TITLE:   GCN CIRCULAR
NUMBER:  29440
SUBJECT: GRB 210104A: LBT observations 
DATE:    21/02/09 15:09:31 GMT
FROM:    Andrea Melandri at INAF-OAB  <andrea.melandri@inaf.it>

A. Melandri (INAF-OAB), and A Rossi (INAF-OAS) report on behalf of the CIBO collaboration:

We observed the field of GRB 210104A (Troja et al., GCN 29233; Malacaria et al., GCN 29246) simultaneously in the r' and z' bands with the LBC imager mounted on LBT (Mt Graham, AZ, USA). We obtained 20min of imaging on 2021-02-05, 31.8 days after the burst trigger. Observations were performed under poor seeing (~1.7") conditions but reached a depth of r~25.5 mag.

At the position of the afterglow (Xin et al., GCN 29235; Hu et al., GCN 29236; Kim et al., GCNs 29238, 29265, 29283; Lipunov et al., GCNs 29239, 29248; Horiuchi et al., GCN 29241; Breeveld et al., GCN 29247; Paek et al., GCN 29254; Smartt et al., GCN 29262; Anandagoda et al., GCN 29273; Mao et al., GCN 29275; Moskvitin & Vlasyuk, GCN 29277; Belikin et al., GCN 29286; Nakamura et al., GCN 29291; Kann et al., GCN 29318), we clearly detect the host galaxy in both filters (RA,DEC = 06:55:05.18,+64:40:33.7; J2000).

After removing the halo of a nearby star, we measure the following AB magnitudes:

r = 24.2 +/- 0.3
z = 22.1 +/- 0.2

calibrated against SDSS field stars.

We acknowledge the excellent support from the LBTO and LBT-INAF staff, particularly B. Rothberg, F. Cusano, and D. Paris, in obtaining these observations.