“`html

Throughout the universe, numerous stars appear in duos, elegantly revolving around each other. Nevertheless, one of the most striking combinations happens between two spinning black holes, which arise after their colossal parent stars detonated in supernova eruptions. Should these black holes be sufficiently close, they will eventually crash into each other and form a significantly larger black hole. Occasionally, a black hole is circled by a neutron star—the compact remnant of a star also generated from a supernova blast, albeit containing less mass than a black hole. When these two entities ultimately merge, the black hole usually engulfs the neutron star entirely.

To gain a better grasp of the extreme physics underlying such a brutal fate, scientists at Caltech are utilizing supercomputers to model black hole–neutron star collisions. In one study published in The Astrophysical Journal Letters, the group, spearheaded by Elias Most, an assistant professor of theoretical astrophysics at Caltech, crafted the most intricate simulation to date of the violent tremors that fracture a neutron star’s exterior approximately one second prior to the black hole consuming it.

“The neutron star’s crust will break apart just like the ground during an earthquake,” Most explains. “The black hole’s gravity first rips the surface, inciting tremors in the star and creating fissures.”

While fractures in the crust of a neutron star had been anticipated previously, this simulation is the first to illustrate the types of light flares astronomers might observe in the future when directing telescopes in space and on Earth at such occurrences.

“This transcends educated theories for the phenomenon—it represents an actual simulation incorporating all the pertinent physics when the neutron star fractures like an egg,” states co-author Katerina Chatziioannou, an assistant professor of physics at Caltech and a William H. Hurt Scholar.

In a subsequent, more recent article in The Astrophysical Journal Letters, released on March 31 of this year, the team employed a supercomputer to simulate what transpires after the neutron star breaks apart—a fleeting milliseconds-long interval when enormous shock waves, predicted to be the most powerful shock waves in the universe, surge outward from the star. These colossal shock waves were only recently predicted by co-author Andrei Beloborodov of Columbia University. Now, this simulation, alongside another from a different study published by the group last year, marks the first time they demonstrate how such waves are generated.

Moreover, the latest simulation does not cease at the formation of the huge shock waves—it continues to show the neutron star being consumed, which then initiates the formation of an unusual entity known as a “black hole pulsar.”

A classic pulsar is a highly magnetized neutron star that emits beams of radiation, sweeping around like a lighthouse signal as the star rotates on its axis. A black hole pulsar is a theoretical object in which a black hole emits magnetic winds that would also circulate around it as it spins, mimicking the appearance of a pulsar. Although black hole pulsars had been conjectured previously, this simulation is the first to illustrate how such a rare object may actually arise in nature from the merger of a neutron star and a black hole.

“When the neutron star plunges into the black hole, the colossal shock waves are generated,” states Yoonsoo Kim (MS ’24), a Caltech graduate student collaborating with Most, and the lead author of the study on giant shock waves and black hole pulsars. “Once the star is absorbed, powerful winds are produced, resulting in the black hole pulsar. However, the black hole cannot maintain its winds and will fall silent again within seconds.”

Similar to the simulation illustrating how a neutron star fractures, this one also anticipates the traits of the resultant flares astronomers might detect through telescopes. During the brief moments when gigantic shock waves surge outward and a black hole pulsar is born, telescopes may catch bursts of radio waves or a combination of X-rays and gamma rays. In summary, the simulations conducted by Most and his associates offer a more profound understanding of the physics propelling some of the most energetic events in the cosmos.

Waves of Space and Time

When two black holes collide, they produce not only shock waves and flares of illumination but also another type of radiation referred to as gravitational waves. These ripples in the essence of space and time itself were initially hypothesized more than a century ago by Albert Einstein. The Caltech- and MIT-led LIGO (Laser Interferometer Gravitational-wave Observatory), which is backed by the National Science Foundation (NSF), famously achieved the first direct observation of gravitational waves, originating from the merger of two black holes, in 2015. This accomplishment later earned three of the collaboration’s foremost members the 2017 Nobel Prize in Physics.

In 2017, LIGO and Virgo, its European counterpart, detected a different type of collision: that of two neutron stars. The explosive event, termed a kilonova, released a spray of metals, including gold. That occurrence emitted both gravitational waves and light. LIGO–Virgo first recorded the blast in gravitational waves and subsequently alerted astronomers worldwide, who used telescopes in space and on the ground to capture a wide spectrum of electromagnetic, or light, wavelengths, from high-energy gamma rays to low-energy radio frequencies.

Whether a neutron star–black hole merger would similarly generate a comparable light display is uncertain, but to date, none have been observed. Nevertheless, it remains plausible that neutron star–black hole collisions, even if they do not create a luminous cloud, may emit brief radio and/or other electromagnetic signals right before and during the mergers. Simulations like those from Most and his colleagues help astronomers identify which electromagnetic signals to watch for.

To assist in the quest for these precursor signals, the LIGO team is working to detect mergers up to a minute prior to their occurrence, which would provide astronomers additional time to aim their telescopes at the explosions and search for telltale indications of an impending impact.

“LIGO can identify mergers before they take place because the pair of colliding entities emit gravitational waves in the frequency spectrum that LIGO detects as they spiral closer and closer together,” explains Chatziioannou, who is also a member of the LIGO team. “At present, we are capable of detecting the collisions just seconds before they happen, and we are progressing towards a full minute. The gravitational waves are

“““html

One component of the conundrum while the electromagnetic radiation is another. We aim to assemble the pieces of the puzzle together.

