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supernovae and their
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A. Supernovae
Supernove, and their relatives, novae, are becoming some of the most frequently observed astronomical events. With the various astronomers and societies of astronomers (amateur and professional) growing by leaps and bounds, so has the study of Variable Stars. Hunting these elusive prey is by no means a new endeavor. The Chinese first record a "Guest Star" as far back as 1064 AD, which shone bright enough to be seen in broad daylight. More recent endeavors have yielded unprecedented numbers of discoveries.
Supernova explosions take place because a massive star, having exhausted its supply of hydrogen fuel, can no longer exert enough outward pressure to counter its own gravitational pull. It collapses upon itself in a thermonuclear explosion that releases more energy in an instant than in the previous 10 billion years. Some fraction of that energy blows away the star's outer envelope of gases, creating the hot shell visible from Earth as a nebula.
Most of the remaining mass of the star, now in effect a giant atomic nucleus composed almost entirely of neutrons, occupies a diameter of no more than a dozen miles, about the size of Manhattan. A teaspoon of neutron star matter, if brought to Earth, would weigh more than a billion tons. As young neutron stars condense, they spin much more rapidly, the effect is called conservation of angular momentum. Radio pulsars rotate with such extreme velocity that they must have the extraordinary density of neutron stars, or they would fly apart. If neutron stars did actually form only in this way, there should be pulsars somewhere within most supernova remnants.
Eleven years ago on Feb. 23, 1987, an event of truly astronomical proportions occurred in in the Large Magellanic Cloud, 167,000 light years from Earth. The great star Sanduleak -69 202, a magnitude 10.2 star, exploded in one of natues greatest cataclysmic events, a supernova. Now known as Supernova 1987A was the brightest visible-wavelength stellar cataclysm seen since Johannes Kepler recorded his observations of a supernova in the year 1604. This event released energy equivalent to that of 100 type G0 stars over their lifetime. Astronomers are now beginning to see the effects of this energy reaching the immense light-year wide ring6.
Astronomers are left with the impression of watching a movie of a hydrogen bomb blast in very slow motion. Events that take milliseconds on Earth take decades in space. The physical shockwave from the destruction of the supernova Sanduleak -69 202 (SN 1987a) is just now reaching the innermost of three gas rings circling the dead star at a distance of two-thirds of a light year. The Harvard-Smithsonian Center for Astrophysics has made the most recent images of the supernova ring.
Shocked by the 40-million mile per hour impact, a 100-billion mile diameter knot of gas in a piece of the ring has already begun to glow as its temperature surges from a few thousand degrees to a million degrees Fahrenheit. Astronomers predict it's only a matter of years before the complete ring becomes ionized as it absorbs the full force of the impact. "This is the first time we have been able to observe these events in the span of a human lifetime," says Ann Kinney of the Space Telescope Science Institute, which manages the orbiting Hubble Space Telescope for the National Aeronautics and Space Administration8.
Astronomers do not think that this inner ring is detrius from the supernova event. They believe that it likely formed about 20,000 years before the star exploded, and only became visible when it was heated by the burst of x-ray and light energy from the supernova. This development lends credence to some theories that there was more than one nova or supernova event - or an even more exotic systemic occurrence. One incident may have triggered the next in a chain reaction of events, since the gas rings had been slowly cooling prior to the impact of the shockwave from SN1987A.
Hubble images recorded in 1994 revealed that what appears to be ejectae from some as yet undiscovered companion star had assumed a shape resembling two wine glasses placed base to base. The supernova is located where the stems of the two glasses would intersect. The inner ring of gas is dispersed around the edges of the bases. Then, there are two more, fainter. rings of gas that would be on the edges of the flutes of the glasses.
Gamma Ray Emission by Stimulated Emission of Radiation ?
In spring of 1997 the newly installed Space Telescope Imaging Spectrograph (STIS) first measured the speed of the supernova debris pushing along the shock wave. "The STIS lets you see the invisible stuff," says George Sonneborn of Goddard Space Flight Center in Greenbelt, MD. "We see the shock happening everywhere around the ring." In July, Hubble Wide Field and Planetary Camera-2 images taken by Robert Kirshner and co-investigators showed that a compact region on the ring had lit up.9
As a result, the outer rings appear to be mirror images, and astronomers speculate that they were superimposed on the expanding envelope of gas by twin jets of high-energy. Some astronomers theorize that a rapidly rotating but unseen companion star, a neutron star or black hole creating a GRASER effect, could have produced these jets. Material from SN1987A that fell within within the Roche lobe limit of the compact companion would have been heated and blasted back into space in two narrow jets, along with a beam of radiation.
It is thought that as this compact object spins, it may wobble or precess about its axis. The twin beams would then spread out randomly from their point of origin. Chris Burrows of the European Space Agency did indeed find a dim object that could be the source of the beams at the predicted location--about one-third light-year from the center of the supernova explosion. Astronomers are searching for a millisecond pulsar at the heart of SN1987A, and theorize that a rapidly spinning, weak-field pulsar/magnetar will eventually reveal itself for observation.
