Binary neutron star mergers: a jet engine for short gamma-ray burst

Picture: These images show the merger of two neutron stars simulated using a new supercomputer model. Redder colors indicate lower densities. Green and white ribbons and lines represent magnetic fields. The orbiting neutron stars rapidly lose energy by emitting gravitational waves and merge after about three orbits, or in less than 8 milliseconds. The merger amplifies and scrambles the merged magnetic field. A black hole forms and the magnetic field becomes more organized, eventually producing structures capable of supporting the jets that power short gamma-ray bursts. Credit: NASA/AEI/ZIB/M. Koppitz and L. Rezzolla

The LIGO and Virgo Collaborations recently reported the first direct detection of a gravitational-wave (GW) signal and demonstrated that it was produced by the inspiral and coalescence of a binary black hole (BHBH) system.


This breakthrough marks the beginning of the era of GW astrophysics.

GW signals are expected to be generated not only by BHBH binaries but also by neutron star-neutron star (NSNS) and black hole-neutron star (BHNS) binaries.
Merging NSNSs and BHNSs are not only important sources of GWs but also the two most popular candidate progenitors of short gamma-ray bursts (sGRBs)

NSNSs and BHNSs may also generate other detectable, transient electromagnetic (EM) signals prior to merger. Combining GW and EM signals from these mergers could test relativistic gravity and constrain the NS equation of state (EOS).

Credit: P.J.T. Leonard (NASA/GSFC)

In a recent paper (Ruiz et al. 2016) the authors perform magnetohydrodynamic simulations in full general relativity (GRMHD) of quasicircular, equal-mass, binary neutron stars that undergo merger.

The initial stars are irrotational, n=1 polytropes and are magnetized.

The authors explore two types of magnetic-field geometries: one where each star is endowed with a dipolar magnetic field extending from the interior into the exterior, as in a pulsar, and the other where the dipolar field is initially confined to the interior.


 In both cases the adopted magnetic fields are dynamically unimportant initially.

The merger outcome is a hypermassive neutron star that undergoes delayed collapse to a black hole immersed in a magnetized accretion disk. About 60(M_NS/1.625M_⊙) ms following merger, the region above the black hole poles becomes strongly magnetized, and a collimated, mildly relativistic outflow - an incipient jet - is launched.

Image: State-of-the-art supercomputer models show that merging neutron stars can power a short gamma-ray burst. Credit: NASA's Goddard Space Flight Center

The lifetime of the accretion disk, which likely equals the lifetime of the jet, is Δt∼0.1(M_NS/1.625M_⊙) s. In contrast to black hole-neutron star mergers, they find that incipient jets are launched even when the initial magnetic field is confined to the interior of the stars.

• Ruiz et al. 2016 (preprint) -  Binary neutron star mergers: a jet engine for short gamma-ray burst (arXiv)