The Line between Neutron Stars and Black Holes


When we detected the very first gravitational wave, a new window was opened to the mysteries of the universe. We knew we’d see things previously thought impossible. And we just did an object on the boundary between neutron stars and black holes, which promises to reveal the secrets of both. By now we’re becoming used to announcements that a new gravitational wave event has been detected. As though it’s no big deal that we regularly read the infinitesimal ripples in the fabric of spacetime due to a cataclysmic collision of black holes billions of light years away. As the LIGO and VIRGO gravitational wave observatories spot event after event. And the latest event is one of the most informative and perhaps most surprising so far. It seemed innocuous enough at first glance two compact bodies spiraling together. From the shape of the gravitational waveform,and based on calculations using Einstein’s general theory of relativity, the masses of those bodies were calculated. One was a hefty 23 times the mass of our Sun making it definitely a black hole, and pretty similar to other LIGO mergers. Its companion was puny by comparison a mere 2.6 solar masses. And there lies the surprise. At that mass, it’s pushing the limit for what was thought possible for a neutron star, and it’s lighter than what was thought possible fora black hole. So what exactly is this thing? Today on Space Time Journal Club we’re going to try to figure it out and we’ll do that by studying the paper that reported this detection,published by the LIGO science collaboration just a few weeks ago. We’ve done gravitational wave astronomy before, but this event is so mysterious we had to cover it. But here’s a quick refresher nonetheless: The LIGO observatories in Washington State and Louisiana and the VIRGO observatory in Italy, consist of kilometers long vacuum tubes set at right angles. A laser beam is split, sent down these tubes,then recombined. The passage of a gravitational wave causes extremely tiny changes in these arm lengths, which in turn causes the peaks and valleys of the laser’s electromagnetic wave to line up differently, and so those changes can be measured. On August 14 2019, a gravitational wave hit the LIGO and VIRGO observatories one after the other in close succession, consistent with a wave traveling through the entire earth at the speed of light. From the shape of the detected waveform, the masses of the merging objects were figured figured out as 23.2 and 2.59 solar masses and we’ll get back to why those are weird. From the arrival times at the three observatories,the location of the event could be narrowed down to some small arcs on the sky. Unfortunately, there are countless galaxies in a region that size, so to start with we have no idea in which galaxy the merger happened. Nonetheless, MANY telescopes quickly swiveled to scan that region, hoping to spot a faint flash of light any indication that the merger of these objects may have been accompanied by an explosive event. Now that’s only been seen once before with the merger of two neutron stars in 2017 which we obviously covered back then. In that case it corresponded to an explosion observed across the electromagnetic spectrum energy released as the neutron stars tore themselves apart in their collision before they collapsed into a black hole. That light carried with it an enormous amount of information about what happens when neutron stars collide and we’re going to be using that information in just a bit. But there was no such luck with the more recent event. No electromagnetic counterpart was found. That’s not entirely surprising that event was 6 times further away than the 2017 neutron star merger, so would probably have been to faint to see anyway. And besides, there may have been no accompanying explosion at all. That would be the case if both objects were black holes, but even if the smaller object was a neutron star it could well have been swallowed whole by the larger black hole without so much as an electomagnetic peep. So, we get to the mystery. What was that smaller body? And why are we all so excited to spot something with this mass at all? To understand that, we have to understand a bit more about black holes and neutron stars. A neutron star is what’s left after some massive stars explode as supernovae. Once it was the burning heart of the star a fusion engine that allowed the star to resist the inward crush of gravity. But when it ran out of fuel, gravity took over and the entire star collapsed. The innermost part of the core turned into an ultra dense nugget of matter while the rest of the in falling core rebounded causing the supernova explosion. So you end up with at least one and a half suns worth of matter locked in a ball that would fit inside a small city. That insane density gives the neutron star a surface gravity around 100 billion times stronger than the surface of the Earth. Scientists believe that it would be very difficult to get out of bed on the surface of a neutron star. And much more difficult to escape the neutron star the escape velocity at the surface