# Why would a black hole have a disk, but not emit x-rays?

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There's a recent paper in Nature about LB-1, a B-class star orbiting a massive black hole.

I don't understand how these two parts of the paper can be reconciled. On page 2, the authors argue that the broad $$Halpha$$ line indicates there's a disk in the system, and then that the disk is around the black hole (as opposed to the star or the entire binary).

This supports the $$Halpha$$ emission line not coming from a circumbinary disk, but from a disk around the black hole.

However in the last paragraph, they also say that this system doesn't emit X-rays:

Unlike every other known stellar black hole, LB-1 has not been detected in X-ray observations. We searched for X-ray emission from this system with a 10-ks observation with the Chandra X-ray Observatory, placing an upper limit for the X-ray luminosity of $$geq 2 × 10^{31} { m erg s}^{−1}$$ (see Methods). This upper limit corresponds to about $$10^{−9}$$ of its Eddington luminosity, and suggests a mass accretion rate ̇$$M leq 10^{-11} M_{odot} { m yr}^{-1}$$ for a conversion efficiency of approximately $$10^{−4}$$ at such low luminosity.

Naively it seems to me that this non-detection is actually an argument that the $$Halpha$$ line isn't coming from a disk around the black hole. If there's a disk around a black hole, it's presumably accreting. If it's accreting, the material presumably gets hot and produces X-rays. If it's producing X-rays, Chandra should have detected them.

What am I missing?

the X rays they emit. How can X rays be coming from a black hole? The X rays come from a highly compressed region in an accretion disk outside the event horizon of the black hole. spaceship and falls toward the black hole.

Hawking radiation is black-body radiation that is predicted to be released by black holes, due to quantum effects near the black hole event horizon. The escaping photon adds an equal amount of positive energy to the wider universe outside the black hole.

## Black hole that doesn't emit x-rays discovered near massive star

Trailed intensity image of the two lines constructed from the phase binned spectra. Two orbital cycles are displayed for clarity. The colour scale indicates counts normalized to the continuum, with the black colour corresponding to 0.98 and the white colour to 1.08 in Fe II and 1.16 in HeII. Credit: Nature 505, 378–381 (16 January 2014) doi:10.1038/nature12916

(Phys.org) —Researchers in Spain have discovered a black hole that doesn't reveal itself through x-ray radiation thrown off by material that is being sucked into it. In their paper published in the journal Nature, team members from several research institutions throughout Spain, report that the black hole appears to exist as a companion (binary) to a massive Be star that spins so fast it's surrounded by a gas disk.

Up until now, virtually all black holes have been discovered via x-ray radiation signals—as material is pulled in past the point of no return, radiation is flung out into space where it is noted by space scientists here on Earth. In this new effort, the research team was able to identify the black hole because of its behavior, rather than its signature.

Many Be stars have been found to have companions—most of the time they are supernova remnants (neutron stars) but never before has a Be star been found to have a black hole as a companion. The star, named MWC 656 is really big—approximately 10 to 16 times as massive as our sun. It spins really fast too (approximately 671,000 mph) which the researchers say, explains why the black hole next to it doesn't emit any radiation. They suggest that because the star is spinning so fast, it casts gas into a disk surrounding its equator which in turn is cast off towards the black hole, but rather than being pulled in, the gas joins an accretion disk that surrounds the "mouth" of the black hole, moving so fast (due to the angular momentum of the gas cast off from the star) that it can't be pulled in. Thus the disk simply continues to grow larger.

The black hole is pretty big too (approximately 3.8 to 6.9 more massive than our sun) which likely puts it in the category of stellar mass black holes—those that come into existence when a star runs out of fuel.

Animation of the system MWC 656. The Be star spins at extremely high speed, ejecting matter through an equatorial disc. Part of this matter falls on to the black hole forming an accretion disc. Animation: Gabriel Pérez - SMM (IAC). On the photo, UB researchers, Marc Ribó and Josep M. Paredes, who have participated in the research.

The discovery of the "silent" black hole suggests that many more like it might exist, which will undoubtedly lead researchers to look for more, now that they know what to look for.

