Added March 21, 2007, RAR, Abridged Article from Scientific American Reports, May 2007
Emphasis added in italics and bold, my comments in red.
This paper presents an impressive set of experimental evidence about Gamma Ray Bursts and Hypernovas, plus some preliminary theoretical ideas as to what might cause them and provide their energy.
THE BRIGHTEST EXPLOSIONS IN THE UNIVERSE
Every time a gamma-ray burst goes off, a black hole is born
This is an assertion based on a guess about the energy release mechanism
By Neil Gehrels, Luigi Piro and Peter J. T. Leonard
Early in the morning of January 23, 1999, a robotic telescope in New
Mexico picked up a faint flash of light in the constellation Corona
Borealis. Though just barely visible through binoculars, it turned
out to be the most brilliant explosion ever witnessed by humanity. We
could see it nine billion light years away, more than halfway across
the observable universe. If the event had instead taken place a
few thousand light-years away, it would have been as bright as the
midday sun, and it would have dosed Earth with enough radiation to
kill off nearly every living thing. The flash was another of the
famous gamma-ray bursts, which in recent decades have been one of
astronomy's most intriguing mysteries.
The first sighting of a gamma-ray burst (GRB) came on July 2, 1967, from military satellites watching for nuclear tests in space. Before 1997, most of what we knew about GRBs was based on observations from the Burst and Transient Source Experiment (BATSE)on board the Compton Gamma Ray Observatory. BATSE revealed that two or three GRBs occur somewhere in the observable universe on a typical day. They outshine everything else in the gamma-ray sky. Although each is unique, the bursts fall into one of two rough categories. Bursts that last less than two seconds are "short," and those that last longer - the majority - are "long." The two categories differ spectroscopically, with short bursts having relatively more high-energy gamma rays than long bursts do. The January 1999 burst emitted gamma rays for a minute and a half. Arguably the most important result from BATSE concerned the distribution of the bursts. They occur isotropically - that is, they are spread evenly over the entire sky.
This is a very interesting aspect of GRBs, they have the same apparent spherical symmetry around Earth as Quasars.
The field took a leap forward in 1996 with the advent of the x-ray spacecraft BeppoSAX, built and operated by the Italian Space Agency with the participation of the Netherlands Space Agency. BeppoSAX was the first satellite to localize GRBs precisely and to discover their x-ray "afterglows." The afterglow appears when the gamma-ray signal disappears. It persists for days to months, diminishing with time and degrading from x-rays into less potent radiation, including visible light and radio waves. Although BeppoSAX detected afterglows for only long bursts - eight more years would pass before counterparts of short bursts would be discovered - it made follow-up observations possible at last. Given the positional information from BeppoSAX, optical and radio telescopes were able to identify the galaxies in which the GRBs took place. Nearly all lie billions of lights years away, meaning that the bursts must be enormously powerful. Extreme energies, in turn, call for extreme causes, and researchers began to associate GRBs with the most extreme objects they knew of: black holes.
The distribution of GRBs is similar to that of Quasars, i. e., far away. The association with black holes is an assertion.
Among the first GRBs pinpointed by BeppoSAX was GRB970508, so named because it occurred on May 8, 1997. Radio observations of its afterglow provided an essential clue. The glow varied erratically by roughly a factor of two during the first three weeks, after which it stabilized and then began to diminish. The large variations probably had nothing to do with the burst source itself; rather they involved the propagation of the afterglow light through space. Therefore, if GRB970508 was scintillating at radio wavelengths and then stopped, its source must have grown from a mere point to a discernible disk. "Discernible" in this case means a few light-weeks across. To reach that size, the source must have been expanding at a considerable rate-close to the speed of light.
This is another set of assertions, to explain how an explosion could have a primary energy release mechanism and then a secondary energy conversion to a different wavelength. But if we compare this to an atomic bomb, we would see a prompt criticality with immediate release of fast neutrons and gamma rays, followed by a churned surrounding atmosphere that produced light, a blast wave, and eventually, long wave electromagnetic radiation that persisted for days.
The latter part of this statement is an attempt to connect the process to a relativistic process.
The BeppoSAX and follow-up observations have transformed astronomers' view of GRBs. The old concept of a sudden release of energy concentrated in a few brief seconds has been discarded. Indeed, even the term "afterglow" is now recognized as misleading: the energy radiated during both phases is comparable. The spectrum of the afterglow is characteristic of electrons moving in a magnetic field at or very close to the speed of light.
So a two step process of fission energy release followed by an explosion is not excluded!
