Tag Archives: accretion disk

This is an artist's impression of planets orbiting a supermassive black hole. (Kagoshima University)

Planets could orbit Supermassive Black Holes

This is an artist's impression of planets orbiting a supermassive black hole. (Kagoshima University)
This is an artist’s impression of planets orbiting a supermassive black hole. (Kagoshima University)

The idea of stars orbiting the supermassive black holes that researchers believe lurk at the centre of most galaxies has been long established as a matter of fact in science. In ‘active galactic nuclei’ or AGNs, these black holes are surrounded by haloes of gas and dust in a violent churning environment. Such clouds of gas and dust have the potential to birth not only stars but planets as well. Yet, the question of whether planets can also orbit these spacetime events has yet to be established. 

Enter Keiichi Wada, a professor at Kagoshima University, and Eiichiro Kokubo, a professor at the National Astronomical Observatory of Japan. These scientists from the distinct fields of active galactic nuclei research and planet formation research respectively have calculated that as a result of gas disc growth, an entirely new class of planets may form around supermassive black holes. 

“With the right conditions, planets could be formed even in harsh environments, such as around a black hole,” Wada points out. 

In their research published in the Astrophysical Journal, the duo of theoreticians propose that protoplanetary discs that surround young stars may not be the only potential site for planet formation. The researchers instead focused calculations and mathematical models on the denser dust discs found around supermassive black holes in AGNs, thus arriving at a surprising conclusion. 

“Our calculations show that tens of thousands of planets with 10 times the mass of the Earth could be formed [at a distance of] around 10 light-years from a black hole,” says Eiichiro Kokubo. 

“Around black holes, there might exist planetary systems of astonishing scale.”

One of the hindrances to the formation of planets in such discs of dust has previously been the amount of energy generated in AGNs, Researchers had believed that this energy output would prevent the coagulation of ‘fluffy ice dust’ that can help the growth of dust grains that can lead to planet formation in protoplanetary discs.

But, what Wada and Kokubo discovered was that the huge density of dust discs around supermassive black holes in AGNs —potentially containing as much as a hundred thousand times the mass of the Sun worth of dust, which is a billion times more massive than a typical protoplanetary disc — helps protect the outer layers from bombardment from high-energy radiation such as gamma rays. 

 A schematic picture of the Active Galactic Nucleus (AGN) and the circumnuclear disc. (Wada, Kokubo, 2019)
A schematic picture of the Active Galactic Nucleus (AGN) and the circumnuclear disc. (Wada, Kokubo, 2019)

This helps form a low-temperature region similar to that found in protoplanetary discs, and thus, in turn, increases the likelihood of fluffy deposits building.

The process would lead to the formation of planets within a period of several hundred million years, according to the pair, and also result in much denser and more populated collections of planets. 

Unfortunately, the limits of current methods of identifying exoplanets would make identifying planets around a supermassive black hole challenging to say the least. 

“ Doppler spectroscopy, transit photometry, gravitational micro-lensing, or direct imaging are hopeless,” warn the duo in their paper. They go on to suggest that a method called photometry with an x-ray interferometer located in space could be a possible solution — if a way of distinguishing the effect caused by such planets from the natural variability of the AGN can be developed. 

For now, researchers will have to look to mathematical models alone to theorise about the potential for planets in orbit around black holes. 


Original research: https://arxiv.org/pdf/1909.06748.pdf

Researchers found a supermassive black hole choking on its meal

Scientists have found a supermassive black hole that seems to have bit more than it can chew. At the center of a galaxy some 300 million light years away from Earth, the black hole is straining to absorb the mass of a star it recently collapsed, “chocking” on its remains.

Artist’s impression of a supermassive black hole at a galaxy’s center. The blue color represents radiation pouring out from material very close to the black hole.
Image credits NASA/JPL-Caltech.

A team of researchers including members from MIT and NASA’s Goddard Space Flight Center have recently reported picking up on a peculiar “tidal disruption flare”, a massive burst of electromagnetic energy released when a black hole collapses a hapless star. The flare, named ASASSN-14li, first hit our sensors on Nov. 11, 2014, and researchers have since pointed all kinds of telescopes towards the source to learn as much as possible about how black holes evolve.