As the shockwave illuminates features that may confirm Burrows' theory and solve other mysteries of dying stars, it will also allow scientists to test new theories about the interaction of shockwaves. "We are beginning to see the signature of the collision - this event will allow us to validate ideas we have built up over the past ten years of observation," Kirshner adds6.
The initial supernova flash only lit up a small part of the gas that surrounds the supernova. Most of it is still invisible, but the light from the crash will allow us a chance to see this invisible matter for the first time.Though astronomers have measured shock effects from the expanding debris of many supernovae which are centuries-old, their impact velocities are at least ten times slower than the ones we see today in supernova 1987a6. The gas and debris visible also tends to elucidate on the spread of heavier atoms throughout the continuum.
1998eq
Last October's detection of SN1998eq represents the most distant supernova yet found. The Supernova Cosmology Project - whose leader, Saul Perlmutter of Lawrence Berkeley National Laboratory, points out that that the anomalous dimness observed in relation to SN1998eq might be caused by a natural consequence of cosmic expansion, which, by stretching light waves, both reddens them and drags out their arrival on the earth. The putative tiring of light might change its color and predicted brightness, but not the apparent passage of time. In addition, a hitherto unknown type of energy may plug this gap, perhaps the infamous cosmological constant - the Hubble Constant (see "Is the Universe Younger than we Think ?" from NASA Science News) - or its inconstant cousin, "quintessence," either of which could exert an antigravity force.
1999am
Astronomers searching for asteroids headed toward Earth have stumbled upon a supernova, named 1999am, which is located in a galaxy about 650 million light-years away. (A light-year is the distance light travels in one year, about 9.5 trillion kilometers or 6 trillion miles.) The star was unknown to astronomers until it was captured by the camera on NASA's Near Earth Asteroid Tracking (NEAT) system on February 18. The NEAT images show the star as it looked just a few weeks after the ancient explosion took place.
Supernova 1999am is a Type Ia supernova - which means that before it exploded, it was a white dwarf star in orbit with a companion star. Near the end of its life, the white dwarf captured so much material from its companion that it became too massive to support itself, and exploded with as much energy as 100 billion suns. 1999am is now nearly as bright as the galaxy surrounding it, which is known as CGCG 060-009.
Lawrence Berkeley National Laboratory scientists found the supernova by comparing images taken in February with previous NEAT data. They could clearly see a change in brightness, indicating the star had exploded and become a supernova. They further confirmed their finding with additional observations by ground-based telescopes. February 18 marked the first time NEAT scientists forwarded new data directly to the Berkeley lab, and as Pravdo pointed out, "We struck paydirt."
Hypernovae
A hypernova explosion, thought of as being one possible source of powerful gamma-ray bursts (GRBs), is about 100 times more energetic than a supernova explosion and is thus the most energetic event known in the universe at the present time - Only the "Big Bang" has been calculated to have produced more energy than any one of these hypernovae. GRBs are elusive flashes of high-energy radiation that appear about three times a day from random directions in space and last typically a few seconds. Hiding within the Pinwheel Galaxy M101 in the constellation Ursa Major are two suspected hypernova remnants, the first such remnants ever identified. Astronomers at Northwestern University and University of Illinois are credited with the first observational evidence for the remnants of these hypernovae.
Remnant NGC5471B is rapidly expanding at a velocity of at least 100 miles per second., and Remnant MF83 is over 850 light-years across and is one of the largest nova remnants known. Both have X-ray luminosities about an order of magnitude brighter than the brightest supernova remnants in our Galaxy. Neither remnant can be seen in optical light; they are only visible in X-ray light and in emission lines spectra. The image of M101 is a combination of optical (blue) and X-ray (red) observations. (Photo credit: Y. H. Chu, R. Fesen, D. Matonick, and Q. D. Wang).
Daniel Wang, research assistant professor of physics and astronomy at Northwestern University, identified two hypernova remnants in galaxy M101. These remnants were previously classified as supernova remnants, but Wang's subsequent detection and analysis of X-ray light from these nebulae pointed to a more energetic process. "These are two of the most unusual remnants known," Wang said. "We see that they are bright in X-ray even at a distance of 25 million light years. They must be from spectacular explosions." Much of Wang's calculations were based on the work of Professor You-Hua Chu of the Astronomy Department at University of Illinois and her collaborators.
Hypernovae were first proposed by Bohdan Paczynski of Princeton University in 1998 as a way to explain GRBs, events initially discovered by U.S. military Vela satellites in the 1960s. The energy emitted is orders of magnitude greater than a supernova, thus hypernovae are thought to be related to the formation of black holes. One possible evolution is thought to be due to the collapse of massive stars and/or their mergers with dense, or compact, objects - which has become popular because of the evidence that GRBs appear close to massive stellar nurseries where such activity is likely to occur. "I suspect GRBs may well be just a tip of an iceberg, as we have no clue why some explosions generate so much gamma-ray emission," Paczynski said.
