Abstract
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 one second in the Swift 15–350 keV energy band exceed 2.6 photons cm−2 s−1, we confirm that there does exist the third class of 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 are between −3 and −2. From these properties, we propose a possible scenario with different central engines: an accreting black hole and a magnetar.
Highlights
The existence of the third class of gamma-ray bursts (GRBs) has been studied by some authors
From the Swift GRB table on the Web2, we obtain the list of long gamma-ray bursts (LGRBs) including the trigger number, the duration T9S0wift for which 90 % of the total photons of the event in the Swift 15-350 keV band are detected, the peak photon flux P in the Swift 15-150 keV band, the time TsXtaRrTt elapsed from the BAT trigger to the first XRT observation, the initial temporal decay index αinit, and spectral index βXRT obtained by XRT observations
Using the complete sample of the bright Swift LGRBs, we have confirmed the subclasses in LGRBs first discovered by Tsutsui et al (2013)
Summary
The existence of the third class of gamma-ray bursts (GRBs) has been studied by some authors. The purpose of this paper is to confirm the third class of GRBs with a completely different method from the previous works (Horvath 1998; Mukherjee et al 1998; Hakkila et al 2000; Horvath et al 2010) In these works, the duration, hardness and/or fluence were used to classify GRBs, but these observed values include some obstacles to identifying the difference between subclasses of GRBs as follows:. The first is to make proper corrections to obtain only intrinsic properties of GRBs (Frail et al 2001; Bloom et al 2003; Ghirlanda et al 2004) This approach needs information on the afterglow emission and the host galaxy in addition to the prompt emission and the number of sample with sufficient information significantly decreases.
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