On the formation and evolution of the first Be star in a black hole binary MWC 656

We find that the formation of MWC 656 (the first Be binary containing a black hole) involves a common envelope phase and a supernova explosion. This result supports the idea that a rapidly rotating Be star can emerge out of a common envelope phase, which is very intriguing because this evolutionary stage is thought to be too fast to lead to significant accretion and spin up of the B star. We predict ∼10–100 of B-BH binaries to currently reside in the Galactic disc, among which around 1/3 contain a Be star, but there is only a small chance to observe a system with parameters resembling MWC 656. If MWC 656 is representative of intrinsic Galactic Be-BH binary population, it may indicate that standard evolutionary theory needs to be revised. This would pose another evolutionary problem in understanding black hole (BH) binaries, with BH X-ray novae formation issue being the prime example. Future evolution of MWC 656 with an ∼5 M⊙ BH and with an ∼13 M⊙ main-sequence companion on an ∼60 d orbit may lead to the formation of a coalescing BH–NS (neutron star) system. The estimated Advanced LIGO/Virgo detection rate of such systems is up to ∼0.2 yr−1. This empirical estimate is a lower limit as it is obtained with only one particular evolutionary scenario, the MWC 656 binary. This is only a third such estimate available (after Cyg X-1 and Cyg X-3), and it lends additional support to the existence of so far undetected BH–NS binaries.

MG and KB acknowledge support from Polish Science Foundation ‘Master2013’ Subsidy. MG acknowledges support by Polish NCN grant Preludium (2012/07/N/ST9/04184). The work of KB was supported by the NCN grant Sonata Bis 2 (DEC-2012/07/E/ST9/01360), the National Science Foundation under grant no. PHYS-1066293 and the hospitality of the Aspen Center for Physics. JC acknowledges support by DGI of the Spanish Ministerio de Educación, Cultura y Deporte under grants AYA2010-18080, AYA2013-42627 and SEV-2011-0187. SdM acknowledges support for early stages of this study by NASA through an Einstein Fellowship grant, PF3-140105 and a Marie Sklodowska-Curie Reintegration Fellowship (H2020-MSCA-IF-2014, project id 661502). The work of IN and AH is partially supported by the Spanish Ministerio de Economía y Competitividad (MINECO) under grant AYA2012-39364-C02-01/02, and the European Union. MR acknowledges support by the Spanish MINECO under grant FPA2013-48381-C6-6-P. IR acknowledges support from the Spanish Ministry of Economy and Competitiveness (MINECO) and the Fondo Europeo de Desarrollo Regional (FEDER) through grant ESP2013-48391-C4-1-R. JMP acknowledges support by the Spanish MINECO under grant AYA2013-47447-C3-1-P and financial support from ICREA Academia. MB acknowledges support from the National Science Foundation under award HRD-1242090 and the hospitality of the Aspen Center for Physics.