GRB990123 - the January 1999 burst - was instrumental in demonstrating the immense power of the bursts. If the burst radiated its energy equally in all directions, it must have had a luminosity of a few times 10 E45 watts, which is 10 E19 times as bright as our sun. In comparison, the photonic emissions of a supernova explosion are spread out over several weeks, with a luminosity that is only a tiny fraction of that of a GRB. Even quasars, which are famously brilliant, give off only about 10 E40 watts.
This is true only if the Quasars are really far away.
If the burst beamed its energy in particular directions rather than in all directions, however, the luminosity estimate would be lower. Evidence for beaming comes from the way the afterglow of GRB990123, among others, dimmed over time. Two days into the burst, the rate of dimming increased suddenly, which would happen naturally if the observed radiation came from a narrow jet of material moving at close to the speed of light. Because of a relativistic effect, the observer sees more and more of the jet as it slows down. At some point, there is no more to be seen, and the apparent brightness begins to fall off more rapidly.
This is another assertion that might explain why the energy release doesn't have to be large to be bright, and to explain why relatively few such events are observed.
For GRB990123 and several other bursts, the inferred jet-opening angle is a few degrees. Only if the jet is aimed along our line of sight do we see the burst. This beaming effect reduces the overall energy emitted by the burst approximately in proportion to the square of the jet angle. For example, if the jet subtends 10 degrees, it covers about one 500th of the sky, so the energy requirement goes down by a factor of 500; moreover, for every GRB that is observed, another 499 GRBs go unseen. Even after taking beaming into account, however, the luminosity of GRB990123 was still an impressive 10 E43 watts.
This is a rationalization that there may be more such events at lower energy releases, but we don't see many of them.
One of the most interesting discoveries has been the connection between GRBs and supernovae. When telescopes went to look at GRB980425, they also found a supernova, called SN1998bw, that had exploded at approximately the same time as the burst. The probability of a chance coincidence was one in 10,000. A firmer case is the association of GRB030329 with SN2003dh. This GRB was localized by NASA's second High Energy Transient Explorer satellite (HETE-2), launched in October 2000. Ground-based observations revealed the broad spectroscopic features of a supernova, basically identical to those of SN1998bw, 10 days after the GRB. The best case by far is GRB060218, which is tied quite nicely to SN2006aj. This GRB was discovered by NASA's Swift satellite, launched in November 2004. Ground-based telescopes were intensely scrutinizing the fading afterglow when the supernova appeared, three days after the GRB.
Of the three cases mentioned above, GRB030329 comes closest to being a normal long GRB; GRB980425 and GRB060218 are unusual in that they are under-luminous, of long duration and predominantly x-ray events. Also, these two bursts occurred in, by GRB standards, relatively nearby galaxies The two have long spectral lags, meaning that the high- and low-energy gamma-ray pulses arrive several seconds apart. These bursts may be best de scribed as "x-ray flashes," which will be explained later.
In addition to GRB030329, there is strong evidence that other normal long GRBs are associated with supernovae GRB970228 was the first BeppoSAX GRB for which an optical afterglow was discovered. At 30 days after the burst, a bump in its optical light curve appeared that looked a lot like a supernova.
A link between GRBs and supernovae has also been suggested by the detection of metals, most notably iron, in the x-ray spectra of several bursts. Iron atoms are known to be synthesized and dumped into interstellar space by supernovae explosions. If these atoms are stripped of their electrons and later hook up with them again, they give off light at distinctive wavelengths, referred to as emission lines. Early detections of such lines by BeppoSAX and the Japanese x-ray satellite ASCA have of emission lines from silicon, sulfur, argon and other elements commonly released by supernovae.
A connection between GRBs and supernovae is now generally accepted by astronomers. Because GRBs are much rarer than supernovae - every day a couple of GRBs go off somewhere in the universe, as opposed to hundreds of thousands of supernovae-not every supernova can be associated with a burst. Perhaps jetting inside a supernova is common, and, in some small fraction of cases, relativistic jets escape from the supernova, and, in some small fraction of those cases, one of the jets is directed toward us, which allows us to observe a GRB. Also, if the jet is pointed just slightly away from us, then we may observe a lower-energy event, with more x-rays than gamma rays.
GRBs of various types are probably connected to supernova type explosions of some kind! The rest of the statement is a rationalization.
Energy Production and Distribution Mechanisms?
Even leaving aside the question of how the energy in GRBs might be generated, their sheer brilliance poses a paradox. Rapid brightness variations suggest that the emission originates in a small region: a luminosity of 10 E19 suns comes from a volume the size of one sun. With so much radiation emanating from such a compact space, the photons must be so densely packed that they should interact and prevent one another from escaping. But if the gamma rays are unable to escape, how can we be seeing GRBs?
This is a guess about how the process works.
