Time Space Science

Astronomers have found the most powerful eruption in the universe using NASA's Chandra X-ray Observatory. A super massive black hole generated this eruption by growing at a remarkable rate. This discovery shows the enormous appetite of large black holes, and the profound impact they have on their surroundings.
The huge eruption was seen in a Chandra image of the hot, X-ray emitting gas of a galaxy cluster called MS 0735.6+7421. Two vast cavities extend away from the super massive black hole in the cluster's central galaxy. The eruption, which has lasted for more than 100 million years, has generated energy equivalent to hundreds of millions of gamma-ray bursts.

This event was caused by gravitational energy release, as enormous amounts of matter fell toward a black hole. Most of the matter was swallowed, but some of it was violently ejected before being captured by the black hole. "I was stunned to find that a mass of about 300 million suns was swallowed," said Brian McNamara of Ohio University in Athens. "This is as large as another super massive black hole." He is lead author of the study about the discovery, which is in the January 6, 2005, issue of Nature.

Astronomers are not sure where such large amounts of matter came from. One theory is gas from the host galaxy catastrophically cooled and was swallowed by the black hole. The energy released shows the black hole in MS 0735 has grown dramatically during this eruption. Previous studies suggest other large black holes have grown very little in the recent past, and that only smaller black holes are still growing quickly.

"This new result is as surprising as it is exciting," said co-author Paul Nulsen of the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass. "This black hole is feasting, when it should be fasting."

Radio emission within the cavities shows jets from the black hole erupted to create the cavities. Gas is being pushed away from the black hole at supersonic speeds over a distance of about a million light-years. The mass of the displaced gas equals about a trillion suns, more than the mass of all the stars in the Milky Way.

The rapid growth of super massive black holes is usually detected by observing very bright radiation from the centers of galaxies in the optical and X-ray wavebands, or luminous radio jets. In MS 0735 no bright central radiation is found, and the radio jets are faint. The true nature of MS 0735 is only revealed through X-ray observations of the hot cluster gas.

"Until now we had no idea this black hole was gorging itself," said co-author Michael Wise of the Massachusetts Institute of Technology, Cambridge, Mass. "The discovery of this eruption shows X-ray telescopes are necessary to understand some of the most violent events in the universe."

The astronomers estimated how much energy was needed to create the cavities by calculating the density, temperature and pressure of the hot gas. By making a standard assumption that 10 percent of the gravitational energy goes into launching the jets, they estimated how much material the black hole swallowed.

Besides generating the cavities, some of the energy from this eruption should keep the hot gas around the black hole from cooling, and some of it may also generate large-scale magnetic fields in the galaxy cluster. Chandra observers have discovered other cavities in galaxy clusters, but this one is easily the largest and the most powerful.

NASA's Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for NASA's Space Mission Directorate, Washington. Northrop Grumman of Redondo Beach, Calif., was the prime development contractor for the observatory. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Center in Cambridge, Mass.

Additional information and images from Chandra are available on the Web at:

Like cold case investigators, astronomers have used NASA's Chandra X-ray Observatory to uncover evidence of a powerful outburst from the giant black hole at the Milky Way's center.
A light echo was produced when X-ray light generated by gas falling into the Milky Way's supermassive black hole, known as Sagittarius A* (pronounced "A-star"), was reflected off gas clouds near the black hole. While the primary X-rays from the outburst would have reached Earth about 50 years ago, the reflected X-rays took a longer path and arrived in time to be recorded by Chandra.

"This dramatic event happened before we had satellites in space that could detect it," said Michael Muno of the California Institute of Technology in Pasadena. "So, it's remarkable that we can use Chandra to dig into the past and see this monster black hole's capacity for destruction."

Previously, scientists have used Chandra to directly detect smaller and more recent outbursts from the black hole. This latest outburst revealed by the X-ray echo was about 1,000 times brighter and lasted well over 1,000 times longer than any of the recent outbursts observed by Chandra.

Theory predicts that an outburst from Sagittarius A* would cause X-ray emission from the clouds to vary in both intensity and shape. Muno and his team found these changes for the first time, thus ruling out other interpretations. The latest results corroborate other independent, but indirect, evidence for light echoes generated by the black hole in the more distant past.

Scientists have long known that Sagittarius A*, with a mass of about 3 million suns, lurked at the center for Milky Way. However, the black hole is incredibly faint at all wavelengths, especially in X-rays.

"This faintness implies that stars and gas rarely get close enough to the black hole to be in any danger," said co-author Frederick K. Baganoff of the Massachusetts Institute of Technology in Cambridge. "The huge appetite is there, but it's not being satisfied."

During the outburst, the area close to the black hole would have been about 100,000 times brighter than it is currently. If such an outburst had occurred more recently, it likely would have been detected by an X-ray instrument, or would have produced similar features in other nearby clouds.

