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If two neutrons stars that are orbiting were to collide, how big would the supernova explosion be?
Would a new black hole or a new supernova remnant be formed?
When two neutron stars collide, the resulting supernova is called a "kilonova," producing much more energy than a regular supernova. Additionally, heavier elements are formed, such as gold, platinum, etc. Other effects of this collision include a gamma-ray burst and/or gravitational waves. The object formed after the event can either be a heavier neutron star, or a black hole (if the mass of the merger exceeds the maximum mass of a neutron star, about 3 solar masses).
As of right now, we have not detected a neutron star merger that is visible to the naked eye; almost all of them occurred in different galaxies, billions of light-years away. I hope this helps.
Largest Supernova Ever Seen Could Rewrite Physics of Stars
An enormous supernova spotted by the Gaia satellite may be the most massive ever seen by astronomers. This tremendous eruption, a billion light years from Earth, could rewrite what we know about the deaths of the largest stars in the Universe.
The star, called SN2016iet, erupted before plant or animal life had evolved on land, at a time when every continent was still huddled together into one massive supercontinent, Rodinia. Light from the event, racing toward Earth at nearly 300,000 kilometers per second (more than 186,000 miles every second), was first seen by the Gaia spacecraft on November 14, 2016.
Following three years of study, researchers have now determined this explosion was the most massive supernova ever recorded. This scale of this supernova may require physicists to rewrite the laws of physics governing these eruptions.
“When we first realized how thoroughly unusual SN2016iet is my reaction was ‘whoa — did something go horribly wrong with our data?’ After a while we determined that SN2016iet is an incredible mystery, located in a previously uncatalogued galaxy one billion light years from Earth,” states Sebastian Gomez, graduate student at Harvard University.
Orbiting 54,000 light years from the center of its galaxy in which it formed, this member of a binary system, once contained more than 200 times the mass of our own Sun.
For millions of year before its explosion, the star shed off much of its mass, including 35 solar masses of material lost in just its final 10 years. At the time of the supernova, the dying star still possessed somewhere between 55 and 120 times as much mass as the Sun. This is created a mammoth explosion, the largest ever recorded by astronomers.
When the shock wave from the titanic explosion reached the shell of material previously cast off by the star, the impact resulted in a second shock wave which raced through the system.
Live Big, Die Young
The natural lifespan of a star is determined solely by its mass, with the most massive stars living out the shortest lives. The behemoth star which formed SN2016iet existed just a few million years before its magnificent demise. By contrast, the Sun formed roughly 4.5 billion years ago,and will shine for about the same amount of time before running out of hydrogen fuel.
The star which formed the SN2016iet supernova was a rogue star — far from its home dwarf galaxy. As SN2016iet reached the end of its life, the supermassive star shed approximately 85 percent of its mass to space over the course of a few million years. This process formed a “cocoon” of gas around the star. When the star exploded, the powerful blast from the event reached the enveloping gas, forming a second shock wave.
Telescopes at the Fred Lawrence Whipple Observatory in Amado, Arizona and the Magellan Telescopes in Chile, were engaged to study this highly-unusual supernova reported by GAIA. Everything about this stellar eruption looked bizarre.
“Everything about this supernova looks different — its change in brightness with time, its spectrum, the galaxy it is located in, and even where it’s located within its galaxy. We sometimes see supernovas that are unusual in one respect, but otherwise are normal this one is unique in every possible way,” Edo Berger, professor of astronomy at Harvard University and co-author of a study on the discovery published in The Astrophysical Journal, states.
Astronomers studying the eruption noted vast quantities of energy released from the event over a long period of time. An unusual chemical makeup and the fact the explosion took place in a metal-poor environment all contributed to the unusual nature of this supernova. Astronomers soon realized this was a type of supernova that was first postulated decades ago, but never before seen.
This explosion was the first pair-instability supernova ever recorded. These long-theorized events occur when electron and positrons (electrons of anti-matter) temporarily reduce internal pressure, leading to runaway nuclear reactions in the core. The resulting explosion completely obliterates the star, leaving nothing behind. Usually when supernovae of this type explode, an ultra-dense neutron star or black hole is left behind. Explosions of this variety are thought to only occur in extremely massive stars living in metal-poor galaxies, like the earliest families of stars.