The brightest GRB yet discovered, the recently observed GRB 990123, represents more energy than any one star could produce, assuming the radiation astronomers saw left its source in all directions. One explanation for all this energy is that the radiation was actually confined into a beam during the explosion. This concentration of energy is known as the beaming effect, which may significantly affect the energy estimate of a GRB. By studying remnants of GRBs or hypernovae in nearby galaxies, it is possible can calculate their explosion energies without the beaming effect," Wang said. "We can also examine the environment of the explosions to infer their true nature."
The author of this paper suspects that IF such beaming is responsible, it will emulate the physics responsible for LASER technology, however, the operational mechanism for this GRASER (Gamma Ray Amplification by Stimulated Emission of Radiation) effect from such systems is as yet unknown. The evidence at this time is indicative of a randomly beamed pulse of high intensity radiation, and as such would mean that the source is either wobbling or has random rotational kinematics.
1987A exhibits some of the criteria for GRASERS, but not all. NGC5471B, on the other hands, seems to be a prime suspect for random GRASER emissions. The environment is that of a star-forming region. The event that produced NGC5471B was therefore most likely the collapse of a massive star. M101 is one of a few nearby galaxies with vigorous ongoing star formation. This explains why one single galaxy could contain two relatively rare hypernova remnants with ages less than about a million years. The mechanism for the formation of these GRBs is still in question, and the determination of the type of supernova (type-I or type-II) will help astrophysicists in this evaluation. At present there are more than 120 theories as to the evolution of GRBs.7a
A Unique Hypernova
The "Pistol star is identified by astronomers using NASA's orbiting Hubble Space Telescope as what may be the brightest star known .47a Approximately 25,000 light-years from Earth near the center of the Milky Way galaxy, this star appears to be spewing out as much energy in six seconds as the sun does in one year, is up to 10 million times more powerful than the sun, and spans the diameter of Earth's orbit. In addition, this star may have been more massive than any other star, formed one to three million years ago, and may have weighed up to 200 Msolar. Since then, it has shed much of its mass in violent eruptions, and may have created the nebula that surrounds it. It has been estimated that the star ejected up to 10 times the mass of the sun in giant outbursts about 4,000 and 6,000 years ago.
Current theoretical morphology indicates that it will continue to lose more material, eventually revealing its bare, hot core, sizzling at 100,000 degrees Fahrenheit at the surface. The Pistol Star is destined for certain death in a brilliant supernova in one to three million years. As these stars evolve, they can eject substantial portions of their atmospheres, producing the nebula and an extreme stellar wind (outflow of charged particles) that is 10 billion times stronger than our Sun's.
Although not visible to the eye, the Pistol Star is located in the direction of the constellation Sagittarius, and is hidden behind the great dust clouds along the Milky Way. Current evidence leads theorists to believe that the star-formation process there may favor stars much more massive than our modest sun. Ten percent of the infrared light leaving the Pistol star reaches Earth, putting it within reach of infrared telescopes, such as the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) on board Hubble. (NICMOS) aboard Hubble, also reveals a bright nebula, created by extremely massive stellar eruptions. The nebula is so big (four light-years) that it would nearly span the distance from the Sun to Alpha Centauri, the nearest star to Earth's solar system.
The Pistol Star was first noted in the early 1990s, however its relationship to the nebula was not realized until 1995. Hubble spectrometer results confirm this conclusion. The star may radiate enough energy to halt the inward fall of material, thus limiting its maximum mass. The Chandrasekhar limit restricts the mass functions of novae/supernovae to 1.4 - 3.0 Msolar. The initial mass of the Pistol Star may have exceeded this theoretical upper limit.
B. Remnants
The High Resolution Camera on the Chandra X-ray Observatory imaged N132D, a remnant of an exploded star also located in the Large Magellanic Cloud on August 30, 1999. The Supernova Remnant N132D in X-Rays (Credit: Chandra X-ray Observatory, NASA) is all that remains of a star that exploded in the Large Magellanic Cloud thousands of years ago. The remnant, which is still expanding, and now covers an area of 80 light years. During its expansion, N132D has included 600 Msolar into its cloud of expanding gasses and particulate matter.
The supernova is notable because it is not symmetrical like most other supernova explosions. Rather, it has a mass of material blowing out of the upper left side. Other supernovas, such as Cassiopeia A, which Chandra imaged several weeks ago, have shockwaves of material that expand outward more-or-less evenly in all directions. One explanation for the uncharacteristic shape could be that the star exploded near a cloud, which forced much of the explosive energy to shoot away from the cloud. Because the supernova is so hot, an extraordinary amount of its total material can not be seen at visible wavelengths, but does shine brightly in the X-ray bands.
The X-ray image shows the result from a collision with an even more massive molecular cloud, indicated by the bright glowing region in the lower right of the image. The molecular cloud, visible with a radio telescope, has the mass of 300,000 Msolar. The relatively weak X-radiation on the upper left of the image shows that the shock wave is expanding into a less dense region on the edge of the molecular cloud. The supernova remnant in the upper left of the image is gas that is expanding more rapidly because that area of space is less densely packed with gas and dust. Another image, one of the first taken with the High Resolution Camera of the Chandra X-ray Observatory shows a highly structured remnant of 10-million-degree K. gas.
