Featured Image: An Outbursting Binary System

hydrodynamic simulations at different angles

Simulations of the Be star (larger green circle) and neutron star (smaller green circle) disks. Lighter colors indicate denser material. Results are shown after 29 (top), 33 (middle), and 38 (bottom) orbital periods of the binary have passed. After the initial mass transfer, an accretion disk forms around the neutron star and elongates. Click to enlarge. [Franchini & Martin 2021]

Alessia Franchini (University of Milano-Bicocca, Italy) and Rebecca Martin (University of Nevada, Las Vegas) study binary systems made up of a neutron star and a Be star — a rapidly rotating high-mass star with hydrogen emission lines in its spectrum. Because the Be star (the letters are pronounced separately as “B e”) spins so quickly, it flings away some of its mass into a decretion disk, from which the neutron star can siphon material. Be star–neutron star binaries tend to emit bursts of X-rays once per orbital period as well as on irregular timescales, with these irregular bursts often occurring in pairs — though it’s not yet clear why. Franchini and Martin use hydrodynamic simulations, shown above and to the right, to understand the behavior of these extreme systems. Their simulations show that the first of the paired X-ray outbursts can be caused by the transfer of mass from the Be star to the neutron star, while the second outburst might be driven by changes in the eccentricity of the neutron star’s disk, though higher-resolution simulations are needed to confirm this result. To learn more, see the full article below.


“Eccentric Neutron Star Disk Driven Type II Outburst Pairs in Be/X-ray Binaries,” Alessia Franchini and Rebecca G. Martin 2021 ApJL 923 L18. doi:10.3847/2041-8213/ac4029