GCN Circular 41279
Subject
GRB 250806A: NOT z-band upper limit and host galaxy candidate
Event
Date
2025-08-08T08:32:02Z (2 months ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
D. B. Malesani (DAWN/NBI and Radboud), L. Izzo (INAF/OAC and DARK/NBI), A. Martin-Carrillo (UCD), G. Corcoran (UCD), Dimple (Birmingham), A. Saccardi (CEA/Irfu), B. Schneider (LAM), D. Xu (NAOC) report on behalf of a larger collaboration:
We observed the location of the SVOM/ECLAIRs GRB 250806A (Xie et al., GCN 41243) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. A total of 25 exposures by 120 s each were secured in the SDSS z band, with mean epoch 2025 Aug 7.00 UT (16.05 hr after the trigger).
There are two galaxies from the Legacy Survey that are consistent with the X-ray position currently listed at https://www.swift.ac.uk/xrt_positions/ (but slightly outside the preliminary error circle reported by Campana et al., GCN 41249).
The first one (RA = 23:13:43.51, Dec = +01:21:22.0) is relatively bright, with (AB) magnitudes from the Legacy survey g = 22.64, r = 21.75, i = 21.45, z = 21.15, and a photometric redshift z = 0.51 +/- 0.09. This object is marginally detected in our NOT stacked z-band image.
The second one (RA = 23:13:43.42, Dec = +01:21:19.8) is fainter (r = 24.10), and is not detected in the NOT image.
No other objects are seen consistent with the XRT position in the NOT z-band images, down to a limiting magnitude z > 22.4 AB, calibrated against nearby Pan-STARRS objects.
Given its low (<2%) chance-association probability with the X-ray source, the brighter galaxy is a promising host galaxy candidate for GRB 250806A, and we invite other observers (e.g., Wu et al., GCN 41244; Fortin et al., GCN 41245; Freeberg et al., GCN 41247; Zheng et al., GCN 41251; Schneider et al., GCN 41253) to examine their data and to monitor this galaxy in order to check for variability. A SN should be also detectable in the next couple of weeks, if the photometric redshift is approximately correct and there is no significant extinction.
We acknowledge expert support from the NOT observing staff, in particular Tapio Pursimo and Laura Fuglsang.