“Prior to this simulation, people assumed you could fracture a neutron star like an egg, but they never inquired if you could perceive the fracture,” Most states. “Our research suggests that, indeed, you could perceive or detect it as a radio signal.”

The Most Sophisticated Computers

A significant contributor to the success of the team’s recent neutron star–black hole simulations is the utilization of supercomputers equipped with GPUs (graphics processing units). For these latest investigations, the team employed the Perlmutter supercomputer situated at the Lawrence Berkeley National Laboratory in Berkeley (named in honor of astronomer Saul Perlmutter, who received the 2011 Nobel Prize in Physics alongside two other scientists for their discovery of the universe’s acceleration). GPUs provide computational power for video games and AI applications such as ChatGPT; in this situation, the immense parallel processing capabilities of GPUs enabled the Perlmutter supercomputer to manage the intricate interactions between a converging neutron star and black hole.

“When you model two black holes merging,” Most explains, “you require the equations of general relativity to illustrate the gravitational waves. However, with a neutron star, there is significantly more physics occurring, incorporating the intricate nuclear physics of the star and the plasma dynamics surrounding it.”

The actual simulations take approximately four to five hours to execute. Most and his team had been engaged in similar simulations for around two years utilizing supercomputers without GPUs before they transitioned to Perlmutter. “That’s what unlocked the issue,” Most notes. “With GPUs, suddenly, everything functioned and aligned with our expectations. Previously, we simply lacked the computing capacity to numerically model these highly intricate physical systems in sufficient detail.”

Secrets of Simulation

The initial cracking simulation unveils the drama of what transpires as the neutron star approaches its companion black hole. Initially, gravitational forces from the colossal black hole tear at the dead star’s surface, leading to its fragmentation. Neutron stars are enveloped by a powerful magnetic field, and when their surface splinters due to these so-called tidal forces, the magnetic field oscillates. This results in magnetic ripples known as Alfvén waves, named after the Swedish physicist Hannes Alfvén, who was awarded the 1970 Nobel Prize in Physics for his contributions to magnetohydrodynamics, a theory that describes the behavior of electromagnetic fields in plasma.

“The magnetic field can be visualized as strings tethered to the neutron star,” Most explains. “The neutron star’s quake sends these strings into violent vibrations similar to a whip, which then creates a cracking sound.”

The Alfvén waves ultimately transition into a blast wave that generates a surge of radio waves approximately one second before the neutron star is engulfed. In the future, Caltech’s proposed Deep Synoptic Array-2000, or DSA-2000—a network of 2,000 radio dishes to be constructed in the Nevada desert—might be capable of detecting these bursts of radio waves, termed fast radio bursts or FRBs, signaling the demise of the neutron star.

“Before this simulation, the assumption was that you could fracture a neutron star like an egg, yet they never considered if the fracture could be heard,” Most states. “Our research asserts that, yes, you could hear or detect it as a radio signal.”

The team’s second simulation illustrates what occurs further along in the neutron star’s decline. When the dead star is consumed by the black hole, some of the most intense shock waves in the cosmos are generated.

“It’s akin to an ocean wave,” Kim remarks. “The sea starts off calm, but as the waves approach the shore, they steepen until they ultimately break. In our simulation, we can observe the magnetic field waves evolve into a colossal shock wave.”

These colossal shock waves would convert into blast waves that exceed the intensity of those produced by the neutron star’s fracture, and they too would generate radio signals. This indicates that astronomers observing a neutron star and black hole in the fleeting moment before their collision might detect two radio signals, one after the other.

“This implies that a neutron star-black hole collision, even though it may not erupt with material like a neutron star-neutron star merger, could generate potent signals that telescopes can acquire,” Most articulates.

Brief Signals

Ultimately, after the neutron star is consumed by the black hole, the second simulation demonstrates how a black hole pulsar is formed.

“When the black hole devours the neutron star, it also absorbs its magnetic field,” Most explains. “And it must dissipate that. The black hole detests the magnetic field; it repels it. What the simulation reveals is that it accomplishes this in a manner that creates a state resembling a pulsar.”

The black hole essentially drags the undesired magnetic field along with it, creating magnetic winds that whirl around the black hole, consequently giving it a pulsar-like appearance for a brief moment of just under a second. The data indicate that this occurrence would emit a fleeting burst of high-energy X-rays and/or even higher-energy gamma rays.

In the future, the researchers aspire to investigate whether this phenomenon applies to other types of binary systems. With the support of supercomputers, they aim to unravel the marvelous physics driving the universe’s most catastrophic events.

The neutron-star cracking study, titled “Nonlinear Alfvén-wave Dynamics and Premerger Emission from Crustal Oscillations in Neutron Star Mergers,” was financed by the NSF and the Simons Foundation. Other contributors include Caltech graduate student Isaac Legred (MS ’24).

The colossal shock waves and black hole pulsar study, titled “Black Hole Pulsars and Monster Shocks as Outcomes of Black Hole–Neutron Star Mergers,” received funding from the Sherman Fairchild Foundation, NSF, NASA, Natural Sciences & Engineering Research Council of Canada, the Canadian Space Agency, and the Simons Foundation. Other contributors include Bart Ripperda from the Canadian Institute for Theoretical Astrophysics.

“`