It All Started with a Big Bang — BANG!
When stars first formed a few hundred million years after the big bang, most were massive objects, existing in small protogalaxies. Measuring just 30 to 100 light years in diameter, these prototype galaxies would have housed vast numbers of massive stars, doomed to end their short lives in gigantic supernova explosions, like SN2016iet.
When the Universe first cooled enough for matter to exist, almost all the material that formed was hydrogen and helium (and a trace of lithium). Every element heavier than that was created in fusion reactions of stars, and everything heavier than iron came from supernova eruptions. These elements were spread to space by titanic explosions of supernovae.
All life on Earth depends on six elements — carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur. Without the elements produced by stars, life (likely) would not exist. We are truly made from stars.
“The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of starstuff.”
― Carl Sagan, Cosmos
Data from this massive supernova could assist astronomers seeking to understand the deaths of massive stars in the ancient Universe.
Most supernovae fade after a few months, but researchers believe energy from this event will be visible for years, providing astronomers a wealth of information about this glorious, rarely-seen, class of supernovae.
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Scientists Watch A Black Hole Collide With A Neutron Star
When a black hole and a neutron star collide, it might shower outer space with precious metals like gold and platinum while spewing out radioactive elements.
Scientists saw this in computer simulations as they were trying to learn what happens when these two dense objects merge. They hope being able to predict the result of these collisions will help them find the real thing as they scan the skies with their instruments, as well as better understand the role neutron stars play in the universe.
For there to be a collision between a black hole and a neutron star, there first has to be a big bang — a neutron star is what remains after a massive, old star explodes in a supernova, the biggest kind of explosion there is in outer space. The collapsed star core that is left behind is the neutron star.
It’s also possible that neutron stars themselves further collapse to create black holes. Either way, both neutron stars and black holes are incredibly dense, with tremendous masses packed into relatively small spaces. When they come together, the result is powerful.
The international team of scientists detailed their findings in two studies in the journal Classical and Quantum Gravity, with one paper focusing on what occurs during the first milliseconds of the collision and the other on what follows, as material is ejected and immediately forms a disk around the crash site.
The results of the simulations varied, with some run-throughs of the collisions showing the black holes completely consuming the neutron stars and others showing it burping out varying amounts of material after its meal, the Department of Energy's Lawrence Berkeley National Laboratory reported. When the two objects join forces, they turn into one larger black hole, and that monster might eat up some of the material that was spewed out or leave some of the radioactive matter hanging there in “an extremely dense, thin, donut-shaped halo of matter” that quickly spreads out.
Heavy elements are likely in that ejected matter, including gold, platinum and radioactive elements.
According to the lab, the simulations are about more than just creating a gruesome image of an enormous crash. They are also geared toward helping detect gravitational waves — disturbances in spacetime that stem from violent events — that result from these collisions so scientists could one day watch the real thing and learn more about our universe. Gravitational waves were recently discovered to be linked to two black holes crashing together, although a collision between a black hole and a neutron star would not create gravitational waves that are as strong.
“We are trying to move more toward actually making models of the gravitational-wave signals produced by these mergers,” researcher Francois Foucart said in the lab statement.
The findings might also offer insight about resulting gamma ray bursts and radioactive material that telescopes might pick up while scanning the skies.
The lab says looking at matter that is spewed out when these simulated objects crash into one another, including its speed, size and composition, will assist astronomers who are searching the universe for real collisions to observe.
Neutron stars are also quite mysterious, and better understanding how they rip apart in a collision with a black hole could give scientists a window into their structure.
“With improved models,” scientist Daniel Kasen said in the statement, “we are better able to tell the observers exactly which flashes of light are the signals they are looking for.”
What Happens When Neutron Stars Collide?
A beautifully elegant visualization of perhaps one of the most ferocious events that can occur in the universe has been released by NASA. This supercomputer simulation, produced by the Albert Einstein Institute, demonstrates what happens when two neutron stars collide and form a black hole.