Abstract
Stellar-mass black holes have all been discovered through X-ray emission, which arises from the accretion of gas from their binary companions (this gas is either stripped from low-mass stars or supplied as winds from massive ones). Binary evolution models also predict the existence of black holes accreting from the equatorial envelope of rapidly spinning Be-type stars (stars of the Be type are hot blue irregular variables showing characteristic spectral emission lines of hydrogen). Of the approximately 80 Be X-ray binaries known in the Galaxy, however, only pulsating neutron stars have been found as companions. A black hole was formally allowed as a solution for the companion to the Be star MWC 656 ( also known as HD 215227), although that conclusion was based on a single radial velocity curve of the Be star, a mistaken spectral classification6 and rough estimates of the inclination angle. Here we report observations of an accretion disk line mirroring the orbit of MWC 656. This, together with an improved radial velocity curve of the Be star through fitting sharp Fe II profiles from the equatorial disk, and a refined Be classification (to that of a B1.5–B2 III star), indicates that a black hole of 3.8 to 6.9 solar masses orbits MWC 656, the candidate counterpart of the γ-ray source AGL J2241+4454 (refs 5, 6). The black hole is X-ray quiescent and fed by a radiatively inefficient accretion flow giving a luminosity less than 1.6 × 10 −7 times the Eddington luminosity. This implies that Be binaries with black-hole companions are difficult to detect in conventional X-ray surveys.

## What exactly may be a black hole?

Although astronomers have observed black holes with telescopes for several years, it wasn’t until March 2021 that scientists saw something really strange while watching a bit of space debris. Using an instrument called a really Expanding Universe Explorer (VEO) at NASA’s Herschel Space Center in Europe, scientists noticed something that appeared to be “hiding” in one among our galaxy’s event horizons. Yes, it had been a region . what’s a region and what are a number of the consequences that cause them to affect the space around them?

When a region starts to soak up matter, it spirals inward. This spiraling gets faster because it gets closer to a black hole’s event horizon – the purpose at which nothing, not even light, can escape. Once the black hole’s event horizon is reached, the spiral collapses and ejects matter into space. One theory is that black holes emit radio waves, x-rays, and gamma rays – and by spotting these emitted waves, researchers were ready to determine that this is often what caused the “aging” of distant stars.

Although astronomers have discovered black holes within the past, they weren’t ready to pinpoint exactly what was causing their emission of radiation. Only over the past few decades have experts been ready to study this sort of radiation, and thus infer different characteristics about black holes supported their presence. a method that they are doing this is often by studying very faint X-rays that come from very accessible galaxies. By detecting the quantity of radiation emitted by matter falling toward or faraway from these black holes, astronomers can pin-point a spread of various processes that are happening .

If you thought that this was all that there was to black holes, then you’d be greatly mistaken. Because black holes also play a serious role within the formation of irregular galaxies, they will help astronomers study the properties of those very rare formations. By observing how black holes and their event horizons differ from the most events within the universe, scientists have gained a far better understanding of how these building blocks of the universe work. they need also determined how they affect the event of other elements, like hydrogen.

Although there’s much to find out about black holes, it’s still unknown what exactly they’re and what they contribute to the structure of the whole universe. Astronomers have learned that they need an impact on very heavy and really light objects, also as gas and mud in space. They also play a big role within the distribution of gas within the gas clouds surrounding our planet. Knowing all of this just goes to point out that understanding black holes is extremely important.

## Researchers solve mystery of X-ray light emitted from black holes

It is a mystery that has stymied astrophysicists for decades: how do black holes produce so many high-power X-rays?

In a new study, astrophysicists from The Johns Hopkins University, NASA, and the Rochester Institute of Technology bridged the gap between theory and observation by demonstrating that gas spiraling toward a black hole inevitably results in X-ray emissions.

The paper, recently published in Astrophysical Journal, states that as gas spirals toward a black hole through a formation called an accretion disk, it heats up to roughly 10 million degrees Celsius. The temperature in the main body of the disk is roughly 2,000 times hotter than the sun and emits low-energy, or "soft," X-rays. However, observations also detect "hard" X-rays, which produce up to 100 times higher energy levels.

