tidal disruptions


We just got an unprecedented look at a black hole ripping apart a star

For the first time ever, astronomers got a close-up peek at a black hole ripping apart a star, a rare event that results in some of the star’s material getting ejected out into space. To research this phenomenon, astronomers used data from a tidal disruption that happened 3.9 billion years ago. Studying tidal disruptions like this one is revealing new information about how black holes behave.

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Spinning black hole swallows star; surpasses all supernovae in brightness

“Almost every galaxy, even quiet, red ones, contain supermassive black holes at their core. When matter approaches – whether an asteroid, planet, gas cloud or a star – the incredible tidal forces stretch and pinch it, tearing it apart into a long, thin strand. Some of these black holes can rotate incredibly rapidly, causing the matter that falls in to accelerate at different rates depending on the orientation and configuration of the infall, which changes over time. The ASASSN-15lh event not only showed an ultraviolet re-brightening, but a rapid temperature spike at late times as well. If the explanation pans out, this would be the first time we’ve ever observed a rare event of this kind: a massive star disrupted and devoured by an ultramassive, rapidly spinning supermassive black hole.”

Last year, a record-shattering event occurred: we saw the brightest supernova ever observed in the Universe. It outshone the previous record holder by more than double, and it reached a peak brightness of more than 20 times the sum total of all the stars in the Milky Way galaxy. Surprisingly, it occurred in a red, quiet galaxy, rather than the bright blue ones famous for them. After 10 months of follow-up observations, it looks like it wasn’t a supernova after all. Instead of fading away, there was a rebrightening months after the peak. Instead of cooling down, something reheated the glow to even greater temperatures. The only thing that fits the data? A tidal disruption event, and even those would only work if it were a supermassive black hole that rotated more quickly than any such event ever observed before.

Come get the full spectacular story – and the science behind it – as we finally learn where the brightest event in history came from!

NASA shows what a black hole eating a star looks like

Previously, we didn’t know much about tidal disruptions because it’s hard to catch a black hole in the act of devouring a star. Now a team of researchers has captured how the dust surrounding black holes absorbs and reflects the flares produced by a tidal disruption. This discovery will help scientists in two areas of study.

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Cosmic ‘Spitballs’ Released From Milky Way’s Black Hole

“Black holes don’t just provide gravity, absorb incoming matter and prevent anything from escaping. They also gravitationally pull on and tear matter that passes nearby, including stars. In a surprising find, a new study out of Harvard shows that torn-apart stars aren’t merely reduced into gas, but they form dense streams that re-condense into planets in just year-long timescales. Moving rapidly away from the central black hole, these 'cosmic spitballs’ represent a brand new population of rogue planets, and are potentially the most catastrophic objects from space careening through our galaxy.”

Imagine you’re a star passing too close to a black hole. What’s going to happen to you? Yes, you’ll be tidally disrupted and eventually torn apart. Some of the matter will be swallowed, some will wind up in an accretion disk, and some will be accelerated and ejected entirely. But quite surprisingly, the ejected matter doesn’t just come out in the form of hot gas, but it condenses into large numbers of rapidly-moving planets. This population should make up approximately one out of every 1000 rogue planets, but should be uniquely identifiable. The vast majority will move at incredible speeds of around 10,000 km/s, be approximately the mass of Jupiter but will be made out of shredded star material, rather than traditional planetary material. As the next generation of infrared telescopes come online, these ‘cosmic spitballs’ should be one of the most exciting novel discoveries of all.

Come get the whole story on cosmic spitballs, fresh from the AAS meeting!


Data suggest black holes swallow stellar debris in bursts.

In the center of a distant galaxy, almost 300 million light years from Earth, scientists have discovered a supermassive black hole that is “choking” on a sudden influx of stellar debris.

In a paper published today in Astrophysical Journal Letters, researchers from MIT, NASA’s Goddard Space Flight Center, and elsewhere report on a “tidal disruption flare” — a dramatic burst of electromagnetic activity that occurs when a black hole obliterates a nearby star. The flare was first discovered on Nov. 11, 2014, and scientists have since trained a variety of telescopes on the event to learn more about how black holes grow and evolve.

