Gravitational Astronomy? How Detecting Gravitational Waves Changes Everything

We’ve now had multiple detections of gravitational waves, opening up a whole new field: gravitational astronomy. We talk about the detections made so far, and how we can see the Universe in a whole new way.

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Weekly Space Hangout – Mar 17, 2017: Stuart McNeill of the Intrepid Sea, Air & Space Museum

Host: Fraser Cain (@fcain) Special Guest: Stuart McNeill is the the Community Engagement specialist in charge of Family Programs and Demonstrations at the Intrepid Sea, Air & Space Museum. Check out their membership site here. Guests: Kimberly Cartier ( KimberlyCartier.org / @AstroKimCartier ) Paul M. Sutter (pmsutter.com / @PaulMattSutter) Their stories this week: The original […]

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Have We Really Just Seen The Birth Of A Black Hole?

Artist's drawing a black hole (Cygnus X-1) as it pulls matter from a blue star beside it.
Credits: NASA/CXC/M.Weiss

For almost half a century, scientists have subscribed to the theory that when a star comes to the end of its life-cycle, it will undergo a gravitational collapse. At this point, assuming enough mass is present, this collapse will trigger the formation of a black hole. Knowing when and how a black hole will form has long been something astronomers have sought out.

And why not? Being able to witness the formation of black hole would not only be an amazing event, it would also lead to a treasure trove of scientific discoveries. And according to a recent study by a team of researchers from Ohio State University in Columbus, we may have finally done just that.

The research team was led by Christopher Kochanek, a Professor of Astronomy and an Eminent Scholar at Ohio State. Using images taken by the Large Binocular Telescope (LBT) and Hubble Space Telescope (HST), he and his colleagues conducted a series of observations of a red supergiant star named N6946-BH1.

To break the formation process of black holes down, according to our current understanding of the life cycles of stars, a black hole forms after a very high-mass star experiences a supernova. This begins when the star has exhausted its supply of fuel and then undergoes a sudden loss of mass, where the outer shell of the star is shed, leaving behind a remnant neutron star.

This is then followed by electrons reattaching themselves to hydrogen ions that have been cast off, which causes a bright flareup to occur. When the hydrogen fusing stops, the stellar remnant begins to cool and fade; and eventually the rest of the material condenses to form a black hole.

However, in recent years, several astronomers have speculated that in some cases, stars will experience a failed supernova. In this scenario, a very high-mass star ends its life cycle by turning into a black hole without the usual massive burst of energy happening beforehand.

As the Ohio team noted in their study – titled “The search for failed supernovae with the Large Binocular Telescope: confirmation of a disappearing star” – this may be what happened to N6946-BH1, a red supergiant that has 25 times the mass of our Sun located 20 million light-years from Earth.

Using information obtained with the LBT, the team noted that N6946-BH1 showed some interesting changes in its luminosity between 2009 and 2015 – when two separates observations were made. In the 2009 images, N6946-BH1 appears as a bright, isolated star. This was consistent with archival data taken by the HST back in 2007.

However, data obtained by the LBT in 2015 showed that the star was no longer apparent in the visible wavelength, which was also confirmed by Hubble data from the same year. LBT data also  showed that for several months during 2009, the star experienced a brief but intense flare-up, where it became a million times brighter than our Sun, and then steadily faded away.

They also consulted data from the Palomar Transit Factory (PTF) survey for comparison, as well as observations made by Ron Arbour (a British amateur astronomer and supernova-hunter). In both cases, the observations showed evidence of a flare during a brief period in 2009 followed by a steady fade.

In the end, this information was all consistent with the failed supernovae-black hole model. As Prof. Kochanek, the lead author of the group’s paper – – told Universe Today via email:

“In the failed supernova/black hole formation picture of this event, the transient is driven by the failed supernova. The star we see before the event is a red supergiant — so you have a compact core (size of ~earth) out the hydrogen burning shell, and then a huge, puffy extended envelope of mostly hydrogen that might extend out to the scale of Jupiter’s orbit.  This envelope is very weakly bound to the star.  When the core of the star collapses, the gravitational mass drops by a few tenths of the mass of the sun because of the energy carried away by neutrinos.  This drop in the gravity of the star is enough to send a weak shock wave through the puffy envelope that sends it drifting away.  This produces a cool, low-luminosity (compared to a supernova, about a million times the luminosity of the sun) transient that lasts about a year and is powered by the energy of recombination.  All the atoms in the puffy envelope were ionized — electrons not bound to atoms — as the ejected envelope expands and cools, the electrons all become bound to the atoms again, which releases the energy to power the transient.  What we see in the data is consistent with this picture.”

