Publications (2)0 Total impact
ABSTRACT: Mergers of double neutron stars are considered the most likely progenitors for short gamma-ray bursts. Indeed such a merger can produce a black hole with a transient accreting torus of nuclear matter (Lee & Ramirez-Ruiz 2007, Oechslin & Janka 2006), and the conversion of a fraction of the torus mass-energy to radiation can power a gamma-ray burst (Nakar 2006). Using available binary pulsar observations supported by our extensive evolutionary calculations of double neutron star formation, we demonstrate that the fraction of mergers that can form a black hole -- torus system depends very sensitively on the (largely unknown) maximum neutron star mass. We show that the available observations and models put a very stringent constraint on this maximum mass under the assumption that a black hole formation is required to produce a short gamma-ray burst in a double neutron star merger. Specifically, we find that the maximum neutron star mass must be within 2 - 2.5 Msun. Moreover, a single unambiguous measurement of a neutron star mass above 2.5 Msun would exclude a black hole -- torus central engine model of short gamma-ray bursts in double neutron star mergers. Such an observation would also indicate that if in fact short gamma-ray bursts are connected to neutron star mergers, the gamma-ray burst engine is best explained by the lesser known model invoking a highly magnetized massive neutron star (e.g., Usov 1992; Kluzniak & Ruderman 1998; Dai et al. 2006; Metzger, Quataert & Thompson 2007). Comment: 3 pages of text + 4 figures: ApJ accepted (some revisions)
ABSTRACT: Using an updated population synthesis code we study the formation and evolution of black holes (BHs) in young star clusters following a massive starburst. This study continues and improves on the initial work described by Belczynski, Sadowski & Rasio (2004). In our new calculations we account for the possible ejections of BHs and their progenitors from clusters because of natal kicks imparted by supernovae and recoil following binary disruptions. The results indicate that the properties of both retained BHs in clusters and ejected BHs (forming a field population) depend sensitively on the depth of the cluster potential. In particular, most BHs ejected from binaries are also ejected from clusters with central escape speeds Vesc < 100 km/s. Conversely, most BHs remaining in binaries are retained by clusters with Vesc > 50 km/s. BHs from single star evolution are also affected significantly: about half of the BHs originating from primordial single stars are ejected from clusters with Vesc < 50 km/s. Our results lay a foundation for theoretical studies of the formation of BH X-ray binaries in and around star clusters, including possible ultra-luminous sources, as well as merging BH--BH binaries detectable with future gravitational-wave observatories.