Abstract
Of the three main types of binaries detectable through ground-based gravitational wave observations, black hole-neutron star (BHNS) mergers remain the most elusive. While candidates BHNS exist in the triggers released during the third observing run of the Advanced LIGO/Virgo collaboration, no detection has been confirmed so far. As for binary neutron star systems, BHNS binaries allow us to explore a wide range of physical processes, including the neutron star equation of state, nucleosynthesis, stellar evolution, high-energy astrophysics, and the expansion of the Universe. Here, we review some of the main features of BHNS systems: the distinction between disrupting and non-disrupting binaries, the types of outflows that BHNS mergers can produce, and the information that can be extracted from the observation of their gravitational wave and electromagnetic signals. We also emphasize that for the most likely binary parameters, BHNS mergers seem less likely to power electromagnetic signals than binary neutron star systems. Finally, we discuss some of the issues that still limit our ability to model and interpret electromagnetic signals from BHNS binaries.
Highlights
The first observation by the LIGO-Virgo collaboration (LVC) of gravitational waves (GWs) coming from merging black holes (Abbott et al, 2016, GW150914) and neutron stars (Abbott et al, 2017c, GW170817) made GW astrophysics a reality
Simulations of post-merger remnants show that the large scale structure of the magnetic field has a significant impact on the jet power and the ejected mass (Christie et al, 2019). This leaves us with important open questions regarding the connection between short gamma-ray bursts (SGRBs) properties and the pre-merger characteristics of a black hole-neutron star (BHNS) binary, as well as regarding the mechanism for the production of relativistic jets, e.g., whether a strong magnetic field outside of the neutron star leads to the production of a jet ∼100 ms after merger (Paschalidis et al, 2015; Ruiz et al, 2018), or a dynamo mechanism within the disk creates a jet later on Christie et al (2019)
The most reliable information that comes from the joint observation of a SGRB and GW signal from a BHNS binary is that the neutron star was disrupted
Summary
Specialty section: This article was submitted to Cosmology, a section of the journal Frontiers in Astronomy and Space. Of the three main types of binaries detectable through ground-based gravitational wave observations, black hole-neutron star (BHNS) mergers remain the most elusive. We review some of the main features of BHNS systems: the distinction between disrupting and non-disrupting binaries, the types of outflows that BHNS mergers can produce, and the information that can be extracted from the observation of their gravitational wave and electromagnetic signals. We emphasize that for the most likely binary parameters, BHNS mergers seem less likely to power electromagnetic signals than binary neutron star systems. We discuss some of the issues that still limit our ability to model and interpret electromagnetic signals from BHNS binaries
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