Cas A exploded more than 9,400 years ago. Any astronomical event is history, but because of the time it take slight to travel across space, so astronomers usually refer to the age of the object as it appears to us - in this case 300 years ago. The result is a remnant of the explosion - a very faint nebula, which Charles Messier apparently missed it in 1784-86 when he compiled his now-famous Messier Catalog.
It was one of the first objects to be discovered when radio astronomy itself was discovered, and has been studied extensively across the spectrum up through gamma rays. In the 1930s, Karl Jansky found not only that the Milky Way was a powerful radio emitter, but that sources in deep space emitted as well. His simple maps showed several regions of the sky that were stronger than others. The brightest source was in the region of Cassiopeia and so was named Cas A.
Inspired by Janksy's discovery, amateur radio operator Grote Reber built the first true radio telescope and plotted intensity maps of a number of sources and published his results in Sky & Telescope in 1938. Among the objects he mapped in detail was Cas A. World War II yielded new technologies that boosted the field in the 1950s, and Cambridge University mapped the skies in detail in radio wavelengths, designating Cas A as 3C 461 in their third catalog. Subsequent observations by the Uhuru Small Astronomy Explorer, launched in 1970, designated the Cas A X-ray source 3U 2321+58 (in the 3rd Uhuru catalog). Cas A has also been designated G111.7-2.1 in the galactic coordinate system.
Extensive observations in radio, infrared, visible, and X-ray wavelengths reveal an incomplete shell of expanding gas with compact knots of material at temperatures up to 28 million K (50 million deg. F). Its outer shell is expanding at 800 km/s (~ 1.73 million mph) - which is fast enough for us to be able to notice, even with our comparatively short life spans. A spectrum of Cas A shows strong lines emitted by oxygen, neon, silicon, sulfur, argon, iron, calcium, and even magnesium silicates, among other elements.
A real oddity is 44Titanium (a radioactive cousin of the prized aerospace metal). Cas A apparently has an unusual abundance of 44Ti as compared to 56Nickel, another product of nucleosynthesis in a star. The Compton Telescope aboard the Compton Gamma Ray Observatory has observed emission lines at 1.156 MeV, corresponding to 44Scandium decaying into 44Calcium, the last step in 44Titanium's life cycle. If correct, the 44Ti is a direct probe of nuclear fusion at the heart of the Cas A source, a sample of stellar material just above the layers that formed the neutron star or black hole at the center of the Cas A remnant - if there one was left behind. One of the mysteries about Cas A is why no central object has yet been found for Cas A.
When the Chandra X-Ray Observatory came on-line, and imaged the Cas A source, scientists readily spotted a small point source just about at dead center, and for that reason Cas A will be studied further to to see if the point source's spectrum, brightness, and time variation correspond with a neutron star. Chandra certainly will provide much finer details about the more than 300 glowing knots of gas that make up the remnant.
The shell of Cas A has two components, the high-temperature blast wave, still plowing outward through the interstellar medium, and a low-temperature reverse shock traveling inward. On the western edge, Cas A plows into a molecular gas cloud. On the opposite side, a high-speed jet moves through relatively empty space.
Cas A is thought to have started its life as a Wolf-Rayet star with an initial mass 30 times that of our sun. Having such mass meant that Cas A would be a short-lived star - its surface temperature of 50,000 K (90,000 deg. F), is more than eight times hotter than the surface of our Sun - it died in the manner of its life - brilliantly, catastrophically, and quickly. This explosion blew off an intense solar wind that reduced Cas A's mass to about 10 Msolar.
When massive stars expend the hydrogen fuel at their cores, they start burning the ash from one round of fusion into still heavier ash to be burned in a shorter, hotter cycle until only iron is left. Fusing iron nuclei into heavier atoms consumes rather than produces energy (See The Death of A Star and Black Holes: Theory and Conjecture). Cas A's furnace turned off - perhaps even earlier, during silicon formation - and the star imploded. Heavier elements were produced by this inward rush and absorbing much of the energy, but not enough to keep the implosion from rebounding and blowing the star apart.
Most of us envision a supernovae as a unilateral explosion, one that propogates from the center outwards, but it is possible that it can explode along the poles as the star's magnetic field becomes more compacted and intense as the outer layers collapse. In such an eventuality, it would be possible for the star's blast to act like an armor-piercing "HEAT" round that forms a plume of fast, molten debris. At this point, we are left with a bright, fragmented shell that is largely the result of Cas A overtaking its own solar wind and then running into the interstellar medium.
One theory concerning the global structure of the interstellar medium says that supernova shock waves will interact with the cold gas and dust of the interstellar medium, eventually forming three or more distinct temperature phases. Although a three-phase model has been popular for the past 20 years, no one had found convincing evidence for one of the model's basic tenets -- a cold cloud evaporating in the hot medium.
The Cassiopeia A supernova remnant keeps raising awkward questions.
There is the neutron star or black hole that most supernovae leave behind?
Why is Cas A a source of cosmic titanium-44 ?
