TITLE:   GCN CIRCULAR
NUMBER:  45418
SUBJECT: GRB 260818B: VLA radio observations
DATE:    26/08/21 22:41:23 GMT
FROM:    Genevieve Schroeder at Cornell University <genevieveschroeder@u.northwestern.edu>

G. Schroeder (U. Toronto), W. Fong (Northwestern), J. Rastinejad (UMD) report:

We observed the location of the short-duration GRB 260818B (Cordier et al., GCN 45398; Wang et al., GCN 45408; Ren et al, GCN 45411) with the Karl G. Jansky Very Large Array (VLA) in D configuration under program 26A-062 (PI Schroeder) for two 0.75 hour epochs at a mean frequency of 6 GHz and mid times of 2026 Aug 20 at 07:58 UT (1.5 days post-burst) and 2026 Aug 21 at 11:23 UT (2.6 days post-burst). The images reached typical rms values of 7-10 uJy.

In preliminary analysis, we detect 13 >= 5-sigma radio sources within the ECLAIRs error region across both epochs. Of them, 9 appear to vary on the 1 sigma level across the two epochs, only 1 of which appears to have faded by 3 sigma across the two epochs (S3). The details of the variable sources are as follows:

| Source | R.A.| Dec. | Error (arcsec) | Fnu e1 (uJy) | Fnu e2 (uJy) | Notes |
| ------------ | ------------ | ------------ | ------------ | ------------ | ------------ | ------------ |
| S2 | 02:59:56.59 | +07:29:28.5 | 2.3 | 83 | 107 | Close to apparent galaxy |
| S3 | 02:59:36.59 | +07:28:16.0 | 3.0 | 96 | 34 | Faded by >= 3sigma |
| S4 | 02:59:51.45 | +07:27:41.8 | 5.2 | 63 | 90 | Close to apparent galaxy |
| S5 | 03:00:00.19 | +07:27:23.4 | 0.4 | 1266 | 1200 | In VLASS/RACS |
| S7 | 02:59:38.44 | +07:26:20.5 | 2.7 | 114 | 91 | |
| S8 | 02:59:49.96 | +07:25:46.9 | 3.5 | 168 | 110 | |
| S9 | 02:59:50.37 | +07:25:41.0 | 4.0 | 169 | 91 | |
| S10 | 02:59:54.25 | +07:25:28.7 | 0.3 | 1232 | 1139 | Coincident with EPF_J025955.0+072535, in VLASS/RACS |
| S11 | 02:59:44.77 | +07:25:25.0 | 1.1 | 200 | 178 | Close to apparent galaxy |


Additionally, we note that EPF_J025955.0+072535 and EPF_J025936.6+072418 (Wu et al., GCN 45407) are coincident with long-lived radio sources present in VLASS and RACS (which are present in our observations as well). There is a faint radio source coincident with EPF_J025951.6+072917, however the flux density doesn’t vary significantly over the two epochs (~50 uJy).

We thank the VLA staff for quickly approving and executing these observations.