On the Subclasses in Swift Long Gamma-Ray Bursts: A Clue to Different Central Engines

Abstract

Analyzing light curves of a complete sample of bright Swift long gamma-ray bursts (LGRBs) of which the peak photon fluxes constructed with the bin width of 1 second in the Swift 15-350 keV energy band exceed 2.6 photons cm$^{-2}$s$^{-1}$, we confirm that there do exist the third class in GRBs in addition to short and long GRBs. Being different from previous works based on the duration, fluence, etc. our classification method is based on two properties both quantified with light curve shapes of the prompt emission: the Absolute Deviation from the Constant Luminosity of their cumulative light curve $ADCL$, and the ratio of the mean counts to the maximum counts $\bar C/C_{\rm max}$. These are independent of the distance and the jet opening angle. A cluster analysis via the Gaussian mixture model detects three subclasses: one consisting of LGRBs with small $ADCL$ and large $\bar C/C_{\rm max}$ values referred to as Type I, one with large $ADCL$ and large $\bar C/C_{\rm max}$ referred to as Type II, and one with intermediate $ADCL$ and small $\bar C/C_{\rm max}$, which is composed of contaminating short GRBs with the extended emission. This result is reinforced by different temporal and spectral indices of their X-ray afterglows. The difference is prominent in the temporal index of the steep decay phase in particular: the indices for Type I LGRBs distribute between $-6$ and $-3$ while those for Type II LGRBs between $-3$ and $-2$. From these properties, we propose a possible scenario with different central engines: an accreting black hole and a magnetar.