Other Cas A Images |
N63A
To study the supernova remnant -- called N63A, scientists obtained optical images from the Hubble Space Telescope and high-resolution X-ray images from the ROSAT X-ray telescope. The X-ray observations reveal the full extent of this huge supernova remnant but the optical images show the features that scientists are most interested in.The supernova remnant lies in the Large Magellanic Cloud, a small neighboring galaxy to our own Milky Way, about 160,000 light-years from Earth.9
The Relationship between Gamma-Ray Bursts and Supernovae
Among those features are three bright clouds of gas and dust, similar in size to the Orion Nebula. Two of the clouds show distinct filamentary structures indicative of shock-wave compression, Chu said. The outward rushing shock wave has not yet reached the third, most distant cloud. Numerous shocked cloudlets were also detected within the supernova remnant. Swept back by high-velocity shock waves, these evaporating cloudlets provide clear support for the three-phase model.
After a massive star is formed, its stellar wind blows much of the surrounding interstellar medium away, creating a huge shell in space called an interstellar bubble.The interstellar medium is not homogeneous, and the denser knots of material are left behind. It has been theorized that the optical emission region of this supernova remnant appears the way it does because the supernova exploded inside an interstellar bubble in a cloudy medium.
The burst was detected on Dec. 14, 1997, by the Italian/Dutch BeppoSAX satellite and NASA's Compton Gamma Ray Observatory satellite. The Compton observatory provided detailed measurements of the total brightness of the burst, designated GRB 971214, while BeppoSAX provided its precise location, enabling follow-up observations with ground-based telescopes and NASA's Hubble Space Telescope11.
For about one or two seconds, this burst was as luminous as all the rest of the entire universe. In a region about a hundred miles across, the burst created conditions like those in the early universe, about one millisecond (1/1,000 of a second) after the Big Bang. This large amount of energy was a surprise to astronomers. Most of the theoretical models proposed to explain these bursts cannot explain this much energy, however, there are recent models, involving rotating black holes, which can work. On the other hand, this is such an extreme phenomenon that it is possible we are dealing with something completely unanticipated and even more exotic.
As the visible light from the burst afterglow faded, an extremely faint galaxy was detected at its location, using one of the world's largest telescopes, the 10-meter Keck II telescope at Mauna Kea, Hawaii. The galaxy is about as faint as an ordinary 100 watt light bulb would be as seen from a distance of a million miles. Subsequent images taken with the Hubble Space Telescope confirmed the association of the burst afterglow with this faint galaxy and provided a more detailed image of the host galaxy. Scientists succeeded in measuring the distance to this galaxy at a redshift of z=3.4, or about 12 billion light years distant (assuming the universe to be about 14 billion years old). From the distance and the observed brightness of the burst, astronomers derived the amount of energy released in the flash.
The burst lasted approximately 50 seconds, and the energy released was hundreds of times larger than the energy given out in supernova explosions - about equal to the amount of energy radiated by our entire Galaxy over a period of a couple of centuries. Scientists say it is possible that other forms of radiation from the burst carried a hundred times more energy than that in the form of neutrinos or gravity waves, which are extremely difficult to detect.
NASA is planning two missions to further investigate gamma- ray bursts: the High Energy Transient Experiment II (HETE II), scheduled to launch in the fall of 1999 (now pushed back into 2000). HETE II will be able to precisely locate gamma-ray bursts in near real - time and quickly transmit their locations to ground-based observatories, permitting rapid follow-up studies. In addition, the Gamma Ray Large Area Space Telescope (GLAST), scheduled to launch in 2005. GLAST will detect only those gamma-ray bursts that emit the highest energy gamma rays, and will be able to locate them with sufficient precision to permit coordinated observations from the ground. Not much is known about the bursts at these high energies, but observations may permit researchers to choose among competing theories for the origin of gamma-ray bursts.
Phi Persei
Within the Catastrophic double-star system of Phi Persei15, an aging, once massive star has been denuded to a lean one-solar mass. Its once moderate-sized companion has bulked up to a hefty nine-solar masses and is spinning so violently that it's flinging gas from its surface. Coincidences are rare in nature, so when one occurs, the natural reaction is to see if the two events are linked by more than just chance...and if that link applies elsewhere in the universe. Thus, some astrophysicists started to rethink the non-linkage between gamma-ray bursts and supernovae after an event in April 1998.24
So far, says Dr. Marc Kippen, the odds are against gamma-ray bursts being associated with supernovas. "They might be related," he said, "but then you must explain how a local supernova produced a gamma ray burst that looks like all the other ones that evidently come from very great distances." Kippen is an astrophysicist with the University of Alabama in Huntsville working at NASA's Marshall Space Flight Center.
"We can almost conclusively say that no bright gamma-ray burst detected so far comes from a known supernova," Kippen said. "We are less certain about weaker bursts because they can't be precisely located. In addition, we miss most supernovas, so about 10 percent of the weaker gamma-ray bursts could come from supernovas."
Gamma-ray bursts have been one of the most mysterious phenomena in the universe since their discovery about 30 years ago. Initially they were thought to be associated with neutron stars within our galaxy. Observations with the Burst and Transient Source Experiment (BATSE) board the Compton Gamma Ray Observatory, launched in 1991, have shifted the scene from our galaxy to deep in the universe.
