A new black hole breaks the record––not for being the smallest or the biggest––but for being right in the middle.
The recently discovered ‘Goldilocks’ black hole is part of a missing link between two populations of black holes: small black holes made from stars and supermassive giants in the nucleus of most galaxies.
In a joint effort, researchers from the University of Melbourne and Monash University––including OzGrav Chief Investigator Eric Thrane––have uncovered a black hole approximately 55,000 times the mass of the sun, a fabled “intermediate-mass” black hole.
The discovery was published today in the paper Evidence for an intermediate mass black hole from a gravitationally lensed gamma-ray burst in the journal Nature Astronomy.
Lead author and University of Melbourne PhD student, James Paynter, said the latest discovery sheds new light on how supermassive black holes form. “While we know that these supermassive black holes lurk in the cores of most, if not all galaxies, we don’t understand how these behemoths are able to grow so large within the age of the Universe,” he said.
The new black hole was found through the detection of a gravitationally lensed gamma-ray burst.
The gamma-ray burst, a half-second flash of high-energy light emitted by a pair of merging stars, was observed to have a tell-tale ‘echo’. This echo is caused by the intervening intermediate-mass black hole, which bends the path of the light on its way to Earth, so that astronomers see the same flash twice.
Powerful software developed to detect black holes from gravitational waves was adapted to establish that the two flashes are images of the same object.
“This newly discovered black hole could be an ancient relic––a primordial black hole––created in the early Universe before the first stars and galaxies formed,” said study co-author Eric Thrane.
“These early black holes may be the seeds of the supermassive black holes that live in the hearts of galaxies today.”
The researchers estimate that some 46,000 intermediate mass black holes are in the vicinity of our Milky Way galaxy.
This article is an edited version of the original media release produced by Lito Vilisoni Wilson at the University of Melbourne.
Also featured in CNet , Cosmos magazine, New Scientist, SciTech Daily, The Independent, Sky News and Space.com
THE EXOTIC LIVES OF BOSON STARS
There are some stars that just don’t look at all like stars. Rather than being composed nearly entirely of hydrogen and helium like other stars, they consist of matter that has no electromagnetic signature. These stars are hypothetical because we’re still refining the techniques to find them.
Boson stars are just one in this family of objects, known as exotic stars, and they’re composed almost entirely of bosons. And what is a boson? It’s one of the two types of fundamental particles, the one that carries forces. The other, the fermion, is what makes up ‘normal’ stars, and all the other matter that we see.
There are a variety of boson stars and sometimes they’re categorised to reflect the type of boson that they are made of. For example, Proca stars are vector boson stars, meaning that their constituent bosons have a spin of one. They’re also unique amongst boson stars because the stars themselves can spin without being disrupted.
A boson star would most likely be shaped like an enormous donut because of the centrifugal forces acting on the bosonic matter, and, bizarrely, they’d be transparent; any matter absorbed by them would be visible at their centres. If boson stars do exist, they might provide the evidence we need for a long sought-after dark matter particle. That’s because the said particle, the axion, is a boson. And we’ve been searching (unsuccessfully) for axions in numerous experiments on Earth for decades.
After the initial excitement of the first-ever observation of an intermediate-mass black hole—gravitational-wave event GW190521—it was quickly realised that the very existence of such an object was not consistent with any of our stellar models. Perhaps it was itself a product of previous, smaller, black hole collisions, or maybe there was something else at play.
Thus, the challenge for scientists was to come up with a theory that could explain the presence of the intermediate-mass black hole progenitor of GW190521, while still being consistent with the original signal. And by assuming that it was caused by merging boson stars, rather than black holes, an international team of scientists, led by OzGrav alumnus Dr Juan Calderón Bustillo at the University of Santiago de Compostela and Dr Nicolás Sanchis-Gual at the University of Lisbon, might have been able to do just that.
Apart from the problems associated with the pair-instability mass gap, any potential hypothesis needed to explain something a bit unusual about the GW190521 signal. Normally gravitational waves that originate in merging binary systems oscillate at higher and higher frequencies as the two progenitors spiral in towards each other. But for GW190521, the inspiral signal before the merger was barely detectable. An extremely abbreviated inspiral could perhaps be explained if two black holes collided head-on rather than by circling into each other, and so that is the first thing that Dr Bustillo and Dr Sanchis-Gual’s team looked at. What they found didn’t help much.
