TITLE:   GCN CIRCULAR
NUMBER:  31864
SUBJECT: GRB 220408A: NOT optical observations and archival detection
DATE:    22/04/09 14:01:01 GMT
FROM:    Zipei Zhu at NAOC  <zpzhu@nao.cas.cn>

Z. Zhu (NAO/CAS, HUST), D. Xu, S.Y. Fu (NAOC/CAS), D. B. Malesani (Radboud Univ and DAWN/NBI), A. de Ugarte Postigo (Obs. Cote d'Azur), A. J. Levan (Radboud univ.), M. Aron (NOT), report on behalf of a larger collaboration:

We observed the field of GRB 220408A detected by Swift (Caputo et al., GCN 31848) and Fermi (Fermi GBM team, GCN 31847) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations were carried out starting at 03:53:08 UT on 2022-04-09 (i.e., 0.92 day since the BAT trigger), and 6x300 s frames in the SDSS r-band were obtained.

The previously reported optical counterpart (e.g., Caputo et al., GCN 31848; Zheng & Filippenko, GCN 31850; Oates et al., GCN 31855; Hu et al., GCN 31859; Strausbaugh & Cucchiara, GCN 31862) is detected in our stacked image with r = 24.1 +/- 0.1 AB, calibrated against nearby stars from the Pan-STARRS catalog.

We note that this field was covered by an image taken in 2007 with CFHT. Consistent with the UVOT coordinates, a faint source is visible with r = 24.5 +/- 0.2 (AB). While the S/N is not large, and we cannot conclusively determine whether this object is extended, it is a viable host galaxy candidate, suggesting a distance to the GRB much larger than M51.

We encourage spectroscopic observations of this Northern target to conclusively measure.