In August last year, the LIGO and Virgo collaborations made a gravitational wave detection, the first of its kind, in what appeared to be a black hole swallowing a neutron star. Now LIGO has confirmed the event, giving it the name of GW190814. And it appears that the neutron star was not actually … a neutron star.
That would mean detection is the first of a different kind: the smallest black hole we’ve detected, narrowing the mysterious ‘mass gap’ between neutron stars and black holes. But, like most of the answers the Universe gives us, it opens a dozen more.
“This is going to change the way scientists talk about neutron stars and black holes,” said physicist Patrick Brady of the University of Wisconsin-Milwaukee and a spokesman for the LIGO Scientific Collaboration.
“In fact, the mass gap may not exist at all, but it may be due to limitations in observation capabilities. Time and more observations will tell.”
In 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 dead and collapsed cores of massive stars. For neutron stars, parent stars are about 8 to 30 times the mass of the Sun; They blow most of their mass before dying, and the nuclei collapse into objects of around 1.4 solar masses.
Meanwhile, progenitor stars of more than 30 solar masses collapse into black holes, with a wide range of masses.
Which brings us to the gap. We have never seen a pre-fusion object between particular upper and lower limits: a neutron star larger than about 2.3 solar masses, or a black hole smaller than 5 solar masses.
GW190814 has now delivered that item. Analysis of the gravitational wave signal has revealed that the larger of the two fused objects, interpreted as a black hole, had 23 solar masses. The smaller of the two had only 2.6 solar masses, nine times smaller than the other.
This mass means it could be the largest neutron star we’ve ever detected; or, much more likely, the smallest black hole.
“It is a challenge for current theoretical models to form fused pairs of compact objects with such a large mass ratio that the low-mass partner resides in the mass gap. This discovery implies that these events occur much more often than that we predicted, which makes this a really intriguing low-mass object, “explained astrophysicist Vicky Kalogera of Northwestern University in Illinois.
“The mysterious object may be a neutron star merging with a black hole, an exciting possibility theoretically expected but not yet confirmed by observation. However, at 2.6 times the mass of our Sun, it exceeds modern predictions for the maximum mass of stars neutron, and it may be the lightest black hole ever detected. “
The limit of neutron stars
The reason astronomers are not sure what lies in the mass gap is that it is 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 large that the external pressure of the neutrons can no longer repel each other against the internal pressure of gravity, and the object collapses into a black hole.
As our observations become more robust, the restrictions on the TOV limit for neutron stars are getting closer. Calculations generally place it somewhere between 2.2 and 2.4 solar masses; and data from GW170817, a 2017 neutron star merger that produced a post-merger black hole of 2.7 solar masses, have reduced it to around 2.3 solar masses.
Uncertainty about the smallest object in GW190814 arises from the maneuvering room at the TOV boundary, but according to the team’s analysis, if the calculation of solar mass 2.3 is taken, there is only a chance of about three percent that the object is a neutron star.
“GW190814 is probably not the product of a merger of neutron stars with black holes, despite its preliminary classification as such,” the researchers wrote in their article. “However, the possibility that the secondary component is a neutron star cannot be completely ruled out due to the current uncertainty in [the TOV limit]”
While a merger of neutron stars with black holes would have been super exciting, the fact that GW190814 probably turned out to have a small black hole is also truly amazing.
For one thing, the finding can now help astronomers limit the mass gap. And, most importantly, it throws our models of neutron star formation and binary systems into a big mess.
You see, astronomers think stellar-mass black holes are produced by really massive stars that turn into a supernova and collapse into a black hole. And we believe that neutron stars are formed in the same way. But theorists were producing training models that fit the mass gap; Now that a mass gap object has been found before the merger, those models will need to be reevaluated.
The other problem is the massive massive discrepancy. Most gravitational wave fusions detected to date involve two objects of roughly equal size. Earlier this year, scientists announced a merger of black holes with a mass ratio of about 3: 1, but GW190814 is much more extreme.
There are two main ways for binary systems to form. Or they are born together from the same piece of interstellar cloud, live together throughout their lives, and then die together; or they get together later in life. But it is really difficult 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 these extreme systems are not even that rare.
“All the common training channels have some deficiency,” astronomer Ryan Foley of the University of California, Santa Cruz told ScienceAlert. Foley was a member of the team that encountered the initial detection of GW190814 and was not involved in this new document.
“Is that the rate [of this kind of event] It is relatively high. [And] it’s not just that you have different masses by a factor of nine. It is also that one of them is in this mass gap. And one of them is very, very massive. So, all those things combined, I don’t think there is a good model that really solves those three problems separately. “
There is plenty in this detection to keep theorists busy for a while, reimagining those formation scenarios to determine how a system like GW190814 and its separate components can emerge, whether the smallest object is a neutron star or black hole .
As for discovering the latter, it will be a matter of more detections. LIGO is currently offline while undergoing updates. It is expected to come back online sometime next year, more responsive than ever, hoping to catch more events like GW190814, helping to solve some of the outstanding questions.
“This is the first glimpse of what a completely new population of compact binary objects might look like,” 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 beginning. As the detectors become increasingly sensitive, we will be looking even more closely at these signals and being able to identify populations of neutron stars and black holes in the Universe.” “
The research has been published in The letters of the astrophysical journal.