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GCN Circular 35505

Subject
GRB 230812B: further radio observations with the VLA
Date
2024-01-09T13:22:01Z (5 months ago)
From
Stefano Giarratana at University of Bologna <s.giarratana@ira.inaf.it>
Via
email
S. Giarratana (University of Bologna, INAF-IRA), M. Giroletti
(INAF-IRA), G. Ghirlanda (INAF-OAB), N. Di Lalla (Stanford Univ.),
N. Omodei (Stanford Univ.)

At 18:39:38 UT on 2023 August 29 (T_mid = 17.0 days post-burst)
and at 17:37:02 UT on 2023 September 19 (T_mid = 38.0 days
post-burst) the Karl G. Jansky VLA started observing the field of
GRB 230812B (Fermi GBM team, GCN 34386) at a central
frequency of 6 and 10 GHz.

The standard J1331+3030 (3C286) was used as bandpass and
flux density calibrator, while J1637+4717 was used as complex
gain calibrator.

From a preliminary analysis, the radio counterpart of GRB 230812B
(Rhodes et al., GCN 34433; de Ugarte Postigo et al., GCN 34468;
Giarratana et al., GCN 34552; Chandra et al., GCN 34735) is no
longer detected. We derive the following upper limits (UL; 3sigma).

=====================================================
T_mid    Freq     UL     r.m.s.    Beam         PA
[days]   [GHz]   [uJy]   [uJy/b]   [arcsec]     [deg]
=====================================================
17.0     6        24     8         0.68x0.29    -71
17.0     10       24     8         0.50x0.18    -65
38.0     6        21     7         0.60x0.29    -74
38.0     10       21     7         0.40x0.18    -70
=====================================================

We note that the 6 GHz upper limit at 17 days post-burst seems
inconsistent with the detection reported by Chandra et al., GCN 34735,
at the same frequency.

We would like to thank the staff of the VLA for approving, executing,
and processing the observations.

The National Radio Astronomy Observatory is a facility of the National
Science Foundation operated under cooperative agreement by Associated
Universities, Inc.

These observations were carried out as part of project SF161095,
approved in the framework of the Fermi - NRAO joint program agreement.

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