Neutron stars are one of several possible endings for a star. They form when a huge star, around 8 to 30 times the mass of our Sun, explodes in a supernova. They’re only city size, around 12 miles (20 kilometers) in diameter, but size isn’t everything and they certainly pack a punch with a mass about 1.4 times that of our Sun. To put that into perspective, a cubic centimeter of neutron star matter would weigh more than Mount Everest.
When neutron stars collide a spectacular event ensues. In this simulation, scientists placed a mismatched pair of neutron stars, weighing 1.4 and 1.7 solar masses, 11 miles apart and watched the fateful event play out. As the stars start to whirl toward each other, immense tidal forces warp the crusts of the stars and the smaller star explodes, spewing its hot and dense contents that then begin spiral around the system. As the stars merge, the overwhelming mass acquired by the larger star causes it to collapse, and a black hole is born.
Watch the mesmerizing simulation unfold in full below:
These events are particularly interesting because scientists believe that they may result in short gamma-ray bursts (GRBs). These short GRBs are immense bursts that emit the same amount of energy as all of the stars in our entire galaxy combined produce in a year, in only around 2 seconds. Since these events are over in the blink of an eye observing them in space has proved quite a challenge, but NASA’s Swift mission has been capturing GRB afterglows which has led to a significant increase in our comprehension of these events.
Black Hole-Neutron Star Collisions Could Finally Settle the Different Measurements Over the Expansion Rate of the Universe
If you’ve been following developments in astronomy over the last few years, you may have heard about the so-called “crisis in cosmology,” which has astronomers wondering whether there might be something wrong with our current understanding of the Universe. This crisis revolves around the rate at which the Universe expands: measurements of the expansion rate in the present Universe don’t line up with measurements of the expansion rate during the early Universe. With no indication for why these measurements might disagree, astronomers are at a loss to explain the disparity.
The first step in solving this mystery is to try out new methods of measuring the expansion rate. In a paper published last week, researchers at University College London (UCL) suggested that we might be able to create a new, independent measure of the expansion rate of the Universe by observing black hole-neutron star collisions.
Let’s back up for a minute and discuss where things stand right now. When we look out into the Universe, galaxies that are further away appear to be moving away from us faster than closer ones, because space itself is expanding. This is expressed by a number known as the Hubble constant, which is usually written as the speed (in kilometers per second) of a galaxy one megaparsec (Mpc) away.
One of the best ways to measure the Hubble constant is to observe objects known as Cepheid Variables. Cepheids are stars that brighten and dim regularly, and their brightness just happens to line up with their period (the time it takes to dim and brighten again). The regularity of these objects makes it possible to estimate their distance, and a survey of many Cepheids gives us a Hubble constant of about 73km/s/Mpc. Type 1A supernovae are another common object with a known brightness, and they also give a Hubble constant hovering around 73km/s/Mpc.
On the other hand, you can measure the expansion of the Universe during its earliest phase by observing the afterglow of the big bang, known as the cosmic microwave background radiation (CMB). Our best measurement of the CMB was taken by the European Space Agency’s Planck spacecraft, which released its final data in 2018. Planck observed a Hubble constant of 67.66km/s/Mpc.
Estimated values of the Hubble constant. Black represent measurements from Cepheids/Type 1A Supernovae (73 km/s/Mpc). Red represents early universe CMB measurements (67 km/s/Mpc). Blue shows other techniques, whose uncertainties are not yet small enough to decide between the two. Credit: Renerpho (Wikimedia Commons).
The difference between 67 and 73 isn’t enormous, and at first, the most likely explanation for the difference seemed to be instrument error. However, through subsequent observations, the error bars on these measurements have been narrowed down enough that the difference is statistically significant. A crisis indeed!
Here is where the UCL researchers hope to step in. They propose a new method of measuring the Hubble constant, which does not rely in any way on the other two methods. It begins with a measurement of gravitational waves: the ripples in spacetime caused by the collision of massive objects like black holes. The first gravitational waves were detected only recently, in 2015, and they haven’t yet been associated with any visible collisions.
