TITLE:   GCN CIRCULAR
NUMBER:  30479
SUBJECT: GRB 210702A: ALMA detection of mm-band rebrightening
DATE:    21/07/22 22:50:41 GMT
FROM:    Tanmoy Laskar at U of Bath  <tanmoylaskar@gmail.com>

T. Laskar (U. of Bath) reports on behalf of a larger collaboration:

"Following the initial detection of the mm counterpart of GRB 210702A (Lien
et al., GCN 30351) with the Atacama Large Millimeter/Submillimeter Array
(ALMA) on 2021 July 03, 26 h after the burst (Laskar & Perley, GCN 30423),
we have been monitoring this source with ALMA at 97.5 GHz. The mm-band
counterpart was observed to fade to a flux density of ~ 0.3 mJy in
observations taken on 2021 July 11 at a mid-time of 8.3 days after the
burst.

We observed GRB 210702A with ALMA again beginning on 2021 July 20 21:02
UTC. Preliminary analysis reveals a strong (~ 50 sigma) detection at 97.5
GHz with a flux density of ~ 0.8 mJy at a mid-time of 18.1 days after the
burst, implying a significant rebrightening of the mm-band afterglow,
contemporaneous to the radio rebrightening observed at 34, 21.2, and 16.7
GHz with ATCA (Laskar & Bhandari, GCN 30477). Further observations are
planned.

We thank the JAO staff, AoD, P2G, and the entire ALMA team for their help
with these observations."