Julian Krolik, professor of physics and astronomy in the Krieger School of Arts and Sciences at JHU, and his fellow scientists used a combination of supercomputer simulations and traditional hand-written calculations to uncover their findings. Supported by 40 years of theoretical progress, the team showed for the first time that high-energy light emission is not only possible, but is an inevitable outcome of gas being drawn into a black hole.

"Black holes are truly exotic, with extraordinarily high temperatures, incredibly rapid motions, and gravity exhibiting the full weirdness of general relativity," Krolik said. "But our calculations show we can understand a lot about them using only standard physics principles."

Krolik's collaborators included Jeremy Schnittman, a research astrophysicist from the NASA Goddard Space Flight Center, and Scott Noble, an associate research scientist from the Center for Computational Relativity and Gravitation at RIT. Schnittman was lead author.

As the quality and quantity of high-energy light observations improved over the years, evidence mounted that photons must be created in a hot, tenuous region called the corona. This corona, boiling violently above the comparatively cool disk, is similar to the corona surrounding the sun, which is responsible for much of the ultra-violet and X-ray luminosity seen in the solar spectrum.

While the team's study of black holes and high-energy light confirms a widely held belief, the role of advancing modern technology should not be overlooked. A grant from the National Science Foundation enabled the team to access Ranger, a supercomputing system at the Texas Advanced Computing Center located at the University of Texas at Austin. Ranger worked over the course of about 27 days, over 600 hours, to solve the equations.

Noble developed the computer simulation solving all of the equations governing the complex motion of inflowing gas and its associated magnetic fields near an accreting black hole. The rising temperature, density, and speed of the inflowing gas dramatically amplify magnetic fields threading through the disk, which then exert additional influence on the gas.

The result is a turbulent froth orbiting the black hole at speeds approaching the speed of light. The calculations simultaneously tracked the fluid, electrical, and magnetic properties of the gas while also taking into account Einstein's theory of relativity.

"In some ways, we had to wait for technology to catch up with us," Krolik said. "It's the numerical simulations going on at this level of quality and resolution that make the results credible."

Schnittman was previously a postdoctoral fellow at Johns Hopkins mentored by Krolik from 2007 to 2010, and Noble was an assistant research scientist and instructor under Krolik from 2006 to 2009.

## Even small black holes emit gravitational waves when they collide, and LIGO heard them

A black hole devouring a star. Credit: NASA

LIGO scientists say they have discovered gravitational waves coming from another black hole merger, and it's the tiniest one they've ever seen.

The findings, submitted to the Astrophysical Journal Letters, could shed light on the diversity of the black hole population - and may help scientists figure out why larger black holes appear to behave differently from the smaller ones.

"Its mass makes it very interesting," said Salvatore Vitale, a data analyst and theorist with the LIGO Lab at the Massachusetts Institute of Technology. The discovery, he added, "really starts populating more of this low-mass region that (until now) was quite empty."

Gravitational waves are ripples in the fabric of space-time that are caused by accelerating or decelerating objects.

They're extremely difficult to detect, but worth searching for because they allow us to directly study extremely powerful cosmic phenomena - including black holes, which can't be seen by conventional means because no light can escape from within the event horizon.

The Laser Interferometer Gravitational-Wave Observatory, or LIGO, can find black hole binaries - a pair of black holes that are bound by gravity - as they spin toward each other and violently merge into a single black hole.

LIGO consists of two L-shaped detectors with 2.5-mile-long arms, one in Hanford, Wash., and the other in Livingston, La.

When a gravitational wave passes through the detectors, squeezing one arm and stretching the other, a finely tuned system of lasers and mirrors inside the arms can pick up those infinitesimally tiny distortions.

Since finding its first black hole merger in September 2015, LIGO has announced the discovery of several more black hole mergers, as well as a merger of two neutron stars - some of which the European Virgo detector picked up as well.

The black hole smashup GW170608 was detected on June 7.

The detectors measured a signal that came from the violent collision of two smaller black holes, about seven and 12 times the mass of the sun, sitting roughly a billion light-years away. The merger left behind a black hole with 18 solar masses the remaining one sun's worth of mass was converted into gravitational waves.

