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Post Info TOPIC: Hypernova GRB
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RE: Gamma-ray bursts
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Observations of a cosmic explosion detected on Feb. 15 by two NASA satellites have thrown into doubt one popular explanation for such explosions and have also seriously weakened the argument for yet another, according to University of Chicago astrophysicist Don Lamb.
But solving the mystery any time soon may be forestalled by plans to shut down one of the satellites in September.
The explosion in question is a powerful burst of X-rays called an X-ray flash that was observed by NASA's Swift and High Energy Transient Explorer-2 satellites.
X-ray flashes seem to be related to gamma-ray bursts, the most powerful explosions in the universe.
"No one understands this relationship at all. It's a complete mystery," - Don Lamb, member of the HETE-2 science team.
Lamb presents some ideas on the relationship of X-ray flashes to gamma-ray bursts during a meeting of the American Astronomical Society in Minneapolis. The co-authors of his paper are Tim Donaghy, a Ph.D. student in physics, and Carlo Graziani, Senior Research Associate in Astronomy & Astrophysics, both at the University of Chicago.

Discovered in 1969, Gamma-ray bursts last anywhere from fractions of a second to many minutes and pack the output of as many as 1,000 exploding stars. They occur almost daily, come from any direction in the sky, and are followed by afterglows that are visible for a few days at X-ray and optical wavelengths.
Discovered in 2000, X-ray flashes seem to form the less powerful end of a continuum of cosmic explosions that progresses to X-ray rich gamma-ray bursts and then culminates in gamma-ray bursts proper. All three phenomena occur in approximately equal numbers.
"We think that regular gamma-ray bursts are all produced by the collapse of massive stars and probably the creation of black holes. I personally think it's essentially a certainty that X-ray flashes are produced by the same kind of event." - Don Lamb.

But exactly how that occurs remains a matter of speculation. One possibility is that a varying rotation rate of the collapsed core of these massive stars produces different opening angles of the jets emitted from the bursts.
"Maybe sometimes they're rotating rapidly and you get narrow jets and other times they're rotating less rapidly and you get wider X-ray rich jets and sometimes they're rotating still more slowly and you get very broad jets that produce the X-ray flashes," - Don Lamb.
The Feb. 15 X-ray flash, designated XRF 050215b, has yielded the best data ever on this phenomenon, thanks to the joint observations of the two NASA satellites. The next-best data come from a flash known as XRF 020427, detected in April 2002 by the Italian BeppoSAX satellite. Three characteristics of both flashes conflict with a popular theory that X-ray flashes are gamma-ray bursts as viewed from slightly off to the side of the jet instead of head on.
First, according to the popular theory, scientists expected the energy levels of an X-ray flash's afterglow to connect smoothly on a gradient with the energy of the burst itself. Second, they expected the afterglow to fade fast. And third, they expected the afterglow to be faint when compared to the original burst.
These expectations all follow from Albert Einstein's theory of special relativity, but none of them have panned out. Scientists apparently cannot rely on special relativity to explain X-ray flashes.

The satellite observations also conflict with the theory that the shape of the jets from a gamma-ray burst are universal, but only look different because of the viewing angle. Based on this theory, scientists would have predicted that the afterglow of the Feb. 15 X-ray flash afterglow would have faded within a day or so following the initial burst. But the afterglow showed now signs of fading even five days following the burst.

One theory suggests that all three types of explosions contain the same amount of energy, but that the opening angle of the jet emitted from the explosions defines their apparent brightness.
In this scenario, narrow jets produce the gamma-ray bursts, wider jets result in X-ray rich gamma-ray bursts, and the broadest lead to X-ray flashes. Lamb and many others view this theory as a possibility.

"There's a lot of people who don't or are not at all sure".
The question could probably be settled within the next few years with more burst observations conducted jointly between the Swift and HETE-2 satellites, which measure slightly different properties of the phenomena. But NASA plans to discontinue the HETE-2 mission this September.
NASA would somehow need to find an additional $1.5 million annually to keep HETE-2 operating. "It's not the best budgetary climate to try to pull this thing off. (But if NASA somehow manages to do it). The HETE mission would leverage the science that Swift could do by a significant amount," - Don Lamb.

