A Way to Produce Neutron Star-Black Hole Binaries

Massive stars in binary systems can potentially evolve to form exotic compact binary objects such as neutron star-black hole (NS-BH) binaries or neutron star-neutron star (NS-NS) binaries. Consider a binary system with two massive stars closely orbiting one another. The more massive star evolves quicker and eventually runs out of nuclear fuel to support itself. Its core collapses to form a neutron star (NS) via a supernova explosion. Meanwhile, the less massive star continues to evolve and swells in size as it builds up a carbon-oxygen (CO) core in its center. The star’s outer layers eventually engulf the nearby NS and cause the NS to spiral inwards, towards the CO core.

As the NS spirals in, it transfers angular momentum to the star’s outer layers. This causes the star’s other layers, comprised mostly of hydrogen and helium, to be expelled into space, leaving behind a massive CO core. What was once a binary system consisting of two massive stars is now a tightly-bound binary system consisting of a massive CO core with ~2 times the Sun’s mass and a NS with more than ~2 times the Sun’s mass. Subsequently, the massive CO core also collapses in a supernova explosion, leaving behind a new neutron star (νNS) with ~1.5 times the Sun’s mass. The supernova explosion ejects ~0.5 Sun’s mass worth of material that was once part of the massive CO core.

The nearby NS grows in mass by accreting matter from the supernova ejecta. Soon, the NS becomes too massive and collapses gravitationally to form a black hole (BH) with at least ~2.5 times the Sun’s mass. An energetic gamma ray burst (GRB) is produced during the collapse. The tightly-bound νNS- BH binary system continues to evolve into an ever tighter configuration via the emission of gravitational waves. Such a νNS- BH binary system may be detectable from its gravitational wave signal. Within a span of 10,000 years or less, the νNS-BH binary eventually merge, driving an ultra-short duration GRB. Two supernovae (i.e. plural of supernova) and two GRBs later, a binary system of two massive stars is now a single black hole drifting through space.

Reference:
C. L. Fryer, F. G. Oliveira, J. A. Rueda, R. Ruffini (2015), “On the Neutron Star-Black Hole Binaries Produced by Binary-driven Hypernovae”, arXiv:1505.02809 [astro-ph.HE]