Black Hole or No Black Hole: Astrophysics of Neutron Star Collisions

Neutron Stars Merger

Artistic representation: In a merger of neutron stars extreme temperatures and densities occur. Credit: Dana Berry, SkyWorks Digital, Inc.

A new study lead by GSI scientists and international colleagues investigates black-hole formation in neutron star mergers. Computer simulations show that the properties of dense nuclear matter play a crucial role, which directly links the astrophysical merger event to heavy-ion collision experiments at GSI and FAIR. These properties will be studied more precisely at the future FAIR facility. The results have now been published in Physical Review Letters. With the award of the 2020 Nobel Prize in Physics for the theoretical description of black holes and for the discovery of a supermassive object at the center of our galaxy the topic currently also receives a lot of attention.

But under which conditions does a black hole actually form? This is the central question of a study lead by the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt within an international collaboration. Using computer simulations, the scientists focus on a particular process to form black holes namely the merging of two neutron stars (simulation animation below).

Neutron stars consists of highly compressed dense matter. The mass of one and a half solar masses is squeezed to the size of just a few kilometers. This corresponds to similar or even higher densities than in the inner of atomic nuclei. If two neutron stars merge, the matter is additionally compressed during the collision. This brings the merger remnant on the brink to collapse into a black hole. Black holes are the most compact objects in the universe, even light cannot escape, so these objects cannot be observed directly.

“The critical parameter is the total mass of the neutron stars. If it exceeds a certain threshold the collapse to a black hole is inevitable” summarizes Dr. Andreas Bauswein from the GSI theory department. However, the exact threshold mass depends on the properties of highly dense nuclear matter. In detail these properties of high-density matter are still not completely understood, which is why research labs like GSI collide atomic nuclei — like a neutron star merger but on a much smaller scale. In fact, the heavy-ion collisions lead to very similar conditions as mergers of neutron stars. Based on theoretical developments and physical heavy-ion experiments, it is possible to compute certain models of neutron star matter, so-call equations of state.

Employing numerous of these equations of state, the new study calculated the threshold mass for black-hole formation. If neutron star matter or nuclear matter, respectively, is easily compressible — if the equation of state is “soft” — already the merger a relatively light neutron stars leads to the formation of a black hole. If nuclear matter is “stiffer” and less compressible, the remnant is stabilized against the so-called gravitational collapse and a massive rotating neutron star remnant forms from the collision. Hence, the threshold mass for collapse itself informs about properties of high-density matter. The new study revealed furthermore that the threshold to collapse may even clarify whether during the collision nucleon dissolve into their constituents, the quarks.

“We are very excited about these results because we expect that future observations can reveal the threshold mass” adds Professor Nikolaos Stergioulas of the department of physics of the Aristotle University Thessaloniki in Greece. Just a few years ago a neutron star merger was observed for the first time by measuring gravitational waves from the collision. Telescopes also found the “electromagnetic counterpart” and detected light from the merger event. If a black hole is directly formed during the collision, the optical emission of the merger is pretty dim. Thus, the observational data indicates if a black hole was created. At the same time the gravitational-wave signal carries information about the total mass of the system. The more massive the stars the stronger is the gravitational-wave signal, which thus allows determining the threshold mass.

While gravitational-wave detectors and telescopes wait for the next neutron star mergers, the course is being set in Darmstadt for knowledge that is even more detailed. The new accelerator facility FAIR, currently under construction at GSI, will create conditions, which are even more similar to those in neutron star mergers. Finally, only the combination of astronomical observations, computer simulations, and heavy-ion experiments can settle the questions about the fundamental building blocks of matter and their properties, and, by this, they will also clarify how the collapse to a black hole occurs.

Reference: “Equation of State Constraints from the Threshold Binary Mass for Prompt Collapse of Neutron Star Mergers” by Andreas Bauswein, Sebastian Blacker, Vimal Vijayan, Nikolaos Stergioulas, Katerina Chatziioannou, James A. Clark, Niels-Uwe F. Bastian, David B. Blaschke, Mateusz Cierniak and Tobias Fischer, 30 September 2020, Physical Review Letters.
DOI: 10.1103/PhysRevLett.125.141103

8 Comments on "Black Hole or No Black Hole: Astrophysics of Neutron Star Collisions"

  1. I think if I was a scientist I’d be more focused on the ocean and the health of the planet. Mother nature is in great danger. Unfortunately we have been dumping crap, testing weapons, spilling oil In the ocean which is the heartbeat of this planet. 95% of life on this planet depends on the ocean. There are two Giant islands of garbage as big as Texas. Our government doesn’t seem to be doing anything about it. And I know for a fact that our military has been dumping s*** in the ocean for years..Why don’t we recycle here? Did you know Switzerland powers the entire country with plastic? Hopefully our scientist will take a trip over to Switzerland so they can see what intelligent people do with their time.

