NASA Telescopes Work Out Black Hole's Snack Schedule

AT2018fyk
Credit Illustration: NASA/CXC/M. Weiss; Image: X-ray: NASA/SAO/Kavli Inst. at MIT/D.R. Pasham;
Optical: NSF/Legacy Survey/SDSS

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By using new data from NASA’s Chandra X-ray Observatory and Neil Gehrels Swift Observatory as well as ESA’s XMM-Newton, a team of researchers have made important headway in understanding how — and when — a supermassive black hole obtains and then consumes material, as described in our latest press release.

This artist’s impression shows a star that has partially been disrupted by such a black hole in the system known as AT2018fyk. The supermassive black hole in AT2018fyk — with about 50 million times more mass than the sun — is in the center of a galaxy located about 860 million light-years from Earth.

Astronomers have determined that a star is on a highly elliptical orbit around the black hole in AT2018fyk so that its point of farthest approach from the black hole is much larger than its closest. During its closest approach, tidal forces from the black hole pull some material from the star, producing two tidal tails of “stellar debris”.

The illustration shows a point in the orbit soon after the star is partially destroyed, when the tidal tails are still in close proximity to the star. Later in the star's orbit, the disrupted material returns to the black hole and loses energy, leading to a large increase in X-ray brightness occurring later in the orbit (not shown here). This process repeats each time the star returns to its point of closest approach, which is approximately every 3.5 years. The illustration depicts the star during its second orbit, and the disk of X-ray emitting gas around the black hole that is produced as a byproduct of the first tidal encounter.

Researchers took note of AT2018fyk in 2018 when the optical ground-based survey ASAS-SN detected that the system had become much brighter. After observing it with NASA’s NICER and Chandra, and XMM-Newton, researchers determined that the surge in brightness came from a “tidal disruption event,” or TDE, which signals that a star was completely torn apart and partially ingested after flying too close to a black hole. Chandra data of AT2018fyk is shown in the inset of an optical image of a wider field-of-view.

X-ray and optical image of AT2018fyk
Credit: X-ray: NASA/SAO/Kavli Inst. at MIT/D.R. Pasham; Optical: NSF/Legacy Survey/SDSS

When material from the destroyed star approached close to the black hole, it got hotter and produced X-ray and ultraviolet (UV) light. These signals then faded, agreeing with the idea that nothing was left of the star for the black hole to digest.

However, about two years later, the X-ray and UV light from the galaxy got much brighter again. This meant, according to astronomers, that the star likely survived the initial gravitational grab by the black hole and then entered a highly elliptical orbit with the black hole. During its second close approach to the black hole, more material was pulled off and produced more X-ray and UV light.

Based on what they had learned about the star and its orbit, a team of astronomers predicted that the black hole’s second meal would end in August 2023 and applied for Chandra observing time to check. Chandra observations on August 14, 2023, indeed showed the telltale sign of the black hole feeding coming to an end with a sudden drop in X-rays. The researchers also obtained a better estimate of how long it takes the star to complete an orbit, and predicted future mealtimes for the black hole.

A paper describing these results appears in the August 14, 2024 issue of The Astrophysical Journal and is available online. The authors are Dheeraj Passam (Massachusetts Institute of Technology), Eric Coughlin (Syracuse University), Muryel Guolo (Johns Hopkins University), Thomas Wevers (Space Telescope Science Institute), Chris Nixon (University of Leeds, UK), Jason Hinkle (University of Hawaii at Manoa), and Ananaya Bandopadhyay (Syracuse).

NASA's Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory's Chandra X-ray Center controls science from Cambridge Massachusetts and flight operations from Burlington, Massachusetts.

NASA Telescopes Work Out Black Hole's Snack Schedule

Source: NASA’s Chandra X-ray Observatory

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Visual Description:

In this digital illustration, a star sheds stellar debris as it orbits a supermassive black hole. This artist’s impression represents the center of a galaxy about 860 million light-years from Earth.

The supermassive black hole sits at our upper left. It resembles an irregular, pitch-black sphere at the heart of an almond-shaped pocket of swirling sand and dirt. Though gritty in texture, the swirling brown and grey pocket is actually a disk of hot gas.

Near our lower right is the orbiting star. In this illustration, the star is relatively close to us, with the black hole far behind it. The star is a blue-white ball that, from this perspective, appears slightly larger than the distant black hole.

Two tapered streaks peel off of the glowing star like the pulled-back corners of a smile. These streaks represent tidal tails of stellar debris; material pulled from the surface of the star by the gravity of the black hole. This partial destruction of the star occurs every 3.5 years, when the star’s orbit brings it closest to the supermassive black hole.

During the orbit, the stellar debris from the tidal tails is ingested by the black hole. A byproduct of this digestion is the X-ray gas which swirls in a disk around the black hole.

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Fast Facts for AT2018fyk:

Scale: Image is about 10 arcsec (42,000 light-years) across.
Category: Black Holes, Quasars & Active Galaxies
Coordinates (J2000): RA 22h 50m 16.1s | Dec -44° 51´ 53.5"
Constellation: Grus
Observation Dates: 2 observations, Oct 10-11, 2023
Observation Time: 14 hours 34 minutes
Obs. ID: 28294, 28972
Instrument: ACIS
References: Pasham, D. et al, 2024, ApJ Letters, Accepted. arXiv:2406.18124
Color Code: X-ray: purple; Optical: red, green, and blue
Distance Estimate: About 860 million light-years

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Scientists identify a black hole choking on stardust

In this artist's rendering, a thick accretion disk has formed around a supermassive black hole following the tidal disruption of a star that wandered too close. Stellar debris has fallen toward the black hole and collected into a thick chaotic disk of hot gas. Flashes of X-ray light near the center of the disk result in light echoes that allow astronomers to map the structure of the funnel-like flow, revealing for the first time strong gravity effects around a normally quiescent black hole. Image: NASA/Swift/Aurore Simonnet, Sonoma State University

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.

Jennifer Chu | MIT News Office

Source: Massachusetts Institute of Technology - MIT/News