In August of last year, the LIGO and Virgo collaborations made a first-of-its-kind gravitational wave detection - what seemed to be a black hole swallowing up a neutron star. Now LIGO has confirmed the event, giving it the name GW190814. And it looks like the neutron star was not actually... a neutron star.
That would mean the detection is the first of a different kind - the smallest black hole we've ever detected, narrowing the mysterious 'mass gap' between neutron stars and black holes. But, like most answers the Universe gives us, it opens up a dozen more.
"This is going to change how scientists talk about neutron stars and black holes," said physicist Patrick Brady of University of Wisconsin-Milwaukee, and the LIGO Scientific Collaboration spokesperson.
"The mass gap may in fact not exist at all but may have been due to limitations in observational capabilities. Time and more observations will tell."
Into the mass gap
The mass gap is a curious exception in our detections of black holes and neutron stars. Both types of objects are the collapsed, dead cores of massive stars. For neutron stars, the progenitor stars are around 8 to 30 times the mass of the Sun; they blow off most of their mass before they die, and the cores collapse down to objects of around 1.4 solar masses.
Meanwhile, progenitor stars larger than around 30 solar masses collapse down into black holes, with a wide range of masses.
Which leads us to the gap. We've never seen a pre-merger object between particular upper and lower limits - a neutron star larger than around 2.3 solar masses, or a black hole smaller than 5 solar masses.
GW190814 has now delivered that object. Analysis of the gravitational wave signal has revealed that the larger of the two merging objects - interpreted as a black hole - was 23 solar masses. The smaller of the two was just 2.6 solar masses, nine times smaller than the other.
This mass means it could be the biggest neutron star we've ever detected; or, much more likely, the tiniest black hole.
"It's a challenge for current theoretical models to form merging pairs of compact objects with such a large mass ratio in which the low-mass partner resides in the mass gap. This discovery implies these events occur much more often than we predicted, making this a really intriguing low-mass object," explained astrophysicist Vicky Kalogera of Northwestern University in Illinois.
"The mystery object may be a neutron star merging with a black hole, an exciting possibility expected theoretically but not yet confirmed observationally. However, at 2.6 times the mass of our Sun, it exceeds modern predictions for the maximum mass of neutron stars, and may instead be the lightest black hole ever detected."
The limit on neutron stars
The reason astronomers aren't sure what resides in the mass gap is that it's really difficult to calculate something called the Tolman-Oppenheimer-Volkoff limit (TOV limit). This is the limit above which the mass of a neutron star is so great, the outward pressure of neutrons can no longer repel each other against the inward pressure of gravity, and the object collapses into a black hole.
As our observations grow more robust, constraints on the TOV limit for neutron stars are closing in. Calculations generally put it somewhere between 2.2 and 2.4 solar masses; and data from GW170817 - a 2017 neutron star merger that produced a post-merger mass-gap black hole of 2.7 solar masses - have narrowed it down to around 2.3 solar masses.
The uncertainty over the smaller object in GW190814 arises from the wiggle room in the TOV limit - but, according to the team's analysis, if the 2.3 solar mass calculation is taken, there's only a chance of around three percent that the object is a neutron star.
"GW190814 is probably not the product of a neutron star-black hole coalescence, despite its preliminary classification as such," the researchers wrote in their paper. "Nonetheless, the possibility that the secondary component is a neutron star cannot be completely discounted due to the current uncertainty in [the TOV limit]."
While a neutron star-black hole merger would have been super exciting, the fact that GW190814 has likely turned out to feature a tiny black hole is really awesome, too.
For one, the finding can now help astronomers to constrain the mass gap. And, importantly, it throws our formation models of both neutron stars and binary systems into quite a disarray.
You see, astronomers think that stellar-mass black holes are produced by really massive stars that go supernova and collapse into a black hole. And we believe neutron stars form the same way. But theorists were producing formation models that fit around the mass gap; now that a pre-merger mass gap object has been found, those models will need to be reevaluated.
The other problem is the huge mass discrepancy. Most of the gravitational wave mergers detected to date involve two objects of more or less equal size. Earlier this year, scientists announced a black hole merger with a mass ratio of roughly 3:1, but GW190814 is way more extreme.
There are two main ways for binary systems to form. Either they are born together out of the same chunk of interstellar cloud, living together for their entire lifespans, and then dying together; or they come together later in life. But it's really hard for these binary formation models to produce systems with such extreme mass ratios.
And the fact that GW190814 was detected just a few years after the first gravitational wave detection in 2015 implies that such extreme systems aren't even that uncommon.
"All of the common formation channels have some deficiency," astronomer Ryan Foley of the University of California, Santa Cruz told ScienceAlert. Foley was a member of the team who found the initial GW190814 detection, and was not involved in this new paper.
"It's that the rate [of this kind of event] is relatively high. [And] it's not just that you have masses that are different by a factor of nine. It's also that one of them is in this mass gap. And one of them is really, really massive. So all those things combined, I don't think that there's a good model that really solves those three separate issues."
There's plenty in this one detection to keep theorists busy for a while, re-imagining those formation scenarios to determine how a system like GW190814, and its separate components, can come into being - whether the smaller object is a neutron star or a black hole.
As for figuring out the latter, that will be a matter of more detections. LIGO is currently offline while it undergoes upgrades. It's expected to come back online sometime next year, more sensitive than ever - hopefully to detect more events like GW190814, which will help resolve some of the outstanding questions.
"This is the first glimpse of what could be a whole new population of compact binary objects," said astrophysicist Charlie Hoy of the LIGO Scientific Collaboration and Cardiff University in the UK.
"What is really exciting is that this is just the start. As the detectors get more and more sensitive, we will observe even more of these signals, and we will be able to pinpoint the populations of neutron stars and black holes in the Universe."
The research has been published in The Astrophysical Journal Letters.