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Konrad Rontgen discovered X-rays in 1895. X-rays are sometimes called Rontgen rays. He was doing an experiment with a fluorescent plate (like a TV screen) and a beam of fast electrons in a tube. By accident he found that the fluorescent plate glowed even when it was a long way from the electron tube. He put his hand in front of the fluorescent plate and saw the first X-ray picture. He saw the bones of his hand because the electron tube was making mysterious, invisible 'X-rays'. Nowadays, X-rays are produced in an X-ray tube by firing a beam of electrons into a Tungsten metal target. X-rays are electromagnetic waves with shorter wavelengths than ultraviolet waves.

The burst of gamma rays detected last December 14, surpassed in power only by the original Big Bang, may be millions of stars being engulfed by a newly born black hole at the edge of the universe. According to a new theory the energy released could be best explained by the formation of a super black hole at least thousands of times more massive than our own sun. A gravitational collapseis the only way to release that much energy.^46b

Generally lasting just about 10 seconds, the amount of energy released by a typical gamma ray burst rivals or even surpasses the total energy output of a supernova - the energy released by the usual supernova is more than a hundred times the energy output of the sun during its lifetime.

Astronomers from the California Institute of Technology detected a cosmic gamma ray burst about 100 times more energetic than previously theorized, located about 12 billion light years from earth when the universe was in its youth. The burst, officially designated GRB 971214 but nicknamed "Big Bang 2," lasted 50 seconds and was detected by the Italian/Dutch BeppoSAX satellite and NASA's Compton Gamma Ray Observatory satellite. The discovery of such a huge burst of energy - unprecedented in astronomy except for the Big Bang itself - sent theorists to their notepads and computers in search of an explanation.

One prevailing model pointed to the merger of two orbiting neutron stars, the end-product of a supernova explosion. According to this model, bursts of gamma rays are released shortly before the orbiting neutron stars, tugged by each other's gravity, plunge into a final death spiral and merge to form a black hole with a mass about three times that of the sun. The resulting hot debris coalesces into a fireball that expands near the speed of light, generating gamma rays.

About a half-dozen such orbiting binary neutron stars have been observed in our own galaxy. It's also recognized that, owing to Einstein's general theory of relativity, these objects will spiral into one another.

However, according to the same theory, their masses must be limited to less than a few times the mass of our sun.

"Such a limited energy cannot explain the gigantic energy of a gamma ray burst," said Shi, "unless the energy is focused into a very narrow beam, just as a search light beam appears to be brighter.

"But how energy can be focused into a very narrow beam at the final moment of a neutron star-neutron star binary is, however, as mysterious as gamma ray bursts themselves."

Prior to last December's discovery, the UCSD astrophysicists began seeking an alternate explanation for gamma ray bursts following the earlier detection of another giant flash last May. "We were very excited when heard the news of 'Big Bang 2'," Shi said, "we felt vindicated."

Their thoughts turned to the formation of a supermassive black hole, at least thousands of times more massive than our sun. In recent years, giant black holes have been observed at the center of many galaxies, including our own Milky Way, and are believed to lie at the centers of extremely energetic quasars billions of light years away.

However, questions surrounding how these gargantuan black holes formed were as profound as the mystery of the gamma ray bursts. The theory proposed by the UCSD researchers, in effect, suggests a solution to both.

"We're suggesting that the events that give rise to distant energetic gamma bursts could also give rise to supermassive black holes in galaxies," said Fuller.

According to the UCSD model, the genesis of a supermassive black hole could begin with the merger of hundreds of thousands or millions of stars drawn together by their own gravitational forces. As the coalescing and colliding stars interact with one another, matter is ripped away from the stellar surfaces until it settles into a single, unstable and short-lived super star, and/or produces a hot plasma of electrons and positrons. In either case, a tremendous amount of mass-more than thousands of times that of the sun-plunges through an "event horizon." A supermassive black hole is born.

In its wake, massive amounts of energy would be released in the form of neutrinos and anti-neutrinos that, when they come into contact with other, annihilate into a huge fireball whose byproduct is the emission of gamma rays.

The UCSD calculations suggested that the energy released by such an event would have no problem matching the energy detected by the recent gamma ray bursts.

"The main problem our model faces is a technical one," said Fuller. "That's getting the neutrino energy deposited in a region where the amount of matter, the density of matter, is very low. It has to be low in order for the conversion to gamma rays to be efficient."

