GCN Circular 43269
Subject
GRB 251230A / EP251230a: NOT observations and spectroscopic redshift upper limit z < 2.2
Event
Date
2025-12-30T13:23:01Z (a day ago)
From
Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>
Via
Web form
L. He (NAOC), L. Izzo (INAF/OACN and DARK/NBI), A. Martin-Carrillo (UCD), A. Saccardi (CEA/Irfu), J. An (NAOC), G. Corcoran (UCD), D. B. Malesani (DAWN/NBI and Radboud), A. Kadela (NOT & NBI) report on behalf of a larger collaboration:
We observed the optical counterpart (Lanava et al., GCN 43252; Li et al., GCN 43255; Busmann et al., GCN 43258; Masi, GCN 43260; Ghosh et al., GCN 43262; Ma et al., GCN 43265) of GRB 251230A / EP251230a (Lanava et al., GCN 43252; Wang et al., GCN 43259; Osborne et al., GCN 43263), using the Nordic Optical Telescope (NOT), equipped with the ALFOSC camera and spectrograph.
We obtained three exposures of 300 s in the r-band starting on 2025 Dec 30 at 06:30:20 UT (5.144 hr after the Swift trigger). The afterglow is clearly detected with an AB magnitude of r = 20.75 +/- 0.02 mag, calibrated against nearby stars from the Pan-STARRS catalogue and not corrected for Galactic extinction. Compared to previous measurements, our photometry suggests rapid optical decay with power-law index ~1.95.
A sequence of 4 spectra of 1200 s each was acquired, covering the wavelength range 3700-9600 AA. In a preliminary reduction of the spectra, we detect a continuum over the entire covered wavelength range. From detection in the blue down to ~3900 AA, and the lack of hydrogen absorption, we set a redshift upper limit z < 2.2. We do not identify clear metal absorption features.
Further analysis is ongoing.