This event was quite small compared with most black hole merger discoveries by LIGO (for example, the first pair in September 2015 weighed about 36 and 29 suns, respectively). The next smallest was found in December 2015, with black hole masses of 7.5 and 14.2 suns, respectively.

As the lowest-mass of LIGO's black hole finds, GW170608's lightweight pair is in the same class as black holes that astronomers have found indirectly via X-rays and other high-energy radiation.

Those X-rays come from outside a black hole, as all the material in its accretion disc spins around, rubs against other material and heats up, emitting high-energy radiation in the process. That material in the disc is pulled from a companion star that's gravitationally locked into a binary pair with the black hole.

But astronomers have really only spotted X-rays coming from lower-mass black holes, not the more massive ones such as those LIGO is finding.

Why haven't larger black holes been found producing X-rays? It's a mystery that researchers have yet to figure out, Vitale said. But GW170608 could help bridge that gap.

LIGO is set to start its next observing run in late 2018, and as it finds more black hole mergers, scientists will start to be able to treat them as a population and study their demographics to further probe these questions.

But Vitale said he was also hoping to see something new, beyond black hole mergers and neutron-star mergers.

"I would love to find a black hole and a neutron star," he said.

Such a hybrid merger would allow scientists to study gravitational waves but would also produce some light that astronomers could study with more conventional telescopes.

## <i>Science</i>: Black Holes Emit Surprisingly Strong Jets

Black holes release more energy into their host galaxies than previously thought, a new study suggests. This finding, reported in the 28 February issue of the journal Science, may help astronomers better understand black holes' effects on their host galaxies.

A black hole grows as gas from space flows onto it. It then releases two kinds of energy from that gas: radiation energy in the form of photons and kinetic energy in the form of wind jets. The more matter falls in, the more energy is released. "The effect of a black hole on its environment depends on how much energy it injects into it," said lead author Roberto Soria, senior research fellow at the International Centre for Radio Astronomy Research at Australia's Curtin University.

###### Artist's impression of a microquasar. Gas falls from a donor star towards the black hole, spreading to form a hot accretion disk. As gas spirals down towards the hole, gravitational energy is radiated partly as photons and partly used to launch a pair of jets. | Courtesy of T.D. Russell, ICRAR-Curtin Using the BINSIM visualization code by R. Hynes, LSU

"Astronomers often assumed the growth rates of powerful black holes could be inferred from the photon power alone, because the jet power was negligible," explained Soria. "Now, we show the jet power is not negligible at all."

Indeed, the authors show that in some cases the jet power actually exceeds the so-called Eddington limit. According to this limit, the radiation energy flowing out of a black hole cannot surpass a certain amount (one based on the black hole's mass), or it will blow the gathering gas away.

Whether a black hole's kinetic energy is constrained in the same way has been unclear. But, an object discovered by Soria and colleagues in a galaxy called M83 may provide some answers. The team is evaluating the energy associated with an object known as a microquasar, a tremendously energetic stellar environment with a black hole at its core.

"Understanding how quasars radiate such immense power has been an important problem," said co-author Frank Winkler, research professor of astrophysics at Middlebury College in Vermont. "Bona fide quasars are so distant that it's hard to observe many of their properties. Microquasars are a lot closer, providing an opportunity to observe what we believe to be the same physical mechanism, in miniature."

Commonly known as the "southern pinwheel," M83 is located 15 million light years away in the southern constellation Hydra. Not only is it one of the closest large spiral galaxies, it also faces almost directly toward Earth and so is particularly amenable to detailed studies by astronomers.

###### Composite image of the spiral galaxy M83 assembled from observations made by NASA's Hubble Space Telescope and the Carnegie Institution of Washington's Magellan telescopes. | NASA ESA and the Hubble Heritage Team, STScI/AURA

The researchers observed the black hole and its environs with different telescopes, including NASA's Chandra X-ray Observatory and Hubble Space Telescope, and the Australia Telescope Compact Array radio complex. They did this at different times spanning several years. By analyzing X-rays produced by gas accreting onto the black hole, the scientists figured out that the black hole's mass was less than 100 times that of the Sun.