Shi noted that it's also possible that supermassive events could account for the most powerful gamma ray bursts, while other less energetic bursts could be triggered by other cosmic events, including the merger of two orbiting neutron stars.

"There may be more than one population of gamma ray bursts," said Shi. "But we think our model explains the most energetic population."

Their model could be tested. For example, the neutrino burst from the birth of a nearby supermassive black hole may be detectable in the proposed ICECUBE neutrino detector in Antarctica, a joint venture of U.S. and several European countries. If a gamma ray burst is seen at the same time as the neutrino burst, "we will be truly vindicated," said Shi.

Support for the study was provided by grants from NASA and the National Science Foundation.

Hardly an astronomical announcement makes the front pages without being said to overturn all existing theories. Gamma-ray bursts are one of the few things for which this has often been true. These seemingly random flashes--which, if you had gamma-ray vision and didn't blink at the wrong time, would outshine the rest of the sky--were long thought to originate in our galaxy. But that theory foundered in 1991, when the Compton Gamma Ray Observatory satellite detected bursts all over the sky, not only in the Milky Way. Another hypothesis, in which bursts occur just outside our galaxy, crumbled last year when the first distance measurement of a burst put it too far away. Although astronomers now agree that bursts are some kind of megaexplosion in distant galaxies, they still appear on most top-10 lists of cosmic mysteries.^46c

This past spring astronomers reported the distances to two new bursts--which, at first glance, seemed to scuttle some of the few remaining plausible explanations. The first of these bursts, spotted last December 14 by the Beppo-SAX satellite, occurred in a galaxy 12 billion light-years away, according to observations by Srinivas R. Kulkarni and S. George Djorgovski of the California Institute of Technology. To be so bright at such a distance, the burst must have shone more brilliantly than any object previously recorded.

Just as astronomers were reconciling themselves to the unexpected distance and brilliance of this burst, along came another on April 25 that was unexpectedly near and dim: nearly 100 times closer and 100,000 times dimmer than the December event. Even stranger, this burst was followed not by the usual afterglow of less energetic radiation but by a supernova--the first time an exploding star and gamma burst have been seen together. The supernova appeared in visible-light images made by Titus J. Galama of the University of Amsterdam and in radio observations by Mark Wieringa of the Australia Telescope National Facility. This was no normal supernova, either. It dimmed more slowly than others with its spectral characteristics, and it spewed out more radio power than any supernova seen before.

Amid these events, a third burst went off--the burst of hype. A widely quoted NASA press release called the December event "the most powerful explosion since the creation of the universe," which slighted cosmic upheavals that have gone undetected. The hyperbole also made gamma bursts out to be something radically different from supernovae, when in fact both entail the death of a star and the conversion of much of its mass into pure energy.

In a supernova the energy conversion involves the thermonuclear detonation of a white dwarf star or the implosion of a massive star. But such cataclysms could not produce gamma bursts: the stellar debris would choke off the gamma rays. Bursts must come from low-density sources. And yet their energy must ultimately originate in high-density volumes at most 100 kilometers across, or else the bursts would not flash and flicker as they do. These two requirements imply that the production of energy and the generation of gammas occur in separate locations.

The second of these steps has been explained by Martin J. Rees of the University of Cambridge and Peter MÈsz·ros of Pennsylvania State University. A jet of radiation and electrons squirting out at near light speeds would outrun any interfering debris. After this fireball had ballooned to several hundred million kilometers in size, shock waves within it would give off the gamma flash; later, collisions with surrounding matter could emit an afterglow. Radio observations by Dale A. Frail of the National Radio Astronomy Observatory have supported this explanation.

But what would produce enough energy to create such a jet? In one scenario, proposed in 1986 by Bohdan Paczy¥nski of Princeton University, two neutron stars or a neutron star and a black hole orbit ever more tightly and eventually unite in a passionate but ruinous embrace. Stan E. Woosley of the University of California at Santa Cruz suggested another possibility in 1993. Perhaps "hypernovae," also called "collapsars"--souped-up supernovae that occur when stars are too massive to undergo normal supernovae--power the cosmic flashes.

Despite some observers' initial claims, the brightness of the bursts may not be enough to decide between these models. Both lead to the same kind of object, a black hole surrounded by a ring of debris. And both leave the same questions unanswered: What triggers the fireball? Is the radiation focused into beams or emitted uniformly? Pinpointing the bursts' locations within their galaxies may supply clues. Neutron stars wander far from their places of birth before merging, whereas hypernovae stay put. So if bursts tend to occur in star-forming regions--as a few shreds of evidence now suggest--hypernovae seem the more likely source.