is up to half the speed of light. In fact neutron stars are on the verge of being black holes, which by definition have an escape velocity at the event horizon equal to the speed of light. If only you could cram a little more matter into the neutron star, the escape velocity would increase and it would become a blackhole. Now in the case of normal matter, you can’t just add mass to make a black hole because as you do so the radius of the object increases. That means the surface gets further away from the center, which means you don’t get the full impact of that extra mass. But neutron stars are NOT made of normal matter. That ball of neutrons is a fundamentally quantum mechanical object. One of its weird properties is that as you add mass the size does not necessarily increase, and at the highest masses the size actually gets smaller. So more mass in a neutron star means higher surface gravity means higher escape velocity. For any given mass, there’s a certain size that if you could crunch an object down below that size it would be a black hole. It’s like a phantom event horizon. In the case of the earth the phantom event horizon is about a centimetre in diameter. In the case of a neutron star it’s several kilometers. As you increase a neutron star’s mass, its phantom event horizon grows while its actual surface shrinks. When they overlap you have a black hole. This basic picture is pretty well accepted,but we still aren’t sure just how massive a neutron star can be before becoming a black hole. It’s not because our theories are wrong it’s because the calculations required to understand the bizarre states of matter in a neutron star are horrendous, and there’s still some stuff that we don’t know. That’s especially true towards the center of the neutron star, where the neutrons themselves probably break down into different types of quark matter. Up and down quarks that comprised the neutrons may even transform into strange quarks something we’ve talked about before. The details of the state of matter in the neutron star determines how a neuron star’s size changes with mass and that’s what determines the maximum possible mass. Those models have predicted maximum masses in the range 2 to 3 times the mass of the sun. Now, we can do better at making this theoretical prediction if we can catch a glimpse of the innards of a neutron star. And we can when they merge. In the 2017 neutron star merger we learned a lot about the structure of these objects by the way they warped as they spiraled together,and by the stuff they spewed out after colliding. We also see the results of these mergers in gamma ray bursts frequent flashes of energetic light from the distant universe. We’ve estimated a maximum neutron star mass of between 2.2 to 2.4 solar masses. More direct measurements of neutron star masses come from pulsars cosmic lighthouses that result from a neutron star’s precessing jets sweeping past the earth. Most pulsars are closer to the minimum neutron star mass of around 1.4 solar masses. But, the most massive so far is around 2.1 Suns. All of these numbers are quite a bit lower the 2.6 solar masses of this new guy. And that’s what makes it so cool. If it IS a neutron star then it puts us at the theoretical limit, and can tell us a lot about the crazy states of matter inside. That’s if it’s a neutron star at all. So why can’t it just be another black hole? Well that would perhaps be even more surprising. So far we’ve never observed a black hole with masses lower than around 5 times that of the Sun. We see those in X-ray binaries when a blackhole is orbiting and cannibalizing another star. It may seem weird that there seems to be a gap in masses between the biggest neuron stars and the smallest black holes, but actually we very much expect this. New black holes are formed when the most massive stars die and the core is too big to become a neutron star. But you don’t get this smooth transition from neutron stars to black holes. Like I said earlier, a neutron star forms when a star’s core collapses, but most of the material rebounds as a supernova explosion. But if that neutron star then becomes a blackhole, some of the in falling material just gets sucked into the black hole. That increases the black hole’s mass quite a bit. Based on our calculations and simulations of how stars die, that minimum black hole mass of 5 Suns seems about right. A black hole with 2.6 solar masses is difficult to explain. If we figure out that this object can't be a neutron star then we’re going to have to rework our models of how stars die or find some other way to make extra tense black holes. But if it is a neutron star then we’ve learned a ton about the most extreme states of matter in the universe. This is just the beginning. With new gravitational wave events coming every week or two, we’re sure to see more of these sorts of mergers. Each will be rich in information on the nature of stars, and gravity, and strange quantum states of matter. Billion year old secrets carried to us on ripples in spacetime.

Thanks For Reading, See you next time

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