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Hypernova GRB
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Hypernova Reveals Hidden Identity As Gamma-Ray Burst :

An international research team, led by astronomers from the University of Tokyo, Hiroshima University, and the National Astronomical Observatory of Japan, used the Subaru telescope to obtain the spectrum of SN2003jd, a hypernova unaccompanied by a gamma-ray burst, and found the first evidence that it is a jet-like explosion viewed off-axis.
Hypernovae are hyper-energetic supernovas that are often associated with gamma-ray bursts. This result provides clear and firm evidence that all hypernovae may be associated with gamma-ray bursts, but that gamma-ray bursts are observable only when jets produced by the hypernova explosion point towards Earth.



Observations by two of the world's largest telescopes provide strong evidence that hypernovas may be the origin of elusive gamma-ray bursts that have puzzled scientists for more than 30 years.
The team, conclude that naked carbon/oxygen stars that flatten as they collapse into a black hole are good candidates for the source of gamma-ray bursts.
Though astronomers have observed a couple of bursts associated with this type of supernova - a Type Ic supernova sometimes called a hypernova - the theory of how a hypernova produces gamma rays is still speculative.
The new observations, though not a smoking gun, provide a major piece of evidence that the theory, called the collapsar model, is correct. The model explains how an asymmetric exploding star produces a tight beam of matter and energy out of each pole that generates an intense burst of gamma rays, while the absence of a hydrogen and helium envelope would allow the blast to escape.
"It appears that to produce a gamma-ray burst, a core-collapse supernova needs to be both asymmetric in its explosion mechanism, so that there is a natural axis along which matter can more easily squirt, and free of a hydrogen envelope, so that the jet doesn't have to pummel through a lot of material," -Alex Filippenko, co-author, UC Berkeley professor of astronomy.
The team, led by Paolo Mazzali of the Trieste Observatory in Italy and the Max-Planck Institute for Astrophysics in Garching, Germany, reported its findings in a paper appearing in the May 27 issue of Science.

The fact that a gamma-ray burst was not observed in association with this supernova is actually in accord with predictions, said UC Berkeley graduate student Ryan Foley, a member of the team.
"These observations suggest that the collapsar model is probably correct and that some of these Type Ic supernovae appear to be off-axis gamma-ray bursts, in which the gamma-ray burst is pointing in some direction other than Earth." - Ryan Foley.

Gamma-ray bursts are brief but bright flashes of X-rays and gamma rays that seem to go off randomly in the sky about once a day, briefly outshining the sun a million trillion times. It took until 1997 to establish that they originate outside our Milky Way Galaxy, and only within the past few years have astronomers gotten tantalizing hints that the bursts are associated with supernovae.
Because they are so bright, gamma-ray bursts have to be a collimated beam, similar to but tighter than the cone of light emitted by a lighthouse. Otherwise, the energy in the explosion would be equivalent to instantaneously converting the mass of several suns into a fireball of energy.
The most popular scenario is that a collapsing star generates two highly collimated beams or jets of particles and energy that flash outward from the poles. The particles and energy generate a shock wave when they hit gas and dust around the star, which in turn accelerates particles to energies at which they emit high-energy light: gamma rays and X-rays. The initial burst fades over a few seconds, but the resulting shock waves (the "afterglow") can be visible to optical, radio and X-ray telescopes for days after the explosion.

A possible candidate for the type of supernova that could produce a gamma-ray burst is the Type Ic supernova. Type Ic supernovae result from massive stars whose winds have shed their outer envelopes of hydrogen and often all their helium, or that have lost these outer layers to a binary companion.
Only the core is left, composed of the elements produced by fusion in the star's centre - mostly carbon and oxygen but other heavy elements as well, down to a solid iron centre.
The collapsar theory proposes that the solid iron sphere at the very core of the star collapses under gravity to a black hole, but that the split-second collapse takes place in a unique way.
As the iron and surrounding matter fall inward, the spin of the core increases, flattening the in-falling material into a disk that flows inward along the equator. The congestion of in-falling matter pushes some of it right back out along the path of least resistance - the two blowholes at either pole.
The matter shot out from the poles rams into the other layers of the star, which it may not be able to penetrate. The lack of a hydrogen and helium envelope presumably increases the chances the jet will punch through.

"It has so much energy that it pushes through these outer layers of the star, which are of relatively small density compared to the disk of in-falling material in the centre of the star. Eventually, if it punches out, you have a gamma-ray jet. Some Type Ic supernovae may be failed gamma-ray bursts, which means the jet tried to push out, but there was too much material in the way, and it never actually broke out. That would explain why we don't see gamma-ray bursts associated with some of these objects." - Ryan Foley.

