GRB 201015A and the nature of low-luminosity soft gamma-ray bursts

Abstract

<p>GRB 201015A is a peculiarly low luminosity, spectrally soft gamma-ray burst (GRB), with <em>T</em>₉₀ = 9.8 ± 3.5 s (time interval of detection of 90 per cent of photons from the GRB), and an associated supernova (likely to be type Ic or Ic-BL). GRB 201015A has an isotropic energy <em>E</em>_{<i>γ</i>,iso}=1.75^+0.60_−0.53×10⁵⁰ erg, and photon index Γ=3.00^+0.50_−0.42 (15–150 keV). It follows the Amati relation, a correlation between <em>E_γ</em>_,iso and spectral peak energy <em>E</em>_p followed by long GRBs. It appears exceptionally soft based on Γ, the hardness ratio of HR = 0.47 ± 0.24, and low-<em>E</em>_p, so we have compared it to other GRBs sharing these properties. These events can be explained by shock breakout, poorly collimated jets, and off-axis viewing. Follow-up observations of the afterglow taken in the X-ray, optical, and radio reveal a surprisingly late flattening in the X-ray from <em>t</em> = (2.61 ± 1.27) × 10⁴ s to <em>t</em>=1.67^+1.14_−0.65×10⁶ s. We fit the data to closure relations describing the synchrotron emission, finding the electron spectral index to be <em>p</em>=2.42^+0.44_−0.30 and evidence of late-time energy injection with coefficient <em>q</em>=0.24^+0.24_−0.18⁠. The jet half opening angle lower limit (<em>θ</em>_j ≥ 16°) is inferred from the non-detection of a jet break. The launch of SVOM and Einstein Probe in 2023 should enable detection of more low-luminosity events like this, providing a fuller picture of the variety of GRBs.<br></p>