As lead researcher Stephen Feeney explains, “we have not yet detected light from these collisions. But advances in the sensitivity of equipment detecting gravitational waves, together with new detectors in India and Japan, will lead to a huge leap forward in terms of how many of these types of events we can detect.”
Gravitational waves allow us to pinpoint the location of these collisions, but we need to measure light from the collisions too if we want to measure their speed. A black hole-neutron star collision might be just the type of event that would produce both.
If we see enough of these collisions, we could use them to produce a new measurement for the Hubble constant.
The LIGO Gravitational Wave detector in Louisiana. Image Credit: Caltech/MIT/LIGO Laboratory.
The UCL team used simulations to estimate how many black hole-neutron star collisions might occur in the next decade. They found that Earth’s gravitational wave detectors might pick up 3000 of them before 2030, and of these, about 100 of them will probably also produce visible light.
That would be enough. As such, by 2030 we just might have a brand-new measurement of the Hubble constant. We don’t know yet whether the new measurement will agree with the CMB measurement, or with the Cepheid/Type 1A measurement, or disagree with both. But the result, whatever it turns out to be, will be an important step in unraveling the puzzle. It could either put the crisis in cosmology to rest, or make it more serious, forcing us to look closer at our model of the Universe, and admit that there is more we don’t know about the Universe than we thought.
Stephen M. Feeney, Hiranya V. Peiris, Samaya M. Nissanke, and Daniel J. Mortlock, “Prospects for Measuring the Hubble Constant with Neutron-Star–Black-Hole Mergers.” Physical Review Letters.
Featured Image: A black hole devouring a neutron star. Credit: Dana Berry/NASA.
Don&rsquot let all this keep you up at night. Kilonovae are relatively rare cosmic phenomena, estimated to occur just once every 10,000 years in a galaxy like the Milky Way. That&rsquos because neutron stars, which are produced by supernovae, hardly ever form as pairs. Usually, a neutron star will receive a hefty &ldquokick&rdquo from its formative supernova sometimes these kicks are strong enough to eject a neutron star entirely from its galaxy to hurtle at high speeds indefinitely through the cosmos. &ldquoWhen neutron stars are born, they&rsquore often high-velocity. For them to survive in a binary is nontrivial,&rdquo Fruchter says. And the chances of two finding each other and merging after forming independently are, for lack of a better term, astronomically low.
The binary neutron stars we know of in our galaxy are millions or billions of years away from merging. Any local merger of neutron stars at all would take LIGO by surprise, given that the events are so rare, and astronomers might not even see the resulting kilonova at all. But if one did occur&mdashsay, in one of the Milky Way&rsquos satellite galaxies&mdashit would be a great reason to run to a telescope to witness the flash and fade of a brief, brilliant new &ldquostar.&rdquo The dangers would be nearly nonexistent, but not the payoff: Our generation of astronomers would have their own supernova 1987A to dissect. &ldquoThis is a once-in-many-lifetimes kind of event,&rdquo Frank says. Thus, she says, we would need to follow something like it with all the world&rsquos astronomical resources. &ldquoWe have to remember to think beyond the initial explosion,&rdquo she adds. &ldquoStuff might still happen and we have to keep a watch out for that.&rdquo
For now astronomers&rsquo attentions are still fixated on the kilonova in NGC 4993. The Earth&rsquos orbital motion has placed the sun between us and the distant galaxy, however, hiding the kilonova&rsquos fading afterglow. When our view clears, in December, many of the world&rsquos telescopic eyes will again turn to the small patch of sky containing the merger. In the meantime papers will be penned and published, careers minted, reputations secured. Science will march on, and wait&mdashwait for the next possible glimpse of a kilonova, the whispers of a neutron star merger or, if we&rsquore lucky, something new altogether.
A normal star is a big ball of gas, its gravity is pulling it together, trying to make it collapse. It's actually held up because it's really, really hot. In the same way that, when a gas is hot it expands, the star's temperature allows it to expand and stay fairly big.
