Scientists Detect First-Ever Collision Between Black Hole and Neutron Star


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Scientists around the world celebrated the first confirmed detection of gravitational waves several years ago, a discovery that resulted in Nobel prizes for Rainer Weiss, Kip Thorne, and Barry Barish in 2017. Since then, The American LIGO and Italian Virgo instruments have spotted many more waves from the collision of pairs of black holes and neutron stars. Now, scientists believe they’ve spotted a black hole gobbling up a neutron star for the first time ever. 

Gravitational waves were predicted in general relativity, but no one had been able to verify their existence before LIGO came online. While these ripples in spacetime come from cataclysmic events like colliding black holes, the waves are extremely faint. That’s why LIGO (Laser Interferometer Gravitational-Wave Observatory) and Virgo use a technique called laser interferometry. They bounce lasers off of reflectors at the end of long tubes — they’re 2.4 miles (4 kilometers) in the case of LIGO. The beams cancel each other out if they bounce back unaltered, but gravitational waves cause tiny fluctuations that produce a signal. LIGO can detect movements as small as a ten-thousandth the charge diameter of a proton. 

It was clear from the start that the LIGO-Virgo detection on August 14th would be an interesting one. At first, it looked like the objects were in the so-called “mass gap,” meaning they were too small to be black holes and too large to be neutron stars. However, human analysis eventually confirmed the event, known as S190814bv, was almost certainly the collision of a black hole and neutron star. 

Gravitational Waves

A simulation of black holes spiraling toward a collision and throwing off gravitational waves.

This is the last type of gravity wave collision scientists have been looking for, having already seen several signals from pairs of black holes and neutron stars. It’s not a sure thing yet, although the initial analysis says S190814bv is a neutron star-black hole merger with greater than 99 percent confidence. The larger of the two objects is well within black hole territory, but the smaller one is a bit south of three solar masses. Between one and two solar masses, and the object is a neutron star. Above three, it may be a black hole. 

Scientists are anxious to spot a neutron star colliding with a black hole because it could reveal much about these super-dense star remnants. The gravitational waves and any electromagnetic component of S190814bv could narrow down the dimensions of the neutron star, which would be a hugely important discovery in nuclear physics. We have only a basic understanding of neutron stars — their structure pushes even the best current models of particle physics to the breaking point. This distant gravitational event could be the first step in improving that understanding.

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Astronomers Spot Mysterious Flash From Our Galaxy’s Supermassive Black Hole


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You don’t usually think of a supermassive black hole as something that can go unnoticed, but many of these interstellar monsters are quite placid. For instance, Sagittarius A* (pronounced “Sagittarius A Star”) in the center of our own Milky Way galaxy. It’s usually very dim, but astronomers recently saw Sagittarius A* flare up — in fact, it just got brighter than we’ve ever seen it. 

Astronomers have detected many active galactic nuclei like quasars around the universe, but Sagittarius A* is on the quiet side despite being four million times more massive than the sun. It’s a bright X-ray radio source due to the heating of matter in the black hole’s accretion disk, but it’s not active enough to be bright across most of the electromagnetic spectrum.

In May of this year, UCLA’s Tuan Do spotted an unusual pulse from Sagittarius A*. That was so unexpected that at first, he believed the flash came from a star in the same part of the sky called S0-2. However, it became apparent over the course of about two and a half hours that the source was variable and was, in fact, Sagittarius A*. At its peak, Sagittarius A* was 75 times brighter than usual in infrared. 

Scientists have been watching Sagittarius A* for decades, but no one was sure what to make of it for much of that time; it was just a strong X-ray source deep in the Milky Way. In the late 1990s and early 2000s, studies of objects near Sagittarius A* demonstrated it had a strong gravity explained best by a supermassive black hole. Today, the evidence for Sagittarius A* as the gravitational center of the Milky Way is quite solid. It’s under constant observation with instruments like the WM Keck Observatory in Hawaii used by the UCLA team. 

Do and his team have speculated on several causes for the spike in brightness. Sagittarius A* itself would not emit radiation detectable on Earth, but objects near it being torn to shreds by gravitational shear would. It’s possible a large volume of matter fell into the black hole’s gravity well, and that caused the flash. The team points to two possibilities. First, the aforementioned S0-2, which is in a long 16-year orbit of Sagittarius A*. It made its closest approach yet last year, coming within 17 light hours of the event horizon. A part of the star may have been pulled away during that pass. There’s also a gas cloud called G2 that swing around Sagittarius A* in 2014. There were no cosmic fireworks at the time, but we could be seeing a delayed reaction.

More observatories should have data from the same time period, so they could shed light on the nature of the flash in the coming months.

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General Relativity Still Holds Up in New Analysis of a Supermassive Black Hole


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While quantum physics continues to raise new questions concerning Einstein’s theory of general relativity, a recent analysis of a giant black hole at the center of the Milky Way demonstrates that general relativity continues to prove itself under more extreme conditions than initially expected.

Space.com spoke with Andrea Ghez, an astronomy professor at the University of California Los Angeles and co-lead author of a research paper that investigated gravitational redshift occurring near the massive black hole known as Sagittarius A* (abbreviated as Sgr A*), about what the results mean for Einstein’s theory:

Einstein’s right, at least for now. Our observations are consistent with Einstein’s theory of general relativity. However, his theory is definitely showing vulnerability. It cannot fully explain gravity inside a black hole, and at some point we will need to move beyond Einstein’s theory to a more comprehensive theory of gravity that explains what a black hole is.

Gravitational redshift is a phenomenon caused by gravity over distance, similar to how the Doppler effect changes characteristics of a wave in motion relative to its observer.  We’re more accustomed to noticing Doppler shifts in audio, like when a police siren’s pitch changes as it speeds past our ears, but the same effect can be seen in other types of waves—such as those produced by photons.  While relative position and motion create Doppler redshift, gravity can also produce redshift when it causes a reduction in frequency in blue light.

Image credit: Keck/UCLA Galactic Center Group

While we’ve proven gravitational redshift on Earth, it remained unclear if the same phenomenon occurred with black holes. Ghez’ team tracked the star S0-2 in its complete orbit in three dimensions using multiple telescopes in different locations. Combined with measurements taken over the last 24 years, the scientists were able to show redshifting as S0-2 passed near Sgr A*’s extreme gravitational field.

Tracking S0-2 is just the beginning of the investigation into gravitational redshift. Sgr A* already provides other candidates to track but future results still may take a while. Out of 3,000 stars near Sgr A*, S0-102 has the shortest orbit of 11.5 years.  Nevertheless, future efforts now have more of an expected outcome and it will require thorough analysis to discover additional detail about one of the universe’s biggest mysteries.

Einstein’s theory of general relativity predates the discovery of black holes, which makes these results all the more impressive. While the party won’t last forever, it’s amazing to see how far an idea can reach across spacetime.

Top image credit: NASA

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