TITLE: GCN CIRCULAR NUMBER: 34301 SUBJECT: GRB 230723A: Detection of candidate millimeter-band radiation by SPT-3G DATE: 23/07/31 19:40:10 GMT FROM: Sam Guns at S Pole Telescope & UC Berkeley sguns@berkeley.edu

S. Guns (UC Berkeley), A. Foster (CWRU), C. Tandoi (UIUC), K. Phadke (UIUC), N. Whitehorn (MSU), G. Holder (UIUC), J. Vieira (UIUC) on behalf of the South Pole Telescope Collaboration:

On 26 July 2023 at 16:48 UTC the South Pole Telescope detected a millimeter-band transient candidate at RA = 43.5275, Dec = -70.4770 (J2000 degrees, uncertainty 12 arcseconds) using the SPT-3G camera in 2 bands centered at 95 GHz and 150 GHz. Peak emission was observed two hours later at 19:14 UTC with flux levels of 18.5 mJy in both bands, after which the SPT slewed away from the observing field. The next series of observations on 28 July showed flux levels consistent with zero. The spatial location of the emission is consistent (1.5 sigma) with GRB 230723A which was detected by Fermi-GBM on 23 July 2023 at RA = 21.0, Dec = -71.2 (J2000 degrees, uncertainty 4.8 degrees). From the SPT-3G transients program, short duration (<1 week) millimeter-band transients that are not associated with nearby stars are rare (<3 events per year). A table of SPT-3G observations is given below.

The lightcurve can be found at: https://pole.uchicago.edu/public/data/transients/lightcurves/SPT-SVJ025406.5...

Observation time | Flux (95 GHz) | Error (95 GHz) | Flux (150 GHz) | Error (150 GHz) 2023-7-22 05:04 UTC | -3.0 mJy | 2.5 mJy | -3.3 mJy | 3.0 mJy 2023-7-26 16:48 UTC | 13.3 mJy | 4.4 mJy | 14.0 mJy | 4.9 mJy 2023-7-26 19:14 UTC | 18.5 mJy | 4.4 mJy | 18.5 mJy | 4.9 mJy 2023-7-28 08:12 UTC | 1.7 mJy | 2.5 mJy | 1.8 mJy | 2.8 mJy

The South Pole Telescope is a 10-meter telescope located at Amundsen-Scott South Pole Station and supported by the National Science Foundation and the US Dept. of Energy. The SPT online transient program providing data in this circular is supported by NSF grants AST-1716965 and OPP 1852617, and observes 1500 square degrees of the southern sky at 95, 150, and 220 GHz with an average revisit cadence of 12 hours. For more details on the SPT transient program and survey strategy, please see https://arxiv.org/abs/2103.06166.

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