But when the star gets really old it can explode: what leads to this is that, eventually, it will have burned most of its fuel and it cools down a bit. It starts to collapse under its own gravity. Stars which are massive enough then start to crush the protons and electrons to form neutrons. These form a huge star-sized atomic nucleus, basically just neutrons, known as a neutron star.
A normal star can collapse into a neutron star. If a neutron star slowly gathered more and more mass then it could collapse again whereby the neutrons couldn't support themselves. It would start to get crushed together and it would get so heavy and dense that it would turn into a black hole.
A black hole is where you get so much mass in one place it distorts space so much that even light can't escape this will happen whatever the internal structure of the black hole.
We don't know anything about the internals of black holes, and in fact they won't affect anyone outside the black hole, so as far as we can tell a black hole is as far as anything can collapse.
How colliding neutron stars could shed light on universal mysteries
An important breakthrough in how we can understand dead star collisions and the expansion of the Universe has been made by an international team, led by the University of East Anglia.
They have discovered an unusual pulsar -- one of deep space's magnetized spinning neutron-star 'lighthouses' that emits highly focused radio waves from its magnetic poles.
The newly discovered pulsar (known as PSR J1913+1102) is part of a binary system -- which means that it is locked in a fiercely tight orbit with another neutron star.
Neutron stars are the dead stellar remnants of a supernova. They are made up of the most dense matter known -- packing hundreds of thousands of times the Earth's mass into a sphere the size of a city.
In around half a billion years the two neutron stars will collide, releasing astonishing amounts of energy in the form of gravitational waves and light.
But the newly discovered pulsar is unusual because the masses of its two neutron stars are quite different -- with one far larger than the other.
This asymmetric system gives scientists confidence that double neutron star mergers will provide vital clues about unsolved mysteries in astrophysics -- including a more accurate determination of the expansion rate of the Universe, known as the Hubble constant.
The discovery, published today in the journal Nature, was made using the Arecibo radio telescope in Puerto Rico.
Lead researcher Dr Robert Ferdman, from UEA's School of Physics, said: "Back in 2017, scientists at the Laser Interferometer Gravitational-Wave Observatory (LIGO) first detected the merger of two neutron stars.
"The event caused gravitational-wave ripples through the fabric of space time, as predicted by Albert Einstein over a century ago."
Known as GW170817, this spectacular event was also seen with traditional telescopes at observatories around the world, which identified its location in a distant galaxy, 130 million light years from our own Milky Way.
Dr Ferdman said: "It confirmed that the phenomenon of short gamma-ray bursts was due to the merger of two neutron stars. And these are now thought to be the factories that produce most of the heaviest elements in the Universe, such as gold."
The power released during the fraction of a second when two neutron stars merge is enormous -- estimated to be tens of times larger than all stars in the Universe combined.
So the GW170817 event was not surprising. But the enormous amount of matter ejected from the merger and its brightness was an unexpected mystery.
Dr Ferdman said: "Most theories about this event assumed that neutron stars locked in binary systems are very similar in mass.
"Our new discovery changes these assumptions. We have uncovered a binary system containing two neutron stars with very different masses.
"These stars will collide and merge in around 470 million years, which seems like a long time, but it is only a small fraction of the age of the Universe.
"Because one neutron star is significantly larger, its gravitational influence will distort the shape of its companion star -- stripping away large amounts of matter just before they actually merge, and potentially disrupting it altogether.
"This 'tidal disruption' ejects a larger amount of hot material than expected for equal-mass binary systems, resulting in a more powerful emission.
"Although GW170817 can be explained by other theories, we can confirm that a parent system of neutron stars with significantly different masses, similar to the PSR J1913+1102 system, is a very plausible explanation.
"Perhaps more importantly, the discovery highlights that there are many more of these systems out there -- making up more than one in 10 merging double neutron star binaries."
Co-author Dr Paulo Freire from the Max Planck Institute for Radio Astronomy in Bonn, Germany, said: "Such a disruption would allow astrophysicists to gain important new clues about the exotic matter that makes up the interiors of these extreme, dense objects.
