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

M. Patel, B. P. Gompertz, P. T. O'Brien, G. P. Lamb, R. L. C. Starling, P. A Evans, L. Amati, A. J. Levan, M. Nicholl, J. Lyman, K. Ackley, M. J. Dyer, K. Ulaczyk, D. Steeghs, D. K. Galloway, V. S. Dhillon, G. Ramsay, K. Noysena, R. Kotak, R. P. BretonL. K. Nuttall, E. Palle, D. Pollacco

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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 Eγ,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 Eγ,iso and spectral peak energy Ep followed by long GRBs. It appears exceptionally soft based on Γ, the hardness ratio of HR = 0.47 ± 0.24, and low-Ep, 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 = 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 = 2.42+0.44−0.30 and evidence of late-time energy injection with coefficient = 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.
Original languageEnglish
Pages (from-to)4923–4937
Number of pages15
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
Early online date7 Jun 2023
Publication statusPublished - 1 Aug 2023


  • gamma-ray burst: general
  • gamma-ray burst: individual: GRB 201015A
  • gamma-ray bursts
  • transients: supernova
  • UKRI
  • STFC
  • ST/T007184/1
  • ST/T003103/1

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