GCN Circular 45931
Subject
GRB 261011A: 10.4m GTC spectroscopic redshift (z = 2.407)
Event
Date
2026-10-13T05:21:34Z (18 hours ago)
From
I. Perez-Garcia at Instituto de Astrofísica de Andalucía <ipg@iaa.es>
Via
Web form
I. Pérez-García (IAA-CSIC), Y.-D. Hu (GXU), A. J. Castro-Tirado (IAA-CSIC), F. J. Aceituno, M.D. Caballero-Garcia, E. Fernandez-Garcia, M. Gritsevich, S. Guziy (IAA-CSIC), D. Garcia-Alvarez, A. Tejero (GRANTECAN, IAC), on behalf of a larger collaboration, report:
We observed the optical afterglow of GRB 261011A detected by SVOM/ECLAIRs (Huang et al. GCN 45895), Swift/BAT (Cenko et al. GCN 45896) and Konus-Wind (Ridnaia et al. GCN 45924), with the 10.4m Gran Telescopio Canarias (equipped with the OSIRIS+ spectrograph) in La Palma (Spain). Long-slit spectra were taken starting on Oct 12, 2026, 00:19 UT (i.e. 10.5 h post burst) with both the R1000R grism (1200 s) and the R1000B grism (600 s), with the slit centred on the optical afterglow position (Cenko et al., GCN 45896; O'Neill et al., GCN 45897; Fu et al., GCN 45898; Izzo et al., GCN 45899; Wu et al., GCN 45900; Yin et al., GCN 45901; Shi et al., GCN 45903; Aryan et al., GCN 45904; Arya et al., GCN 45907; Izzo et al., GCN 45909; Huang et al., GCN 45913; Bochenek et al., GCN 45914; Li et al., GCN 45915; Zhao et al., GCN 45916; Takahashi et al., GCN 45917; Cheng et al., GCN 45922).
A faint continuum of the afterglow is detected across the full range. On top of it, we identify low S/N ratio absorption features consistent with Fe II (2344, 2374, 2382 A), Al II (1670 A) and Al III (1854/1862 A), all at a common redshift z = 2.407 +/- 0.001.
A joint analysis of all the expected lines gives the above mentioned, unique solution, at this redshift over the range 1.5 < z < 4.2. Moreover, we notice that the flux drops strongly below ~4270 A, consistent with the damped Lyman-alpha absorption trough at this redshift.
This confirms the tentative redshift z = 2.405 derived by Monsalves et al. (GCN 45920) from the damped Lyman-alpha trough in the VLT/X-shooter spectrum.
In addition to this, we notice that the Legacy Survey DR10 shows a faint source (r = 25.4 AB), coincident with the afterglow position within 0.1 arcsec, a likely host galaxy candidate.
We thank the staff at GTC for their excellent support.