1000 days of lowest frequency emission from the low-luminosity GRB 171205A

Abstract

We report the lowest frequency measurements of gamma-ray burst (GRB) 171205A with the upgraded Giant Metrewave Radio Telescope (uGMRT) covering a frequency range from 250-1450 MHz and a period of 4 − 937 days. It is the first GRB afterglow detected at 250-500 MHz frequency range and the second brightest GRB detected with the uGMRT. Even though the GRB is observed for nearly 1000 days, there is no evidence of transition to non-relativistic regime. We also analyse the archival Chandra X-ray data on day ∼ 70 and day ∼ 200. We also find no evidence of a jet break from the analysis of combined data. We fit synchrotron afterglow emission arising from a relativistic, isotropic, self-similar deceleration as well as from a shock-breakout of wide-angle cocoon. Our data also allow us to discern the nature and the density of the circumburst medium. We find that the density profile deviates from a standard constant density medium and suggests that the GRB exploded in a stratified wind like medium. Our analysis shows that the lowest frequency measurements covering the absorbed part of the light curves are critical to unravel the GRB environment. Our data combined with other published measurements indicate that the radio afterglow has contribution from two components: a weak, possibly slightly off-axis jet and a surrounding wider cocoon, consistent with the results of Izzo et al. (2019). The cocoon emission likely dominates at early epochs, whereas the jet starts to dominate at later epochs, resulting in flatter radio lightcurves.

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

“…The flux-density levels in the fourth and fifth SED, sampled after the passage of 𝜈 sa through the band, were also very similar; this is consistent with the wind scenario, where the flux density is predicted to remain constant in the 𝜈 sa < 𝜈 < 𝜈 m spectral segment (GS02). Our conclusion that the afterglow is produced by the relativistic outflow decelerating in a circumburst medium with a wind density profile is in agreement with the conclusion obtained by the independent analysis of Maity & Chandra (2020) using observations from the upgraded Giant Metrewave Radio Telescope.…”

“…The very late-time SED (850 days post-burst) enabled us to constrain the slope of the electron spectrum 𝑝, where the distribution of electron energies is 𝑁 (𝐸) ∝ 𝐸 − 𝑝 . According to Maity & Chandra (2020), even at ∼ 1, 000 days post-burst, the blast wave had not transitioned to the non-relativistic (Newtonian) regime. A spectral inversion occurred between the fifth and sixth SED (76 and 850 days post-burst), where the spectral index transitioned from 𝛼 = 1/3 to 𝛼 = −0.92.…”