TITLE: GCN CIRCULAR NUMBER: 40208 SUBJECT: IC-250421A: LAST optical observations of the IceCube neutrino candidate DATE: 25/04/22 13:56:21 GMT FROM: Simone Garrappa at Weizmann Institute of Science <simone.garrappa@weizmann.ac.il> S. Garrappa (WIS), R. Konno (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), P. Chen (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 the observations of the IceCube Bronze neutrino candidate IC-250421A with the Large Array Survey Telescope (LAST, Ofek et al. 2023; Ben-Ami et al. 2023). We observe the field of IC-250421A using four 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. We observed for a total of 19 epochs under bright moon conditions, with an average limiting magnitude of 19.70 (AB). We find one candidate transient optical counterpart in our automatic transient-detection pipeline (Konno et al., in prep.). This is SN2025cbj, a known SN candidate classified as Type IIn (Sollerman et al., 2025) at redshift z = 0.07, which was discovered by ZTF on 20 02 2025 (~60 days before T0, https://alerce.online/object/ZTF25aagbrpb). SN2025cbj is located at ~1.8 deg distance from the IceCube best-fit localization, within the 90% localization error. We began observations on 21 04 2025 at 22:19:54 UTC (T-T0 = 5.23 hours). The observations cover part of the 90% neutrino footprint reported in GCN#40195 with predefined LAST observation fields. The following LAST field was observed: | Field| RA | Dec | Time | Limiting Mag (AB) | | ---- | ------ |-------- |----------- |------------ | | 1294 | 242.22 | 27 | 2459326.43 | 19.70 | Here we report the average brightness during the night: | RA (J2000)|Dec (J2000)| Mag (AB) | MagErr | Comment | | ---------- |-----------|-----------|--------|-------------| | 239.916993 | 27.111324 | 18.53 | 0.07 | SN 2025cbj | On 04 04 2025 (~17 days before T0) LAST observed the same field during nominal all-sky survey operations and detected SN2025cbj. We report the average source brightness from these archival observations. | RA (J2000)|Dec (J2000)| Mag (AB) | MagErr | Comment | | ---------- |-----------|-----------|--------|-------------| | 239.916993 | 27.111324 | 18.52 | 0.06 | SN 2025cbj | The reported magnitudes are in the AB system, calibrated using the LAST absolute photometric calibration pipeline (Garrappa et al. 2024). All reported errors are statistical only. The monitoring of this field will continue during LAST sky survey operations. We encourage multi-wavelength follow-up observations of this source. 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/40208. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...