Bad Astronomy | More mergers of black holes and neutron stars observed



A very large team of astronomers announced they discovered 35 new mergers of compact objects – from black holes and / or neutron stars – on top of those previously found in recent years, which now makes a total of 90 such events observed. These are some of the most colossally energetic explosions the Universe is capable of, but, oddly enough, literally emit no light that has been seen.

The mergers were detected by the LIGO, Virgo and KAGRA collaborations, three observatories around the world designed, built and dedicated to the research and analysis of these events.

This new round of mergers has also had some fun. One was a black hole that ate a neutron star, and not just any neutron star, but one with the lowest mass ever. Another created what could be an elusive intermediate-mass black hole with more than 100 times the mass of the Sun. Another may come from either the lowest mass black hole ever seen or the highest mass neutron star.

These observatories do not seek the light. Instead, they are looking for gravitational waves: The very literal jerk of the fabric of space-time as two extremely massive but extremely small objects spiral towards each other and then merge. Einstein’s general relativity predicts that when an object is accelerated in space-time, it creates these waves, much like ripples in a sheet if you shake it. The strength of these waves depends on the mass of the object and the force with which it is accelerated.

This is where black holes come in. If two holes are orbiting each other, the waves they create at the start are small, but it takes energy to make them. It steals energy from their obit, so they come closer. But if they are closer, they orbit faster, so they generate more gravitational waves, which steal more energy… it’s a runaway effect. It’s excruciatingly slow at first, but when black holes get close enough they whirl around each other at a speed very close to the speed of light – two massive objects accelerating. hard. The waves emitted are sharp and powerful, and in the final seconds – after perhaps billions of years of this dance – the black holes are finally approaching and coalescing, releasing an immense amount of gravitational wave energy.

These waves then move away at the speed of light, crossing the Universe. As they pass through Earth, they cause space-time to expand and contract on a very small scale, but enough to measure whether your instruments are sensitive enough.

And this is where the United States LIGO (Observatory of gravitational waves of laser interferometry), European Virgin, and Japanese KAGRA Collaborations (KAmioka gravitational wave detector) are coming. They use an array of lasers and mirrors that are several kilometers apart. When a gravitational wave passes through, it changes the distance between mirrors – usually by about one thousandth of the diameter of a single proton – which can be detected by minute changes in the way laser photons interact with each other.

The wave pattern tells us what the two fused objects looked like: their masses, something about their orbit, the total mass of the final object left behind (usually a larger black hole), and the energy that came in. in the event.

The energy is astounding. For example, in this new race, they saw an event called GW191109 (a gravitational wave detection on November 9, 2019) where two large black holes with 65 and 47 times the mass of the Sun merged, creating a single black hole more large with 107 solar masses, making it the bottom of what we call intermediate mass black holes, which have been proven to be difficult to detect. The 5 times the mass of the Sun in addition has been completely converted to gravitational waves. This amount of energy will melt your brain: This is roughly the same amount of energy emitted by all the stars in the entire Universe during this time.

This merger of black holes was literally the most energetic event in the Universe, but completely invisible. These mergers do not emit any light. Everything takes place in shaking space-time. Unbelievable.

Another interesting example they saw was GW191219, where a black hole of 31 solar masses merged with an object with only 1.17 times the mass of the Sun. The detected waves are consistent with the fact that the second is a neutron star, which would be the lowest mass ever! Normally, they form when a massive star explodes, creating a supernova, and the star’s core collapses (or two white dwarfs of lower mass collide, but they rarely leave behind anything other than a huge explosion), and the neutron star has at least 1.4 times the mass of the Sun. Lower masses are possible but very difficult to produce.

Two other black hole / neutron star mergers have been observed (which I mentioned at the time) but new analysis shows that one of them (GW200105) may be a false alarm, although it is not clear.

A third very interesting one is GW200210, where a solar mass of 24 has merged with an object that is only 2.83 times the mass of the Sun. Whether at very upper limit tippy to the mass of a neutron star, but is very weak for a black hole as well. It could be either, and I’m not sure how I would bet. The uncertainty about the mass of the second object is about half the mass of the Sun, so if it’s heavier than expected, it’s definitely a black hole, and if it’s lighter it’s a neutron star. Unfortunately, that’s the best we can say at the moment.

These observatories go through “observation runs”, where they are activated and listen for signals for a few months at a time. They are then turned off for a short time for maintenance, upgrades, cleaning of mirrors, etc. This is the third run (called O3) and has seen a dramatic increase in detection over the first two due to all of these upgrades. They are now in maintenance mode, and the fourth round, O4, is expected to start at the end of 2022. Given the progress made since the start – the first gravitational wave event was observed on September 15, 2015 – O4 is likely to see a lot of things Continued.

This type of astronomy is extremely important. We know a lot about how stars die and form neutron stars and black holes, but detecting them this way tells us even more, including what happens next. Black holes get bigger this way, which we know very little about. We see supermassive black holes a billion times the mass of the Sun in galaxies that are only a billion years old. How did this black hole grow so quickly? Have mergers been involved? Why are some like this so huge, while others like our Milky Way’s central black hole – which has only 4 million solar masses – are so small?

Gravitational wave astronomy has presented us with a whole new way of looking at the Universe, and many answers to very big questions are indeed possible to be found now. We are just getting started.


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