The resolution of this conundrum, developed over the past 10 years, is that the gammas are not emitted immediately. Instead the initial energy release of the explosion is stored in the kinetic energy of a shell of particles - a fireball moving at close to the speed of light. The particles include electrons and their antimatter counterpart, positrons. This fireball expands to a diameter of 10 billion to 100 billion kilometers, by which point the density has dropped enough for the gamma rays to escape unhindered. The fireball then converts some of its kinetic energy into electromagnetic radiation, yielding a GRB.
Again, we have an assertion about how the process works, and this puts it in the relativistic range.
The initial gamma-ray emission is most likely the result of internal shock waves within the expanding fireball. Those shocks are set up when faster blobs in the expanding material overtake slower blobs. Because the fireball is expanding so close to the speed of light, the timescale witnessed by an external observer in the path of the fireball is vastly compressed, according to the principles of relativity. So the observer in the path of the fireball sees a burst of gamma rays that lasts only a few seconds, even if it took a day to produce. The fireball continues to expand, and eventually it encounters and sweeps up surrounding gas. Another shock wave forms, this time at the boundary between the fireball and the external medium, and persists as the fireball slows down. This external shock nicely accounts for the GRB afterglow emission and the gradual degradation of this emission from gamma rays to x-rays to visible light and, finally, to radio waves.
This assertion is wholely unjustified by the experimental evidence! There is a dispute, mainly among NPA members, about whether or not time itself compresses, so that what we might be seeing is an effect of time varying without time itself changing.
Although the fireball can transform the explosive energy into the observed radiation, what generates the energy to begin with? That is a separate problem, and astronomers have yet to reach a consensus.
Amen! They don't really know how it works.
One family of models, referred to as hypernovae or collapsars, involves stars born with masses greater than about 20 to 30 times that of our sun. Simulations show that the central core of such a star eventually collapses to form a rapidly rotating black hole encircled by a disk of leftover material.
Simulations are notoriously affected by what laws and features are put into the model. They are almost infinitely adjustable to give almost any answer desired. The key problem here is how does the collapse produce so much energy in so short a time that is able to flee the black hole? Won't there be some problems of inertia to overcome?
A second family of models invokes binary systems that consist of two compact objects, such as a pair of neutron stars (which are ultra-dense stellar corpses) or a neutron star paired with a black hole. Such a system loses orbital energy as a result of the emission of gravitational radiation, and so the two objects spiral toward each other and merge into one, just as in the collapsar scenario, the result is the formation of a single black hole surrounded by a disk.
Now we are getting somewhere! Maybe all of these events involve collisions of two separate bodies: neutron stars with black holes; neutron stars with neutron stars; neutron stars with ordinary stars; ordinary stars with ordinary stars. The relative abundance of these events would depend on the relative abundances of the constituent parts, going from rare to common in the above order.
It is easy to visualize a neutron star doing something to release a portion of its mass as energy while it is in the process of being gobbled up by a black hole, since it is already very dense, and some fraction would lie outside the event horizon during the merger.
Many celestial phenomena involve a hole-disk combination. What distinguishes this particular type of system is the sheer mass of the disk (which allows for a gargantuan release of energy) and the lack of a companion star to re-supply the disk (which means that the energy release is, more or less, a one-shot event). The black hole and disk have two large reservoirs of energy: the gravitational energy of the disk and the rotational energy of the hole. Exactly how these are converted into a fireball is not fully understood. It is possible that a magnetic field, 10 E15 times more intense than Earth's magnetic field, builds up during the formation of the disk. In so doing, it heats the disk to such high temperatures that it unleashes a fireball of photons, neutrinos and plasma. The fireball is funneled into a pair of narrow jets that flow out along the rotational axis.
They don't have the foggiest idea of how the disc idea works! They are working on the assumption that the only energy release mechanisms available are gravity and spin. Perhaps if there were a nuclear fission process available, the rest wouldn't be so hard to explain.
In addition to collapsar and compact-object merger models, it should be noted that there are other models for the central engine of a GRB. One involves the extraction of energy from an electrically charged black hole. In this scenario, both the prompt and afterglow GRB emissions are a result of the fireball sweeping up the external medium.
Another stab in the dark. I firmly believe black holes are neutral.
There is quite a bit of evidence to support the hypothesis that collapsars account for the long GRBs. In particular, the association of long GRBs with supernovae is a point in favor of collapsars, which, after all, are essentially large supernovae. Furthermore, long GRBs are usually found just where collapsars would be expected to occur - namely, in areas of recent star formation within galaxies. A massive star blows up fairly soon (a few million years) after it is born, so its deathbed is close to its birthplace.
