Starts With A Bang

Seeing One Example Of Merging Neutron Stars Raises Five Incredible Questions (Synopsis)

Neutron stars, when they merge, can exhibit gravitational wave and electromagnetic signals simultaneously, unlike black holes. But the details of the merger are quite puzzling, as the theoretical models don't quite match what we've observed. Image credit: Dana Berry / Skyworks Digital, Inc.

“O. Hahn and F. Strassmann have discovered a new type of nuclear reaction, the splitting into two smaller nuclei of the nuclei of uranium and thorium under neutron bombardment. Thus they demonstrated the production of nuclei of barium, lanthanum, strontium, yttrium, and, more recently, of xenon and caesium. It can be shown by simple considerations that this type of nuclear reaction may be described in an essentially classical way like the fission of a liquid drop, and that the fission products must fly apart with kinetic energies of the order of hundred million electron-volts each.” -Lise Meitner

Now that we’ve observed merging neutron stars for the first time, in many different wavelengths of light as well as in gravitational waves, we’ve got a whole new world of data to work with. We’ve independently confirmed that gravitational waves are real and that we can, in fact, pinpoint their locations on the sky. We’ve demonstrated that merging neutron stars create short gamma ray bursts, and shown that the origin of the majority of elements heavier than the first row of transition metals comes primarily from neutron star-neutron star mergers.

This color-coded periodic table groups elements by how they were produced in the universe. Hydrogen and helium originated in the Big Bang. Heavier elements up to iron are generally forged in the cores of massive stars. The electromagnetic radiation captured from GW170817 now confirms that elements heavier than iron are synthesized in large amounts the aftermath of neutron star collisions. Image credit: Jennifer Johnson / SDSS.

But the new discovery raises a ton of questions, too. Seeing this event has presented theorists with a number of new challenges, ranging from the event rate being some ten times as great as expected to much more matter being ejected than we’d thought. And what was it that was left behind? Was it a neutron star? A black hole? Or an exotic object that’s in its own class?

We knew that when two neutron stars merge, as simulated here, they create gamma-ray burst jets, as well as other electromagnetic phenomena. But whether you produce a neutron star or a black hole, as well as how much of a UV/optical counterpart is produced, should be strongly mass-dependent. Image credit: NASA / Albert Einstein Institute / Zuse Institute Berlin / M. Koppitz and L. Rezzolla.

There are some great advances that the future will hold for gravitational wave and neutron star astronomy, but it’s up to theorists to explain why these objects behave as they do. Here are five burning questions we now have.