What Is a Neutron Star Collision?
Theorized for countless years, a neutron star collision has been an elusive target for the astronomical community. We have had lots of ideas about them and their relationship to the known Universe, but simulations only take you so far. That’s why 2017 was an important year, for after all the frustrating null results, a neutron star collision was finally spotted. Let the good times roll.
The Universe is full of merging stars, falling in through a complicated tango of gravitational effects and drag. Most stars that fall into each other become more massive but still remain what we would call a traditional star. But provided enough mass, some stars end their life in a supernova, and depending on that mass either a neutron star or a black hole will remain. Getting a binary set of neutron stars, therefore, should be difficult because of the condition that arise in making them. Provided that we do have such a system, two neutron stars falling into each other can either become a more massive neutron star or a black hole. Radiation and gravity waves should roll out of the system as this happens, with material emanating as jets from the poles as the incoming objects spin faster and faster before finally becoming one (McGill).
All of this should make the hunt for these collisions extremely difficult. This is why the detection of GW170817 was so amazing. Found on August 17, 2017, this gravity wave event was found by the LIGO/Virgo gravity wave observatories. Less than 2 seconds later, the Fermi Space Telescope picked up a gamma ray burst from the same location. The scramble was on now, as 70 other telescopes across the world joined in to see this moment in visual, radio, X-rays, gamma rays, infrared, and the ultraviolet. In order to be detected, such an event needs to be close (within 300 million light-years) to Earth otherwise the signal is too weak for detection. At just 138 million light-years away in NGC 4993, this fit the bill.
Also, because of that weak signal, pinpointing a specific location is tough unless you have multiple detectors operating at once. With Virgo just recently becoming operational, a few weeks difference may have meant poorer results due to a lack of triangulation. For over 100 seconds, the event was recorded by our gravitational wave detectors and it became clear quickly that this was a coveted neutron star collision. Prior observations indicate that the neutron stars were 1.1 to 1.6 solar masses each, which meant they spiraled in slower than a massive pair such as black holes, allowing for a longer merger time to be recorded (Timmer 2017, Moskovitch, Wright).
One of the first things scientists realized was that short gamma ray burst detected by Fermi, just as theory predicted. This burst occurred nearly at the same time as the gravitational wave detection (following them in only 2 seconds after traveling 138 million light-years!), meaning those gravitational waves were moving at nearly the speed of light. Heavier elements not traditionally thought to come from supernovas were also spotted, including gold. This was a validation of predictions arising from GSI scientists whose work gave the theoretical electromagnetic signature that such a situation would result in. These mergers could be a factory for producing these higher-mass elements rather than the traditionally assumed supernovas, for some paths to element synthesis require neutrons under the conditions that only a neutron star merger could provide. This would include elements on the periodic table from tin up to lead (Timmer 2017, Moskovitch, Wright, Peter “Predictions”).
As the months after the event continued on, scientists kept observing the site to see the conditions around the merger. Surprisingly, the X- rays around the site actually increased according to sightings by the Chandra Space Telescope. This could be because the gamma rays hitting the material around the star gave enough energy to have many secondary collisions that show off as X-rays and radio waves, indicating a dense shell around the merger.
It’s also possible that those jets instead came from a black hole, which does have jets from the newly formed singularity as it feeds on the material surrounding it. Further sightings have shown a shell of heavier materials around the merger and that the peak brightness occurred 150 days post-merger. The radiation fell off very fast after that. As for the resultant object, while there was that evidence for it being a black hole, further evidence of the LIGO/Virgo and Fermi data indicated that as the gravity waves fell off, the gamma rays picked up and with a frequency of 49 Hz pointing to a hyper-massive neutron star instead of a black hole. This is because such a frequency would come from such a spinning object rather than a black hole (McGill, Timmer 2018, Hollis, Junkes, Klesman).
Some of the best results from the merger were those which negated or challenged theories of the Universe. Because of that nearly-instantaneous reception of gamma rays and gravity waves, several dark energy theories based on scalar-tensor models were struck a blow because they predicted a much larger separation between the two (Roberts Jr.).
Future Neutron Star Collision Studies
Well we have certainly seen how neutron star collisions have a great data set to them, but what will future events be able to help us resolve? One mystery they can contribute data to is the Hubble Constant, a debated value that determines the expansion rate of the Universe. One way to find it is to see how stars at different points in the Universe were moving away from each other while another method involves looking at the shifting of densities in the cosmic microwave background.