The evidence is growing that compact-star coalescence accounts for the short-duration GRBs. This mechanism is not expected to produce a supernova and, indeed, an association between short GRBs and supernovae has not been found. Also, the decay of the orbits of a pair of compact stars is a process that occurs on a range of timescales from tens of millions to billions of years. In the former case, the merge will occur close to where the stars in the compact pair were born. In the latter case, the pair will drift around its host galaxy, and so the final coalescence is unlikely to have an association with any star-forming region. Such a mixed association of short GRBs with star formation is exactly what was found after Swift and HETE-2 discovered and localized the x-ray afterglows of several short bursts in 2005.
We still do not completely understand the differences between long and short GRBs. For example, the recent burst GRB060614 was a bright, well observed, nearby event that does not fit cleanly into either category.
One outstanding question concerns the dark, or "ghost," GRBs. Of the roughly 200 GRBs that have been localized and studied at wavelengths other than gamma rays, about 90 percent have been seen in x-rays. In contrast, only about 50 percent have been seen in visible light. Why do some bursts fail to shine in visible light?
There are several effects that can make a burst dark. One explanation is that these GRBs lie in regions of star formation, which tend to be filled with dust. Dust blocks visible light but not x-rays. Another intriguing possibility is that some of the ghosts are GRBs that happen to be very far away. The relevant wavelengths of light produced by these bursts would be absorbed by intergalactic gas. To test this hypothesis, measurement of the distance via x-ray or infrared spectra will be crucial. A third possibility is that ghosts are optically faint by nature.
High-sensitivity optical and radio investigations have identified the probable host galaxies of some dark GRBs. Most of them are at moderate distances, favoring - for these events - the dust explanation. But one of them, pinpointed by Swift, is at high redshift in the dark universe region.
Another mystery concerns a class of events known as the x-ray-rich GRBs, or simply the x-ray flashes. Discovered by BeppoSAX, later confirmed by HETE-2 observation and reanalysis of BATSE data, and currently observed by Swift, these bursts represent 20 to 30 percent of GRBs. They give off more x-radiation than gamma radiation; indeed, extreme cases exhibit no detectable gamma radiation at all.
There are three possible explanations for the x-ray flashes. One is that the fireball is loaded with a relatively large amount of baryonic matter such as protons, making for a "dirty fireball." These particles increase the inertia of the fireball, so that it moves more slowly and is less able to boost photons into the gamma-ray range. Alternatively, the x-ray flashes could be typical GRBs with jets that are pointing just out of our view, so that only the less collimated and less energetic x-rays reach us. A third possibility is that at least one of the x-ray flashes appears to be associated with a less extreme supernova explosion than is usual for normal long GRBs. There is speculation that, in this case, a neutron star, not a black hole, was formed in the supernova.
It might be that all three classes could be explained by collisions between different types of stars and black holes, with the biggest and fastest bursts coming from the most massive pairs!
The next step for GRB astronomy is to accumulate observations of hundreds of bursts of all varieties to flesh out the data on burst, afterglow and host-galaxy characteristics. This effort is being spearheaded by the Swift satellite, thanks to its multi-wavelength capabilities and its ability to quickly and autonomously reorient itself to better observe a burst with its high-resolution instruments. Swift's sensitivity to short-duration bursts has been a major factor in understanding this poorly studied class.
Another goal is to probe extreme gamma-ray energies. GRB940217, for example, emitted high-energy gamma rays for more than an hour after the burst, as observed by the Energetic Gamma Ray Experiment Telescope on the Compton Gamma Ray Observatory. Astronomers do not understand how such extensive and energetic afterglows can be produced. The Italian Space Agency's AGILE satellite, expected to launch in 2007, will observe GRBs at these high energies. The supersensitive Gamma-Ray Large Area Space Telescope mission, also scheduled for launch in 2007, will be key for studying this puzzling phenomenon.
Other missions, though not designed solely for GRB discovery, will also contribute. The International Gamma-Ray Astrophysics Laboratory, launched on October 17, 2002, is detecting more than 10 GRBs a year. The proposed Energetic X-ray Imaging Survey Telescope will have a sensitive gamma-ray instrument capable of detecting thousands of GRBs. The Explorer of Diffuse Emission and GRB Explosions (EDGE) is proposed to observe GRBs as cosmic beacons - to study the early stages of the universe and its evolution over time.
This latter idea assumes that the Big Bang model is valid.
The field has experienced a series of breakthrough years, with the discovery that GRBs are immense explosions occurring throughout the universe. Space and ground-based observations over the coming years should allow us to uncover the detailed nature of these most remarkable beasts. Astronomers can no longer talk of bursts as utter mysteries, but that does not mean the puzzle is completely solved.
Our greatest hope is that with enough experimental data, we might eventually sort the whole process out. However, if we continue to only allow gravity to supply the energy, and continue to believe in certain relativistic mechanisms, we will never find the answer.