An international team of astronomers have announced today (July 8, 2020) in Nature the breakthrough discovery of an unusual neutron star binary system. A rapidly spinning neutron star (i.e. pulsar), named PSR J1913+1102, is locked in a tight orbit with another densely packed stellar remnant, set to collide in around 470 million years – relatively soon in cosmic timescales. When they do so, the event will release extraordinary amounts of energy as gravitational waves and light.
But the heightened interest in the duo comes from the difference in their masses. In fact, the system, observed by the Arecibo radio telescope in Puerto Rico, is the most asymmetric merging neutron star binary system ever discovered. Its existence suggests that there are plenty of similar systems out in space whose catastrophic collisions could provide new insights into the mysterious make-up of neutron stars and even help to determine a more accurate measure of the expansion rate of the Universe (the Hubble constant).
Interestingly, the researchers believe that the first-ever neutron star merger detected, back in 2017, could have been a result of such an asymmetric binary system.
“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,” lead researcher Dr Robert Ferdman, from the University of East Anglia, UK, said in a statement. “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."
The inequality of the star’s masses in these binary systems can produce an even more spectacular merger than that of equal-mass systems. On top of the phenomenal power released in the fraction of a second when the two stars collide, estimated to be tens of times larger than all the stars in the Universe combined, enormous amounts of mass are ejected, brightening the event further.
“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,” Ferdman explained. “This 'tidal disruption' ejects a larger amount of hot material than expected for equal-mass binary systems, resulting in a more powerful emission.”
“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,” co-author Dr Paulo Freire, from the Max Planck Institute for Radio Astronomy in Bonn, Germany, continued. “This matter is still a major mystery – it's so dense that scientists still don't know what it is actually made of.”
However, the interior of neutron stars is not the only mystery that could be probed. As the asymmetrical system would brighten the material ejected, both gravitational wave detectors (such as LIGO and VIRGO) and conventional telescopes would be able to pinpoint the collision.
“Excitingly, this may also allow for a completely independent measurement of the Hubble constant – the rate at which the Universe is expanding,” Ferdman added. “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.”