Naturally, the team considered all available possibilities to explain the sudden “disappearance” of the star. This included the possibility that the star was shrouded in so much dust that its optical/UV light was being absorbed and re-emitted. But as they found, this did not accord with their observations.

“The gist is that no models using dust to hide the star really work, so it would seem that whatever is there now has to be much less luminous then that pre-existing star.” Kochanek explained. “Within the context of the failed supernova model, the residual light is consistent with the late time decay of emission from material accreting onto the newly formed black hole.”

Naturally, further observations will be needed before we can know whether or not this was the case. This would most likely involve IR and X-ray missions, such as the Spitzer Space Telescope and the Chandra X-ray Observatory, or one of he many next-generation space telescopes to be deployed in the coming years.

In addition, Kochanek and his colleagues hope to continue monitoring the possible black hole using the LBT, and by re-visiting the object with the HST in about a year from now. “If it is true, we should continue to see the object fade away with time,” he said.

Needless to say, if true, this discovery would be an unprecedented event in the history of astronomy. And the news has certainly garnered its share of excitement from the scientific community. As Avi Loeb – a professor of astronomy at Harvard University – expressed to Universe Today via email:

“The announcement on the potential discovery of a star that collapsed to make a black hole is very interesting. If true, it will be the first direct view of the delivery room of a black hole. The picture is somewhat messy (like any delivery room), with uncertainties about the properties of the baby that was delivered. The way to confirm that a black hole was born is to detect X-rays. 

“We know that stellar-mass black holes exist, most recently thanks to the discovery of gravitational waves from their coalescence by the LIGO team. Almost eighty years ago Robert Oppenheimer and collaborators predicted that massive stars may collapse to black holes. Now we might have the first direct evidence that the process actually happens in nature.

But of course, we must remind ourselves that given its distance, what we could be witnessing with N6946-BH1 happened 20 million years ago. So from the perspective of this potential black hole, its formation is old news. But to us, it could be one of the most groundbreaking observations in the history of astronomy.

Much like space and time, significance is relative to the observer!

Further Reading: arXiv

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Can We Now Predict When A Neutron Star Will Collapse?

New research indicates that it may now be possible to predict when a neutron star will collapse to form a new black hole. Credit and Copyright: Paramount Pictures/Warner Bros.

A neutron star is perhaps one of the most awe-inspiring and mysterious things in the Universe. Composed almost entirely of neutrons with no net electrical charge, they are the final phase in the life-cycle of a giant star, born of the fiery explosions known as supernovae. They are also the densest known objects in the universe, a fact which often results in them becoming a black hole if they undergo a change in mass.

For some time, astronomers have been confounded by this process, never knowing where or when a neutron star might make this final transformation. But thanks to a recent study by a team of researchers from Goethe University in Frankfurt, Germany, it may now be possible to determine the absolute maximum mass that is required for a neutron star to collapse, giving birth to a new black hole.

As with everything else relating to neutron stars, the process by which they become black holes has long been a source of fascination and bewilderment for astronomers. As the densest of all objects in the known universe, their mass cannot grow without bound – meaning that any increase in mass will also cause an increase in their density.

Normally, this process will cause a neutron star to simply achieve a new state of equilibrium, or will result in a non-rotating neutron star beginning to  spin. This latter effect will allow it to remain stable for longer than it could otherwise, since the additional centrifugal force can help to balance out the intense gravitational force at work in its interior.

However, even this process cannot last forever. As Professor Luciano Rezolla of Goethe University told Universe Today via email:

“If the star is nonrotating, then this mass is not too difficult to compute and is called the maximum nonrotating mass, or M_TOV. However, this is not the largest mass possible because if the star is rotating, it can sustain more mass than if is not rotating. Even in this case, however, there is a limit because there is a limit to how much a star can rotate before being broken apart from the centrifugal force. Hence, the absolute largest mass that a neutron star can achieve is known as the “maximum mass of a maximally rotating configuration”, M_max.  This is the largest possible mass of the most rapidly rotating model. Suppose you have built such a model: if you added a single atom onto it, it would collapse to a black hole, while it would break apart if you spun it a bit more.”

As neutron stars accumulate mass, the speed of their rotation will increase; and here too, there is a limit. Basically, sooner or later, a neutron star will reach its absolute maximum mass and beyond this, it will inevitably collapse in on itself to become a black hole. Unfortunately, in the past, astronomers have had a hard time determining what the value of this limit was.

The reason for this is because such a maximum value is dependent on the equation of state of the matter composing the star. This thermodynamic equation describes the state of matter under a given set of physical conditions – i.e. temperature, pressure, volume, or internal energy. And while astronomers have been able to ascertain within a degree of certainty what the maximum mass of a nonrotating neutron stars would be, they have been less successful in calculating what the maximum mass is for those that are rotating.

