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 $T_{90}=9.8\pm 3.5$ s (time interval of detection of 90% of photons from the GRB), and an associated supernova (likely to be type Ic or Ic-BL). GRB 201015A has an isotropic energy $E_{\gamma ,\mathrm{i}\mathrm{s}\mathrm{o}}=1.75_{-0.53}^{+0.60}\times {10}^{50}$ erg, and photon index $\mathrm{\Gamma }=3.00_{-0.42}^{+0.50}$ (15-150 keV). It follows the Amati relation, a correlation between $E_{\gamma ,\mathrm{i}\mathrm{s}\mathrm{o}}$ and spectral peak energy $E_{\mathrm{p}}$ followed by long GRBs. It appears exceptionally soft based on $\mathrm{\Gamma }$, the hardness ratio of HR = $0.47\pm 0.24$, and low-$E_{\mathrm{p}}$, 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 $t=(2.61\pm 1.27)\times {10}^4$ s to $t=1.67_{-0.65}^{+1.14}\times {10}^6$ 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 (${\theta }_j\ge {16}^{\circ }$) 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.