TITLE:   GCN CIRCULAR
NUMBER:  20991
SUBJECT: GRB 170405A: Watcher optical detection
DATE:    17/04/06 07:36:55 GMT
FROM:    Antonio Martin-Carrillo at UCD,Space Science Group  <antonio.martin-carrillo@ucd.ie>

A. Martin-Carrillo (UCD), D. Murphy (UCD), L. Hanlon (UCD), H. J. van Heerden (UFS), B. van Soelen (UFS) and P. J. Meintjes (UFS)

We observed the field of GRB 170405A (Troja et al, GCN 20984; Hui & Meegan, GCN 20986; Vianello & Kocevski, GCN 20987) using the 40cm UCD Watcher telescope at Boyden Observatory in South Africa.

Due to bad weather observations started at 19:31:13 UT, about 51.4 minutes after the burst trigger (T0=18:39:48 UT). A faint source consistent with the UVOT coordinates reported by Troja et al. is detected on our combined images taken with the SDSS r’ filter. At a mid-time of 20:00 UT, we derive a preliminary magnitude of r’=18.0 (AB system), confirming that the source has faded since the UVOT detection.

Magnitudes were calibrated using several nearby APASS stars. No correction for Galactic extinction in the direction of the GRB has been applied.