‘We first tried to fit the data to head-on collisions of black holes, but these happen to produce a final black hole whose spin is too low to reproduce the GW190521 signal. The reason is that the lack of an inspiral diminishes a lot of the spin of the final black hole, and the individual spins of the black holes, which also contribute to the spin of the final one, are bounded by a limit called the Kerr limit,’ says Dr Bustillo.
That’s when the team started looking at boson stars, or Proca stars to be exact. They compared the GW190521 signal to computer simulations of Proca star mergers and found that statistically they were a considerably better fit to the data than when it was assumed that the progenitors were black holes.
‘First, we would not be talking about colliding black holes anymore, which eliminates the issue of dealing with a forbidden black hole,’ explains Dr Bustillo. ‘Second, because boson star mergers are much weaker, we infer a much closer distance than the one estimated by LIGO and Virgo. This leads to a much larger mass for the final black hole, of about 250 solar masses, so the fact that we have witnessed the formation of an intermediate-mass black hole remains true.’
This is an exciting result as the final black hole formed by the merger in this case would have to be about 62% larger than previously thought. And rather than the signal originating from a point that is now some 17-billion light-years from us, it would have been just over 1.8-billion light-years away.
‘Of course, there are potentially many ways in which this event may be explained, as this is an event for which we have very little information about what produced the final black hole we observe. The best we can say right now is that the data tells us that a collision of Proca stars is approximately 8 times more likely than the black hole collision scenario.’
And what of the implications of discovering the first boson stars?
‘That would be dramatic,’ says Dr Bustillo. ‘Boson stars and their building blocks—the ultralight bosons—are one of the most solid candidates for forming what we know as dark matter. If our result is further confirmed by future observations, it would represent the first actual measurement of the particle responsible for dark matter.’
‘Gravitational-wave astronomy is still very much in its infancy,’ says Dr Rory Smith—an OzGrav researcher from Monash University and one of the collaborators in this research. ‘However, the fact that we’re already starting to draw connections between gravitational-wave observations and fundamental particle physics is a remarkable sign of how powerful this new field is. Even if future observations rule out boson stars as real astronomical objects, we should expect many new and exciting discoveries in the future.’
This is an edited extract from the original article featured in Space Times, written by Dan Lambeth.
RESEARCH BRIEF: Simulating the complicated history of Eta Carinae - Ryosuke Hirai, Monash University
‘Eta Carinae’ is an extraordinary star that has fascinated mankind for decades. It’s surrounded by an expanding ‘Homunculus nebula’, shaped like an hourglass. This nebula was expelled in an energetic explosion called the ‘Great Eruption’ that occurred in the 1840s, when Eta Carinae became the second brightest star in the sky and was visible to the naked eye for over a decade.
There are other clusters of bullets outside the Homunculus nebula, that were shot out several centuries before the Great Eruption. Eta Carinae itself is extremely massive with a mass more than 100 times the Sun and is orbited by another star that has a mass about 30 times the Sun on a highly eccentric orbit.
With all these and many more peculiar features, scientists have been puzzled for a long time on how the star exploded and created the surrounding messy nebula.
Out of many other proposed models, our recently published study focused on one hypothesis that the star system used to be a triple system that eventually became unstable and caused a stellar merger. As more detailed observations are made, this scenario is becoming increasingly popular but has lacked detailed theoretical investigations so far.
In this work we performed the first comprehensive set of detailed theoretical calculations for this scenario. We first carried out three-body dynamical simulations to see how a triple system becomes unstable and eventually two of the stars collide. We started with a stable system in which one star is in a wide orbit around the other two stars which are in close orbit. Once the most massive star approaches the end of its life, it expands and starts transferring matter to its companion. This makes the system unstable and causes two of the stars to merge within a few thousand years. We found that before the final merger, the stars can wildly swap their positions and encounter each other at close distances, grazing each other’s surfaces.
We carried out additional N-body simulations to see how a star responds to these close encounters. Part of the surface material can be peeled off and sent away as narrow sprays. Combining the orbital dynamics and close-encounter simulations, we found that the multiple grazing encounters—that occur centuries before the merger—can reproduce the messy structure outside the Homunculus nebula.
