Black Hole Bends Light Back on Itself

This illustration shows how some of the light coming from a disk around a black hole is bent back onto the disk itself due to the gravity of the hefty black hole. The light is then reflected back off the disk. Astronomers using data from NASA's now-defunct Rossi X-ray Timing Explorer (RXTE) mission were able to distinguish between light that came straight from the disk and light that was reflected. The bluish material coming off the black hole is an outflowing jet of energetic particles. Credit: NASA/JPL-Caltech/R. Hurt (IPAC)/R. Connors (Caltech)

New study proves a theory first predicted more than 40 years ago

You may have heard that nothing escapes the gravitational grasp of a black hole, not even light. This is true in the immediate vicinity of a black hole, but a bit farther out—in disks of material that swirl around some black holes—light can escape. In fact, this is the reason actively growing black holes shine with brilliant X-rays.

Now, a new study accepted for publication in The Astrophysical Journal offers evidence that, in fact, not all of the light streaming from a black hole's surrounding disk easily escapes. Some of it gives in to the monstrous pull of the black hole, turns back, and then ultimately bounces off the disk and escapes.

"We observed light coming from very close to the black hole that is trying to escape, but instead is pulled right back by the black hole like a boomerang," says Riley Connors, lead author of the new study and a postdoctoral scholar at Caltech. "This is something that was predicted in the 1970s, but hadn't been shown until now."

The new findings were made possible by combing through archival observations from NASA's now-defunct Rossi X-ray Timing Explorer (RXTE) mission, which came to an end in 2012. The researchers specifically looked at a black hole that is orbited by a sun-like star; together, the pair is called XTE J1550-564. The black hole "feeds" off this star, pulling material onto a flat structure around it called an accretion disk. By looking closely at the X-ray light coming from the disk as the light spirals toward the black hole, the team found imprints indicating that the light had been bent back toward the disk and reflected off.

"The disk is essentially illuminating itself," says co-author Javier Garcia, a research assistant professor of physics at Caltech. "Theorists had predicted what fraction of the light would bend back on the disk, and now, for the first time, we have confirmed those predictions."

The scientists say that the new results offer another indirect confirmation of Albert Einstein's general theory of relativity, and also will help in future measurements of the spin rates of black holes, something that is still poorly understood.

"Since black holes can potentially spin very fast, they not only bend the light but twist it," says Connors. "These recent observations are another piece in the puzzle of trying to figure out how fast black holes spin."

The new study, titled, "Evidence for Returning Disk Radiation in the Black Hole X-ray Binary XTEJ1550-564," was funded by NASA, the Alexander von Humboldt Foundation, and the Margarete von Wrangell Fellowship. Other co-authors are Thomas Dauser, Stefan Licklederer, and Jörn Wilms of The University of Erlangen-Nüremberg in Germany; Victoria Grinberg of the Universität Tübingen in Germany; James Steiner of the MIT Kavli Institute for Astrophysics and Space Research and Harvard University; Navin Sridhar of Columbia University; John Tomsick of UC Berkeley; and Fiona Harrison, the Harold A. Rosen Professor of Physics at Caltech and the Kent and Joyce Kresa Leadership Chair of the Division of Physics, Mathematics and Astronomy.

Written by Whitney Clavin

Contact

Whitney Clavin
(626) 395‑1944
wclavin@caltech.edu

Source: Caltech (California Institute of Technology)/News

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RXTE Homes in on a Black Hole's Jets

XTE J1550-564 is a binary system in which an evolved star orbits -- and donates matter to -- a black hole estimated at 10 times the sun's mass. Credit: ESO/L. Calçada - View Hi-Res

In April 2000, XTE J1550-564 erupted. The blue line indicates the energy and brightness of X-rays from the system as detected by NASA's Rossi X-ray Timing Explorer. Insets show where the X-rays are thought to originate in the vicinity of the black hole. From June to September, the system's particle jets produced most of the X-rays. Credit: NASA/RXTE. View Hi-Res - Print Resolution

The Rossi X-ray Timing Explorer spacecraft undergoes pre-launch tests in 1995. Credit: NASA/GSFC - View Hi-Res

For decades, X-ray astronomers have studied the complex behavior of binary systems pairing a normal star with a black hole. In these systems, gas from the normal star streams toward the black hole and forms a disk around it. Friction within the disk heats the gas to millions of degrees -- hot enough to produce X-rays. At the disk's inner edge, near the black hole, strong magnetic fields eject some of the gas into dual, oppositely directed jets that blast outward at about half the speed of light.

