{
  "subject": "GRB 250925A: Mondy I-band optical observations",
  "eventId": "GRB 250925A",
  "submittedHow": "web",
  "bibcode": "2025GCN.42025....1P",
  "createdOn": 1759151359366,
  "circularId": 42025,
  "submitter": "Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com>",
  "format": "text/plain",
  "body": "N. Pankov (HSE, IKI), A. Pozanenko (IKI), E. Klunko (ISTP) report on behalf of GRB-IKI-FuN:\n\nWe observed the field of GRB 250925A detected by Swift (Dichiara et. al, GCN 41985), at z=3.899 (Castro-Tirado et. al, GCN 41997), with the AZT-33IK 1.5m telescope of the Sayan Solar Observatory (Mondy) starting on 2025-09-27 at 14:45 UT, i.e. ~1.93 days since trigger. In total we obtained 45x120 s exposures in the I-band using CMOS photometer. The optical afterglow initially detected by NOT/ORM (Corcoran et. al, GCN 41990) and also observed by (Saikia et. al, GCN 41986; Pavoni & Moretti et. al, GCN 41989; Bochenek & Perley, GCN 41991; Masi et. al, GCN 41992; Mendez et. al, GCN 41995; Castro-Tirado et. al, GCN 41997; Liu et. al, GCN 41998; Ochi et. al, GCN 41999; Xin et. al., GCN 42002; Rakotondrainibe et. al, GCN 42004; Pankov et. al, GCN 42005) is not visible in the stacked image. Preliminary photometry and observation properties are presented below:\n\nDate       UTstart  Exptime   t-T0      Filter OT   Err.   UL      FWHM\n                     (nxs)  (mid, days)                   (3sigma) (\")\n2025-09-27 14:45:07  45x120  1.93451       I   n/d  n/d    22.3    1.6\n\nThe photometry was calibrated against nearby USNO-B1.0 stars (I-mags) and not corrected for the Galactic extinction."
}