  Long gamma-ray bursts (GRBs) are believed to be related to the explosion of
type Ic supernovae, which have been stripped of their hydrogen and helium
envelopes. There appear to be two types of these explosions: those which are
approximately spherical (GRB980425/1998bw), and which are associated with weak
bursts, and the classical GRBs which generate ultrarelativistic jets
(GRB030329/SN2003dh). If this bimodality is real Swift will provide a clear
evidence for it.
  We propose that classical powerful GRBs, which generate ultrarelativistic
outflows, are a result of a formation of quark stars. Quark stars may provide
an additional energy for the explosion of SN Ic, but far more important is a
creation of a surface which acts as a membrane which cannot be penetrated by
baryons. A surface of a quark star allows only ultrarelativistic matter to
escape: photons, neutrinos, electron -- positron pairs and magnetic fields. The
formation of a quark star follows the initial core collapse in several minutes.
Possible evidence for this time delay is provided by BATSE precursors to GRBs,
as analyzed by Lazzati (2005).
