Identifying Subclasses of Long Gamma-Ray Bursts with Cumulative Light-Curve Morphology of Prompt Emissions

Abstract

Abstract We argue for a new classification scheme of long gamma-ray bursts (LGRBs) using the morphology of the cumulative light curve of prompt emission. We parametrize the morphology by an absolute deviation from their constant luminosity (ADCL), and derive the value for 36 LGRBs that have spectropic redshifts, spectral parameters determined by the Band model, 1-s peak fluxes, fluences, and 64-ms resolution light curves whose peak counts are 10-times larger than background fluctuations. Based on a possible bimodal distribution of the value of ADCL, we divide the sample into two groups (ADCL$ <$ 0.17 and ADCL$ >$ 0.17), and for each group derive the spectral peak energy $ E_{\rm p}$ –peak luminosity $ L_{\rm p}$ correlation and the Fundamental Plane of LGRBs, which is a correlation between $ E_{\rm p}$ , the luminosity time $ T_{\rm L}$ ($ \equiv$$ E_{\rm iso}/L_{\rm p}$ where $ E_{\rm iso}$ is isotropic energy), and $ L_{\rm p}$ . We find that the Fundamental Plane at $ E_{\rm p}$$ =$ 10$ ^{2.71}$ keV and $ T_{\rm L}$$ =$ 10$ ^{0.86}$ s for large ADCL events are 2.8-times brighter than that for small ADCL events with 5-$ \sigma$ levels. The Fundamental Planes with small and large ADCL are given by $ L_{\rm p}$$ =$ 10$ ^{52.53\pm 0.01}$ ($ E_{\rm p}/$ 10$ ^{2.71}$ keV)$ ^{1.84\pm 0.03}$ ($ T_{\rm L}/$ 10$ ^{0.86}$ s)$ ^{0.29\pm0.08}$ with $ \chi^2_{\nu}$$ =$ 10.93$ /$ 14 and $ L_{\rm p}$$ =$ 10$ ^{52.98\pm0.08}$ ($ E_{\rm p}/$ 10$ ^{2.71}$ keV)$ ^{1.82\pm 0.09}$ ($ T_{\rm L}/$ 10$ ^{0.86}$ s)$ ^{0.85\pm 0.27}$ with $ \chi^2_{\nu}$$ =$ 7.58$ /$ 8, respectively. This fact implies the existence of subclasses of LGRBs characterized by the value of ADCL. The other significant difference between the relations for the two subclasses consists in the exponent of $ T_{\rm L}$ , which might provide a hint of the physics of the relations, but the significance is now only at 2-$ \sigma$ levels. Also, there is a hint for the existence of an intermediate-ADCL class, which deviates from both fundamental planes. Both relations are so tight that our result provides a new accurate distance measurement scheme up to the high-redshift universe.