Led by MIT postdoc at the Kavli Institute for Astrophysics and Space Research Dheeraj Pasham, the team looked at data obtained with two different telescopes and found a strange pattern in the energy levels of the flare. As the supermassive black hole (I’ll just call it a SBH from not on) first began absorbing the former star’s matter, the team picked up on slight variations in the visible and ultraviolet intervals of the electromagnetic spectrum. Which in itself isn’t that weird — we’ll get to it in a moment. But the same pattern of fluctuations was picked up again 32 days later, this time in the X-ray band.

A flare of gluttony

So first off let’s get to know what these flares are and how they usually behave.

As I’ve said, tidal disruption flares are huge bursts of energy released when a black hole’s immense gravitational pull rips a star apart. The bursts propagate all over the electromagnetic spectrum, from radio, visible, and UV all the way to X-ray and gamma ray intervals. They’re pretty rare, so we didn’t witness that many of them despite the fact that they really stand out. But when we do, it’s a dead give away for hidden black holes — which would be almost impossible to spot otherwise.

“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, who’s also the paper’s first author, says.

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

So in a way we were lucky, but our sensors were also ready for it. The ASASSN-14li flare was picked up by the ASASSN (All Sky Automated Survey for SuperNovae) network of automated telescopes. Soon after, researchers pointed other telescopes towards the black hole, including the X-ray telescope aboard NASA’s Swift satellite — designed to monitor the sky for bursts of extremely high energy.

Artist’s rendering of the supermassive black hole that generated the flare and its accretion disk.
Image credits NASA / Swift / Aurore Simonnet, Sonoma State University.

“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.”

By looking at all the data they gathered on the event, Pasham and his team answered a long-standing mystery: where did these bursts of light originate in flares? By modeling a black hole’s dynamics, scientists have previously been able to explain that as a black hole rips its star apart, the resulting material can produce X-ray emissions very close to the event horizon. But the source for the visible and UV light proved elusive.

The team studied the 270 days after ASASSN-14li was first detected, with particular emphasis on the X-ray and optical/UV data taken by the Swift satellite and the Las Cumbres Observatory Global Telescope. Two broad peaks in the X-ray band were identified (one around day 50, and the other around day 110), and one short dip (around day 80). This was the exact same pattern they recorded for the visible/UV spectrum just 32 days earlier.

Their next step was to run simulations of the flare produced by a star collapsing next to a black hole and the resulting accretion disc (similar to how planets get them) — along with its presumed speed, size, and the rate which material falls onto the black hole.

Tug of war

 

The results suggest these energy fluctuations are a kind of electromagnetic echo. After the star was torn apart, its remains started swirling the supermassive black hole. As it drew nearer to the event horizon, the cloud of matter accelerated and became more tightly packed, releasing bursts of UV and visible light when its particles collided at high speeds. As the matter was pulled closer to the black hole it got even faster and denser, which also made it heat up. In this excited state of matter close to absorption into the event horizon, the collisions produced X- and gamma ray bursts instead of the lower-energy visible and UV bursts.

In the case of ASASSN-14li, this process happened much more slowly that usually because the great quantity of matter proved a bit too much for the black hole to chew in a single bite.

 

“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.” Pasham explains. “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.”

“For supermassive black holes steadily accreting, you wouldn’t expect this choking to happen. The material around the black hole would be slowly rotating and losing some energy with each circular orbit,” he adds.

“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.”

The full paper “Optical/UV-to-X-Ray Echoes from the Tidal Disruption Flare ASASSN-14li” has been published in the journal Astrophysical Journal Letters.

 

 

Mercury iron might be the result of cosmic collision

The Earth contains a lot of iron, but it is not alone in the solar system in that aspect. Venus, Mars, the Moon and asteroids such as Vesta all have iron in their structure, but Mercury is the champion in that aspect: about 70 percent of its mass is iron! Now, researchers believe they have found why Mercury is so rich in this metal – the planet is the result of a cosmic ‘hit and run’.

The main proposed reason for the lunar iron is that the Moon was formed as a result of a giant impact with proto-Earth – but that can’t account for the much vaster Mercurian iron. Such a scenario requires that proto-Mercury was blasted apart with far greater specific energy than required for lunar formation, but in such a way that it retained substantial volatile elements and did not reaccrete its ejected mantle – in other words, something struck Mercury so hard that the planet lots half its mantle in a collision with proto-Earth or proto-Venus, leaving behind the iron-rich body we see today. The mantle which was torn from Mercury also didn’t re-accrete on to the planet.