They then compared the mass of the black hole with its outgoing kinetic power, which they were able to infer in part by looking at a combination of the X-ray, radio, optical and near-infrared light energy being radiated into the surrounding region of space. This revealed that the kinetic energy from the black hole was higher than the Eddington limit for a black hole of this mass.

In other words, the kinetic energy was higher than the radiation energy, suggesting that some of the fastest growing black holes may emit more energy by way of jets than they do via light energy.

Critically, the team's observations show that the black hole has been producing this very high kinetic power for a long time, not just for short, explosive events. "If a black hole reaches super-Eddington power only for a short burst of a few seconds, it's interesting but doesn't really affect the galaxy," Soria explained. "But black holes such as this one seem to have been in a powerful state for tens of thousands of years, during which they ejected a lot of energy."

The work of this research team will help astronomers better model the evolution of black holes over time. "Super powerful black holes like this one are rare today, almost extinct," Soria explained, "but they were common in the early universe when big galaxies had a lot more gas."

"Energy production at this level for tens of thousands of years…may trigger ongoing star formation or have other large-scale effects," explained co-author William Blair, an astrophysicist and research professor in the department of physics and astronomy at Johns Hopkins University.

"It's quite astounding to think that, out of the millions or even billions of stars in a given galaxy, one X-ray binary can have this kind of large-scale impact on a galaxy's evolution," Blair n oted.

## Mystery of X-ray light from black holes solved

It is a mystery that has stymied astrophysicists for decades: how do black holes produce so many high-power X-rays?

In a new study, astrophysicists from The Johns Hopkins University, NASA and the Rochester Institute of Technology conducted research that bridges the gap between theory and observation by demonstrating that gas spiraling toward a black hole inevitably results in X-ray emissions.

The paper states that as gas spirals toward a black hole through a formation called an accretion disk, it heats up to roughly 10 million degrees Celsius. The temperature in the main body of the disk is roughly 2,000 times hotter than the sun and emits low-energy or "soft" X-rays. However, observations also detect "hard" X-rays which produce up to 100 times higher energy levels.

Julian Krolik, professor of physics and astronomy in the Zanvyl Krieger School of Arts and Sciences, and his fellow scientists used a combination of supercomputer simulations and traditional hand-written calculations to uncover their findings. Supported by 40 years of theoretical progress, the team showed for the first time that high-energy light emission is not only possible, but is an inevitable outcome of gas being drawn into a black hole.

"Black holes are truly exotic, with extraordinarily high temperatures, incredibly rapid motions and gravity exhibiting the full weirdness of general relativity," Krolik said. "But our calculations show we can understand a lot about them using only standard physics principles."

The team's work was recently published in the print edition of Astrophysical Journal. His collaborators on the study include Jeremy Schnittman, a research astrophysicist from the NASA Goddard Space Flight Center, and Scott Noble, an associate research scientist from the Center for Computational Relativity and Gravitation at RIT. Schnittman was lead author.

As the quality and quantity of the high-energy light observations improved over the years, evidence mounted showing that photons must be created in a hot, tenuous region called the corona. This corona, boiling violently above the comparatively cool disk, is similar to the corona surrounding the sun, which is responsible for much of the ultra-violet and X-ray luminosity seen in the solar spectrum.

While the team's study of black holes and high-energy light confirms a widely-held belief, the role of advancing modern technology should not be overlooked. A grant from the National Science Foundation enabled the team to access Ranger, a supercomputing system at the Texas Advanced Computing Center located at the University of Texas in Austin. Ranger worked over the course of about 27 days, over 600 hours, to solve the equations.

Noble developed the computer simulation solving all of the equations governing the complex motion of inflowing gas and its associated magnetic fields near an accreting black hole. The rising temperature, density and speed of the inflowing gas dramatically amplify magnetic fields threading through the disk, which then exert additional influence on the gas.

The result is a turbulent froth orbiting the black hole at speeds approaching the speed of light. The calculations simultaneously tracked the fluid, electrical and magnetic properties of the gas while also taking into account Einstein's theory of relativity.

"In some ways, we had to wait for technology to catch up with us," Krolik said. "It's the numerical simulations going on at this level of quality and resolution that make the results credible."

