Milky Way's black hole wakes up

Milky Way's black hole wakes up

Our galaxy's giant black hole erupted 300 years ago.

Provided by NASA's Goddard Space Flight Center

 

 

April 15, 2008

Using NASA, Japanese, and European X-ray satellites, a team of Japanese astronomers has discovered that our galaxy's central black hole let loose a powerful flare 3 centuries ago.



The finding helps resolve a long-standing mystery: why is the Milky Way's black hole so quiescent? The black hole, known as Sagittarius A* (pronounced "A-star"), is a certified monster, containing about 4 million times the mass of our Sun. Yet the energy radiated from its surroundings is billions of times weaker than the radiation emitted from central black holes in other galaxies.



"We have wondered why the Milky Way's black hole appears to be a slumbering giant," says team leader Tatsuya Inui of Kyoto University in Japan. "But now we realize that the black hole was far more active in the past. Perhaps it's just resting after a major outburst."



The new study, which will appear in the Publications of the Astronomical Society of Japan, combines results from Japan's Suzaku and ASCA X-ray satellites, NASA's Chandra X-ray Observatory, and the European Space Agency's XMM-Newton X-ray Observatory.



The observations, collected between 1994 and 2005, revealed that clouds of gas near the central black hole brightened and faded quickly in X-ray light as they responded to X-ray pulses emanating from just outside the black hole. When gas spirals inward toward the black hole, it heats up to millions of degrees and emits X-rays. As more and more matter piles up near the black hole, the greater the X-ray output.

 

These X-ray pulses take 300 years to traverse the distance between the central black hole and a large cloud known as Sagittarius B2, so the cloud responds to events that occurred 300 years earlier. When the X-rays reach the cloud, they collide with iron atoms, kicking out electrons that are close to the atomic nucleus. When electrons from farther out fill in these gaps, the iron atoms emit X-rays. But after the X-ray pulse passes through, the cloud fades to its normal brightness.

Amazingly, a region in Sagittarius B2 only 10 light-years across varied considerably in brightness in just 5 years. These brightenings are known as light echoes. By resolving the X-ray spectral line from iron, Suzaku's observations were crucial for eliminating the possibility that subatomic particles caused the light echoes.



"By observing how this cloud lit up and faded over 10 years, we could trace back the black hole's activity 300 years ago," says team member Katsuji Koyama of Kyoto University. "The black hole was a million times brighter 3 centuries ago. It must have unleashed an incredibly powerful flare."



This new study builds upon research by several groups who pioneered the light-echo technique. Last year, a team led by Michael Muno, who now works at the California Institute of Technology, used Chandra observations of X-ray light echoes to show that Sagittarius A* generated a powerful burst of X-rays about 50 years ago — about a dozen years before astronomers had satellites that could detect X-rays from outer space. "The outburst three centuries ago was 10 times brighter than the one we detected," says Muno.



The galactic center is about 26,000 light-years from Earth, meaning we see events as they occurred 26,000 years ago. Astronomers still lack a detailed understanding of why Sagittarius A* varies so much in its activity. One possibility, says Koyama, is that a supernova a few centuries ago plowed up gas and swept it into the black hole, leading to a temporary feeding frenzy that awoke the black hole from its slumber and produced the giant flare.

 

 

Source: Newsletter from Astronomy

 

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Supersized stellar blackhole prompts model rewrite

Supersized stellar blackhole

Boffins go back to the drawing board

Researchers have located the most massive stellar black hole ever discovered, just three million light-years away in a nearby galaxy. The stellar remnant is in a binary system known as M33, orbiting a huge companion star. The researchers say the find is "intriguing", because of what it suggests about stellar evolution.

A stellar black hole is what is left after the death-by-collapsing-core of a massive star. The star that formed this one must have been huge. The scientists used the Chandra X-Ray observatory and the Gemini telescope on Mauna Kea in Hawaii to measure the mass of the black hole, and discovered the remnant still has 15.7 times the mass contained in our own modest, yellow sun. Its companion star is also a humdinger - checking in at roughly 70 solar masses, it is the largest known companion star to a black hole. Eventually it too will go supernova, leaving a binary system containing only black holes.

"This discovery raises all sorts of questions about how such a big black hole could have been formed," said Jerome Orosz of San Diego State University, lead author of a paper appearing in the 18 October issue of Nature. Conventional models of black hole formation suggest that the star would have been much larger even than its 70-solar-mass companion. It would have been so big that its radius would have been larger than the current separation between the two bodies, NASA's boffins explain. This means the two stars must have drawn closer together while sharing a common outer atmosphere. But if this were the case, according to conventional models, the black hole shouldn't have retained such a large mass.

