TITLE:   GCN CIRCULAR
NUMBER:  20606
SUBJECT: GRB 170202A: SMARTS optical/IR afterglow observations
DATE:    17/02/05 16:55:09 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 170202A
(GCN 20575, Racusin et al.) at three different epochs: 0.41671
[2017-02-03 04:28 UT], 0.53096 [2017-02-03 07:13 UT] and 2.39175
[2017-02-05 03:52 UT] days post-burst. At each epoch, total summed
exposure times amounted to 36 minutes in I and 30 minutes in J.

The afterglow of GRB 170202A (e.g. GCN 20575, Racusin et al.;
GCN 20576, Xin et al.; GCN 20578, Lipunov et al.) is detected
in our early images, with the preliminary magnitudes (and 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 +/-0.3 magnitudes, which has
not been included in the errors quoted below. The IR photometry
is calibrated against the single 2MASS star in our field-of-view,
so similarly may include a large (but unknown) calibration error.

mid-exposure time
(days post-burst)  I mag             J mag
0.41671                20.1+/-0.1      18.9+/-0.1
0.53096                20.6+/-0.1      >18.5
2.39175                >21.0             >18.6