GCN Circular 41550
Subject
GRB 250806A: HiPERCAM/GTC limits on a possible supernova
Event
Date
2025-08-26T16:15:04Z (a month ago)
From
Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo@gmail.com>
Via
email
A. de Ugarte Postigo (LAM), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), C. C. Thoene (AbAO), J. F. Agui Fernandez (CAHA), M. A. Aloy (UV), J. P. U. Fynbo (DAWN/NBI), L. Galbany (IEEC-CSIC), S. Geier (GTC), L. Izzo (INAF/OACN and DARK/NBI), G. Lombardi (GTC), N. A. Rakotondrainibe (LAM), B. Schneider (LAM), N. R. Tanvir (Univ. Leicester), and D. Gonzalez Gonzalez (GTC) report:
We observed the field of GRB 250806A (Xie et al., GCN 41243) with the 5-band HiPERCAM imager mounted on the 10.4 m GTC telescope, at the Roque de los Muchachos Observatory (La Palma, Spain). The observation consisted of 20 x 60 s exposures in the u, g, r, i, and z bands, with mean epoch 2025-08-23 01:51:07 UT (16.75 days after the burst). Our images have 3-sigma detection limits of u > 26.1, g > 26.4, r > 25.9, i > 25.6, and z > 24.7 mag as compared to SDSS field stars (all the magnitudes in this GCN are in the AB system).
Within the refined XRT error box (Campana et al., GCN 41249) we clearly detect the two galaxies mentioned by Malesani et al. (GCN 41279), the brightest of which has been proposed as the host of GRB 250806A and has a measured redshift of 0.367 (Sevilla et al., GCN 41499). Under the assumption that the GRB is indeed associated with this galaxy, its low redshift prompted us to perform a search for a possible emerging supernova (SN). The time of our observation would correspond to 14.7 days after the burst in the rest frame, close to the time of expected maximum light.
No other significant new source is detected within the XRT error box beyond the mentioned galaxies. Image subtraction with respect to the Legacy Survey (LS) does not reveal any significant residuals, although we note that the reference images are significantly shallower than our data. A deeper search for the SN component will require obtaining deeper templates once the possible SN has faded away.
Assuming as limits for the subtraction images the depth of the Legacy Survey, we would not be detecting a SN component down to r > 25.0 mag and i > 24.5 mag. At a redshift of 0.367, this is equivalent to absolute magnitudes of M_4500AA (g-band) > -16.1 and M_5500AA (V-band) > -16.6, or approximately 2.7 magnitudes fainter than SN 1998bw in each of these bands. This could indicate a SN intrinsically fainter than SN 1998bw by a comparable amount, or might indicate line-of-sight extinction of A_V > 2.7 mag. The latter option would be consistent with the lack of optical/NIR afterglow detection (Wu et al., GCN 41244; Fortin et al., GCN 41245; Freeberg et al., GCN 41247; Xin et al., GCN 41250; Zheng et al., GCN 41251; Schneider et al., GCN 41253; Malesani et al., GCN 41279; Pankov et al., GCN 41285).
As an alternative, the galaxy at z = 0.367 might be of course a chance association, and the GRB might be simply further away.