TITLE: GCN CIRCULAR NUMBER: 33707 SUBJECT: GRB 230427A: OHP/T193 spectroscopy observations of WISEA J101316.01+481158.7 DATE: 23/04/28 15:40:52 GMT FROM: Benjamin Schneider at MIT bschn@mit.edu

C. Adami, M. Garnichey, F. Michel (LAM), B. Schneider (MIT), S. Basa (LAM), S. D. Vergani (GEPI, Obs. de Paris), D. Turpin, D. Götz, E. Le Floc'h (CEA Paris-Saclay), report on behalf of a larger collaboration:

We re-observed the field of GRB 230427A (Troja et al., GCN 33687) using the T193cm telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager. This time, we targeted in spectroscopy the galaxy close to the GRB line of sight: WISEA J101316.01+481158.7, at r'SDSS(PSF) = 19.6 (AB). Two exposures of 900s were obtained in the blue configuration (4200A-8000A) from 2023 28 April 00:25:02 UT to 2023 28 April 00:55:59 UT. The wavelength calibration was performed using the MISTRAL calibration lamp, exposed just after the science observation. Data were reduced using the ASPIRED package (https://github.com/cylammarco/ASPIRED) configured for MISTRAL data.

In the combined frame, we clearly detect the [NII]@6548A, Halpha, [NII]@6583A, and [SII]@6718A. Halpha is the dominant line of the spectrum. Based on these lines, we inferred a redshift of z = 0.1524 +/- 0.0005, in good agreement with the SDSS photometric redshift of z = 0.13 +/- 0.03.

The GRB is at an offset of ~18 arcsec from the WISEA J101316.01+481158.7 center. At the redshift of this galaxy, the burst would have a projected offset of ~48 kpc and a probability of chance coincidence of ~0.16, which is not commonly observed for the long GRB population. For this reason, this object is probably not the host galaxy of GRB 230427A.

We acknowledge the excellent support from Observatoire de Haute-Provence, in particular Jean Balcaen.