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