Then came a burst and a supernova both on April 25, 1998, both in the same region of the sky. The gamma-ray burst was observed by BATSE and the Beppo SAX satellite. The precise position of the burst provided by instruments on Beppo SAX allowed ground-based optical telescopes to discover that the burst was coincident with a new supernova - SN1998bw - within the same small section of the sky. This presented a new challenge because the gamma-ray burst was average in its properties. Nothing distinguished it from the other bursts routinely detected by BATSE. However, sn1998bw was extraordinary - the intrinsically brightest supernova ever observed in its category.
"All of this may lead to a revolution in our thinking about how core-collapse supernovae are produced," wrote Dr. Eddie Baron of the University of Oklahoma. Questions to be answered include what causes "ordinary" supernovae, is there a limit in their energy release, and when does core collapse cause a gamma-ray burst?
.It could be a coincidence - BATSE has recorded more than 2,000 bursts since 1991, about one a day. Only 1 supernova in 10 is actually detected. So, a burst and a supernova are bound to coincide in time and apparent location. If they're related, it could send astrophysicists back to rethink the mystery. "I looked to see if there were any more of these coincidences," Kippen said. "The chances of observing a supernova within a small bit of sky over the span of a few days is pretty small, about 1 in 10,000." Kippen divided the BATSE burst catalog into two groups.
First he looked for bursts that were seen both by BATSE and Ulysses. BATSE has eight detector modules. Measuring the brightness of a burst as seen by the three or four modules that are actually triggered will describe a large error box in the sky. Triangulating the arrival time of the burst with the time of arrival at Ulysses, located deep within our solar system, reduces the error box to a short, thin arc across the sky.
Kippen then compared these with supernovas that had been detected at about the same time as the burst. He allowed a generous margin - up to a month - since the dates of supernova explosions aren't always precisely known. He came up empty handed. The 415 bursts and 585 supernovas all had separate locations. Still, there is the possibility that the supernovas might cause weaker bursts that were detected by BATSE and not by Ulysses which carries a much smaller instrument. This gave him a set of 1,222 bursts.
"At some level you expect gamma rays to come from a supernova," Kippen said. But they should be less powerful than the events that cause gamma-ray bursts at cosmological distances, and their light profiles - how they brighten and dim - also should be different. "The result for using just BATSE locations was that there's no significant excess," Kippen said. "It was consistent with random locations. "
A plot of recent supernova locations (red diamonds) shows that none coincides with gamma-ray bursts seen by both BATSE and a detector on the Ulysses international solar polar spacecraft (purple lines). The supernova locations are shown in diamonds for clarity; their actual locations are known to much greater precision. The BATSE/Ulysses lines are the overlap between a great circle formed by timing triangulation between BATSE and Ulysses, and the rough position defined by BATSE alone. this plot also depicts the random nature of gamma-ray bursts. All of these sources are outside our galaxy. For reference, the Milky Way lies along the equator of this map. Links to 1320x615-pixel, 381K JPG. Credit: Dr. Marc Kippen, UAH and NASA/Marshall.
The issue likely will remain open for some time, until the right combination of instruments happens to be pointing in the direction of a supernova that also yields a burst, or until more advanced telescopes are built. "The next-generation gamma-ray detectors could have the capability of observing hundreds of bursts per year with very accurate positions," Kippen said. "Then you could start to make more definitive statements. Then we could also precisely localize weak bursts and say whether any of them occur with supernovas."
In the meantime, astrophysicists are left with three possibilities.
For now, scientists may have to live with an uncomfortable coincidence. "One event is just one event," Kippen said. "It's not overwhelming evidence."
A Different Viewpoint
In a paper to appear today in the international journal Nature, the international team led by the California Institute of Technology presents evidence that the gamma-ray burst of March 26, 1998 (GRB 980326) is apparently associated with a supernova explosion. Cosmic gamma-ray bursts, the brightest known explosions in the universe, may come from the fiery deaths of very massive stars in supernova explosions, a team of astronomers said today.
This would then indicate that some gamma-ray bursts are associated with the formation of black holes during the fiery deaths of very massive stars. If true, this would be some of the first direct evidence for what produces gamma-ray bursts. As a consequence, the team suggests that a burst of gamma rays are seen when one of the jets from the supernova's central black hole is pointed directly toward Earth.
Despite the strides, scientists were still left wondering what produces these spectacular explosions. Various theories of their possible origins are still vigorously debated. There are currently two popular models, both suggesting that the bursts originate in a formation of a black hole. In one model, two massive objects such as neutron stars or black holes (both of which may be end-products of previous supernova explosions) coalesce, forming a single massive black hole.
In the second model, such a black hole is produced in a catastrophic collapse of the core of a massive star. In this model, one then expects two sources of light: the "afterglow'' emission from the gamma-ray burst itself and light from the exploding star, a supernova. The afterglow rapidly declines whereas the supernova explosion gains in brightness over a period of a few weeks, and then gradually fades away.
