GCN Circular 41061

A.J. Tetarenko (ULethbridge), J. Bright (Oxford), G. Bower (EAO/JCMT), S. Graves (EAO/JCMT):

The James Clerk Maxwell Telescope (JCMT) observed the position of GRB 250702B,D,E/EP250702A (GCNs 40906, 40883, 40885, 40886, 40890, 40891, ATels 17259, 17261) on 2025 July 08 between ~08:50 - 12:30 UTC for 3 scans totalling 0.55 hrs on-source and 2025 July 09 between ~08:15 - 09:20 UTC for 2 scans totalling 1.05 hrs on-source. The observations were made at a central frequency of 850 um (350 GHz) with the SCUBA-2 instrument. We do not detect a target source at the previously reported VLA radio position (RA: 18:58:45.565, Dec: -07:52:26.42; GCN 41053), yielding a 3-sigma upper limit of 6.3 mJy from stacking all of the data.

Extrapolating the previous radio detections with MeerKAT/VLA at 1.28/10 GHz of ~0.1/0.5 mJy (which imply a spectral index of ~1.3; GCN 41053), we would expect a clear detection at 350 GHz of ~50 mJy, suggesting a break in the spectrum between the cm and mm/sub-mm regions for this source. However, the recent ALMA detection at 97.5 GHz of ~2 mJy (GCN 41059), taken nearly simultaneously with our JCMT observations, implies a flatter spectral index of ~0.6. Therefore, a more accurate estimate of the expected 350 GHz flux density at the time of our JCMT observations would be ~4 mJy, consistent with our 3-sigma upper limit. Given these constraints, the source appears to likely have an inverted optically thick spectrum extending into the sub-mm bands.

We thank the JCMT staff for rapidly scheduling these observations. The JCMT is operated by the EAO on behalf of ASIAA and NARIT. Additional funds for the construction of SCUBA-2 were provided by the Canada Foundation for Innovation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.