TITLE:   GCN CIRCULAR
NUMBER:  20508
SUBJECT: GRB 170121B: LCO FTN observations
DATE:    17/01/23 08:41:54 GMT
FROM:    Cristiano Guidorzi at Ferrara U,Italy  <guidorzi@fe.infn.it>

C. Guidorzi (U. Ferrara), S. Kobayashi, I.A. Steele (LJMU), A. Gomboc 
(U. Nova Gorica), C.G. Mundell (U. Bath) on behalf of a large 
collaboration report:

We observed Fermi GRB 170121B (Hamburg et al. GCN 20505) on January 23, 
07:21 UT (1.7 days since the GRB) with the 2-m LCO Faulkes Telescope 
North in Hawaii with SDSS r and i filters. We do not detect the MASTER 
optical counterpart (Lipunov et al. GCN 20502) down to the following limits:

Mid Time      Exposure       Filter       Magnitude (AB)
(days)           (s)
-------------------------------------------------------
1.69            2x120        SDSS-R        > 21.0
1.69            2x120        SDSS-I        > 19.5
-------------------------------------------------------

as calibrated against nearby USNO-URAT1 sources.