TITLE:   GCN CIRCULAR
NUMBER:  45740
SUBJECT: GRB 260924A: 2.2m CAHA spectroscopic redshift z = 3.51
DATE:    26/09/25 08:46:21 GMT
FROM:    I. Perez-Garcia at Instituto de Astrofísica de Andalucía <ipg@iaa.es>

A. J. Castro-Tirado, I. Perez-Garcia, A. Sota, E. Fernandez-Garcia, M.D. Caballero-Garcia, S. Guziy and M. Gritsevich (IAA-CSIC), Y.-D. Hu (GXI), A. Fernandez-Martin and S. Gongora (CAHA) on behalf of a larger collaboration, report:

Following the detection of GRB 260924A by Fermi/GBM (Fermi Team, GCN 45712, McDermott, GCN 45723), Swift/BAT (D'Elia et al., GCN 45713, Gupta et al., GCN 45733) and Swift/XRT (enhanced XRT position, Evans et al., GCN 45715), AstroSAT (Salunke et al., GCN 45720) and NuSTAR (Waratkar et al. GCN 45732), we observed the optical afterglow (Hu et al. GCN 45717, Izzo et al. GCN 45718) with the 2.2m telescope at the Calar Alto Observatory (Almeria, Spain) equipped with CAFOS .

We obtained an acquisition image in the r’-band and long-slit spectroscopy with the grisms G-200 (2 x 500 s, mid-time 2026-09-25 04:16 UT, i.e. T0 + 27.0 h) and B-100 (3 x 500 s, mid-time 04:44 UT, T0 + 27.4 h), with a 2.0 arcsec slit set at the parallactic angle. The airmass decreased from 2.6 to 1.9 and the seeing was 1.1 arcsec.

In the acquisition image (30 s, 26.3 h post trigger) the afterglow is detected at  r’ = 19.04 +/- 0.18 (AB, calibrated against Pan-STARRS DR1) which, compared with r' = 18.68 +/- 0.04 at  21.50 h post trigger (GROWTH-India, GCN 45728), implies a decay index alpha = 1.6 +/- 0.8.

The combined spectrum covers 3700-8800 A and shows a well-detected continuum above 5500 A. Below 5480 +/- 40 A the flux drops by a factor of about 2-3, and below ~4100 A it falls to a few per cent of the extrapolated continuum. We interpret these as the Lyman-alpha break and the Lyman limit of the same absorber, giving a redshift estimate of z = 3.51 +/- 0.10

The break is present independently in both grisms, and is also seen in the raw count ratio between the afterglow and a field star that fell on the same slit, so it does not depend on the flux calibration. No metal absorption lines are detected due to the low signal-to-noise of these data.

This redshift confirms the photometric estimate z = 3.6 +/- 0.3 that we can derive from BVRI imaging with the 1.5m telescope of the Sierra Nevada Observatory (OSN) at 3.2 h post trigger, where the afterglow had R = 15.51 +/- 0.01 (Hu et al. , GCN 45717).

At z = 3.51 this corresponds to an absolute magnitude of about -31 at a rest-frame wavelength of 1400 A and a rest-frame epoch of 0.7 h, placing GRB 260924A among the most luminous optical afterglows observed till date.

We thank the staff of the Calar Alto and Sierra Nevada observatories for their excellent support.