GCN Circular 41419
Subject
GRB 250818B: Keck redshift of the optical afterglow
Event
Date
2025-08-18T22:13:52Z (2 days ago)
From
Wen-fai Fong at Northwestern University <wfong@northwestern.edu>
Via
Web form
W. Fong, A. C. Gordon (Northwestern), A. J. Levan (Radboud), N. R. Tanvir (Leicester), Y. Dong, A. Suresh, C. Liu (Northwestern) report:
We observed the position of the SVOM short-duration GRB 250818B (Wang et al., GCN 41405) with a Swift/XRT counterpart (Ferro et al., GCN 41407). We obtained spectroscopy of the optical afterglow candidate detected by GOTO (Kumar et al., GCN 41406) and SVOM/VT (Yao et al., GCN 41409) using the Low-Resolution Imaging Spectrograph (LRIS) mounted on the Keck I telescope atop Mauna Kea in Hawaii (PI: Chang Liu; Program O397). We obtained 3x300-sec exposures using the 400/3400 grism on the blue side and the 400/8500 grating on the red side at an airmass of 1.2 in 0.9” seeing and clear conditions, starting on 2025 Aug 18 at 14:10 UT (approximately 10.68 hr post-burst). The spectrum spans approximately 3500 to 9500 Ang.
The continuum is well-detected along with several prominent absorption features. In particular, we identify absorption features of FeII (2344, 2374, 2382), MgII (2796, 2803), MgI (2852), CaII H&K, and possibly AlII (1670), at a common redshift of z=1.216 which we consider to be the redshift of the GRB. This implies an optical luminosity of ~5e45 erg/s at 0.67 hr after the burst (using Kumar et al., GCN 41406), which is on the bright side compared to the handful of short GRBs observed at these epochs.
We thank Keck Observatory staff Rita Morris and Josh Walawender for their assistance with our observing night.