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GCN Circular 34409

Subject
GRB 230812B: Redshift from OSIRIS+/GTC
Date
2023-08-14T01:49:35Z (8 months ago)
From
Antonio de Ugarte Postigo at OCA <deugarte@oca.eu>
Via
email
A. de Ugarte Postigo (OCA-CNRS), J.F. Agui Fernandez (IAA-CSIC), C. C Thoene (ASU-CAS) and  L. Izzo (INAF-OACN and DARK/NBI) report:

We have observed the afterglow of GRB 230812B (Roberts et al. GCN34391, Scotton et al. GCN 34392, Zheng & Filippenko GCN 34395, Beardmore et al. GCN 34400) with OSIRIS+ mounted on the 10.4m GTC telescope at Roque de los Muchachos Observatory in La Palma (Spain). The observation consisted of spectroscopy with an exposure time of 3x900s and grism R1000B, with a wavelength coverage between 3600 and 7800 AA. The first spectrum started at 21:37 UT, 1.110 days after the burst.

In a preliminary reduction using old calibrations, the spectrum shows a strong trace with both emission and absorption lines which we identify as MgII, MgI, CaII, CaI in absorption, and [OII] and [OIII] in emission, at a common redshift of 0.360, which we interpret as the redshift of the GRB.

At this redshift, and assuming a fluence of 3.27e-4 erg/cm^2 as reported by Fermi/GBM (Roberts et al. GCN34391), the burst would have an Eiso = 8.3e52 erg. Together with a Ep = 273 keV (Roberts et al. GCN34391), GRB 230812B is consistent with the Amati relation for long GRBs.

We acknowledge excellent support from the GTC staff.
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