The new study reports on the observations of GRB 980326 carried out at the W. M. Keck Observatory's 10-m telescope located atop Mauna Kea, Hawaii. As in many other cases, a visible light afterglow was found following the burst, which then rapidly faded away. However, the Caltech-led team discovered something never previously observed - a dramatic rebrightening of optical emission at the position of the gamma-ray burst.
Normally, the optical light of a gamma-ray burst vastly outshines its host galaxy for weeks. When the light from the gamma-ray burst fades, the apparent total brightness remains constant: all that remains is the light from the host galaxy. Shrinivas R. Kulkarni of the Caltech team explains, "A month after GRB 980326, it looked as though the host galaxy was dominating the light." However, the next time the team observed, some eight months after the burst, the "galaxy" was gone.
"Galaxies do not just disappear, so we were astonished," Kulkarni said. "Clearly, what we were seeing is a new source of light brightening one month and then fading away. This is something quite new." This unexpected rebrightening is now believed to be due to the underlying supernova created in the explosion of the massive star. The team had also obtained spectra of the object at different times, and that provided additional clues.
"The spectrum of the source right after the burst was blue, which is common," said S. George Djorgovski of Caltech. "But after a month it was very red, which was unexpected. "That alone suggested that we were looking at some different phenomenon happening at the same location, but with a time delay of a few weeks."
Both the rebrightening and the spectrum changes are naturally explained by the presence of a supernova. The intensity of the apparent re-burst matches the peak brightness of a supernova seen in a distant galaxy, and its red spectrum also has the right color. This represents the most direct evidence to date in favor of the massive supernova model. In this scenario, a black hole is quickly formed in the center of a massive star whose core is unable to support itself against gravity.
When the star explodes, powerful jets from the central black hole emerge along the original axis of rotation, and gamma rays are created by the jets. If the jets are not pointed toward Earth, then we see only a supernova and the effects of the exploding star. But gamma rays as well as the light from the supernova arrive at Earth if the jets are pointing in our direction.
Joshua S. Bloom, a graduate student at Caltech and lead author of the paper said, "This appears to be the smoking gun for the origin of some gamma-ray bursts, a perfect marriage of the two brightest events in the universe. It is wonderful to be a part of such a discovery." Gamma-ray bursts, since their discovery some 30 years ago, have over 150 theoretical models about their possible origins, but only a handful can come close to describing the true trigger of the bursts.
"It is possible that there are other causes for gamma-ray bursts such as the coalescence of neutron stars," Bloom said. "Undoubtedly, astronomers will focus on unearthing new classes in the years to come." Early reports of the results created some excitement in the astronomical community. Two other groups, from universities of Amsterdam and Chicago, in view of the work presented by the Caltech team, have reanalyzed the data on some other gamma-ray bursts. They appear to find good evidence for an underlying supernova in another well-studied gamma-ray burst.
"It is encouraging to have had such a resounding reception to an unexpected result," said Kulkarni. "Even some of the initial skeptics seem to be converted by these results." Other members of the Caltech team are graduate student A. C. Eichelberger; postdoctoral scholars P. Cote, J. P. Blakeslee, and S. C. Odewahn; and Assistant Professor F. A. Harrison.
In addition to the members of the Caltech team, the other coauthors include M. Feroci of the BeppoSAX team; D. A. Frail of the National Radio Observatory; A. V. Filippenko, D. C. Leonard, A. G. Reiss, H. Spinrad, D. Stern, A. Bunker, B. Grossan, S. Perlmutter, and R. A. Knop of the University of California at Berkeley; A. Dey of the National Optical Astronomy Observatory; and I. M. Hook of the European Southern Observatory.
And Another...
Astrophysicists at the University of Texas at Austin and the Naval Research Laboratory (NRL) in Washington, D.C., have developed a new theory of how supernovae explode, based on observations made at the University of Texas at Austin McDonald Observatory.52b The results were published in the Astrophysical Journal Letters on October 20 by Alexei Khokhlov, Elaine Oran, and Almadena Chtchelkanova of NRL and Peter Hoeflich, Lifan Wang, and J. Craig Wheeler of the University of Texas. "Combining Texas observations with the cutting-edge numerical techniques at NRL has pointed the way to a new idea," says Wheeler, the Samuel T. & Fern Yanagisawa Regents Professor in Astronomy at the University of Texas at Austin. "We think that jets cause a major class of supernova explosions."
Supernovae are caused by the explosion of a massive star, and the explosions have been thought to arise through one of two mechanisms. In the first type, called Type Ia, massive stars can explode like a stick of dynamite, leaving no collapsed remnant. Astronomers use Type Ia supernovae as "standard candles" to measure distances in the Universe, and studies of Type Ia supernovae have suggested that the expansion of the Universe is accelerating.
Other types of supernovae involve the collapse of the center of an especially massive star to form an extremely dense object, either a neutron star or, perhaps in some circumstances, a black hole. The formation of a neutron star is thought to be more common. These types of supernovae are called Type Ib and Ic and Type II. Some scientists have focused on determining whether the light of supernovae is polarized - that is, if the light waves given off by supernovae are aligned in certain directions. If a supernova's light is expanding uniformly in all directions, there is no polarization. There will be measurable polarization if light from the parts of the supernova is spreading asymmetrically.