The MIT-led team looked through data collected by two different telescopes and identified a curious pattern in the energy emitted by the flare: As the obliterated star’s dust fell into the black hole, the researchers observed small fluctuations in the optical and ultraviolet (UV) bands of the electromagnetic spectrum. This very same pattern repeated itself 32 days later, this time in the X-ray band.

The researchers used simulations of the event performed by others to infer that such energy “echoes” were produced from the following scenario: As a star migrated close to the black hole, it was quickly ripped apart by the black hole’s gravitational energy. The resulting stellar debris, swirling ever closer to the black hole, collided with itself, giving off bursts of optical and UV light at the collision sites. As it was pulled further in, the colliding debris heated up, producing X-ray flares, in the same pattern as the optical bursts, just before the debris fell into the black hole.

“In essence, this black hole has not had much to feed on for a while, and suddenly along comes an unlucky star full of matter,” says Dheeraj Pasham, the paper’s first author and a postdoc in MIT’s Kavli Institute for Astrophysics and Space Research. “What we’re seeing is, this stellar material is not just continuously being fed onto the black hole, but it’s interacting with itself — stopping and going, stopping and going. This is telling us that the black hole is ‘choking’ on this sudden supply of stellar debris.”

Pasham’s co-authors include MIT Kavli postdoc Aleksander Sadowski and researchers from NASA’s Goddard Space Flight Center, the University of Maryland, the Harvard-Smithsonian Center for Astrophysics, Columbia University, and Johns Hopkins University.

A “lucky” sighting

Pasham says tidal disruption flares are a potential window into the universe’s many “hidden” black holes, which are not actively accreting, or feeding on material.

“Almost every massive galaxy contains a supermassive black hole,” Pasham says. “But we won’t know about them if they’re sitting around doing nothing, unless there’s an event like a tidal disruption flare.”

Such flares occur when a star, migrating close to a black hole, gets pulled apart from the black hole’s immense gravitational energy. This stellar obliteration can give off incredible bursts of energy all along the electromagnetic spectrum, from the radio band, through the optical and UV wavelengths, and on through the X-ray and high-energy gamma ray bands. As extreme as they are, tidal disruption flares are difficult to observe, as they happen infrequently.

“You’d have to stare at one galaxy for roughly 10,000 to 100,000 years to see a star getting disrupted by the black hole at the center,” Pasham says.

Nevertheless, on Nov. 11, 2014, a global network of robotic telescopes named ASASSN (All Sky Automated Survey for SuperNovae) picked up signals of a possible tidal disruption flare from a galaxy 300 million light years away. Scientists quickly focused other telescopes on the event, including the X-ray telescope aboard NASA’s Swift satellite, an orbiting spacecraft that scans the sky for bursts of extremely high energy.

“Only recently have telescopes started ‘talking’ to each other, and for this particular event we were lucky because a lot of people were ready for it,” Pasham says. “It just resulted in a lot of data.”

A light-on collision

With access to these data, Pasham and his colleagues wanted to solve a longstanding mystery: Where did a flare’s bursts of light first arise? Using models of black hole dynamics, scientists have been able to estimate that as a black hole rips a star apart, the resulting tidal disruption flare can produce X-ray emissions very close to the black hole. But it’s been difficult to pinpoint the origin of optical and UV emissions. Doing so would be an added step toward understanding what happens when a star gets disrupted.

“Supermassive black holes and their host galaxies grow in-situ,” Pasham says. “Knowing exactly what happens in tidal disruption flares could help us understand this black hole and galaxy coevolution process.”

The researchers studied the first 270 days following the detection of the tidal disruption flare, named ASASSN-14li. In particular, they analyzed X-ray and optical/UV data taken by the Swift satellite and the Las Cumbres Observatory Global Telescope. They identified fluctuations, or bursts, in the X-ray band — two broad peaks (one around day 50, and the other around day 110) followed by a short dip around day 80. They identified this very same pattern in the optical/UV data some 32 days earlier.

