TITLE: GCN CIRCULAR NUMBER: 40520 SUBJECT: GRB 250521D: LAST tentative optical counterpart candidate DATE: 25/05/22 04:07:48 GMT FROM: Ruslan Konno at Weizmann Institute of Science <ruslankonno@gmail.com> R. Konno (WIS), S. Garrappa (WIS), E. A. Zimmerman (WIS), A. Horowicz (WIS), E. O. Ofek (WIS), S. Ben-Ami (WIS), D. Polishook (WIS), O. Yaron (WIS), S. Fainer (WIS), A. Krassilchtchikov (WIS), Y. M. Shani (WIS), E. Segre (WIS), A. Gal-Yam (WIS), S. Spitzer (WIS), and K. Rybicki (WIS) on behalf of the LAST Collaboration We report observations of GRB 250521D (GCN #40516), detected by Fermi-GBM, with the Large Array Survey Telescope (LAST; Ofek et al. 2023; Ben-Ami et al. 2023). We observe the field of GRB 250521D using 20 telescopes, each with a FoV of 7.4 deg^2 and no filter (clear - similar to the GAIA Bp band) over several epochs. In each epoch, we coadd 20x20s images. The standard limiting magnitude of a single 20x20s coadd is about 20.5 (AB mag). We began observations at 2025-05-21 22:47:38 UTC (T-T0 = 25.7 min). The observations are performed by tiling the 90% confidence area, supplied by the glg_healpix_all_bn250521932.fit map, with predefined LAST observation fields. We cover a total area of about 240 deg^2, corresponding to an integrated localization probability of about 56%. We found one new optical transient candidate, reported as AT 2025kze to TNS, in our automatic transient-detection pipeline (Konno et al., in prep.) which showed a brightening during observations. The candidate coordinates are RA, Dec = 278.4350830, 17.5896389 deg, which is 1.95 deg away from the best-fit position reported in GCN #40516. The associated photometry is | JD | T-T0 (h) | Mag | |----------------|----------|-------------------| | 2460817.45206228 | 0.48 | 20.50 (UL) | | 2460817.46226134 | 0.73 | 20.50 (UL) | | 2460817.48502633 | 1.27 | 20.105 ± 0.098 | | 2460817.50906663 | 1.85 | 19.722 ± 0.083 | The reported magnitudes are in the AB system, calibrated against the GAIA DR3 catalog, with no filter (clear). The position shows no LAST detections prior to the GRB trigger time, with the latest observations dating to 2025-04-21 01:33:59 UTC. Neither do we find any evidence of this source prior to the GRB trigger time in the ZTF observations provided by the Lasair broker (Smith et al. 2019), or the ATLAS forced photometry server (Shingles et al. 2021). There is a faint ambiguous extended underlying source at the candidate position visible in survey images provided by the SDSS SkyServer. Follow-up observations are encouraged to determine the nature of this candidate or the underlying source present in survey images. LAST is a survey telescope array of the Weizmann Astrophysical Observatory (https://www.weizmann.ac.il/wao/). View this GCN Circular online at https://gcn.nasa.gov/circulars/40520. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...