All the supernovae examined that are thought to arise from core collapse-the Type Ib and Ic and Type II supernovae-have been substantially polarized, and hence their "halo" has been substantially "out-of-round." At the same time, all the Type Ia supernovae have shown little or no polarization. For the polarized supernovae, a trend suggesting that the closer one looks to the center of a supernova explosion, the larger the asymmetry found. Data suggest, the explosion must be occurring strongly along a preferred axis, and thus the explosion must be bipolar. Observations cannot be explained by current theory, so a new theory was needed.
Ith is thought that when the core collapses, a neutron star forms before any explosion can occur. Up to now, the theory of core-collapse supernovae has been focused on the production of neutrinos that are generated within the newly formed neutron star. These ephemeral particles carry off more than a hundred times the energy required to trigger the explosion of the star. The question has been whether they carry too much and spoil the explosion, or leave enough energy behind to cause the explosion.
To help with a new theory that explains supernova formation and takes polarization into account, scientists used computer modeling to test scenarios that could explain the newfound polarization of these supernovae. Their models tested the idea that collapsing supernovae begin by expelling mass and energy from the new neutron star in a strongly directional process.
Moving mass and energy in a single direction is the operational definition of a jet, and these are jet-induced explosions. If the new jet theory is right, the traditional questions about neutrinos and supernovae may be irrelevant. In their calculations, scientists found that the jet punches out of the star, but also sends shock waves sideways, spreading the energy throughout the star. The result is that the entire star is blown up by the jet and the neutrinos do not need to play any obvious role. The ejected matter is sent out in the jet and in a pancake containing other star material. This satisfactorilly explains the polarization of light waves.
Numerical techniques to compute the effect of a jet on a star is fully three dimensional and has an "adaptive-mesh" capability, so that it automatically computes most carefully just where the need is greatest. This code was used to compute the propagation of a jet from near the surface of a newly formed neutron star to its eruption into space. The next task is to better understand the origin of the jet. Scientists think the most plausible cause is the rapid rotation of the neutron star and its strong magnetic field, however, they have just begun to look into how the newly formed neutron star can channel its energy up the rotation axis by magnetic jets or intense pulsar radiation.
Supernova Breathes Out Oxygen For
Future Worlds
Atlanta - January 17, 2000
- NASA's Chandra X-ray Observatory
has revealed an expanding ring-like structure of oxygen and neon
that was hurled into space by the explosion of a massive star.
The image of E0102-72 provides unprecedented details about the
creation and dispersal of heavy elements necessary to form
planets like Earth.
The results were reported by Professor Claude Canizares of the Massachusetts Institute of Technology (MIT), Cambridge, at the 195th national meeting of the American Astronomical Society in Atlanta last week. Drs. Kathryn Flanagan, David Davis, and John Houck of MIT collaborated with Canizares in this investigation.
E0102-72 is the remnant of a supernova explosion located in our neighbor galaxy, the Small Magellanic Cloud, nearly 200,000 light years away. It was created by the explosion of a star that was more than ten times as massive as our Sun. We are seeing the aftermath of the explosion a thousand or more years after the outburst. Shock waves are heating gas to temperatures of nearly 10 million degrees, so it glows with X-rays that are detected by Chandra's instruments.
By using the High Energy Transmission Grating Spectrometer (HETG), astronomers were able to pinpoint the distribution of each chemical element individually and measure the velocities of different parts of the expanding ring. They also show the shock wave in a kind of "freeze-frame," revealing the progressive heating of the stellar matter as it plows into the surrounding gas. This is the first time such detailed X-ray information has ever been obtained for a supernova remnant, and should provide critical clues to the nature of supernovae.
The grating spectrometer, which was built by an MIT team led by Canizares, spreads the X-rays according to their wavelength, giving distinct images of the object at specific wavelengths characteristic of each chemical element. Small wavelength shifts caused by the Doppler effect are used to measure the expansion velocities of each element independently.
"We've been studying these supernova remnants for decades, but now we're getting the kind of information we need to really test the theories," said Canizares.
"Understanding supernovae helps us to learn about the processes that formed chemical elements like those which are found on Earth and are necessary for life," said Flanagan.
Most of the oxygen in the universe, for example, is synthesized in the interiors of relatively few massive stars like the one being studied here. When they explode, they expel the newly manufactured elements which become part of the raw material for new stars and planets. The amount of oxygen in the E0102-72 ring is enough for thousands of solar systems.
By measuring the expansion velocity of the ring, the team can estimate the amount of energy liberated in the explosion. The expansion energy would be enough to power the sun for 3 billion years. The ring has more complex structure and motion than can be explained by current simplified theories, suggesting complexity in the explosion itself or in the surrounding interstellar matter.
The supernova remnant also provides a laboratory for atomic physics. The observations show how the atoms in the expelled matter behave when heated to such high temperatures. The images reveal the progressive stripping of electrons from the atoms after the super-sonic shock wave has passed.
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