  A large fraction of 100-km-class low-inclination objects in the classical
Kuiper Belt (KB) are binaries with comparable mass and wide separation of
components. A favored model for their formation was capture during the
coagulation growth of bodies in the early KB. Instead, recent studies suggested
that large objects can rapidly form in the protoplanetary disks when swarms of
locally concentrated solids collapse under their own gravity. Here we examine
the possibility that KB binaries formed during gravitational collapse when the
excess of angular momentum prevented the agglomeration of available mass into a
solitary object. We find that this new mechanism provides a robust path toward
the formation of KB binaries with observed properties, and can explain wide
systems such as 2001 QW322 and multiples such as (47171) 1999 TC36. Notably,
the gravitational collapse is capable of producing 100% binary fraction for a
wide range of the swarm's initial angular momentum values. The binary
components have similar masses (80% have the secondary-over-primary radius
ratio >0.7) and their separation ranges from ~1,000 to ~100,000 km. The binary
orbits have eccentricities from e=0 to ~1, with the majority having e<0.6. The
binary orbit inclinations with respect to the initial angular momentum of the
swarm range from i=0 to ~90 deg, with most cases having i<50 deg. Our binary
formation mechanism implies that the primary and secondary components in each
binary pair should have identical bulk composition, which is consistent with
the current photometric data. We discuss the applicability of our results to
the Pluto-Charon, Orcus-Vanth, (617) Patroclus-Menoetius and (90) Antiope
binary systems.