We also carried out hydrodynamical simulations to see how the outflow from the stellar merger is shaped into the hourglass shape we see today. We proposed a new scenario that takes similar ideas for how the triple-ring nebula for the supernova SN1987A was formed. As the stars merge, a huge amount of energy is released inside the star, causing the Great Eruption. But unlike supernovae, a large fraction of the energy and mass remains in the star. This energy slowly leaks out over the following century as strong bipolar winds. The wind sweeps up the inner parts of the explosion ejecta and forms a hollow shell-like structure. Our simulations show that with this scenario, we can closely reproduce the shape and size of the Homunculus nebula.
Our combination of simulations successfully reproduces the main features of Eta Carinae’s surrounding nebula and provides strong support to the stellar-merger-in-a-triple scenario. This not only gives us insight into the origin of Eta Carinae, but also many other astronomical objects that can be created through mergers in triple systems. For example, some massive black holes found by LIGO (GW190521) are considered to have been created this way. Using the rich information from Eta Carinae, we can learn much more about the formation of exotica in our Universe.
Written by OzGrav researcher Ryosuke Hirai, Monash University
An artist’s impression of the Cygnus X-1 system. This system contains the most massive stellar-mass black hole ever detected without the use of gravitational waves, weighing in at 21 times the mass of the Sun. Credit: International Centre for Radio Astronomy Research. Credit: International Centre for Radio Astronomy Research.
New observations of the first black hole ever detected have led astronomers to question what they know about the Universe’s most mysterious objects. Published in the journal Science, the research shows the system known as Cygnus X-1 contains the most massive stellar-mass black hole ever detected without the use of gravitational waves.
Cygnus X-1 is one of the closest black holes to Earth. It was discovered in 1964 when a pair of Geiger counters were carried on board a sub-orbital rocket launched from New Mexico. The object was the focus of a famous scientific wager between physicists Stephen Hawking and Kip Thorne, with Hawking betting in 1974 that it was not a black hole. Hawking conceded the bet in 1990. In this latest work, an international team of astronomers used the Very Long Baseline Array—a continent-sized radio telescope made up of 10 dishes spread across the United States—together with a clever technique to measure distances in space.
OzGrav Chief Investigator and study co-author Prof Ilya Mandel, from Monash University, says the black hole is so massive it’s actually challenging how astronomers thought they formed. ‘Stars lose mass to their surrounding environment through stellar winds that blow away from their surface. But to make a black hole this heavy, we need to dial down the amount of mass that bright stars lose during their lifetimes,’ says Prof Mandel. ‘The black hole in the Cygnus X-1 system began life as a star approximately 60 times the mass of the Sun and collapsed tens of thousands of years ago,’ he says. ‘Incredibly, it’s orbiting its companion star—a supergiant—every five and a half days at just one-fifth of the distance between the Earth and the Sun. These new observations tell us the black hole is more than 20 times the mass of our Sun—a 50 per cent increase on previous estimates.’
Second study author Dr Arash Bahramian from the Curtin University node of the International Centre for Radio Astronomy Research (ICRAR) says this was an exciting discovery, resulting from a collaboration between astronomers focused on different observational and theoretical aspects of black holes, coming together for a new extensive and rigorous look at a known but previously elusive black hole. ‘It is exciting that we can measure so precisely so many aspects of the system, like its distance from us, its motion and speed through the Galaxy, and the binary motion of the black hole and the star around each other,’ says Dr Bahramian. ‘Our new distance estimate caused an interesting domino effect, leading us to new measurements for the mass and spin of the black hole, which in turn led to fascinating new insights about how stars evolve and how black holes form.’
Lead researcher James Miller-Jones also from ICRAR says over six days the researchers observed a full orbit of the black hole and used observations taken of the same system with the same telescope array in 2011. ‘This method and our new measurements show the system is further away than previously thought, with a black hole that’s significantly more massive,’ says Prof Miller-Jones.
In a separate but related development University of Birmingham PhD candidate Coenraad Neijssel, affiliated with OzGrav and Monash, led a companion paper to this work simultaneously published in the Astrophysical Journal. ‘Using the updated measurements of the system properties, we were able to unwind the previous history of the binary as well as predict its future,’ says Coenraad. ‘Precise observations like this are critical for improving our understanding of the evolution of massive stars.
This article is an edited of the original media release written by Silvia Dropulich at Monash University Media Office. Also featured in the New York Times and The Daily Mail.