Hypotheses

Hypotheses

Still under development, however changes are made regularly - check back by later.

* Astronomers are not certain what produces gamma ray bursts, but possible causes include the mergers of two neutron stars, two black holes, or a neutron star and a black hole, or the explosion of a so-called hypernova^51c.
-- A hypernova is a theorized type of supernova or exploding star.

* If indeed a galaxy--as current theories would have--it must be very far away, near the outer reaches of the observable universe^51d.
-- In that case, gamma-ray bursts must represent the most powerful explosions in the universe.

* By the mid-1980s the consensus was that the bursts originated on nearby neutron stars in our galaxy.
-- In particular, theorists were intrigued by dark lines in the spectra (component wavelengths spread out, as light is by a prism) of some bursts, which suggested the presence of intense magnetic fields.
-- The gamma rays, they postulated, are emitted by electrons accelerated to relativistic speeds when magnetic-field lines from a neutron star reconnect.

* The first challenge is to conceive of circumstances that would create a sufficiently energetic fireball.

* Most theorists favor a scenario in which a binary neutron-star system collapses [see "Binary Neutron Stars," by Tsvi Piran; SCIENTIFIC AMERICAN, May 1995].
-- Such a pair gives off gravitational energy in the form of radiation.
-- Consequently, the stars spiral in toward each other and may ultimately merge to form a black hole.

* Theoretical models estimate that one such event occurs every 10,000 to one million years in a galaxy.
-- There are about 10 billion galaxies in the volume of space that BATSE observes
-- that yields up to 1,000 bursts a year in the sky, a number that fits the observations.

* Variations on this scenario involve a neutron star, an ordinary star or a white dwarf colliding with a black hole.
-- The details of such mergers are a focus of intense study.
-- theorists agree that before two neutron stars, say, collapse into a black hole, their death throes release as much as 10⁵³ ergs.
-- This energy emerges in the form of neutrinos and antineutrinos, which must somehow be converted into gamma rays.
-- That requires a chain of events: neutrinos collide with antineutrinos to yield electrons and positrons, which then annihilate one another to yield photons.
-- Unfortunately, this process is very inefficient, and recent simulations suggest it may not yield enough photons.
-- if too many heavy particles such as protons are in the fireball, they reduce the energy of the gamma rays.
-- Such proton pollution is to be expected, because the collision of two neutron stars must yield a potpourri of particles.
-- then all the energy ends up in the kinetic energy of the protons, leaving none for radiation.

* As a way out of this dilemma, Peter Meszaros of Pennsylvania State University and Martin J. Rees of the University of Cambridge have suggested that when the expanding fireball--essentially hot protons--hits surrounding gases, it produces a shock wave.
-- Electrons accelerated by the intense electromagnetic fields in this wave then emit gamma rays.

* A variation of this scenario involves internal shocks, which occur when different parts of the fireball hit one another at relativistic speeds, also generating gamma rays.
-- Both the shock models imply that gamma-ray bursts should be followed by long afterglows of x-rays and visible light.
-- Mario Vietri of the Astronomical Observatory of Rome has predicted detectable x-ray afterglows lasting for a month--and also noted that such afterglows do not occur in halo models.
-- GRB 970228 provides the strongest evidence yet for such a tail.
-- the binary collapse does not explain some long-lasting bursts. Last year, for instance, BATSE found a burst that endured for 1,100 seconds and possibly repeated two days later.

* There are other ways of generating the required gamma rays.
-- Nir Shaviv and Arnon Dar of the Israel Institute of Technology in Haifa start with a fireball of unknown origin that is rich in heavy metals.
-- Hot ions of iron or nickel could then interact with radiation from nearby stars to give off gamma rays.
-- Simulations show that the time profiles of the resulting bursts are quite close to observations, but a fireball consisting entirely of heavy metals seems unrealistic.

* Another popular mechanism invokes immensely powerful magnetic engines, similar to the dynamos that churn in the cores of galaxies.
-- Theorists envision that instead of a fireball, a merger of two stars--of whatever kind--could yield a black hole surrounded by a thick, rotating disk of debris.
-- Such a disk would be very short-lived, but the magnetic fields inside it would be astounding, some 10¹⁵ times those on the earth.
-- Much as an ordinary dynamo does, the fields would extract rotational energy from the system, channeling it into two jets bursting out along the rotation axis.
-- The cores of these jets--the regions closest to the axis--would be free of proton pollution.
-- Relativistic electrons inside them can then generate an intense, focused pulse of gamma rays.
-- Although quite a few of the details remain to be worked out, many such scenarios ensure that mergers are the leading contenders for explaining bursts.

* To further constrain the models, we will need to look at radiation of both higher and lower frequency than that currently observed.
-- The Energetic Gamma Ray Experiment Telescope (EGRET), which is also on the Compton satellite, has seen a handful of bursts that emit radiation of up to 10 billion electron volts, sometimes lasting for hours.
-- Better data in this regime, from the Gamma Ray Large Area Space Telescope (GLAST), a satellite being developed by an international team of scientists, will greatly aid theorists.

