When two black holes or two neutron stars come close to each other and eventually merge, a strong gravitational wave signal is generated, which is a wave of space and time. These gravitational wave signals can be captured by probes on Earth and provide a new window into studying these extreme objects. But do these merging events produce electromagnetic radiation in addition to gravitational waves?In particular, when a black hole merges with a neutron star, does it emit high-energy gamma rays?
Gamma rays are very short-wavelength and high-energy electromagnetic waves, and they usually come from the most violent phenomena in the universe, such as supernova explosions, active galactic nuclei, pulsars, etc. One particular type of gamma-ray source is the short gamma-ray burst (SGRB), which is an intense gamma-ray flash that lasts for a few seconds, usually accompanied by subsequent low-energy radiation such as X-rays, optics, and radio waves. The origin of the SGRBs has always been a mystery, but there is a popular hypothesis that they were the result of relativistic jets produced by binary mergers. A relativistic jet is a beam of high-energy particles moving at nearly the speed of light, which is ejected along the axis of rotation of a black hole or neutron star and interacts with the surrounding medium to produce gamma rays and other radiation.
So, can a black hole-neutron star merger produce such a relativistic jet?It depends on the structure and physical processes after the merger. When a black hole and a neutron star meet, there are two possible outcomes: either the black hole directly engulfs the neutron star, or the neutron star is torn apart into some pieces, forming a disk-like structure around the black hole, called an accretion disk. If it is the former, then it is likely that no electromagnetic radiation is generated, because the black hole will swallow up all the matter and energy. If the latter is the case, then it is possible to produce a relativistic jet, because the accretion disk transports matter and angular momentum to the black hole and converts a portion of the energy into a jet through a magnetic field.
However, there are a few conditions that need to be met in order for a jet stream to form. First, a black hole needs to have a certain spin, which is the speed at which it rotates, so that it can provide enough energy and magnetic field to drive the jet. Secondly, the accretion disc needs to have a certain thickness in order to support the amplification and transmission of the magnetic field. Thirdly, the accretion disc needs to have a certain lifespan in order to maintain the duration of the jet. Finally, the jet stream needs to have an opening angle so that it can penetrate the accretion disk and other material and reach the observer's direction.
So, how easy are these conditions to achieve in a black hole-neutron star merger?To answer this question, some scientists have conducted numerical simulations that trace the evolution of a black hole-neutron star merger, from before the merger to a few seconds after the merger. They considered different black hole spins, neutron star radii, and magnetic field configurations, as well as the effects of relativistic hydrodynamics and magnetohydrodynamics. They found some interesting results, which I will briefly cover below.
First, they found that if the black hole spin was smaller, then the neutron star would be swallowed up directly, without the formation of accretion disks or jets. If the black hole spins greater, then the neutron star will be torn apart, forming a mass of about 003 The accretion disk of the solar mass, as well as some material that is thrown outward, is called the tidal tail. These substances cool down in a matter of seconds and form some heavy elements such as gold, platinum, etc., which may be one of the main ** of these elements in the universe.
Second, they found that if the merged magnetic field is predominantly hooped, the formation of the jet stream is delayed until the magnetic field is converted to polar through a process called magnetic vortex instability. This process takes a certain amount of time, so the start-up time of the jet stream is about seconds, not milliseconds. If the magnetic field after the merger is predominantly polar, then the formation of the jet stream will be rapid, almost starting after the merger. This process does not require waiting for the magnetic field to be converted, so the jet stream starts up in about a few milliseconds.
Third, they found that the power and duration of the jet are related to the black hole's magnetic flux, which is a measure of the strength of the magnetic field on the black hole's surface. If the magnetic flux is less, then the jet stream will have less power and duration because the black hole cannot extract energy and magnetic field efficiently. If the magnetic flux is larger, then the power and duration of the jet are greater because the black hole is able to extract energy and magnetic field efficiently. However, if the magnetic flux is too large, the power of the jet will drop after a certain amount of time because the black hole enters a state called magnetic field bondage, where the magnetic field blocks the accretion of matter, thus reducing the energy of the jet**.
Finally, they found that the opening angle and shape of the jet stream are related to the interaction of the jet stream with surrounding materials, including accretion disks, tidal tails, and accretion disk winds. If the jet is strong, then it penetrates these substances, creating a narrow opening angle (about 10 degrees). If the jet is weak, then it will be blocked by these substances, creating a wide opening angle (about 30 degrees). The interaction of the jet and matter also creates a structure called an air sac, which is a thin crust that wraps around the jet that reflects and scatters the radiation from the jet, making the jet appear wider and brighter.
These results suggest that black hole-neutron star mergers can produce different relativistic jets, which are characterized by the parameters and physical processes of the merger. These jets may be one of the candidates for SGRB**, but more observational and theoretical studies are needed to verify this. For example, what are the radiation mechanisms, polarization, and light curves of jets?What kind of subsequent radiation will be generated by the interaction of the jet stream and the surrounding material?How much does the direction of the jet deviate from the direction of the observer?These questions are all worth further exploration.