The scientists are all familiar with each other as their paths have all crossed with Krolik during graduate school at Johns Hopkins. Schnittman was previously a postdoctoral fellow mentored by Krolik from 2007 to 2010 while Noble was an assistant research scientist and instructor also under Krolik from 2006 to 2009.

The work was supported by the National Science Foundation Grants AST-0507455, AST- 0908336 and AST-1028087.

## Scientists observe huge black hole shred a star, emit x-rays

They’re known as tidal disruption events: when a black hole consumes a nearby star.

And now, scientists have observed a dramatic tidal disruption event in a galaxy nearly 4 billion light years away, involving a black hole that’s approximately a million times as massive as our sun. The matter that this supermassive black hole swallowed created what’s called an accretion disk around the black hole.

The supermassive black hole, called Swift J1644+57, was dormant— as 90 percent of black holes are— until it sucked in the star, shredding it. That event allowed scientists to observe the black hole.

“Tidal disruption events offer us this rare view at the most common kind of supermassive black hole in the universe— these so-called dormant supermassive black holes,” Erin Kara, a Hubble postdoctoral fellow at the University of Maryland, said in a video explaining the discovery. “Tidal disruption events, where the stellar debris causes the formation of a temporary accretion disk, offers us a way to probe this population of supermassive black holes.”

What’s more, that accretion disk amplified x-ray flares— and the location of where those x-rays are coming was surprising.

“Previously, astronomers had thought that the x-ray emission is coming from far out in a jet,” Kara said. “But what we’re finding with these observations, is that the x-ray emission is coming from flares very close to the supermassive black hole. And we can use these observations to probe properties of the black hole itself.”

“For instance, we found that the mass of the black hole is something on the order of a million times the mass of the sun,” she added.

## X-ray pulse detected near event horizon as black hole devours star

On Nov. 22, 2014, astronomers spotted a rare event in the night sky: A supermassive black hole at the center of a galaxy, nearly 300 million light-years from Earth, ripping apart a passing star. The event, known as a tidal disruption flare, for the black hole’s massive tidal pull that tears a star apart, created a burst of X-ray activity near the center of the galaxy. Since then, a host of observatories have trained their sights on the event, in hopes of learning more about how black holes feed.

Now researchers at MIT and elsewhere have pored through data from multiple telescopes’ observations of the event, and discovered a curiously intense, stable, and periodic pulse, or signal, of X-rays, across all datasets. The signal appears to emanate from an area very close to the black hole’s event horizon — the point beyond which material is swallowed inescapably by the black hole. The signal appears to periodically brighten and fade every 131 seconds, and persists over at least 450 days.

The researchers believe that whatever is emitting the periodic signal must be orbiting the black hole, just outside the event horizon, near the Innermost Stable Circular Orbit, or ISCO — the smallest orbit in which a particle can safely travel around a black hole.

Given the signal’s stable proximity to the black hole, and the black hole’s mass, which researchers previously estimated to be about 1 million times that of the sun, the team has calculated that the black hole is spinning at about 50 percent the speed of light.

The findings, reported today in the journal Science, are the first demonstration of a tidal disruption flare being used to estimate a black hole’s spin.

The study’s first author, Dheeraj Pasham, a postdoc in MIT’s Kavli Institute for Astrophysics and Space Research, says that most supermassive black holes are dormant and don’t usually emit much in the way of X-ray radiation. Only occasionally will they release a burst of activity, such as when stars get close enough for black holes to devour them. Now he says that, given the team’s results, such tidal disruption flares can be used to estimate the spin of supermassive black holes — a characteristic that has been, up until now, incredibly tricky to pin down.

“Events where black holes shred stars that come too close to them could help us map out the spins of several supermassive black holes that are dormant and otherwise hidden at the centers of galaxies,” Pasham says. “This could ultimately help us understand how galaxies evolved over cosmic time.”

Pasham’s co-authors include Ronald Remillard, Jeroen Homan, Deepto Chakrabarty, Frederick Baganoff, and James Steiner of MIT Alessia Franchini at the University of Nevada Chris Fragile of the College of Charleston Nicholas Stone of Columbia University Eric Coughlin of the University of California at Berkeley and Nishanth Pasham, of Sunnyvale, California.

