  Evolution of first population of massive metal-free binary stars is followed.
Due to the low metallicity, the stars are allowed to form with large initial
masses and to evolve without significant mass loss. Evolution at zero
metallicity, therefore, may lead to the formation of massive remnants. In
particular, black holes of intermediate-mass (100-500 Msun) are expected to
have formed in early Universe, in contrast to the much lower mass stellar black
holes (10 Msun) being formed at present. Following a natural assumption, that
some of these Population III stars have formed in binaries, the physical
properties of first stellar binary black holes are presented. We find that a
significant fraction of such binary black holes coalesces within the Hubble
time. We point out that burst of gravitational waves from the final
coalescences and the following ringdown of these binary black hole mergers can
be observed in the interferometric detectors. We estimate that advanced LIGO
detection rate of such mergers is at least several events per year with high
signal to noise ratio (>10).