* In the range of 0.1 to 10 keV, there is a good chance of discovering absorption or emission lines that would tell volumes about the underlying fireball and its magnetic fields.
-- Such lines might also yield a direct measurement of the redshift and, hence, the distance.
-- Sensitive instruments for detecting soft x-rays are being built in various institutions around the world.

* most astronomers were convinced that bursts originated in our own Galaxy, on or near objects called neutron stars^51e.

* It was believed that with BATSE's increased sensitivity, we would be able to see the faint gamma-ray bursts map out the Milky Way on the sky.

* The combined angular and brightness distributions of bursts eliminates the possibility that GRBs come from the disk of the Milky Way, and left astronomers with a choice between one of two possibilities:

* The galactic halo would have to be very big, about a million light years across, which is much bigger than the diameter of the known Milky Way system (which is less than a hundred thousand light years), much less the halo that the Milky Way is known to have.

* In the case of the other distance scale (referred to by astronomers as "cosmological") it is the smoothness of the GRB sky distribution that tells us that their distances must exceed the length scales over which the distribution of matter appears clumpy, i.e., larger than clusters and superclusters of galaxies, and the voids between them.

* At such large distances the effect of the expansion of the Universe becomes noticeable.
-- An important effect of this expansion is the redshift of light (and gamma rays) and the apparent slowing down of clocks, both of which would cause a change in the brightness distribution very similar to what BATSE actually observes.
-- some scientists have reported seeing time dilation (weak bursts, which are presumably farther away, are, on average, longer than the strong, nearby bursts) and others report energy shifting, where the distribution of gamma-rays in faint bursts appears shifted to lower energies compared to strong bursts.
-- In general the BATSE data seemed to be hinting at a cosmological origin, but it couldn't be proved more directly.

* The key to nailing the cosmological hypothesis seemed to hinge on the ability to find a counterpart to a burst in a region of the spectrum outside of the gamma-rays.
-- These counterparts would have the positional accuracies to link the GRB, for instance, to a faint galaxy, or allow the measurement of a redshift from an optical spectrum.
-- Conversely, these would not be seen if GRBs came from a big galactic halo.

* For all their differences from other bursts, SGRs have at least one similarity: we don't know what causes them^51f.
-- Among the possibilities are young neutron stars - only 6,000 years old - energizing a large cloud of gas cast off in a supernova explosion.
-- They are temporary phenomena, dying around their 10,000th birthday.
-- Another possibility is X-ray binary stars that accrete matter at irregular intervals and belch gamma rays when the matter lands.

* Another topic that has grown enough to warrant its own discussion is Soft Gamma Repeaters (SGRs). Where true gamma-ray bursters are distant and never repeat (the blast is so energetic that it shreds the source object), SGRs are within our galaxy and repeat at unpredictable times.

* "There is a really intense debate as to whether these optical flashes happen with all bursts," Connaughton noted.

* The basic idea is that a tremendous explosion ejects a shock wave of material that accelerates charged particles, like electrons and protons, to velocities near the speed of light^51g.

* If we assume that all gamma-ray bursts are caused by the same thing, this means that none can be due to a synchrotron shock.

* gamma-ray bursts can't be caused by a synchrotron shock.
-- Interestingly though, there is strengthening evidence that the optical counterparts, the glow from fireballs that appear to be the aftermath of gamma-ray bursts in distant galaxies, are caused by synchrotron shock waves.
-- Whatever makes the fireball glow is apparently different from the mechanism that makes gamma-ray bursts. It's yet another mystery in the fantastic saga of gamma-ray bursters.

* Some astrophysicists started to rethink the non-linkage between gamma-ray bursts and supernovae after an event in April 1998^51h.

* 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."
-- "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."

* 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.

* "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 in the Oct. 15 issue of the prestigious science journal, Nature. 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? The full story is covered in Science@NASA's story, When stars go hyper.

* This would induce rapid starbirth as gas clouds were heated and compressed, precipitating millions of newborn stars.

* The presence of this so-called starburst activity is strongly supported by Hubble and Keck telescope images that show the host galaxy is exceptionally blue.

* Space Telescope’s observations further support the idea that these mysterious powerful explosions happen where vigorous star formation takes place.

* Gamma-ray bursts may be created by the mergers of a pair of neutron stars or black holes, or a hypernova, a theorized type of exceptionally violent exploding star.

* "Most of the theoretical models proposed to explain these bursts cannot explain this much energy," said Kulkarni. "However, there are recent models, involving rotating black holes, which can work⁵¹ⁱ.
-- On the other hand, this is such an extreme phenomenon that it is possible that we are dealing with something completely unanticipated and even more exotic."

Blasts From the Past: High-Redshift Burst is the Latest Piece in 30-year Trail of Discovery
May 6, 1998: ^51e - NONE

* this is contradicted by the observed brightness distribution of GRBs, which shows a distinct dearth of very weak GRBs: it is as though in all directions we do see that edge (see "the angular distribution could lead one to argue...").

Gamma-ray Bursters cross the 'Line of Death' - Fireballs and gamma-ray bursts are not the same thing
October 13, 1998: ^51g - NONE

Astrophysicists puzzle over intergalactic coincidence - Gamma-ray burst and supernova may have no relation^51h - NONE

I am still in the process of concatenating the plethora of data that I have accumulated - please be patient !
-- This should change within the next couple of weeks.

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