Researchers have developed a new method to predict some of the universe’s most catastrophic mergers.
The oceans of highly charged particles surrounding neutron stars can experience tidal waves as the incredibly dense cores of neutron stars spiral toward each other or into a black hole. The researchers discovered that these tidal waves manifest themselves as regular flashes of electromagnetic radiation, which can act as an early warning system for approaching fusions.
Neutron stars are arguably the most extreme objects in the universe. Yes, black holes might be more exotic, but they’re relatively simple — they’re just very, very much gravity. In contrast, neutron stars are essentially gigantic atomic nuclei, and that brings with it a lot of interesting, complex physics that black holes can’t share.
A typical neutron star is only a few kilometers across but can weigh a few solar masses. They consist almost entirely of neutrons (hence the name), but contain populations of loose electrons, protons, and ions from heavy nuclei. They are born from supernovas — explosions of dying massive stars — and some can harbor the strongest magnetic fields in the entire universe.
The outer crust of a neutron star is thought to be made up of superfluid electrons and neutrons, which give way to a crystal lattice the closer you get to the surface. Finally, there is an ocean — a layer of liquid electrons, neutrons and ions at a depth of 10 to 100 meters (33 to 330 feet).
The interiors of neutron stars are the most mysterious, as the pressures and densities are so extreme that they exceed our current physical knowledge. Some models suggest that the nuclei are simply a homogeneous clump of neutrons, while others suggest that the neutrons themselves decay into their constituent quarks. Beyond the inner core lies a solid, smooth neutron mass that slowly transitions into more complex patterns, such as clumps and strands, collectively known as core noodles.
Observing the exotic behavior of neutron stars
The extremely exotic nature of matter under these conditions—superfluid neutrons are not usually found just lying around—makes neutron stars prime candidates for the study of extreme physics. This idea has been improved since the discovery of GW 170817, a gravitational-wave signal detected along with electromagnetic emission from two merging neutron stars. Joint detection, called multimessenger astronomy, allows physicists to study the hearts of neutron stars like never before.
But we haven’t seen any other neutron star merger events since that first gravitational-wave detection in 2017 — which is frustrating because neutron stars are among nature’s best laboratories for testing high-energy physics.
But now, a new way of observing the exotic behavior of neutron stars could mean we don’t have to wait much longer. The new work, published in the arXiv preprint database in May, focuses on the oceans of neutron stars, which can contain carbon, oxygen and iron in addition to free electrons and neutrons. Although the oceans are relatively shallow compared to the neutron star’s overall depth, they are the outermost layer (without an incredibly thin “atmosphere”) and the part of the neutron star that is most easily responsive to the outer universe. In particular, the researchers found that these shallow oceans can support tides, just like oceans on Earth. But raising a tide on a neutron star requires a lot more gravitational force to overcome all that extreme gravity. Tides on neutron stars only occur when the neutron star is close enough to a massive, dense object, like another neutron star or a black hole.
Fortunately, these types of binary pairs are relatively common, as stars tend to form in multiple systems and then go through their life cycles, eventually leaving black hole/neutron star combinations in their wake. Exotic lighthouses.
When a neutron star begins merging with another neutron star or a black hole, the objects spend a few years slowly spiraling towards each other. As they orbit, gravitational waves carry energy away from the system, pulling the pair closer. Finally, in the final moments, the fusion is over in a matter of seconds.
But before that happens, the orbiting companion can create a series of resonance tides on the neutron star. These tides can sustain frequencies up to 100 megahertz and carry up to a whopping 10^29 joules of energy. To give you a sense of how awesome this number is, all of humanity uses just 10^20 joules each year. A single resonant tidal wave from a neutron star has more energy than the total output of the Sun shining for 10,000 years. Unlike ocean waves, these tides are made up of an ocean of plasma. The extreme electrical charges mean that as the tides slosh about, they can emit intense bursts of electromagnetic radiation that can appear to us as flashes of X‑rays and gamma rays.
Based on their calculations, the researchers estimate that space-based observatories such as the Fermi Gamma-ray Space Telescope and the Nuclear Spectroscopic Telescope Array (NuSTAR) may detect a handful of neutron star spirals each year, and that these signals will appear up to a few years before the eventual merger . With this kind of warning, astronomers can prepare their telescopes and observatories ready to catch the moment of the merger itself and dig into even more valuable electromagnetic and gravitational-wave data.
Summary of the news:
- Tidal flashes from neutron stars can signal approaching mergers