m SCICAM QHY600 is equipped with 9576 x 6388 pixel CCD (FOV: 1.9 x 1.2 degrees, scale: 0.74 arcsec/pixel) but we only used the FOV of 30 x 30 arcmin for our observation. Observations began on January 29, 2025, starting 1.17 hours after the GRB trigger.

We clearly detect the optical transient (OT) reported by GCNs ( Francile, GCN 39065; Schneider et al., GCN 39071; Belkin et al., GCN 39072; Izzo et al., GCN 39073, Izzo et al., GCN 39074) in our V, r band images. Follow-up observations are on going.

|Date| |UTstart| |t-T0 (hours)| |Exp (sec)| |Filter| |Magnitude| --------------------------------------------------------------------------------------------------- 2025-01-29 05:55:21.79 1.17 1 x 600 r r = 17.05 +/- 0.01

2025-01-29 06:22:59.80 1.63 1 x 600 V V = 17.34 +/- 0.01

The field was calibrated against nearby APASS stars, with magnitudes converted using Lupton (2005) equations, and has not been corrected for Galactic extinction.

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