Thanks to two orbiting X-ray observatories, astronomers have the first strong evidence of a supermassive black hole ripping apart a star and consuming a portion of it.
The event, captured by NASA's Chandra and ESA's XMM-Newton X- ray Observatories, had long been predicted by theory, but never confirmed.
Astronomers believe a doomed star came too close to a giant black hole after being thrown off course by a close encounter with another star. As it neared the enormous gravity of the black hole, the star was stretched by tidal forces until it was torn apart. This discovery provides crucial information about how these black holes grow and affect surrounding stars and gas.
"Stars can survive being stretched a small amount, as they are in binary star systems, but this star was stretched beyond its breaking point," said Stefanie Komossa of the Max Planck Institute for Extraterrestrial Physics (MPE) in Germany, leader of the international team of researchers. "This unlucky star just wandered into the wrong neighborhood."
While other observations have hinted stars are destroyed by black holes (events known as "stellar tidal disruptions"), these new results are the first strong evidence. Evidence already exists for supermassive black holes in many galaxies, but looking for tidal disruptions represents a completely independent way to search for black holes. Observations like these are urgently needed to determine how quickly black holes can grow by swallowing neighboring stars.
Observations with Chandra and XMM-Newton, combined with earlier images from the German Roentgen satellite, detected a powerful X-ray outburst from the center of the galaxy RXJ1242-11. This outburst, one of the most extreme ever detected in a galaxy, was caused by gas, heated to millions of degrees Celsius, from the destroyed star being swallowed by the black hole. The energy liberated in the process was equivalent to a supernova.
"Now, with all the data in hand, we have the smoking gun proof that this spectacular event has occurred," said coauthor Guenther Hasinger, also of MPE.
The black hole in the center of RXJ1242-11 is estimated to have a mass of about 100 million times Earth's sun. By contrast, the destroyed star probably had a mass about equal to the sun, making it a lopsided battle of gravity. "This is the ultimate David versus Goliath battle, but here David loses," said Hasinger.
The astronomers estimated about one percent of the star's mass was ultimately consumed, or accreted, by the black hole. This small amount is consistent with predictions the momentum and energy of the accretion process will cause most of the destroyed star's gas to be flung away from the black hole.
The force that disrupted the star in RXJ1242-11 is an extreme example of the tidal force caused by differences in gravity acting on the front and back of an object. The tidal force from the moon causes tides in Earth's oceans. A tidal force from Jupiter pulled Comet Shoemaker-Levy apart, before it plunged into the giant planet.
The odds stellar tidal disruption will happen in a typical galaxy are low, about one in 10,000 annually. If it happened at the center of the Milky Way Galaxy, 25,000 light-years from Earth, the resulting X-ray outburst would be about 50,000 times brighter than the brightest X-ray source in our galaxy, beside the sun, but it would not pose a threat to Earth.
Other dramatic flares have been seen from galaxies, but this is the first studied with the high-spatial resolution of Chandra and the high-spectral resolution of XMM-Newton. Both instruments made a critical advance. Chandra showed the RXJ1242-11 event occurred in the center of a galaxy, where the black hole lurks. The XMM-Newton spectrum revealed the fingerprints expected for the surroundings of a black hole, ruling out other possible astronomical explanations.Information and images about the event are available on the Internet at:
More information is available in ESO PR 26/03:
One of the most enigmatic stellar systems in our Milky Way Galaxy has been shown to harbour a very massive black hole. With 14 times more mass than the Sun , this is the heaviest known stellar black hole in the Galaxy.
Using the ISAAC instrument on the VLT 8.2-m ANTU telescope at the ESO Paranal Observatory, an international team of astronomers  peered into a remote area of the Milky Way to probe the binary system GRS 1915+105, located almost 40,000 light-years away.
They were able to identify the low-mass star that feeds the black hole by means of a steady flow of stellar material. A detailed follow-up study revealed how this star revolves around its hungry companion. The analysis of the orbital motion then made it possible to estimate the mass of the black hole.
The observation of the heavy black hole in GRS 1915+105 is opening up fundamental questions about how massive stellar black holes form, and whether or not such objects rotate around their own axes.
The Gemini Observatory press release on the deepest mid-infrared
image ever of the core and jet of M87 has been made public at:
Slight advances could make event horizon of Milky Way's central black hole "visible"
A "picture" of the massive black hole thought to be lurking at the heart of our home galaxy may be within astronomers' reach in the next few years, according to a report in the Jan. 1, 2000 edition of Astrophysical Journal Letters.
The paper predicts that upcoming improvements in scientific techniques could permit astronomers to see how a narrow escape from the black hole's clutches twists, dims, and amplifies radio waves.
Such observations should reveal a circular shadow at the heart of the galaxy the first image of a black hole's event horizon according to a computer model created by theorists at The Johns Hopkins University, the Max-Planck-Institut fuer Radioastronomie in Germany, and the University of Arizona.
The event horizon is thought to be the defining feature of a black hole, a point-of-no-return surrounding the hole inside which even light cannot escape the black hole's gravity. Imaging this would be a final step in the black hole's journey from curious theoretical oddity to cosmic reality.
