[vsnet-grb-info 15435] GRB140819A: Deep GROND Upper Limits

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  16724
SUBJECT: GRB140819A: Deep GROND Upper Limits
DATE:    14/08/20 01:31:51 GMT
FROM:    Sebastian Schmidl at TLS Tautenburg  <schmidl at tls-tautenburg.de>

S. Schmidl (TLS Tautenburg), K. Varela (MPE Garching), D. A. Kann (TLS
Tautenburg) and J. Greiner (MPE Garching) report on behalf of the GROND
team:

We observed the field of GRB 140819A (Fermi trigger 430148973; optical
candidate: Lipunov et al., GCN #16720) simultaneously in g'r'i'z'JHK with
GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2m MPG
telescope at ESO La Silla Observatory (Chile).

Observations started on August 19, 2014, at 23:42 UT, 9.9 hrs after the
GRB trigger. They were performed at an average seeing of 1.0" and at an
average airmass of 1.1.

We do not detect a source at the postion of the afterglow candidate
reported by Lipunov et al. (GCN #16720).

Based on a total exposure of 25 minutes in g'r'i'z'and 20 minutes in JHK,
at a midtime of 0.42 days after the burst, we measure the following
preliminary upper limits (AB magnitudes system):

g' > 25.0,
r' > 25.0,
i' > 24.1,
z' > 23.6,
J  > 20.9,
H  > 20.7, and
K  > 18.8.

These derived upper limits are significantly deeper than those reported
beforehand (Espartero et al., GCN #16772, Dichiara et al., GCN #16773).

The decay slope between the 10th magnitude detection at a minute after the
GRB and r' > 25 mag at 10 hours implies a decay slope > 2. Such a steep
slope over a prolonged period of time is unlikely. Note that if the
transient is not seen already in the second image taken by MASTER, an even
steeper slope is implied (assuming a magnitude limit of 12.6 for the
image).

We therefore propose the optical transient is unassociated with the GRB,
and may be due to, e.g., a glint off a tumbling object in near Earth
orbit.

Given magnitudes are calibrated against GROND zeropoints as well as
2MASS field stars and are not corrected for the expected Galactic
foreground extinction corresponding to a reddening of E_(B-V) = 0.58 mag
in the direction of the burst (Schlegel et al. 1998).