"This matter is still a major mystery -- it's so dense that scientists still don't know what it is actually made of. These densities are far beyond what we can reproduce in Earth-based laboratories."
The disruption of the lighter neutron star would also enhance the brightness of the material ejected by the merger. This means that along with gravitational-wave detectors such as the US-based LIGO and the Europe-based Virgo detector, scientists will also be able to observe them with conventional telescopes.
Dr Ferdman said: "Excitingly, this may also allow for a completely independent measurement of the Hubble constant -- the rate at which the Universe is expanding. The two main methods for doing this are currently at odds with each other, so this is a crucial way to break the deadlock and understand in more detail how the Universe evolved."
How does a supernova completely destroy a star?
A supernova does not completely destroy a star. Supernovae are the most violent explosions in the universe. But they do not explode like a bomb explodes, blowing away every bit of the original bomb. Rather, when a star explodes into a supernova, its core survives. The reason for this is that the explosion is caused by a gravitational rebound effect and not by a chemical reaction, as explained by NASA. It is true that within most stars there are violent hydrogen fusion reactions churning away, but these do not cause the supernova. Stars are so large that the gravitational forces holding them together are strong enough to keep the nuclear reactions from blowing them apart. It is the gravitational rebound that blows apart a star in a supernova.
Consider the typical momentum transfer exhibit found in many science museums, as depicted in the animation on the right. Rubber balls of different sizes are held at different heights. The balls are then let go at the same moment. Gravity pulls them all down and they all fall towards the ground. In the next few moments, the bottom ball hits the ground and bounces back, and then the balls start colliding. Momentum equals mass times velocity. This means that a heavy object going slow has as much momentum as a light object going fast. When two objects collide, they transfer some momentum. When a heavy slow object collides with a light object, it can give it a very high velocity because of the conservation of momentum. As this animation shows, by arranging the rubber balls from heaviest on the bottom to lightest on the top, momentum is transferred to ever lighter objects, meaning ever higher speeds. As a result, even though gravity is pulling all the balls downwards, the upper balls rebound at incredible speeds. This is all in keeping with the law of conservation of momentum. The lower balls are too heavy and too slow to fly off. They remain behind as the surviving core of the original system. On the other hand, the upper balls are blown away (in a science museum exhibit, they are captured at the top of the apparatus so that the demonstration can be rerun). This explosion of rubber balls occurs without any significant chemical or nuclear reactions taking place. This explosion is simply due to gravity and momentum transfer, i.e. a gravitational rebound. If you look closely at the animation, you see that the rebound takes the form of an outward shock wave that gains in intensity as it spreads.
A supernova is the same kind of explosion as this rubber-balls demonstration. An aging star is composed of denser layers down towards the center, and thinner layers near the surface. The star's nuclear reactions typically balance out the force of gravity. But when the star runs out of fuel, the nuclear reactions slow down. This means that gravity is no longer balanced. Gravity begins collapsing the star. After the core of a collapsing star reaches a critical density, its pressure becomes strong enough to hold back the collapse. But, like the rubber balls, the star has been falling inwards and now bounces back. The outer layers are blown off into space in a giant explosion, spreading fertile dust clouds through-out the universe . But because of the momentum transfer, the star's core survives. The collapsing event has so intensely squeezed the star's core, that it transforms into something exotic. If the star started out with between 5 and 12 times the mass of our sun, the core becomes a big ball of neutrons called a neutron star. If the star started out with more than 12 times the mass of our sun, the core becomes a black hole. You may be tempted to argue that when a star explodes so that all that remains is a black hole, there is nothing left and the star has therefore been completely destroyed. But a black hole is not nothing. Black holes have mass, charge, angular momentum, and exert gravity. A black hole is just a star that is dense enough, and therefore has strong enough gravity, to keep light from escaping. The black hole created by a supernova is the leftover core of the star that exploded.
Not all stars experience a supernova. Stars that have less than 5 times the mass of our sun are too light to experience this violent transformation. They simply don't have enough gravity to collapse and rebound so violently. Instead, when lighter stars run out of nuclear fuel, they go through a series of stages and then settle down as long-lived white dwarfs. Whether stars end up as neutron stars, black holes, or white dwarfs, they never go completely away.