Erik Asphaug from Arizona State University and Andreas Reufer of the University of Bern developed a statistical scenario for how planets merge and grow; apparently, Mars and Mercury lucked out, but in different ways.

“How did they luck out? Mars, by missing out on most of the action – not colliding into any larger body since its formation – and Mercury, by hitting the larger planets in a glancing blow each time, failing to accrete,” explains Asphaug.

Their model showed that this was unlikely, but not extremely unlikely.

“It’s like landing heads two or three times in a row – lucky, but not crazy lucky. In fact, about one in 10 lucky.”

The rather surprising result the model projected was that hit and run collisions are might not have been that uncommon in our solar system.

“The surprising result we have shown is that hit-and-run relics not only can exist in rare cases, but that survivors of repeated hit-and-run incidents can dominate the surviving population. That is, the average unaccreted body will have been subject to more than one hit-and-run collision,’ explains Asphaug. We propose one or two of these hit-and-run collisions can explain Mercury’s massive metallic core and very thin rocky mantle.”

Scientific Reference: Mercury and other iron-rich planetary bodies as relics of inefficient accretion.

Artist's impression of a spinning supermassive black hole with a surrounding accretion disk and relativistic jets. (c) NASA/JPL

Supermassive black hole spin measured for first time – nears the speed of light

Artist's impression of a spinning supermassive black hole with a surrounding accretion disk and relativistic jets. (c) NASA/JPL

Artist’s impression of a spinning supermassive black hole with a surrounding accretion disk and relativistic jets. (c) NASA/JPL

Astronomers have made the first accurate measurement of a supermassive black hole’s spin, providing new insights that might help scientists probe the mysteries the surround them.

Supermassive black holes have an incredibly huge gravitational pull that doesn’t let anything in its surroundings escape its hungry maw, be it dust, rock or even light. Some are as massive as 10 billion times the mass of the Sun; typically most galaxies have a supermassive black hole residing at their center, including own Milky Way galaxy.

“It’s the first time that we can really say that black holes are spinning,” study co-author Fiona Harrison, of Caltech in Pasadena said. “The promise that this holds for being able to understand how black holes grow is, I think, the major implication.”

Previous studies have hinted towards the idea that supermassive black holes spin very fast, but until recently no evidence has been found to support these claims. Astronomers using uSTAR‘s super-sensitive measurements of high-energy X-rays have now for the first demonstrated that supermassive black holes spin, and quite fast too.

Image of the spiral galaxy NGC 1365 was taken by the powerful HAWK-I infrared camera on ESO’s Very Large Telescope at Paranal Observatory in Chile. (c)  ESO/P. Grosbøl

Image of the spiral galaxy NGC 1365 was taken by the powerful HAWK-I infrared camera on ESO’s Very Large Telescope at Paranal Observatory in Chile. (c) ESO/P. Grosbøl

The X-ray instrument was directed to peer into the guts of the NGC 1365 galaxy, located about 56 million light-years from Earth in the constellation Fornax, where a huge black hole lies at its center. Like most supermassive black holes, it forms an accretion disk of matter around it that funnels gas and dust. The motion of this accretion disk can tracked by telescopes that analyze the  high-energy light emitted by iron atoms, emissions that are highly distorted.

“We selected (NGC 1365) because it is bright in X-rays, and previous observations with less powerful satellites suggested that this could be a good candidate for such a study,” said astronomer Guido Risaliti, of the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass., and the Italian National Institute for Astrophysics, and lead author of research published in the journal Nature.

Spinning right round

To explain this behavior, scientists have hypothesized that either the black hole is spinning very fast, or a cloud of dust lying between the telescope and the black hole is obstructing observation. Using the uSTAR telescope, launched in 2012, astronomers zeroed in on high-energy X-rays emitted by the black hole at NGC 1365 and found the purported gas clouds would have to be incredibly thick to cause these levels of distortions. So thick as to make the idea extremely unlikely. This means that the black hole’s spin the cause.

“To shine through these thick clouds, the black hole would have to be so bright it would basically blow itself apart,” Harrison said.

Based on these measurements, the astronomers assert that  this gigantic black hole is spinning at about 84% of the speed that Einstein’s general theory of relativity will allow.

“What excites me is the fact that we are able to do this for the very massive black holes at the centers of galaxies but we can also make the same measurement for black holes in our galaxy … black holes that resulted from the explosion of a star … The fact we can extend this from billions of solar masses to 10 solar masses is pretty cool,” Harrison concluded.

via Discovery News