    • Torbjörn Larsson | November 22, 2020 at 10:23 am | Reply

      Science covers a wide area and is mutually reinforcing. E.g. the discoveries of X-rays and positrons are used in medicine today – who knows what discoveries in quark phsyics can do tomorrow.

      Earth science programs are moving apace in both Europe and US. Why, earlier today I started to watch a launch video of yesterday’s Sentinel-6 Michael Freilich satellite since it was a SpaceX land return and stayed for the science coverage led by NASA. The unique name was adopted from the late former director of NASA Earth Science Division, Michael Freilich, who was primary in merging US and Europe satellite altimeter programs and place the ever better altimeters to follow the older USA Jason-3 so that the data capture can be seamlessly and calibrated handed over to the newer satellite. (In 5 years time the next Sentinel will go up.)

      The program was – for NASA – family friendly and internationally cooperative, and included Freilich’s surviving children, also scientists and educators, and a young grandchild that spoke on the urgency of the science. For a US media arena it was open with that the sea level measurements are vital due to the man made global warming and the correlated observations of ever more rapid sea rise. And Sentinel-6 will also for the first time acceptably resolve the coast areas and provide better hurricane/tsunami forecasts for the dominating near sea urban part of the global population.

      Science and society can walk and chew bubble gum at the same time – in both cases the results will be better. Why, your mentioning the irrelevancies of individual governments and militaries in the face of global issues illustrate that. One can well ask why science (or governments or militaries) are focused here or there, but recycling is not the issue for computer astrophysicists – it is the issue of their national and university data centers that provide the energetically costly equipment.

  2. Could a special atomic clock detect space time changes ???

    • Torbjörn Larsson | November 22, 2020 at 10:32 am | Reply

      Yes, that experiment has been done in the 70’s.

      “The Hafele–Keating experiment was a test of the theory of relativity. In October 1971, Joseph C. Hafele, a physicist, and Richard E. Keating, an astronomer, took four cesium-beam atomic clocks aboard commercial airliners. They flew twice around the world, first eastward, then westward, and compared the clocks against others that remained at the United States Naval Observatory. When reunited, the three sets of clocks were found to disagree with one another, and their differences were consistent with the predictions of special and general relativity.” [“Hafele–Keating experiment” @ Wikipedia]

      In fact, your GPS does it for you in order to keep its time stamps correct.

      “In 1955, Friedwardt Winterberg proposed a test of general relativity – detecting time slowing in a strong gravitational field using accurate atomic clocks placed in orbit inside artificial satellites. Special and general relativity predict that the clocks on the GPS satellites would be seen by the Earth’s observers to run 38 microseconds faster per day than the clocks on the Earth. The GPS calculated positions would quickly drift into error, accumulating to 10 kilometers per day (6 mi/d). This was corrected for in the design of GPS.” [“Global Positioning System” @ Wikipedia]

      If you mean for these neutron star mergers, I’m sure that various relativistic effects come into it from quark physics all the way up to the merger simulations and observations. But that is a lot of science to cover!

  3. Lea’s comment is awfully pertinent. The blue planet owes life to water in liquid state due to température condition on its surface. We humains are destroying our environment by excess of pollution, all that is well documented now. The only problem is the very difficult turn toward more mastered way of life, especially with so many people and growing so fast. Probably all scientists would not be enough to solve this problem and even if they could, it is not sure their solution would be acceptable, simply because with the very fast avance in knowledge, better solution will inevitably pop up sooner or later. We just have to be patient and confident these guys are doing their best, even if some seem to wander nobody knows where, in present case so far in remote stellar objects. But in fact they look at the finest level at which a huge power is released in the Universe (the so-called quarks deconfinement) in the hope that this could be adapted to our situation. Recall that this is today becoming reality with the next higher level of thermonuclear fusion as you m’ayant gavé seen in the newspapers.

    • Torbjörn Larsson | November 22, 2020 at 10:40 am | Reply

      Thanks for an insightful comment! Scientists are not tasked to solve societal problems all on their own – though sometimes they more or less do [say, pandemic rapid vaccine development] – but can suggest solutions, and support them given the resources.

      By the way, interesting mistranslation – is your first language French (but I don’t think it is entirely, I only recognize “m’ayant”] or did a software act up on you?

  4. Torbjörn Larsson | November 22, 2020 at 9:59 am | Reply

    “Employing numerous of these equations of state, the new study calculated the threshold mass for black-hole formation.”

    The threshold is illustrated here [ ]as the gray object mass region between 2-5 solar masses of ingoing or outgoing objects in gravitational wave observations of mergers [“Masses of the Stellar Graveyard” ; “The graphic shows black holes (blue), neutron stars (orange) and compact objects of uncertain nature (gray) detected through gravitational waves. “.]

  5. I’m very interested in space, but especially in black holes

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