Astronomers spot two neutron stars swallowed by black holes


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One of the best things about being an astronomer is being able to discover something new about the universe. In fact, maybe the only better thing is to find out twice. And that’s exactly what my colleagues and I did, making two separate observations, just ten days apart, of a whole new kind of astronomical phenomenon: a neutron star circling a black hole before being engulfed.

Both observations were made in January 2020, by the Laser interferometer Gravitational wave observatory and the Observatory of the Virgin, both of which detect gravitational waves from the distant cosmos.

After 18 months of careful analysis, our findings are Posted in Letters from the journal of astrophysics. The new observations open up new avenues for studying the life cycle of stars, the nature of space-time, and the behavior of matter at extreme pressures and densities.

The first observation of a neutron star system and black holes was made on January 5, 2020. The Laser Interferometer Gravitational Wave Observatory and Virgo observed gravitational waves – distortions in the very fabric of the space-time – produced by the last 30 seconds of death the orbit of the neutron star and the black hole, followed by their inevitable collision. The find is named GW200105.

Remarkably, just ten days later, the Laser Interferometer Gravitational Wave Observatory and Virgo detected gravitational waves from a second collision between a neutron star and a black hole. This event is named GW200115. The two collisions happened around 900 million years ago, long before the first dinosaurs appeared on Earth.

Artist’s impression of a neutron star orbiting and colliding with a black hole.

Neutron stars and black holes are among the most extreme objects in the universe. These are the fossil relics of dead massive stars. When a star more than eight times the size of the Sun runs out of fuel, it experiences a spectacular explosion called a supernova. What remains may be a neutron star or a black hole.

Neutron stars are generally between 1.5 and twice as massive as the Sun, but are so dense that their entire mass is lumped together in an object the size of a city. At this density, atoms can no longer maintain their structure and dissolve into a flow of free quarks and gluons: the building blocks of protons and neutrons.

Black holes are even more extreme. There is no upper limit to the size of a black hole, but all black holes have two things in common: a point of no return on their surface called an “event horizon”, hence the light cannot escape and a point at their center called “singularity”, where the laws of physics as we understand them collapse.

It’s fair to say that black holes are an enigma. One of the holy grails of 21st century physics and astronomy is to better understand the laws of nature by observing these strange and extreme objects.

New star system

Neutron stars orbiting black hole companions have long been thought to exist. The laser interferometer gravitational wave observatory and Virgo had been looking for them for over a decade, but they have remained elusive until now.

So why are we so confident that we have now seen not one of these systems, but two?

When the Laser Interferometer Gravitational Wave Observatory and Virgo observe gravitational waves, the first question that comes to mind is “what caused them?” To find out, we use two things: our observational data and supercomputer simulations of different types of astronomical events that could plausibly explain these data.

By comparing the simulations to our actual observations, we look for the characteristics that best match our data, focusing on the most likely and discarding the most unlikely.

For the first discovery (GW200105), we determined that the most likely source of gravitational waves was the final few orbits, and the eventual collision, between an object approximately 8.9 times the mass of the Sun, with an object d ‘about 1.9 times the mass of the sun. Considering the masses involved, the most plausible explanation is that the heaviest object is a black hole and the lightest is a neutron star.

Likewise, from the second (GW200115), we determined that its most probable source was the last orbits and the collision of a black hole of solar mass of 5.7 with a neutron star of solar mass of 1, 5.

There is no definitive proof that the lightest objects are neutron stars, and in principle, they could be very light black holes, although we consider this explanation unlikely. The best guess by far is that our new observations are consistent with the merger of neutron stars and black holes.

Stellar fossil hunt

Our findings have several intriguing implications. Star neutron-black hole systems allow us to reconstruct the evolutionary history of stars. Gravitational wave astronomers are like stellar fossil hunters, using the relics of exploded stars to understand how massive stars form, live and die.

We have been doing this for several years with the observations of the Observatory of gravitational waves by laser interferometer / Virgo of pairs of black holes and pairs of neutron stars. The rarer, recently discovered pairs, containing one of each, are fascinating pieces of the stellar fossil record.

For the first time, we have directly measured the speed at which neutron stars merge with black holes: we believe there will likely be tens or hundreds of thousands of such collisions across the universe per year. With more observations, we will measure the rate more precisely.

What happens to neutron stars after being swallowed up? Now we are really looking at the laws of nature up to 11. When neutron stars merge with black holes, they are distorted, imprinting information about their exotic form of matter on the gravitational waves we observe on Earth.

This can reveal the makeup of neutron stars, which in turn tells us about the behavior of quarks and gluons at extreme pressures and densities. It doesn’t tell us what’s going on behind the black hole’s event horizon, although another aspect of our findings is that we can look for clues of new physics in black holes in wave signals. gravitational.

When the Laser Interferometer Gravitational Wave Observatory and Virgo resume their observations in mid-2022 after upgrading to further increase their sensitivity, we will see more collisions between neutron stars and black holes. Over the next decade, we plan to accumulate thousands of more gravitational wave detections.

Over time, we hope to piece together the laws of nature that will help us understand the inner workings of the universe’s most extreme and impenetrable objects.

Rory smith is a lecturer in astrophysics at Monash University.

This article first appeared on The conversation.

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