In short, they have been unable to determine how much mass is needed before a rotating neutron star will surpass its maximum speed of rotation and finally form a new black hole. As Rezolla explained:

“What made it difficult in the past to calculate M_max is its value will differ from what composes the neutron star (i.e. its “equation of state”) and this is something we don’t really know. Neutron-star matter is so different from the one we know that we can only make educated guesses; and unfortunately, there are many guesses because there are several different ways to compute the properties of the equation of state. So one ended up up with a situation in which not only the maximum mass was different for different equations of state, but even the maximum rotation speed was different for different equations of state.”

However, in their study, titled “Maximum mass, moment of inertia and compactness of relativistic stars” – which appeared recently in the Monthly Notices of the Royal Astronomical SocietyRezzolla and Cosima Breu (a Masters student in theoretical physics at Goethe University and co-author of the study) argue that it may now be possible to infer what the maximum mass of a rapidly rotating star would be.

For the sake of their research, Rezolla and Breu relied on recent work by astronomers that has shown that it is possible to express the properties of stellar equilibrium configurations that does not depend on the specific equation of the state of their mass. In short, these studies have shown that there are certain “universal relations” when it comes to the equilibrium of stars.

As a result, they were able to show that it is possible to predict the maximum mass a rapidly rotating neutron star can attain by simply considering what the maximum mass is of a neutron star in a corresponding, non-rotating configuration. But as Rezolla indicated, even with these data sets available, what was needed was a fresh perspective:

“Universal relations simply state that objects that are apparently different actually share many things in common. For example, although we are different from other mammals, say pigs, our genome has a huge amount of common features, essentially because we have to synthesize the same proteins, breath the same air, etc. Hence, if we learn of hemoglobin actually works for one mammal, we have learned for many more mammals. This seems to happen also for neutron stars so that although there are many equations of state that predict different results for M_max, they all show there is a universal relation between M_max and M_TOV. More specifically, we have found that M_max = 1.203 +- 0.022 M_TOV.”

These findings are likely to have interesting implications when it comes to future astronomical research. For starters, knowing the maximum mass a neutron star can achieve will be useful when analyzing the gravitational-wave signals produced by neutron stars, allowing astronomers to extract information on the equation of state before the object collapses into a black hole.

Second, it will be useful in determining the moment of inertia for neutron stars, i.e. knowing how much mass is required before it begins to rotate. In short, scientists will be able to know with greater accuracy what it takes to set a neutron star to spinning and will able to predict with greater accuracy when a spinning neutron star will be on the verge of collapsing, and thus knowing when and where a new black hole will be.

Al this, in turn, is likely to be a boon for research into black holes, the one object in the universe that is arguably more awe-inspiring and less understood than neutron stars. One step closer to understanding this grand, mysterious thing known as the Universe!

Further Reading: phys.org

 

 

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18 Billion Solar Mass Black Hole Rotates At 1/3 Speed Of Light

Black-hole-powered galaxies called blazars are the most common sources detected by NASA's Fermi Gamma-ray Space Telescope. As matter falls toward the supermassive black hole at the galaxy's center, some of it is accelerated outward at nearly the speed of light along jets pointed in opposite directions. When one of the jets happens to be aimed in the direction of Earth, as illustrated here, the galaxy appears especially bright and is classified as a blazar. Credits: M. Weiss/CfA

Way up in the constellation Cancer there’s a 14th magnitude speck of light you can claim in a 10-inch or larger telescope. If you saw it, you might sniff at something so insignificant, yet it represents the final farewell of chewed up stars as their remains whirl down the throat of an 18 billion solar mass black hole, one of the most massive known in the universe.

Astronomers know the object as OJ 287, a quasar that lies 3.5 billion light years from Earth. Quasars or quasi-stellar objects light up the centers of many remote galaxies. If we could pull up for a closer look, we’d see a brilliant, flattened accretion disk composed of heated star-stuff spinning about the central black hole at extreme speeds.

As matter gets sucked down the hole, jets of hot plasma and energetic light shoot out perpendicular to the disk. And if we’re so privileged that one of those jet happens to point directly at us, we call the quasar a “blazar”. Variability of the light streaming from the heart of a blazar is so constant, the object practically flickers.

A recent observational campaign involving more than two dozen optical telescopes and NASA’s space based SWIFT X-ray telescope allowed a team of astronomers to measure very accurately the rotational rate the black hole powering OJ 287 at one third the maximum spin rate — about 56,000 miles per second (90,000 kps) —  allowed in General Relativity  A careful analysis of these observations show that OJ 287 has produced close-to-periodic optical outbursts at intervals of approximately 12 years dating back to around 1891. A close inspection of newer data sets reveals the presence of double-peaks in these outbursts.

