“This will change the way scientists speak of neutron stars and black holes,” said the physicist Patrick Brady, of the University of Wisconsin-Milwaukee, and the LIGO Scientific Collaboration spokesperson.
“The mass of the gap may in fact not exist at all, but may have been due to limitations in observation capabilities. Time and more observations will tell. “
In the mass of the gap
The mass deviation is a curious exception in our detections of black holes and neutron stars. These two types of objects are collapsed, the death of the cores of massive stars. For neutron stars, the progenitor stars are around 8 to 30 times the mass of the Sun; they blow most of their mass before they die, and the cores collapse down to the objects of 1.4 solar masses.
During this time, the progenitor stars of more than about 30 solar masses collapse down into black holes, with a wide range of masses.
This brings us to the gap. We have never seen a pre-merge of the object between the upper and lower limits – a neutron star more than about 2.3 solar masses, or a black hole of less than 5 solar masses.
GW190814 presented this item. The analysis of the gravitational wave signal revealed that the larger of the two merging objects is interpreted as a black hole – is 23 solar masses. The smaller of the two was 2.6 solar masses, nine times smaller than the other.
This mass meant that it could be the largest neutron star, we have never detected; or, much more likely, the lesser black hole.
“It is a challenge for current theoretical models to form the fusion of pairs of compact objects with such a mass ratio in which the low-mass partner lies in the mass gap. This discovery implies that these events occur much more often than we predicted, which makes it a very intriguing low-mass of the object “, explains the astrophysicist Vicky Kalogera of Northwestern University in Illinois.
“The mystery of the object may be a neutron star merging with a black hole, an exciting possibility should, in theory, but not yet confirmed by observation. However, at 2.6 times the mass of our Sun, it exceeds modern estimates for the maximum mass of neutron stars, and may be the lightest black hole ever detected. “
The limit on the neutron stars
The reason astronomers are not sure what lies in the mass gap, is that it is very 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 huge, 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 growth more robust, the constraints on the TOV limit for neutron stars are closing in. The calculations usually put it somewhere between 2.2 and 2.4 solar masses and the data GW170817 – 2017 neutron star merger produces a post-fusion mass gap black hole of 2.7 solar masses – have narrowed it down to about 2.3 solar masses.
The uncertainty on the smallest object in GW190814 arises from the margin of manoeuvre in the TOV limit – but, according to the analysis team, if the 2.3 solar mass calculation is taken, there is a chance of close to three percent that the object is a neutron star.
“GW190814 is probably not the product of a neutron star-black hole coalescence, in spite of its preliminary classification as such,” the researchers wrote in their paper. “Nevertheless, the possibility that the second component is a neutron star cannot be completely ruled out 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 probably turned out to have a tiny black hole is really awesome, too.
For one, can now help astronomers to constrain the mass of the gap. And, most importantly, he casts our models of the formation of two neutron stars and binary systems in quite a disarray.
You see, the 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 that neutron stars form in the same way. But the theorists of the summer the production of the formation of a model that fits around the mass of the gap; now that pre-merger variance of mass of the object has been found, these models need to be reevaluated.
The other problem is the huge mass difference. Most of the gravitational wave mergers detected to date contain two objects of more or less of equal size. Earlier this year, scientists announced a black hole merger with a mass ratio of about 3:1, but GW190814 is more extreme.
There are two main methods for binary systems to form. Either they are born together of the same piece of interstellar cloud, live together for their entire life, and then die together, or they come together later in life. But it is really difficult for these binary formation models to produce systems with mass ratios.
And the fact that GW190814 has been detected in only a few years after the first gravitational wave detection in 2015 implies that the extreme, such systems are not even rare.
“All the training in the common channels have certain deficiencies,” the astronomer Ryan Foley of the University of California, Santa Cruz, told ScienceAlert. Foley was a member of the team that found the first GW190814 detection, and was not involved in this new book.
“It is that the rate of [of this kind of event] is relatively high. [And] it is not just that you have masses that are different by a factor of nine. It is also one of them is in this mass gap. And one of them is really, really huge. Therefore, all of these things combined, I don’t think there is a good model that really solves these three separate questions. ”
There is a lot in this a the detection of the theorists busy for some time, re-imagining those of the training scenarios in order to determine how a system such as GW190814, and its components, may come to be – if the smaller object is a neutron star or a black hole.
To understand this last, which will be a subject of more detections. LIGO is currently offline while it undergoes an upgrade. It is expected to come back online during the next year, more sensitive than ever, we hope to detect more events like GW190814, which will help you to resolve some of the outstanding issues.
“This is the first glimpse of what could be a new population of compact binary objects,” said the astrophysicist Charlie Hoy of the LIGO Scientific Collaboration and the University of Cardiff in the united KINGDOM.
“What is really exciting is that this is just the beginning. As the detectors more sensitive, we’re going to see even more of these signals, and we will be able to identify populations of neutron stars and black holes in the Universe. ”
The research has been published in The Astrophysical Journal Letters.