But there is a third possibility. The diversity of these latest bursts, Woosley says, suggests that each model accounts for some bursts. This time, rather than overturning all the theories, the observations may have done the opposite: confirmed them, all of them.

The star - GM Sgr - is in the constellation Sagittarius, in the direction of the center of our galaxy. GM Sgr is a visible star discovered in 1927. It's actually part of a binary system. Orbiting GM Sgr is a massive, compact body, most likely a neutron star or a black hole, that's causing all the fuss, starting with a modest announcement early this year.

"On Feb. 20, Beppo SAX and the Rossi X-ray Timing Explorer observed an X-ray source that was variable and noted that it was coincident with GM Sgr," said Dr. Mike McCollough, a high-energy astrophysicist working with the Universities Space Research Association at NASA's Marshall Space Flight Center. Beppo SAX is a Dutch-Italian X-ray astronomy satellite. Rossi X-ray Timing Explorer (RXTE) is operated by NASA's Goddard Space Flight Center.

While observing the galactic center, the Beppo SAX team observed an X-ray transient, an object that became almost 1/10th as bright as the Crab, and then faded away. The neutron star at the heart of the Crab Nebula is so regular in its rotation and brightness than astronomers use it as a standard candle for calibrating their instruments and for measuring the brightness of other sources.

Both teams calculated that the source - named J1819.3-2525 by the Beppo SAX team and J1819-254 by the RXTE team, both using its location in the sky - was associated with GM Sgr. Then it faded and attention went elsewhere.

Fast forward to Sept. 15: Scientists using the Rossi X-ray Timing Explorer (RXTE) announced that the spacecraft's All-Sky Monitor had seen a strong X-ray transient apparently associated with GM Sgr.

"The All-Sky Monitor was seeing nothing and then, boom! it goes off, up to 12 Crab," McCollough explained. "It was pretty bright."

The brightness started at 1.6 to 3.7 Crab, then spiked to 10 and then 12.2 times the Crab before suddenly dropping off the scale.

As soon as the RXTE team sent out a circular announcing the event, McCollough, Dr. Mark Finger, a high-energy astrophysicist with NASA/Marshall, and Pete Woods, a graduate student with the University of Alabama in Huntsville, started checking through the data from the Burst and Transient Source Experiment (BATSE) aboard the Compton Gamma Ray Observatory.

BATSE comprises eight detectors that stare at the entire sky so they can record gamma-ray bursts from deep space. But it also sees every bright source in the 20,000 to 100,000 electron volt (20-100 keV) range, if the right computer codes are used to filter the data. "Now you see it, now you don't" actually becomes a way of seeing discrete sources.

X-Ray Emission Images

This is done by making the Earth part of the instrument. If a compact body's location is known, then the BATSE team can watch for changes in the data as the source rises and sets as BATSE's orbits the Earth.

"We can take measurements from about two minutes before it sets, then as it drops below the horizon, and then get two minutes of background after it sets," McCollough explained. "You subtract the background from the observations before it set, and you know how bright the source was." The same is done for sources as they rise. Thus, a source can be checked every 45 minutes. (Gamma-ray bursts, however, last from a few seconds to a few minutes, at most, and never make a repeat performance.)

Checking through their data, McCollough and Finger found J1819-2525 acting up.

"In about 2 hours this source went from not being there for BATSE to being 5 times brighter than the Crab," McCollough said. "And an hour and a half later, it's gone."

He noted that "you always worry when you get hits like that" because an energetic cosmic ray can have the same effect in a detector. But the rise and fall in the data were seen by two detectors that differed only to the extent that one detector was pointed at a slightly different angle than the other, so it was not a cosmic ray event.

"What we saw later makes it look more real," McCollough continued. Ten hours later, "nothing, and then a major jump about the same time that RXTE saw it. This things takes off, it goes to 8 times the Crab. Then it comes down very dramatically, but is intensely active for about a half of a day, then it drops off the scale."

The question on everyone's mind now is, What caused the outburst? The X-rays themselves are a result of material falling into the compact body which probably is a neutron star or a black hole.

"There are two ways to feed this beast," McCollough said. The first is a wind flowing out from the larger star and being partially captured by the compact companion.

