TITLE: GCN CIRCULAR NUMBER: 39465 SUBJECT: LIGO/Virgo/KAGRA S250223dk and Swift/BAT-GUANO ID 762004910: Magellan upper limits. DATE: 25/02/25 02:44:55 GMT FROM: Harsh Kumar at Center for Astrophysics | Harvard & Smithsonian <harshkosli13@gmail.com> H. Kumar, E. Berger, A. Villar, D. Hiramatsu, P. K. Blanchard, K. D. Soto, S. K. Yadavalli, A. Gagliano, C. Ransome, Anya Nugent, Y. Dong (Harvard) report: We obtained imaging with the IMACS imager on the 6.5m Magellan Baade telescope to search for an optical counterpart following the announcement of Swift/BAT-GUANO 762004910 (GCN #39443) event, coincident with gravitational wave event S250223dk. We obtained 4 x 120-second images in each of g- and r-band, starting about 16.6 hours after the GW trigger, targeting the Swift/BAT-GUANO localization of 5 arcmin around RA(J2000)= 85.341 deg, Dec(J2000)= -47.554, covering ~95% of the joint LVK-Swift/BAT localization 50% credible region. We do not detect new transients above 5-sigma level up to ~23.7 mag in the g-band and ~23.5 mag in the r-band after performing image subtraction with Legacy Survey-DECam images as a reference. We checked for the DECam DESGW candidates (GCN #39462) in our images. One source (AT2025cpq) falls in the area covered by Magellan. The source is marginally detected at <3 Sigma. We performed forced photometry and obtained the following magnitudes: —------------------------------------------ Name | Filter | Magnitude +/- e_magnitude —------------------------------------------ AT2025cpq | g | 24.04 +/- 0.48 AT2025cpq | r | 23.78 +/- 0.28 —------------------------------------------- We thank Yuri Beletsky for the rapid execution of these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/39465. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...