TITLE:   GCN CIRCULAR
NUMBER:  21003
SUBJECT: GRB 170405A: SMARTS optical/IR afterglow observations
DATE:    17/04/09 00:35:06 GMT
FROM:    Bethany Cobb at GWU  <bcobb@gwu.edu>

B. E. Cobb (GWU), reports:

Using the ANDICAM instrument on the 1.3m telescope at CTIO, we
obtained optical/IR imaging of the error region of GRB 170405A
(GCN 20984, Troja et al.) at two epochs (with mid-exposure times
of 2017-04-06 02:51 UT & 06:52 UT). For each epoch, several
dithered images were obtained with total summed exposure times
of 15 min in V and I and 12 min in J and K.

The fading afterglow of GRB 170405A (e.g. GCN 20984, Troja et al.;
GCN 20988, Malesani et al.; GCN 20989, Klotz et al.) was detected
with the preliminary magnitudes (or 3-sigma limits) listed below.
Note that the optical photometry is calibrated against USNO-B1.0
stars in the field, so suffers from a large photometric calibration
error of about +/-0.2 magnitudes, which has not been included in
the errors quoted below. The IR photometry is calibrated against
2MASS stars. Additionally, the nearby source noted by Malesani
et al. (GCN 20988) is seen and is likely affecting the afterglow
photometry due to its proximity.

mid-exposure
time post-burst
(hours)     I mag          J mag     K mag
8.19000    20.4+/-0.1   >18.5     >16.9
12.1975    21.1+/-0.1   >18.8     >16.8