TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3064 SUBJECT: GRB 050219b: candidate NIR afterglow DATE: 05/02/24 22:27:28 GMT FROM: Paolo D'Avanzo at INAF-OAB P. D'Avanzo, D. Fugazza, S. Covino, G. Tagliaferri, D. Malesani, L.A. Antonelli, S. Campana, G. Chincarini, M. Della Valle, F. Fiore, L. Stella, F.M. Zerbi, on behalf of the MISTICI collaboration report : Optical and near-infrared monitoring of the field of GRB050219b (Cummings et al., GCN 3044) was performed at the ESO-VLT, with the FORS2 and ISAAC instruments respectively. Observations were carried out under a clear sky, starting 0.18 days after the GRB. The log is reported below. Mean date (UT) t-t0 (days) filter seeing (") ---------------------------------------------- Feb 20.062 0.18 R 1.0 Feb 21.062 1.18 R 1.0 Feb 21.088 1.21 Ks 0.5 Feb 24.024 4.14 Ks 0.5 ---------------------------------------------- t0 = Feb 19.879 UT (GRB trigger). In the refined XRT position and error circle (Burrows et al., GCNs 3043; Racusin et al., GCN 3049) we detect three sources in the Ks images: 1. RA: 05:25:15.99 Dec: -57:45:28.7 2. RA: 05:25:16.13 Dec: -57:45:30.5 3. RA: 05:25:15.59 Dec: -57:45:25.4 Source #2 and #3 do not exhibit significant variations (the latter being also present in the 2MASS catalogue). Source #1 is pointlike and is fading by 1.09 +- 0.19 mag in the Ks band between 1.21 and 4.15 days after the burst. We therefore identify this is the NIR afterglow of GRB050219b. The flux decay index (F = K*t^-delta) is delta = 0.81 +- 0.14. The candidate afterglow has a magnitude Ks = 21.47 +- 0.18 on Feb 24.02, based upon field calibration with standard stars. We caution that we get Ks = 16.45 +- 0.04 for the 2MASS star quoted above (source #3), which has a tabulated magnitude K=15.345 in the 2MASS catalog. In the R band, no source is detected at the position of the candidate afterglow, down to a limiting magnitude R~23, based on calibration against the USNO catalog. Further observations are planned at the VLT. This message can be cited.