"Regardless of the structure of the region around the black hole that we tried in our computer models, we saw a shadow in the simulated images," says Eric Agol, a postdoctoral researcher at Hopkins and an author of the paper. "This paper is our way of trying to interest astronomers in working together to perform the actual observations, which could produce very exciting results."
Agol cautions that the same plasma that emits radio waves near the black hole might also block the radio waves needed to "see" the hole--an effect not included in the models. This could be circumvented by observing at even shorter wavelengths where the plasma becomes transparent and the black hole shadow will appear. "This would make it harder to see it from the ground, but it should always be possible to see it from space," Agol says, noting that some shorter wavelengths are blocked by Earth's atmosphere.
So far scientists have only been able to indirectly detect black holes by observing their effects on the orbits of nearby stars or by detecting the powerful radiation given off by gas and other material being pulled into the black hole.
Astronomers have seen these effects in the centers of other galaxies. The Milky Way's center can't be seen in visible light because there's too much interstellar gunk in the 25,000 light years between Earth and the galactic center. But longer-wavelength radiation like infrared radiation and radio waves can make it through relatively unscathed.
"At Sagittarius A star [Sagittarius A*], a point at or near our galaxy's center, astronomers have found a compact source of very strong radio emission, perhaps created by highly ionized gas surrounding a black hole," says Heino Falcke, research scientist at Max-Planck-Institut and lead author on the paper. "Infrared observations of the same region show rapidly moving stars pulled around by a very concentrated mass at the same position as the radio source Sagittarius A*. This is probably the best evidence that we have for a black hole so far, but not decisive proof."
To zoom in further on the radio wave emission in this area, scientists have used a technique known as Very Long Baseline Interferometry (VLBI). By coordinating and comparing the results they receive from different radio telescopes, they can produce an image with greater detail and resolution than the individual radio telescopes could on their own.
"The resolving power is equivalent to what you'd get if you had a radio telescope as large as the telescopes you're combining and the area between them," says Falcke. "This can be as large as the size of the Earth."
Astronomers at the Max-Planck-Institut and elsewhere have been working to use VLBI to observe shorter wavelengths of radio emission, a technique known as millimeter-VLBI. By pushing VLBI to the shortest wavelengths and highest spatial resolutions available in astronomy, they have already come very close to the resolution that should be needed to see the shadow.
"I think we didn't realize before how close the technique is to detecting this shadow," Falcke says. "With the currently available resolution, we could 'see' from Berlin Germany a radio source in Los Angeles the size of a mustard seed. Now we have to improve things just to the point where we can image a dent on the seed."
"The improvements necessary to test this prediction are within reach and should become feasible over the next few years," says Anton Zensus, director at the Max-Planck-Institut and leader of the VLBI group.
For the paper, the authors took what astronomers currently know about the mass of Sagittarius A* and plugged it and other potential features of the black hole, such as its rotation, into a "relativistic ray-tracing" program Agol had developed. The program traces the path of electromagnetic radiation through space warped by the tremendous gravity of a black hole.
"You can think of it as taking each photon of radiation emitted somewhere near the black hole and following its path to the observer," explains Fulvio Melia, astrophysicist from the University of Arizona and co-author on the paper. "The program calculates the effects of the black hole on the radiation's path and wavelength, effects that are very precisely predicted by Einstein's Theory of General Relativity."
"A similar, simplified calculation was made by physicist James Bardeen in the 1970s," says Agol. "At that time, we didn't have as much information on the galactic center, so his work was considered by many to be a purely theoretical exercise."
Given the resolution achievable at short radio wavelengths, the new calculations showed a distinctive pattern in radiation from Sagittarius A*: a circular shadow.
"With the major observatories working together, and a further improvement of millimeter-VLBI, we should soon be able to actually image the shadow of a black hole. This would be the final test of whether black holes and event horizons exist," says Falcke.
Since demand is high for time at radioastronomy observatories, he acknowledges, that would take no small amount of money, effort and sacrifice. But because of the potentially tremendous step forward this effort might produce, he and the other authors strongly feel the challenge is worthwhile.
This research was supported by Melia's Sir Thomas Lyle Fellowship and grants from NASA, DFG (Deutsche Forschungsgemeinschaft), and the National Science Foundation.
For More Information:
"The Black Hole in the Galactic Center," a slideshow by Heino Falcke:
Included in the show are an MPEG movie simulating a zooming view into the Galactic Center and the black hole (slide 14), and color images (GIFs) of the shadow seen in the author's calculations (slides 11 and 12)
Electronic preprint available at:
Black holes are the stuff of science fiction. Or are they? Certainly one of the most compelling phenomena in the universe, black holes have inspired fantasies and nightmares alike. For years NASA's Hubble Space Telescope has been hard at work hunting for and learning about these awe-inspiring celestial voids. What are they? What do they look like? Is there one at the center of our Milky Way galaxy? Are they really "the point of no return"? Students can now find the answers to these and other questions in a fun and interactive way. In or out of the classroom, visit "No Escape: The Truth about Black Holes" on the Space Telescope Science Institute's "Amazing Space" Web site:
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