"Our data show it has been 50 years or so since the black hole had its last decent meal," said Muno. "This is nothing like the feasting that black holes in other galaxies sometimes enjoy, but it gives unique knowledge about the feeding habits of our closest supermassive black hole."

The details of how Sagittarius A* feeds remain unclear. For example, one possibility is that the black hole grows by pulling in matter from the winds of nearby young stars. Also, if there is a disk of material swirling around Sagittarius A*, it might be unstable in such a way that material migrates toward the black hole's edge in clumps, emitting X-rays before disappearing from the universe forever. The theoretical work is still being developed.

Studying this light echo is also important because it illuminates and probes the poorly understood molecular clouds near the center of the galaxy. In particular, it gives information about the dense cores of these clouds where new stars may be forming.

Variability in the X-ray emission between three Chandra observations in 2002, 2004 and 2005 argues against an alternate source for the light echo, which is that it came from a neutron star or black hole pulling matter away from a binary companion. This explanation is not favored because the data show the outburst would have been unusually long and bright for such a binary.

These results were presented at the American Astronomical Society meeting in Seattle, Wash., and will appear in an upcoming issue of The Astrophysical Journal Letters. NASA's Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for the agency's Science Mission Directorate. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Center, Cambridge, Mass.


Adapted from materials provided by Chandra X-ray Center.

Scientists have found new evidence that black holes are performing the disappearing acts for which they are known.
A team from MIT and Harvard has found that a certain type of X-ray explosion common on neutron stars is never seen around their black hole cousins, as if the gas that fuels these explosions has vanished into a void.

This is strong evidence, the team said, for the existence of a theoretical border around a black hole called an event horizon, a point from beyond which nothing, not even light, can escape.

Ron Remillard of the Kavli Institute for Astrophysics and Space Research at MIT led the analysis and is discussing his team's result Jan. 9 at a press conference at the 207th meeting of the American Astronomical Society in Washington, D.C. His colleagues are Dacheng Lin of MIT and Randall Cooper and Ramesh Narayan of the Harvard-Smithsonian Center for Astrophysics in Cambridge.

The scientists studied a complete sample of transient X-ray sources detected with NASA's Rossi X-ray Timing Explorer during the last nine years. They detected 135 X-ray bursts from the 13 sources believed to be neutron stars, but none from the 18 suspected black holes.

Gas released by a nearby star can accumulate on the hard surface of a neutron star, and it will eventually erupt in a thermonuclear explosion. The more massive compact objects in this study suspected of being black holes appeared to have no surface. Gas falling toward the black hole seems to disappear.

"Event horizons are invisible by definition, so it seems impossible to prove their existence," said Remillard. "Yet by looking at dense objects that pull in gas, we can infer whether that gas crashes and accumulates onto a hard surface or just quietly vanishes. For the group of suspected black holes we studied, there is a complete absence of surface explosions called X-ray bursts."

A black hole forms when a very massive star runs out of fuel. Without energy to support its mass, the star implodes. If the star is more than 25 times more massive than our sun, the core will collapse to a point of infinite density with no surface. Within a boundary of about 50 miles from the black hole center, gravity is so strong that not even light can escape its pull. This boundary is the theoretical event horizon.

Stars of about 10 to 25 solar masses will collapse into compact spheres about 10 miles across, called neutron stars. These objects have a hard surface and no event horizon.

Black holes and their neutron star cousins are sometimes located in binary systems, orbiting a relatively normal star companion. Gas from these stars, lured by strong gravity, can flow toward the compact object periodically. This process, called accretion, releases large amounts of energy, predominantly in the form of X-rays.

Gas can accumulate on a neutron star surface, and when conditions are ripe, the gas will ignite in a thermonuclear explosion that is visible as a one-minute event called a Type I X-ray burst. The suspected black holes -- that is, the more massive types of compact objects in this study -- behave as if they have no surface and are located behind event horizons.

The idea of using the absence of X-ray bursts to confirm the presence of event horizons in black holes was proposed in 2002 by Harvard's Narayan and Jeremy Heyl of the University of British Columbia in Vancouver.

The Rossi Explorer, launched on Dec. 30, 1995, is operated by NASA Goddard Space Flight Center in Greenbelt, Md.

According to two astrophysicists from Paris Observatory, the fate of stars that venture too close to massive black holes could be even more violent than previously believed. Not only are they crushed by the black hole’s huge gravity, but the process can also trigger a nuclear explosion that tears the star apart from within. In addition, shock waves in the pancake star carry a brief and very high peak of temperature outwards, that could give rise to a new type of X-ray or gamma-ray bursts.
Scientists have long understood that massive black holes lurking in galactic nuclei and weighing millions of Suns can disrupt stars that come too close. Due to intense tidal forces, the black hole’s gravity pulls harder on the nearest part of the star, an imbalance that pulls the star apart over a period of hours, once it gets inside the so-called “tidal radius”.

