GRB 071227: an additional case of a disguised short burst
  (shortened) [...] In the context of the fireshell model, [...] a new family
of disguised short bursts has been identified: long bursts with a protracted
low instantaneous luminosity due to a low density CircumBurst Medium (CBM). In
the 15-150 keV energy band GRB 071227 exhibits a short duration (about 1.8s)
spike-like emission followed by a very soft extended tail up to one hundred
seconds after the trigger. It is a faint (E_{iso}=5.8x10^{50}) nearby GRB
(z=0.383) that does not have an associated type Ib/c bright supernova (SN). For
these reasons, GRB 071227 has been classified as a short burst not fulfilling
the Amati relation holding for long burst. We check the classification of GRB
071227 provided by the fireshell model. In particular, we test whether this
burst is another example of a disguised short burst, after GRB 970228 and GRB
060614, and, for this reason, whether it fulfills the Amati relation. We
simulate GRB 071227 light curves in the Swift BAT 15-50 keV bandpass and in the
XRT (0.3-10 keV) energy band within the fireshell model. We perform simulations
of the tail in the 15-50 keV bandpass, as well as of the first part of the
X-ray afterglow. This infers that: E_{tot}^{e^\pm}=5.04x10^{51} erg,
B=2.0x10^{-4}, E_{P-GRB}/E_{aft}~0.25, and <n_{cbm}>=3.33 particles/cm^3. These
values are consistent with those of "long duration" GRBs. We interpret the
observed energy of the first hard emission by identifying it with the P-GRB
emission. The remaining long soft tail indeed fulfills the Amati relation.
Previously classified as a short burst, GRB 071227 on the basis of our analysis
performed in the context of the fireshell scenario represents another example
of a disguised short burst, after GRB 970228 and GRB 060614. Further
confirmation of this result is that the soft tail of GRB 071227 fulfills the
Amati relation.
