GCN Circular 45560
Subject
GRB 260818A: Continued ATCA observations
Event
Date
2026-09-14T03:26:21Z (3 days ago)
From
Tao An at USTC <antao2008@ustc.edu.cn>
Via
Web form
Tao An (USTC), Ailing Wang (YNAO, CAS) report on behalf of a large collaboration:
We report a second-epoch Australia Telescope Compact Array (ATCA) observation of the candidate radio counterpart to GRB 260818A (Yin et al., GCN 45391; EP-FXT X-ray afterglow: Wang et al., GCN 45400; Plasse et al., GCN 45434; first ATCA detection: An & Wang, GCN 45494).
The new observation started at 2026-09-12 14:01 UTC (~25.5 days after the SVOM/ECLAIRs trigger). Data were obtained simultaneously at central frequencies of 6.53 GHz and 10.37 GHz. For a homogeneous comparison, we also re-imaged the first-epoch data (starting 2026-08-30 10:49 UTC; T0+12.4 d) with the same spectral-window selection.
A radio source is detected at both epochs near R.A. (J2000) = 23:20:25.422, Dec. (J2000) = −32:05:49.60, consistent with the fading X-ray afterglow EPF_J232025.8-320549. Preliminary peak brightnesses and image rms values are:
Epoch T0+ (d) Peak_6GHz rms S/N Peak_10GHz rms S/N
1 12.41 71.9 8.2 8.8 50.2 7.5 6.7
2 25.54 61.7 7.7 8.0 32.3 7.0 4.6
The peak flux density and rms are in unit of μJy/beam.
Between the two epochs, the 6.53 GHz peak declined by ~14% and the 10.37 GHz peak by ~36%. Approximating the unresolved peak brightness as the flux density, the two-point spectral index (S_ν ∝ ν^α) evolved from α = −0.78 ± 0.41 to α = −1.40 ± 0.54. The negative spectral indices are consistent with optically thin synchrotron emission. The apparent fading and spectral steepening are in the direction expected for a GRB radio afterglow. Absolute flux-calibration systematics are not included in the quoted uncertainties.
We thank the CSIRO Space and Astronomy staff, in particular Jamie Stevens, for supporting these observations. The Australia Telescope Compact Array is part of the Australia Telescope National Facility which is funded by the Australian Government for operation as a National Facility managed by CSIRO.