TITLE: GCN CIRCULAR NUMBER: 45458 SUBJECT: GRB 260818B: VLT optical and nIR observations DATE: 26/08/28 14:56:05 GMT FROM: muskan.yadav@students.uniroma2.eu Muskan Yadav, Massine El Kabir, Nicolás Monsalves, Hira Waseem, Yu-Han Yang and Eleonora Troja (U Rome) report on behalf of the ERC BHianca team: We observed the field of short duration GRB 260818B detected by SVOM/ECLAIRs (Cordier et al., GCN 45398) with the FORS2 imager on the ESO VLT UT1 (Antu) and the HAWKI imager on the ESO VLT UT4 (Yepun). Both observations began 9.5 days after the trigger and were carried out at an average airmass ~1.2 and with an average seeing ~0.8 arcsec. At the position of the candidate optical counterpart reported by COLIBRÍ (Ducoin et al., GCN 45450) we detect a faint (~24.7 AB) source in the z-band, but not in the J-band down to a 3-sigma limit >24.2 AB mag. Magnitudes are calibrated using nearby stars in the Pan-STARRS1 DR1 catalog (Chambers et al. 2016) and the 2MASS catalog (Skrutskie et al. 2006). Reported magnitudes are not corrected for Galactic extinction. Our observations show that the optical source faded significantly between the two epochs, by at least ~2 mag between 6.5 d and 9.5 d after the burst. This would correspond to a power-law decay slope of ~-4.6, steeper than expected for standard GRB afterglows. The lack of a nIR detection suggests that its red color (r-z~1.8; Ducoin et al., GCN 45450) could be due to dust along the line of sight or, alternatively, that the late observations are dominated by the host galaxy light. Further observations are planned. We thank the staff at the VLT for the rapid execution of these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/45458. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...