FORMATION OF BLACK HOLE X-RAY BINARIES IN GLOBULAR CLUSTERS

Abstract

Inspired by the recent identification of the first candidate BH-WD X-ray\nbinaries, where the compact accretors may be stellar-mass black hole candidates\nin extragalactic globular clusters, we explore how such binaries could be\nformed in a dynamical environment. We provide analyses of the formation rates\nvia well known formation channels like binary exchange and physical collisions\nand propose that the only possibility to form BH-WD binaries is via coupling\nthese usual formation channels with subsequent hardening and/or triple\nformation. Indeed, we find that the most important mechanism to make a BH-WD\nX-ray binary from an initially dynamically formed BH-WD binary is triple\ninduced mass transfer via the Kozai mechanism. Even using the most optimistic\nestimates for the formation rates, we cannot match the observationally inferred\nproduction rates if black holes undergo significant evaporation from the\ncluster or form a completely detached subcluster of black holes. We estimate\nthat at least 1% of all formed black holes, or presumably 10% of the black\nholes present in the core now, must be involved in interactions with the rest\nof the core stellar population.\n