That's the big picture, but the details have been elusive. For example, do most of the X-rays arise from the jets? The disk? Or from a high-energy region on the threshold of the black hole?

Now, astronomers using NASA's Rossi X-ray Timing Explorer (RXTE) satellite, together with optical, infrared and radio data, find that, at times, most of the X-rays come from the jets.

"Theoretical models have suggested this possibility for several years, but this is the first time we've confirmed it through multiwavelength analysis," said David Russell, lead author of the study and a post-doctoral researcher at the University of Amsterdam.

Russell and his colleagues looked at a well-studied outburst of the black-hole binary XTE J1550-564. The system lies 17,000 light-years away in the southern constellation of Norma and contains a black hole with about 10 times the sun's mass. The usually inconspicuous binary was discovered by RXTE in 1998, when the system briefly became one of the brightest X-ray sources in the sky.

Between April and July 2000, the system underwent another outburst. RXTE monitored the event in X-rays, with some additional help from NASA's Chandra X-ray Observatory. Optical and infrared observations covering the outburst came from the YALO 1-meter telescope at Cerro Tololo Inter-American Observatory in Chile, while radio observations were collected by the Australia Telescope Compact Array.

Drawing on these data, Russell and his team reconstructed a detailed picture of X-ray emission during the outburst. The study appears in the July 1 edition of Monthly Notices of the Royal Astronomical Society.

"We suspect that these outbursts are tied to increases in the amount of mass falling onto the black hole," explained Russell. "Where and how the emission occurs are the only clues we have to what's going on."

As the outburst began in mid-April 2000, the system's brightest X-ray emission was dominated by higher-energy ("hard") X-rays from a region very close to the black hole.

"We think the source of these X-rays is a region of very energetic electrons that form a corona around the innermost part of the disk," Russell said. When these electrons run into photons of visible light, the collision boosts the photons to hard X-ray energies, a process known as inverse Compton scattering. The jets were present, but only minor players.

Over the next couple of weeks, the peak X-ray emission moved to lower ("softer") energies and seems to have come from the dense gas in the accretion disk. At the same time, the hot disk quenched whatever process powers the jets and shut them down.

By late May 2000, XTE J1550-564's accretion disk was cool enough that the jets switched on again. Most of the X-rays, which were fainter but higher in energy, again came from scattering off of energetic electrons close to the black hole.

In early June, as the system faded and its peak emission gradually softened, the jets emerged as the main X-ray source. In the jet, electrons and positrons moving at a substantial fraction of light speed emit the radiation as they encounter magnetic fields, a process called synchrotron emission.

The jets require a continuous supply of particles with energies of a trillion electron volts -- billions of times the energy of visible light. "The total energy bound up in the jet is enormous, much larger than previously thought," Russell said.

As summer wore on, the jets gradually faded and their X-ray emission softened. By September, the system's brightest X-rays came from high-speed blobs of matter that the jets had hurled into space during previous eruptions.

"We're really beginning to get a handle on the 'ecology' of these extreme systems, thanks in large part to RXTE," Russell added. "We can apply what we've learned in nearby binaries like XTE J1550 to the supersized black holes and jets found at the centers of galaxies."

Launched in 1995, RXTE is still going strong. "Of currently operating NASA missions, only Hubble has been working longer," said Tod Strohmayer, the mission's project scientist at NASA's Goddard Space Flight Center in Greenbelt, Md. RXTE's unique capabilities provide insight into accreting black holes and neutron stars and allow it to detect short, faint outbursts that are easily missed by other current missions exploring the X-ray regime.

Related links:

Rossi X-ray Timing Explorer

XTE J1550-564: Chandra Tracks Evolution of X-Ray Jets

X-rays from Free Electrons

Francis Reddy
NASA's Goddard Space Flight Center