EP250702a/GRB 250702B,C,D,E: Archival radio observations from ASKAP VAST
ATel #17269; Akash Anumarlapudi (UNC Chapel Hill), David Kaplan (UWM), Igor Andreoni (UNC Chapel Hill), Dougal Dobie (U. Sydney/OzGrav), Tara Murphy (U. Sydney) on behalf of the VAST collaboration.
on 5 Jul 2025; 03:36 UT
Credential Certification: Akash Anumarlapudi (aakash@uwm.edu)
Subjects: Radio, X-ray, Transient
Fermi Gamma-ray Burst Monitor (GBM) reported the discovery of four possible long-durational GRBs, GRB 250702B,C,D,E, that might likely be coming from the same source (GCN #40891). Further X-ray observations by the Einstein Probe (EP; GCN #40906), MAXI (GCN #40910), Konus-WIND (GCN #40914), and SVOM (GCN #40923) resulted in the discovery of an X-ray counterpart. Data from the Wide-field X-ray Telescope (WXT) aboard EP revealed that the source was active in X-rays even a day before the Fermi GBM discovery. Follow-up observations from the Follow-up X-ray Telescope on EP (FXT, GCN #40906) constrained the position of the source to 20 arcseconds (90% confidence). The inferred line of sight column density is slightly higher than the expected column density, not ruling out a Galactic origin. No optical counterparts were reported down to i=22.0 (GCN #40918), but infrared observations have resulted in the discovery of a candidate counterpart (GCN #40924) with Ks~17.3 Vega, which is very red in color and rapidly fading (GCN #40961). Subsequent analysis of GBM data suggested that the burst GRB 250702C is likely not related to GRB 250702 B/D/E, but an unrelated short GRB (GCN #40931).
Here we report the archival radio observations of this source taken at the Australian SKA Pathfinder (ASKAP) observations as part of the Variable And Slow Transients Survey (VAST; Murphy et al. 2021) at 887.5 MHz.
The position of the source was observed for 13 epochs, roughly with a two-week cadence between March 2024 and December 2024. Each observation lasted approximately 12 minutes, reaching a sensitivity (noise level) of 250 uJy. The source was not detected in any of the single-epoch images. We stacked the individual exposures, which also resulted in a non-detection. The 1-sigma noise level at the location of the source is 75 uJy, and hence we place a limit on the persistent radio flux density (5-sigma) of the source to be < 375 uJy.
