TITLE:   GCN CIRCULAR
NUMBER:  46081
SUBJECT: GRB 261029A: NOT imaging and spectroscopic redshift z = 3.27
DATE:    26/10/29 05:41:22 GMT
FROM:    Laura Cotter at Cornell <laura.cotter@ucdconnect.ie>

L. Cotter (Cornell), L. B. He (NAOC), D. O’Neill (Birmingham), D. Xu (NAOC), N. R. Tanvir (Leicester), D. B. Malesani (DAWN/NBI and Radboud), J. An (NAOC) and K. Jhass (Birmingham) report on behalf of a larger collaboration:

We observed the optical and near-infrared counterpart (O’Neill et al., GCN 46073; He et al., GCN 46078; Wang et al., GCN 46079; De et al., GCN 46080) of the Fermi and SVOM GRB 261029A (Fermi GBM team, GCN 46071; Goto et al., GCN 46072) using the Nordic Telescope (NOT) equipped with the ALFOSC imager. Observations began on 2026-10-29 at 02:52:39 UT, approximately 86 min post-trigger. We obtained 3x300 s exposures in the SDSS-r band and 5x200 s exposures in the SDSS-z band. The optical counterpart candidate is well detected in our images; photometry yields the following preliminary magnitudes:

r: 16.61 +/- 0.01 AB at a mid time of 93.5 min post trigger;
z: 16.62 +/- 0.01 AB at a mid time of 114 min post trigger.

Photometry was calibrated against Pan-STARRS DR2 catalog stars and is not corrected for Galactic extinction.

Spectroscopic observations began at 03:53:31 UT consisting of 4x1200 s exposures using grism 4, which covers the wavelength range between 3200 and 9600 Å. Based on a preliminary reduction of the spectra using IRAF, we observe a number of absorption features matching well Lyman delta, gamma, beta, alpha, Si II, O I, C II, Si IV, C IV, Al II, all at a common redshift of z = 3.27, which we consider to be the redshift of GRB 261029A.