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 T 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 E-gamma,E-iso = 1.75(-0.53)(+0.60) x 10(50) erg, and photon index Gamma = 3.00(-0.42)(+0.50) (15-150 keV). It follows the Amati relation, a correlation between E-gamma,E-iso and spectral peak energy Ep followed by long GRBs. It appears exceptionally soft based on Gamma, 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) x 10(4) s to t = 1.67(-0.65)(+1.14) x 10(6) s. We fit the data to closure relations describing the synchrotron emission, finding the electron spectral index to be p = 2.42(-0.30)(+0.44) and evidence of late-time energy injection with coefficient q = 0.24(-0.18)(+0.24). The jet half opening angle lower limit (theta(j) >= 16 degrees) 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.