The second is called Roche lobe overflow. The Roche lobe is a teardrop-shaped region of space where the gravitational pulls from the two stars balance each other. Periodically, gas from the larger star will overflow the lobe and squirt through the balance point between the two to feed an accretion disk of material swirling into the compact body.

In either case, the X-ray outburst is powered by an incredibly fast fall down the steep gravitational field of the compact body. At the end, matter from the primary body either slams into a neutron star's tougher-than-diamond surface, or is super-heated in an incredible traffic jam at the event horizon before it disappears into a black hole. The kinetic energy from the superhot matter is turned into X-rays.

"You don't want to be anywhere near this," McCollough cautioned. GM Sgr and J1819-2525 are estimated to be up to 10 times farther away than the Crab Nebula. Since brightness drops with the square of the distance, the source was as much as 100 x 12 or 1,200 times more luminous than the Crab at its peak.

"Just in X-rays, this is 100,000 times more luminous than the entire output of the sun at the object's peak," McCollough estimates. That peak output is estimated at about 10³⁸ ergs per second.

To put that in perspective, 1 erg is a mosquito bumping into your hand. The proverbial apple hitting Newton on the head had a total energy of 10⁷ ergs. The 10-megaton fusion device that obliterated part of the Bikini Atoll was 10²³ ergs. So, for a short period of time, J1819-2525 was equivalent 1 million billion high-yield nuclear bombs every second.

But what prompted GM Sgr to feed the compact object?

That remains a mystery because the exact nature of the visible star remains uncertain, McCollough noted, even though several detailed measurements have been made. And right now is a difficult time to get a visible-light spectrum that would help identify the star because the compact body's X-ray burst would have energized the outer atmosphere of the visible star.

Still, measurements are being made that eventually help delineate both objects. The observatory at the University of Kyoto captured images that, when compared with images taken a few days earlier, show GM Sgr has brightened. And the Very Large Array of radiotelescopes has detected a "reasonably bright" radio source where none was detected in 1996.

While J1819-2525 has faded below detectable levels for now, the story is not over. The American Association of Variable Star Observers has put GM Sgr near the top of its watch list. Both the RXTE and BATSE teams are checking their data to see what J1819-2525 may do next.

And McCollough is going back in time.

"There's a chance that this guy has gone off before and we didn't catch it," he said. Buried in more than eight years of observations by BATSE may be other outbursts that may reveal clues to the true nature of J1819-2525.

An intense wave of gamma rays, emanating from a catastrophic magnetic flare on a mysterious star 20,000 light years away, struck the Earth's atmosphere on August 27, 1998, providing important clues about some of the most unusual stars in the Universe. Scientists said the gamma radiation posed no health risk to humans.

The wave hit the night side of the Earth and ionized (or knocked electrons out of) the atoms in the upper atmosphere to a level usually seen only during daytime. This astonishing blast of ionization was detected by Prof. Umran Inan of Stanford University. "It is extremely rare for an event occurring outside the solar system to have any measurable effect on the Earth," Inan said.

It was so powerful that it blasted sensitive detectors to maximum or off-scale on at least seven scientific spacecraft in Earth orbit and around the solar system.

The wave of radiation emanated from a newly discovered type of star called a magnetar. Magnetars are dense balls of super-heavy matter, no larger than a city but weighing more than the Sun. They have the greatest magnetic field known in the Universe, so intense that it powers a steady glow of X-rays from the star's surface, often punctuated by brief, intense gamma-ray flashes, and occasionally by cataclysmic flares like the one observed on August 27. Astronomers think that all these effects are caused by an out-of-control magnetic field -- a field capable of heating, mixing, and sometimes cracking the star's rigid surface to bits.^46d

In June a team of scientists led by Dr. Chryssa Kouveliotou of NASA's Marshall Space Flight Center in Huntsville, AL, used NASA's Compton Gamma Ray Observatory to detect a series of about 50 flashes from the star, a type called a Soft Gamma Repeater (SGR), known as "SGR1900+14" in the constellation Aquila. During the flashing episode, Kouveliotou's team, in collaboration with Dr. Tod Strohmayer and his colleagues at NASA's Goddard Space Flight Center, Greenbelt, MD, pointed sensitive X-ray detectors aboard NASA's Rossi X-ray Timing Explorer satellite toward the star. They found faint X-rays coming from the star, which pulsed regularly in intensity every 5.16 seconds.