Depending on how one goes about measuring the value of this universal constant, we can get two different values that are off from each other by about 8%. Clearly, something is wrong here. Either one (or both) of our methods have flaws to them and so a third method would be useful in guiding our efforts. Neutron star collisions are therefore a great tool because their gravity waves not being impacted by material along their routes like traditional distance measurements nor do the waves depend on a ladder of built up distances like the first method. Using GW170817 along with red shift data, scientists found their Hubble Constant to be between the two methods. More collisions will be needed so don’t read too much into this result (Wolchover, Roberts Jr., Fuge, Greenebaum).
Then we start to get real wild with our ideas. It’s one thing to say that two objects merge and become one, but it’s totally different to say the step-by-step process. We have the general brushstrokes, but is there a detail in the painting we are missing? Beyond the atomic scale lies the realm of quarks and gluons, and in the extreme pressures of a neutron star it could be possible for them to break down into these constituent parts. And with a merger being even more complex, a quark-gluon plasma is even more likely. Temperatures are several thousands of times more than the Sun and densities exceeding that of basic atomic nuclei being compacts. It should be possible, but how would we know? Using supercomputers, researchers from Goethe University, FIAS, GSI, Kent University, and Wroclaw University were able to map out such a plasma forming in the merger. They found that only isolated pockets of it would form but it would be enough to cause a flux in the gravity waves that could be detected (Peter “Merging”).
It’s a new field of study, in its infancy. It’s going to have applications and results that surprise us. So check in often to see the latest news in the world of neutron star collisions.
- Fuge, Lauren. “Neutron star collisions hold key to universe expansion.” Cosmosmagazine.com. Cosmos. Web. 15 Apr. 2019.
- Greenebaum, Anastasia. “Gravitational waves will settle cosmic conundrum.” Innovations-report.com. innovations report, 15 Feb. 2019. Web. 15 Apr. 2019.
- Hollis, Morgan. “Gravitational waves from a merged hyper-massive neutron star.” Innovations-report.com. innovations report, 15 Nov. 2018. Web. 15 Apr. 2019.
- Klesman, Allison. “Neutron Star Merger Created a Cocoon.” Astronomy, Apr. 2018. Print. 17.
- Junkes, Norbert. “(Re)solving the jet-cocoon riddle of a gravitational wave event.” 22 Feb. 2019. Web. 15 Apr. 2019.
- McGill University. “Neutron-star merger yields new puzzle for astrophysicists.” Phys.org. Science X Network, 18 Jan. 2018. Web. 12 Apr. 2019.
- Moskovitch, Katia. “Neutron-Star Collision Shakes Space-Time and Lights up the Sky.” Quantamagazine.com. Quanta, 16 Oct. 2017.Web. 11 Apr. 2019.
- Peter, Ingo. “Merging neutron stars – How cosmic events give insight into fundamental properties of matter.” Innovations-report.com. innovations report, 13 Feb. 2019. Web. 15 Apr. 2019.
- ---. “Predictions by GSI scientists now confirmed: Heavy elements in neutron star mergers detected.” Innovations-report.com. innovations report, 17 Oct. 2017. Web. 15 Apr. 2019.
- Roberts Jr., Glenn. “Star mergers: A new test of gravity, dark energy theories.” Innovaitons-report.com. innovations report, 19 Dec. 2017. Web. 15 Apr. 2019.
- Timmer, John. “Neutron stars collide, solve major astronomical mysteries.” Arstechnica.com. Conte Nast., 16 Oct. 2017. Web. 11 Apr. 2019.
- ---. “Neutron-star merger blasted a jet of material through the debris.” Arstechnica.com. Conte Nast., 05 Sept. 2018. Web. 12 Apr. 2019.
- Wolchover, Natalie. “Colliding Neutron Stars Could Settle the Biggest Debate in Cosmology.” Quantamagazine.com. Quanta, 25 Oct. 2017. Web. 11 Apr. 2019.
- Wright, Matthew. “Neutron star merger directly observed for the first time.” Innovations-report.com. innovations report, 17 Oct. 2017. Web. 12 Apr. 2019.
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