When neutron stars collide — and reveal the Universe's mysteries
An asymmetric binary pulsar system discovered by our team with the Arecibo radio telescope has shown how neutron-star mergers can shed important light over the widest of astrophysical scales: from the exotic nuclear matter that makes up these extreme objects, to the expansion of the Universe itself.
In 2017, the first discovery was made of two neutron stars that crashed into each other 130 million years ago. This massive collision released gravitational-wave ripples that permeated the fabric of spacetime — precisely as predicted by Albert Einstein over a century ago. Known as GW170817, this spectacular event was also observed with many traditional telescopes, leading to the identification of its precise location in a distant galaxy. Thanks to these multi-messenger observations, the suspicion that the phenomenon of short gamma-ray bursts was confirmed to be due to the merger of two neutron stars. These are now thought to be the factories that produce much of the heaviest elements in the Universe, such as gold.
Neutron stars are the fast-spinning, highly magnetic, remnants left over from supernova explosions of massive stars. They can be observed when their emission beams pass through our line of sight, much like a cosmic lighthouse. These observable neutron stars are called pulsars. Neutron stars have masses that are about one and a half times that of our Sun, but squeezed into an area about half the size of Greater London. This results in extreme densities, similar to that of an atomic nucleus. The energy released when two neutron stars are ripped apart in a merger is enormous, estimated to reach up to that of 100 million Suns!
However, the source of the massive amounts of matter ejected from the merger, responsible for heavy element production, has been a mystery. The amount of matter released in the GW170817 event as determined from observation is estimated to be around five times higher than expected. This is because most theoretical models and predictions of these collisions have assumed that neutron stars locked in binary systems are very close to each other in mass.
Our recent discovery of the pulsar PSR J1913+1102 changes these assumptions. This pulsar is in a binary system with yet another neutron star. We calculate that this system will merge within 500 million years, which is well within the age of the Universe. We are therefore witnessing a double neutron collision in the making. We have found this binary system to be particularly unique, as the two neutron star within it have very different masses.
Because of this mass asymmetry, the extreme gravitational influence of the more massive neutron star will distort the shape of its companion, stripping away large amounts of matter just before they actually merge. This will produce more hot material than expected for equal-mass binary systems, and will be observable with radio, optical, and X-ray telescopes. Although GW170817 can be explained with several non-traditional theoretical models, we now confirm that a parent system of neutron stars of very different masses is a plausible explanation.
Perhaps more importantly, this discovery points to many more of these systems out there — more than 1 out every 10 merging double neutron star binaries are asymmetric. As a result, this may provide unique opportunities to unlock some of the Universe's most stubbornly elusive mysteries.
The mangling of a less-massive stellar companion (known as tidal distortion) will cause the merger event to have an enhanced brightness as seen by gravitational wave detectors such as LIGO and Virgo, as well as by conventional light-detecting telescopes. This will allow astrophysicists to gain important new clues about the exotic nuclear matter that makes up the enigmatic interiors of these extreme, dense objects.
A very exciting prospect is that future discoveries of asymmetric mergers will also lead to a completely independent measurement of the Hubble constant, the current rate at which the Universe is expanding. The two prominent methods for doing so are currently at odds with each other, and the detection of several such mergers will be a vital way to “break the deadlock” and understand how the Universe has evolved in unprecedented detail.
Our discovery of PSR J1913+1102 was made with with the Arecibo radio telescope in Puerto Rico, as part of the PALFA pulsar survey of the Galactic plane.