Neutron star collisions as a heavy element source
Half is a lot. I'm curious. What mechanisms distribute those heavy elements throughout the galaxy?
Type 1a supernovas create a different mix of elements from type II and type 1b.
Type 1a SN produce a lot of nickel and iron. Massive stars create iron by burning lighter elements. Massive star cores can not explain abundances seen in the Milky Way because the center of a star's core stays inside the neutron star.
How much nucleosynthesis does the outer core experience during the bounce, in that scenario?
As I understand it, the bounce is the only time that can happen, and the outer core is the only part with weight A >
50 that escapes the star (and maybe not most of that). So adding neutrons to to the outer core seems like the only way the SN can put neutron-rich nuclei into the ISM. Even then, I'd imagine most of the detritus (after radioactive decay) is only modestly enhanced in neutrons, so it wouldn't add much for, say, A > 100. True? Or do the simulations show a more favorable outcome?
You and @alantheastronomer have both made statements of this type, but I haven't seen any references or calculations to back up these statements, which makes them personal speculations. In addition to the paper I referenced in the OP, you can also look at this paper from the LIGO collaboration. Both make detailed estimates of the rate of NS-NS mergers, and come to the conclusion that NS-NS mergers happen at a rate sufficient to explain the R process elements. I've included a figure from this latter paper below. Of course, the rate is still uncertain, but I suspect that after another 5-10 years of LIGO/VIRGO observations it will be clear that there are more than enough of these events.
No, I'm afraid this doesn't clear things up. Where does your estimate of NS-NS mergers of 1-2 per year for the entire universe come from? There are approximately 10^11 galaxies in the observable universe, so your estimate of NS-NS mergers is about 10^-11 per galaxy per year. Many people have done these estimates, and I won't list them all here, but almost all of these estimates have been in the range of 1-100 per galaxy per Myr, which is 10^-4 - 10^-6 per galaxy per year, which is 10^5-10^7 times greater than your estimate. Let me ask you two questions:
(1) If the rate is 10^-11 per galaxy per year, what would be the odds that LIGO, which was sensitive to around 10^6 galaxies, would detect a NS-NS merger in a few months of observing? Did they just get really really really lucky?
(2) If the rate is 10^-11 per galaxy per year, how can it be that even in the small portion of the Milky Way that we have surveyed, we already have found six verified NS-NS binaries, four of which will merge in the next few hundred million years? (See below)
Perhaps your confusion lies in calling these events "collisions". These are not NS wandering through space that randomly collide. These are binary star systems which evolve into binary NS, then spiral in and merge due to the emission of gravitational waves.
Narrowing down the binary neutron star merger rate is a vital step in determining if these events are a viable candidates for heavy element enrichment of the MW.. It is relevant to take into account that binary NS merger is one of the few environments believed even capable of supporting r-process nucleosynthesis. So, it is perfectly understandable detecting such an event would be of immense interest The number of related papers published is no coincidence. The two big questions relate to: 1] them amount of heavy nuclei liberated and, 2] mergers frequency. This paper appears to cover the first count, but, falls a little short on the second.
The lack of well founded alternatives to NS mergers as a source of r-process elements appears to offer circumstantial encouragement for fine tuning projections of r-process element output and frequency of NS mergers. This paper, https://arxiv.org/pdf/1710.02142.pdf, points out that r-process contributions from single massive stars may still be necessary by noting
"Observations of lowest metallicity stars in our Galaxy and (ultra-faint) dwarf galaxies show substantial ”pollution” by r-process elements, indicating a production site with a low event rate and consistent high amount of r-process ejecta in order to explain solar abundances. This is also underlined by the large scatter of Eu/Fe (Eu being an r-process element and Fe stemming from CCSNe at these low metallicities) seen in the earliest stars of the Galaxy, indicating that in a not yet well mixed interstellar medium the products of regular CCSNe and these rare events vary substantially."