Now, Matthieu Brassart and Jean-Pierre Luminet of the Observatoire de Paris-Meudon, France, say the strain of these tidal forces can also trigger a nuclear explosion powerful enough to destroy the star from within. They carried out computer simulations of the final moments of such an unfortunate star’s life, as it penetrates deeply into the tidal field of a massive black hole.

When the star gets close enough the black hole (without falling into), the tidal forces flatten it into a pancake shape. Previous studies already performed by Luminet and collaborators twenty years ago had suggested this flattening would increase the density and temperature inside the star enough to trigger intense nuclear reactions that would tear it apart. But other studies had suggested that the picture would be complicated by shock waves generated during the flattening process, and that no nuclear explosion should occur.

The new simulations investigate the effects of shock waves in detail, and find that even when their effects are included, the conditions favour a nuclear explosion which will completely destroy the star, and which will be powerful enough to hurl much of the star’s matter out of the black hole’s reach.

Stellar fireworks

The tidal disruption of stars by black holes may already have been observed by X-ray telescopes such as GALEX, XMM and Chandra, although at a much later stage : several months after the event that rips the star apart, its matter starts swirling into the hole, heats up and releases ultraviolet light and X-rays. However, if pancake stars really do explode, then they could in principle allow these events to be detected at a much earlier stage. Future observatories, such as the Large Synoptic Survey Telescope (LSST), which will detect large numbers of supernovae, could turn up some explosions of this type.

But this might be not the only hazard facing the doomed star. Brassart and Luminet calculated that the shock waves inside the stellar pancake carry a brief (< 0.1 s) but very high (above 10^9 K) peak of temperature outwards from the centre to the surface of the star. This last result is very promising since it could give rise to a new type of X-ray or gamma-ray burst, making it possible to see the disruption of the star immediately if it gets hot enough.

The rate of such "flambeed pancake stars" is estimated to about 0.00001 event per galaxy. Since almost every galaxy – including our own Milky Way – harbors a massive black hole in its centre, and since the universe is transparent to hard X and gamma radiation, several events of this kind per year should be detectable within the full observable universe.

Conclusion

The planned high-energy, all-sky surveys are the best suited to detect more flares of this type because of their large sky coverage. By providing a quick localization of flambeed stellar pancakes, followed by the detection of the corresponding afterglows in the optical, infrared, and radio bands, these missions could bring as much to the understanding of stellar disruptions by black holes as the Beppo-Sax and Swift telescopes did for the comprehension of gamma-ray bursts.

Journal reference: Shock Waves in Tidally Compressed Stars by Massive Black Holes,

Stellar explosions called novæ are caused by nuclear reactions between the star's atoms. In order to better understand such violent phenomena, astrophysicists study the radiation emitted by certain types of atom, and in particular the fluorine-18 produced by these reactions. Now, researchers at GANIL ,in collaboration with teams from the UK, Belgium, Romania and France, have determined that fluorine-18 appears to be less abundant than expected.This discovery therefore reduces the chances of observing the radiation emitted by this atom. It implies new constraints for the observation and understanding of novæ.

Observed since ancient times, novæ are stellar explosions which occur in our galaxy around 20 times a year. Today, physicists think that they take place in stellar binary systems, which are made up of two stars, a red giant and a small, hot companion called a white dwarf. "Matter is torn off the red giant and falls onto the surface of the white dwarf," explains François de Oliveira Santos, a physicist working at GANIL. "This stellar matter accumulates on the surface of the white dwarf, leading to an increase in its temperature and density. A number of nuclear reactions, transforming one or more atomic nuclei into other particles, then take place: stable atomic nuclei (carbon, oxygen, etc) in the star are transformed into radioactive nuclei, such as fluorine-18." It is by observing the radiation emitted by these particles that researchers hope to better understand the physical processes taking place during novæ.

Fluorine-18 is a radioactive atom whose unstable nucleus is deficient in neutrons compared to its stable form, fluorine-19. When it disintegrates, fluorine-18 emits specific electromagnetic radiation that astrophysicists study in order to get a better understanding of what goes on inside novæ. "The amount of radiation emitted during the explosion depends on the amount of fluorine-18 present," de Oliveira Santos explains. In order to show this, researchers have tried to identify all the nuclear reactions that lead to the creation and destruction of fluorine-18. Since these reactions depend on the structure of the nuclei, they have been studied with the use of particle accelerators.

An experiment carried out at Louvain-la-Neuve University in Belgium, as part of an international collaboration, has led scientists to revise downwards their estimate of the amount of fluorine-18 present in novae. The conclusion is that nuclear reactions involving fluorine-18 in these explosions lead to its destruction to a greater degree than had previously been estimated. "Our result is in agreement with recent theoretical work," de Oliveira Santos points out. "We obtained this result thanks to a new experimental technique that uses beams of accelerated radioactve nuclei." It leads to new constraints for the observation and understanding of stellar explosions.