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

GRB 201015A is a peculiarly low luminosity, spectrally soft gamma-ray burst (GRB), with T₉₀ = 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 E_γ,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 E_γ,iso and spectral peak energy E_p followed by long GRBs. It appears exceptionally soft based on Γ, the hardness ratio of HR = 0.47 ± 0.24, and low-E_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 t = (2.61 ± 1.27) × 10⁴ s to t = 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 p = 2.42+0.44−0.30 and evidence of late-time energy injection with coefficient q = 0.24+0.24−0.18. The jet half opening angle lower limit (θj ≥ 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.