To explain these emission “echoes,” the team ran simulations of a tidal disruption flare produced from a black hole obliterating a star. The researchers modeled the resulting accretion disc — an elliptical disc of stellar debris swirling around the black hole — along with its probable speed, radius, and rate of infall, or speed at which material falls onto the black hole.

From simulations run by others, the researchers conclude that the optical and UV bursts likely originated from the collision of stellar debris on the outer perimeter of the black hole. As this colliding material circles closer into the black hole, it heats up, eventually giving off X-ray emissions, which can lag behind the optical emissions, similar to what the scientists observed in the data.

“For supermassive black holes steadily accreting, you wouldn’t expect this choking to happen,” Pasham says. “The material around the black hole would be slowly rotating and losing some energy with each circular orbit. But that’s not what’s happening here. Because you have a lot of material falling onto the black hole, it’s interacting with itself, falling in again, and interacting again. If there are more events in the future, maybe we can see if this is what happens for other tidal disruption flares.”

This research was supported, in part, by NASA.


Mostly Mute Monday: Crater Chains of the Moon

“While craters young and old litter its surface, large numbers of catenae, or crater chains, can be found as well on both the near and far sides. While about 20 have been known since the 1990s, often extending for hundreds of kilometers, many more have been discovered with the advent of LROC and citizen science projects like Moon Zoo.”

You might think that your odds of getting 3, 5, or even 10 or more craters all next to each other and in a row on an object like the Moon are astronomically small. Yet, we’ve identified dozens of features that show exactly this! Here are some of the most spectacular, along with the redux of the leading ideas of where they came from, including secondary impacts, tidally disrupted impactors and volcanic and geologic explanations.

(NASA)  Arp 188 and the Tadpole’s Tail
Image Credit: Hubble Legacy Archive, ESA, NASA; Processing & Copyright: Joachim Dietrich

Why does this galaxy have such a long tail? In this stunning vista, based on image data from the Hubble Legacy Archive, distant galaxies form a dramatic backdrop for disrupted spiral galaxy Arp 188, the Tadpole Galaxy. The cosmic tadpole is a mere 420 million light-years distant toward the northern constellation Draco. Its eye-catching tail is about 280 thousand light-years long and features massive, bright blue star clusters. One story goes that a more compact intruder galaxy crossed in front of Arp 188 - from right to left in this view - and was slung around behind the Tadpole by their gravitational attraction. During the close encounter, tidal forces drew out the spiral galaxy’s stars, gas, and dust forming the spectacular tail. The intruder galaxy itself, estimated to lie about 300 thousand light-years behind the Tadpole, can be seen through foreground spiral arms at the upper right. Following its terrestrial namesake, the Tadpole Galaxy will likely lose its tail as it grows older, the tail’s star clusters forming smaller satellites of the large spiral galaxy.


Most massive collection of giant stars ever revealed by Hubble

“The single greatest is R136a1: 250 times our Sun’s mass. Nine total stars over 100 solar masses, as well as dozens over 50, are found inside. These nine largest stars, combined, outshine the Sun by 30,000,000 times. All will die in catastrophic supernovae, creating massive black holes when they do.”

When we look for the brightest, bluest, most massive individual stars, we’re restricted to looking nearby, since it’s impossible to resolve individual stars at distances that extend much beyond our own galaxy. So how surprising is it, then, when the most massive stars we’ve ever found aren’t in our own galaxy, nor in any of the monster galaxies we’ve found nearby, but in a small, satellite dwarf of our own: the Large Magellanic Cloud? The tidal disruption of the Milky Way causes a huge spike in star formation among the neutral gas, and has led to an incredibly rich region of new stars, including dozens of stars over 50 solar masses, nine over 100, four over 150 and the most massive one, R136a1, coming in at an incredible 250 times the mass of our Sun. It’s the most massive collection of hot, young stars in the entire known Universe.