These 5.16-second pulses already had been detected in April, when Dr. Kevin Hurley, University of California, Berkeley, aimed the Japanese/NASA Advanced Satellite for Cosmology and Astrophysics (ASCA) at the star. Comparisons of the ASCA and RXTE data showed that the X-ray pulses were gradually slowing down.

The finding implies that the Soft Gamma Repeater has a magnetic field about 800 trillion times stronger than Earth's magnetic field, and about 100 times stronger than any found anywhere in the Universe. Kouveliotou and her team had earlier found that another SGR was also a magnetar. This was exactly what Dr. Robert Duncan, University of Texas, Austin, and Dr. Christopher Thompson, University of North Carolina, Chapel Hill, predicted in 1992 when they originated the "magnetar" theory.

Before the NASA team could announce these conclusions, SGR1900+14 emitted the tremendous flare of August 27, which was observed by almost every spacecraft with a high-energy radiation detector in space.

"Magnetars seem to answer several mysteries about the structure and evolution of stars," said Kouveliotou. "We think magnetars spend their first 10,000 years as Soft Gamma Repeaters. As they weaken with age and slow their rotation, they become Anomalous X-ray Pulsars -- stars that do not have enough 'juice' to flash anymore, but which emit a steady flow of X-rays for perhaps another 30,000 years. After that, they fade to black and drift for eternity through the heavens. The absence of observable pulsars in some supernova remnants just means that the pulsar's lights have gone out sooner than we expected."

A magnetar forms from the explosion, or supernova, of a very large, ordinary star. The star's heavy center collapses under its own gravity into a dense ball of super-compressed matter 12 miles across. This "neutron star" consists mostly of neutrons in a dense fluid, but the outer layers solidify into a rigid crust of atoms about 1 mile deep, with a surface of iron.

Even with this solid crust, a magnetar is incredibly unstable. Almost unimaginable magnetic fields, about 800 trillion times that of Earth's, cause the crust to crack and ripple in powerful starquakes. The energy released in these explosive starquakes streams out into space as intense flashes of gamma-rays. In the August 27 flare, pure magnetic energy was also released, as the star's entire crust was broken to bits.

"A magnet this strong could erase the magnetic strip on the credit cards in your wallet or pull the keys out of your pocket from a distance halfway to the Moon," said Duncan.

The latest images from NASA’s newest space telescope reveal the fine detail of extremely long X-ray jets erupting from the center of a galaxy thought to harbor a massive black hole.

The jets, detected by the $1.5 billion Chandra X-ray Observatory that has been orbiting Earth since July, span a distance of more than 25,000 light years. (A light year is the distance light travels in a year -- almost 6 trillion miles.) The galaxy, Centaurus A, is about 11 million light years from Earth.

"For twenty years we have been trying to understand what produced the X rays seen in the Centaurus A jet," Schreier said. "Now we at last know that the X-ray emission is produced by extremely high energy electrons spiraling around a magnetic field."

Schreier said the length of the longest X-ray jet racing from the galaxy is comparable to half the diameter of our Milky Way galaxy.

Astronomers think the X-ray sources detected in Centaurus A are a by-product of a collision between galaxies several hundred million years ago, said Ethan Schreier of the Space Telescope Science Institute.

This collision is believed to have triggered a burst of star formation and supplied gas to fuel the activity of the central black hole.

Centaurus A has long been a favorite target of astronomers because it is the nearest example of a class of galaxies called "active galaxies." These are noted for their explosive activity, which is thought to result from black holes at their centers. The Chandra image shows intense X-ray activity at the center of Centaurus A, which jibes with the belief that its center harbors a black hole.

The bright jet extending out from the nucleus to the upper left in the Chandra image is due to explosive activity around the black hole which ejects matter at high speeds. A fainter "counter-jet" extending to the lower right can also be seen.

Scientists are intrigued by the apparent fact that black holes do not suck up all the matter that falls within their sphere of influence, as demonstrated by the jets shooting from Centaurus A.

This composite of optical and X-ray observations shows an X-ray jet extending from the center of Centaurus A out to a distance of more than 25,000 light years. Credit: NASA/CXC/SAO. Click to enlarge.	This image shows Centaurus A at optical wavelengths. Credit: AURA/NOAO/NSF. Click to enlarge
This artist's rendering shows how a black hole may be powering X-ray jets. Credit: NASA/CXC/SAO. Click to enlarge	Chandra Takes X-ray of Nebula and Star X-ray Eye Sees Skeleton of a Crab Chandra Finds Oddly-Shaped Supernova X-ray Probe in Place at Last