If the theory is true, astronomers should see different things depending on whether the jet is aimed toward Earth or away from it. If the jet is coming out perpendicular to our line of sight, for example, no gamma-ray burst would be visible, but other aspects of the expanding supernova blast wave should be observable. In particular, the spectrum of the supernova a year or so after its explosion should show emission lines of elements, such as oxygen, that are split, one shifted slightly to lower wavelengths and the other shifted to higher wavelengths. The two lines would come from opposite sides of the expanding disk around the equatorial region of the remnant black hole, one Doppler shifted toward the red because it is moving away from us, the other blue shifted because it is moving toward us. Such split or double lines would not be visible from a polar perspective.

About two years ago, on Oct. 25, 2003, UC Berkeley researchers had discovered a Type Ic supernova using Filippenko's automated supernova search telescope, the Katzman Automatic Imaging Telescope (KAIT) at the University of California's Lick Observatory. Called SN 2003jd, the supernova was about 260 million light years away in the constellation Aquarius.


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Position(2000): RA 23 21 03.38 Dec - 4 53 45.5 16.1 magnitude

Though no associated gamma-ray burst was recorded, the supernova appeared to be as bright as the supernovae previously associated with gamma-ray bursts, so the international team reporting this week in Science decided to look again at the supernova, taking its spectrum in search of double-peaked emission lines.
"These observations were actually guided by our theoretical predictions. The idea was that a bright Type Ic supernova, not accompanied by a gamma-ray burst, could be just what we were looking for: an off-axis event which could confirm our predictions." - Paolo Mazzali

Koji Kawabata from Hiroshima University, Ken'ichi Nomoto of the University of Tokyo and his colleagues observed the remnant nebula with the 8.2-meter Subaru telescope on Sept. 12, 2004, about 330 days after it blew. Subsequently, Filippenko and Foley turned the 10-meter Keck telescope on the nebula on Oct. 19, 2004, about 370 days after the initial explosion, to obtain spectral images with the Low Resolution Imaging Spectrometer (LRIS). Both telescopes sit atop Mauna Kea volcano on the island of Hawaii. Subaru is operated by the National Astronomical Observatory of Japan, while the Keck Observatory is operated by the California Association for Research in Astronomy, whose board of directors includes representatives from the California Institute of Technology (Caltech) and the University of California.

Kawabata, Mazzali and his team analyzed the spectra, revealing that they exhibit split oxygen and magnesium emission lines exactly as would be expected if the collapsar model of gamma-ray production were correct. This was the first Type Ic supernova to show split oxygen lines.
"Jets are a signature of the model, which means that not all explosions will be pointed directly at us. If every time we looked at these objects they appeared to be pointing at us, which would mean the model is probably flawed. The model predicts that a certain percentage of these objects should look like this supernova (SN 2003jd). Now that we've found one of these, the credibility of the model has increased." - Ryan Foley.



To see such double oxygen lines, the supernova nebula would have to be viewed within 20 degrees of the expanding disk, a rare situation that could explain why other Type Ic supernovae, including some associated with a gamma-ray burst, do not show the split oxygen line.

"(Our observations) strengthen the connection between gamma-ray bursts and Type Ic supernovae by showing that the Type Ic SN 2003jd appears to indeed have been an asymmetric explosion whose main axis of ejection happened not to be pointing at us." - Alex Filippenko.

Other co-authors of the paper are Keiichi Maeda, Jinsong Deng and Nozomu Tominaga of the University of Tokyo; Enrico Ramirez-Ruiz of the Institute for Advanced Study in Princeton, New Jersey; Stefano Benetti of the Astronomical Observatory of Padova, Italy; Elena Pian of the Trieste Observatory; Youichi Ohyama of the Subaru Telescope; Masanori Iye of Japan's National Astronomical Observatory; Thomas Matheson of the National Optical Astronomy Observatory in Tuscon, Ariz.; Lifan Wang of Lawrence Berkeley National Laboratory; and Avishay Gal-Yam of Caltech.

The work was supported in part by the National Science Foundation, the Japan Society for the Promotion of Science and Japan's Ministry of Education, Culture, Sports, Science and Technology.

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-- Edited by Blobrana at 12:20, 2005-05-28

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