Still, it did, so the models are being re-thought. The researchers say the star must have lost mass roughly 10 times more slowly than they expected before it exploded. The discovery could help explain an incredibly bright supernova, observed in 2006. The progenitor of this supernova is thought to have been about 150 solar masses when it exploded, which would make more sense if more massive stars lose their mass more slowly.The system is also interesting because it is an eclipsing black hole. This unusual property is what allowed researchers to make "unusually accurate" estimates of the mass of both the black hole and its companion.

Acknowledgements: Report by Lucy Sheriff through Yahoo news

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The loneliest black holes

The loneliest black holes
Supermassive black holes are actively growing in even the emptiest regions of the universe.
Provided by Drexel University

This artist's impression of a supermassive black hole highlights the accretion disk of gas and stars swirling around the black hole, and the jets of material ejected along the poles. Supermassive black holes are found even where galaxies are sparse and interaction is minimal. These black holes accrete matter at a slower rate than black holes in denser galactic environments. A. Kamajian/NASA

This is a tiny extract from a newsletter to me from the Astronomy magazine. The readers may further enrich their knowledge by subscribing to it.

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June 6, 2007

In a study of more than 1,000 void galaxies, using data from the Sloan Digital Sky Survey (SDSS-II), astronomers from Drexel and Widener Universities announced that the growth of these monster black holes — with masses millions to hundreds of millions times that of our sun — are found where galaxies are sparse and interact very little with each other. The researchers also found that the accretion of matter onto these void black holes is slower than in denser galactic environments.These findings shed light on the black hole formation and evolution process by showing that the environment does affect how quickly galaxies proceed through their evolutionary cycle. The simple presence of growing supermassive black holes in the rural outposts of the universe challenges the current theoretical models of galaxy and structure formation and evolution, explained Anca Constantin of Drexel University, lead author of the paper delivered last week at the American Astronomical Society meeting in Honolulu. "Interestingly, we see actively accreting galactic black holes in all phases of evolution in these sparse regions," said Constantin. "This means that the black hole growth process is quite similar in what could be compared to the most reclusive countrysides and in the crowded urban regions of the universe." The void regions, nearly empty, three-dimensional fields hundreds of millions of light-years across, fill half of the universe. Only five percent of all galaxies live in these bubble-like regions. The other 95 percent of galaxies live together in communities, crowded into clusters, filaments, and walls: the cities and suburbs of the universe. Studying a 700-million light year wide 'slice' of the universe, the researchers found that spectra of the centers of void galaxies show hot gases ionized by light emitted from matter swirling around supermassive black holes. Constantin adds that, "the more isolated accreting black holes are however not as active as the ones in more populous environs, and the fuel seems less available for accretion in voids than in 'urban' galaxies." Astronomer Fiona Hoyle, a member of the discovery team from Widener University added: "This is strange given that these reclusive galaxies are forming stars at higher rates than their counterparts in denser regions; this means there is plenty of fuel, but it is not efficiently channeled toward the central engine."


Star formation requires the presence of large amounts of gas and so there must be more than enough gas in the void galaxies if their star forming rates are high, explained Hoyle. The smaller accretion rate observed in void galaxies means that this gas is just not getting down to the nuclear region where accretion happens. Interactions with other galaxies are thought to disturb the gravitational potential, which drives some gas into the nuclear region. "These interactions are not as frequent in voids, so the 'feeding' of the black hole is slower." These rugged individuals in voids do not need to compete with their neighbors for fuel, and their life cycle is rarely bothered, noted Constantin. In contrast, life is more hectic in crowded regions where galaxy interactions are frequent. As a consequence, galaxies are either stripped of their gas or more material is funneled toward the central engine. This means that there are many more chances the accretion onto black holes is enhanced or turned off in more 'urban environments.' "On the other hand, the void galaxy black holes might take longer to reach the mature, low accretion rate phase, which might explain why the most massive, lazy black holes are less frequent in voids," she noted. The data studied by Constantin may also show that active black holes appear to be more common in voids but only among small (less massive) galaxies, while less common among massive galaxies. This is also a clue that the life cycle of black hole growth in voids is delayed or slower compared to that in denser regions. Discovery team member Michael Vogeley of Drexel said that it's particularly puzzling that the few most massive and sluggishly accreting void systems live within the most secluded sub-regions, while their "urban" counterparts are found in the most populated neighborhoods. "Perhaps because massive objects are prone to accreting material around them, such a 'cleaning' process would contribute to emptying the already rarefied neighboring space in voids," Vogeley noted. "This would leave little or insufficient material for future formation of other nearby massive, bright galaxies." In contrast, within galaxy clusters where there is plenty of stuff around, accretion of surrounding material would make a small difference. These results have been possible only because of the sheer number of void regions and void galaxies found in the SDSS-II data, the most ambitious survey of the universe ever undertaken, the researchers said. The sample used in the analysis announced last week comprises more than 1,000 void galaxies. Previously, the black hole accretion in centers of void galaxies had been studied in only a handful of objects contained in only one void region, the Bootes Void.

Acknowledgements: Astronomy magazine's newsletter to me

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