A search for radio afterglows from gamma-ray bursts with the Australian Square Kilometre Array Pathfinder

Abstract

We present a search for radio afterglows from long gamma-ray bursts using the Australian Square Kilometre Array Pathfinder (ASKAP). Our search used the Rapid ASKAP Continuum Survey, covering the entire celestial sphere south of declination +41 • , and three epochs of the Variables and Slow Transients Pilot Survey (Phase 1), covering ∼ 5, 000 square degrees per epoch. The observations we used from these surveys spanned a nine-month period from 2019 April 21 to 2020 January 11. We crossmatched radio sources found in these surveys with 779 well-localised (to ≤ 15 ) long gamma-ray bursts occurring after 2004 and determined whether the associations were more likely afterglow-or host-related through the analysis of optical images. In our search, we detected one radio afterglow candidate associated with GRB 171205A, a local low-luminosity gamma-ray burst with a supernova counterpart SN 2017iuk, in an ASKAP observation 511 days post-burst. We confirmed this detection with further observations of the radio afterglow using the Australia Telescope Compact Array at 859 days and 884 days post-burst. Combining this data with archival data from early-time radio observations, we showed the evolution of the radio spectral energy distribution alone could reveal clear signatures of a wind-like circumburst medium for the burst. Finally, we derived semi-analytical estimates for the microphysical shock parameters of the burst: electron power-law index 𝑝 = 2.84, normalised wind-density parameter 𝐴 * = 3, fractional energy in electrons 𝜖 𝑒 = 0.3, and fractional energy in magnetic fields 𝜖 𝐵 = 0.0002.

Radio Constraints on $r$-process Nucleosynthesis by Collapsars

“…The flux detected for GRB 171205A Leung et al (2021) prior to the latest non-detection reported here does not meaningfully constrain the possible collapsar ejecta mass. If we assume that the latest detection at ∼ 900 days after the GRB is fully from the collapsar ejecta, i.e.…”

“…Assuming a constant-density interstellar medium around the collapsars, we found that ejecta with 0.2 M mass are inconsistent with observations for our fiducial model. Considering GRB 171205A and a wind profile for the circum-burst density (Leung et al 2021), we obtain a mass constraint of 0.5 M ( 0.05 M ) for 1 pc (0.01 pc) assumed shock termination radius.…”

“…Only one of the considered GRBs had a reported radio counterpart: GRB 171205A. Leung et al (2021) reported a radio afterglow candidate associated with GRB 171205A, detectable at least until 900 days post-burst. Comparing to early afterglow detections they found that all radio observations can be explained by afterglow emission propagating in a wind medium and with an unusually high electron Lorentz factor power law index p ≈ 2.8.…”

“…For each GRB in our sample, the time of and obtained radio flux limit from the most constraining observation is presented in Table . 1. In the case of GRB 171205A, the Table presents the latest non-detection obtained after the reported detection in Leung et al (2021).…”

“…model with density decreasing with r −2 . For this case we adopted the density obtained by Leung et al (2021) through their fit of the afterglow emission of GRB 171205A. With this model, assuming a terminal shock radius of 1 pc (0.01 pc) beyond which the medium density is taken to be 1 cm −3 , we could constrain the ejecta mass to 0.5 M ( 0.05 M ).…”