To explain the blazar’s behavior, Prof. Mauri Valtonen of the University of Turku (Finland) and colleagues developed a model that beautifully explains the data if the quasar OJ 287 harbors not one buy two unequal mass black holes — an 18 billion mass one orbited by a smaller black hole.

OJ 287 is visible due to the streaming of matter present in the accretion disk onto the largest black hole. The smaller black hole passes through the larger’s the accretion disk during its orbit, causing the disk material to briefly heat up to very high temperatures. This heated material flows out from both sides of the accretion disk and radiates strongly for weeks, causing the double peak in brightness.

The orbit of the smaller black hole also precesses similar to how Mercury’s orbit precesses. This changes when and where the smaller black hole passes through the accretion disk.  After carefully observing eight outbursts of the black hole, the team was able to determine not only the black holes’ masses but also the precession rate of the orbit. Based on Valtonen’s model, the team predicted a flare in late November 2015, and it happened right on schedule.

The timing of this bright outburst allowed Valtonen and his co-workers to directly measure the rotation rate of the more massive black hole to be nearly 1/3 the speed of light. I’ve checked around and as far as I can tell, this would make it the fastest spinning object we know of in the universe. Getting dizzy yet?

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If You’re Going to Fall Into a Black Hole, Make Sure It’s Rotating

In "Interstellar" Matthew McConaughey saves the day by traveling into a black hole. New research suggests this could be possible. (Image © Paramount Pictures/Warner Bros.)

It’s no secret that black holes are objects to be avoided, were you to plot yourself a trip across the galaxy. Get too close to one and you’d find your ship hopelessly caught sliding down a gravitational slippery slope toward an inky black event horizon, beyond which there’s no escape. The closer you got the more gravity would yank at your vessel, increasingly more on the end closest to the black hole than on the farther side until eventually the extreme tidal forces would shear both you and your ship apart. Whatever remained would continue to fall, accelerating and stretching into “spaghettified” strands of ship and crew toward—and across—the event horizon. It’d be the end of the cosmic road, with nothing left of you except perhaps some slowly-dissipating “information” leaking back out into the Universe over the course of millennia in the form of Hawking radiation. Nice knowin’ ya.

 

That is, of course, if you were foolish enough to approach a non-spinning black hole.* Were it to have a healthy rotation to it there’s a possibility, based on new research, that you and your ship could survive the trip intact.

 

A team of researchers from Georgia Gwinnett College, UMass Dartmouth, and the University of Maryland have designed new supercomputer models to study the exotic physics of quickly-rotating black holes, a.k.a. Kerr black holes, and what might be found in the mysterious realm beyond the event horizon. What they found was the dynamics of their rapid rotation create a scenario in which a hypothetical spacecraft and crew might avoid gravitational disintegration during approach.

 

“We developed a first-of-its-kind computer simulation of how physical fields evolve on the approach to the center of a rotating black hole,” said Dr. Lior Burko, associate professor of physics at Georgia Gwinnett College and lead researcher on the study. “It has often been assumed that objects approaching a black hole are crushed by the increasing gravity. However, we found that while gravitational forces increase and become infinite, they do so fast enough that their interaction allows physical objects to stay intact as they move toward the center of the black hole.”

 

Read more: 10 Amazing Facts About Black Holes

 

Because the environment around black holes is so intense (and physics inside them doesn’t play by the rules) creating accurate models requires the latest high-tech computing power.

 

“This has never been done before, although there has been lots of speculation for decades on what actually happens inside a black hole,” said Gaurav Khanna, Associate Physics Professor at UMass Dartmouth, whose Center for Scientific Computing & Visualization Research developed the precision computer modeling necessary for the project.

 

 

Like science fiction movies have imagined for decades—from Disney’s The Black Hole to Nolan’s Interstellar—it just might be possible to survive a trip into a black hole, if conditions are right (i.e., you probably still don’t want to find yourself anywhere near one of these.)

 

Of course, what happens once you’re inside is still anyone’s guess…

 

The team’s paper “Cauchy-horizon singularity inside perturbed Kerr black holes” was published in the Feb. 9, 2016 edition of Rapid Communication in Physical Review D. You can find the full text here. The research was supported by the National Science Foundation.

 

Sources: UMass Dartmouth and Georgia Gwinnett College

 

*A true non-rotating “Schwarzschild” black hole would not, due to angular momentum etc., be readily found in the real world, thus making this research on rotating black holes all the more essential.

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