A real signal

Theoretical models of tidal disruption flares show that when a black hole shreds a star apart, some of that star's material may stay outside the event horizon, circling, at least temporarily, in a stable orbit such as the ISCO, and giving off periodic flashes of X-rays before ultimately being fed by the black hole. The periodicity of the X-ray flashes thus encodes key information about the size of the ISCO, which itself is dictated by how fast the black hole is spinning.

Pasham and his colleagues thought that if they could see such regular flashes very close to a black hole that had undergone a recent tidal disruption event, these signals could give them an idea of how fast the black hole was spinning.

They focused their search on ASASSN-14li, the tidal disruption event that astronomers identified in November 2014, using the ground-based All-Sky Automated Survey for SuperNovae (ASASSN).

“This system is exciting because we think it’s a poster child for tidal disruption flares,” Pasham says. “This particular event seems to match many of the theoretical predictions.”

The team looked through archived datasets from three observatories that collected X-ray measurements of the event since its discovery: the European Space Agency’s XMM-Newton space observatory, and NASA’s space-based Chandra and Swift observatories. Pasham previously developed a computer code to detect periodic patterns in astrophysical data, though not for tidal disruption events specifically. He decided to apply his code to the three datasets for ASASSN-14li, to see if any common periodic patterns would rise to the surface.

What he observed was a surprisingly strong, stable, and periodic burst of X-ray radiation that appeared to come from very close to the edge of the black hole. The signal pulsed every 131 seconds, over 450 days, and was extremely intense — about 40 percent above the black hole’s average X-ray brightness.

“At first I didn’t believe it because the signal was so strong,” Pasham says. “But we saw it in all three telescopes. So in the end, the signal was real.”

Based on the properties of the signal, and the mass and size of the black hole, the team estimated that the black hole is spinning at least at 50 percent the speed of light.

“That’s not super fast — there are other black holes with spins estimated to be near 99 percent the speed of light,” Pasham says. “But this is the first time we’re able to use tidal disruption flares to constrain the spins of supermassive black holes.”

Illuminating the invisible

Once Pasham discovered the periodic signal, it was up to the theorists on the team to find an explanation for what may have generated it. The team came up with various scenarios, but the one that seems the most likely to generate such a strong, regular X-ray flare involves not just a black hole shredding a passing star, but also a smaller type of star, known as a white dwarf, orbiting close to the black hole.

Such a white dwarf may have been circling the supermassive black hole, at ISCO — the innermost stable circular orbit — for some time. Alone, it would not have been enough to emit any sort of detectable radiation. For all intents and purposes, the white dwarf would have been invisible to telescopes as it circled the relatively inactive, spinning black hole.

Sometime around Nov. 22, 2014, a second star passed close enough to the system that the black hole tore it apart in a tidal disruption flare that emitted an enormous amount of X-ray radiation, in the form of hot, shredded stellar material. As the black hole pulled this material inward, some of the stellar debris fell into the black hole, while some remained just outside, in the innermost stable orbit — the very same orbit in which the white dwarf circled. As the white dwarf came in contact with this hot stellar material, it likely dragged it along as a luminous overcoat of sorts, illuminating the white dwarf in an intense amount of X-rays each time it circled the black hole, every 131 seconds.

The scientists admit that such a scenario would be incredibly rare and would only last for several hundred years at most — a blink of an eye in cosmic scales. The chances of detecting such a scenario would be exceedingly slim.

“The problem with this scenario is that, if you have a black hole with a mass that’s 1 million times that of the sun, and a white dwarf is circling it, then at some point over just a few hundred years, the white dwarf will plunge into the black hole,” Pasham says. “We would’ve been extremely lucky to find such a system. But at least in terms of the properties of the system, this scenario seems to work.”

The results’ overarching significance is that they show it is possible to constrain the spin of a black hole, from tidal disruption events, according to Pasham. Going forward, he hopes to identify similar stable patterns in other star-shredding events, from black holes that reside further back in space and time.

“In the next decade, we hope to detect more of these events,” Pasham says. “Estimating spins of several black holes from the beginning of time to now would be valuable in terms of estimating whether there is a relationship between the spin and the age of black holes.”