New Energy Release Explanation added, 12/4/07, New Conventional Explanation added 2/15/07, New Scientific American Reports article added 3/21/07

Revised, 1/14/04, plus added report on 2/18/06 Supernova on 3/8/06

Italics and bold added for emphasis

A Self-Consistent Explosive Energy Release Mechanism for Supernova SN-1987a, Hypernovae, Gamma Bursters and Eta Carinae

 

Originally Submitted to the 1999 Gravity Research Foundation Competition

by Roger A. Rydin and Charles A. Bly

University of Virginia, Charlottesville, VA

Summary

The energy release mechanisms in supernovae type-II, gamma bursters and hypernovae can be explained as two different forms of neutrino-induced supercritical chain reactions, leading to atomic-bomb-like explosions. This explanation for supernovae is consistent with experimental evidence and resolves current difficulties in modeling these phenomena when a direct mass-to-energy conversion mechanism is not included. This explanation for a hypernova is the first suggested mechanism that can realistically explain the tremendous gamma-ray energy release observed in such an event.

 

Experimental Observations and Conventional Explanations

In January of 1999, Astronomers detected an extremely energetic burst of gamma radiation that was quickly pinpointed to be coming from a far galaxy. The sheer magnitude of the energy release was impressive compared to other observed galactic phenomena. The exact location was subsequently refined, leading to the inescapable conclusion that the gamma ray source was coming from a very distant supernova! The original speculation was that the triggering event was the collision of a pair of binary neutron stars. The result was an almost total conversion of the mass of these stars to gamma ray energy. The name "hypernova" was coined for this event.

On February 22, 2001, another flash of high-energy gamma rays, more powerful than what our sun will produce in its entire lifetime, was observed in the direction of Bootes, some 8 billion light years from Earth. This was attributed to the death of a giant star that catastrophically collapsed to a black hole. The cause of the event was not considered to involve colliding neutron stars because the galaxy is young, and is actively giving birth to new stars. Therefore, the hypernova burst was called the signature of black hole formation in the universe.

At 6:37 a.m. on March 29, 2003, astronomers captured an incredible burst of light from a giant star as it exploded and then collapsed into a black hole. During the first minute, the explosion "emitted energy at a rate more than a million times the combined output of all the stars in the Milky Way," said Carl W. Akerlof of the University of Michigan, leader of an international team of fast-response astrophysicists that use a network of space- and ground-based telescopes to study these fleeting displays. The optical brightness of the gamma ray burst was about 100 times more intense than anything seen before, he said. Gamma ray bursts are the most powerful explosions known, visible across the universe, but they are extremely difficult to study because they are so distant, random and brief. This one, a mere 2 billion light years away, in the direction of the constellation Leo, was unusually bright and close, so it could be studied in rare detail.

Astronomers have speculated that the dazzling gamma ray bursts are generated by explosive stellar deaths known as supernovae and their flashier cousins known as hypernovae - which are 10 times as powerful. But researchers have had a hard time catching a gamma ray burst and the triggering event together. Their observations of the fading afterglow from this nearby burst--tentatively classified as a hypernova have provided the best direct evidence so far, said Thomas Matheson of the Harvard Smithsonian Center for Astrophysics. "There should no longer be doubt in anybody's mind that Gamma Ray Bursts and Supernovae [and hypernovae] are connected."

Approximately 3000 lower energy output, "gamma bursters" have been observed. These events have not been satisfactorily explained either. The current theory is that the detected energy is released by matter as it is torn apart when it is absorbed into a nearby black hole. This explanation leaves much to be desired, since only a small fraction of the matter would be able to escape the black hole. Probably the most difficult problem to explain is how so much energy could be emitted in so short a time from the outskirts of a gravitational collapse, in other words, how does the gamma energy escape a black hole that does not allow light to escape?

There is a new explanation for how the gamma rays are generated, put forward by Stan Woosley of UC Santa Cruz [4]. "Rotation is the name of the game," says Woosley. Without spin, there would be no disk, and without a disk, there'd be no burst. Friction heats the disk, whipping around the black hole thousands of times a second, to 40 billion degrees, while new material keeps cascading in. Moments after the black hole forms, jets of superheated gas blowtorch outward.

Each jet may draw its energy directly from this friction in the disk, or from the newborn black hole, via the magnetic fields that link it to its surroundings. Like the original star, the black hole spins frenetically, which could cause the fields to stretch, twist, and snap like rubber bands, dumping vast amounts of energy into the disk.

Either way, the jet shoots outward, reaching the surface of the star in a mere ten seconds. If the star has retained its original, puffy envelope of hydrogen gas, the jet stops dead and the gamma-ray burst may fizzle. But if the powerful winds that blow from some massive star have stripped away the hydrogen earlier in the star's life, the jet escapes, arrowing into space a more than 99 percent of the speed of light.

 Now comes the burst. High-speed collision between blobs of material in each jet produce a cascade of speedy electrons. The electrons whirl around the jet's magnetic fields, flinging out gamma rays. Over many days, as the jet plows into the thin gas between the stars, it generates an afterglow at visible, infrared, and radio wavelengths.

Even after the jets have erupted, the star has not yet exploded. "The jet gets to the surface of the star minutes beforehand," says Woosley. "The burst is a herald of the supernova."

It's not enough, however, to cause the explosion. "Just running a jet through a star won't make a very good supernova;" says Woosley. "It will unbind some of the star, but most of it will fall back." To make a collapsing star explode, he says, "there needs to be something else."

Woosley suggests that the same sequence of events - an implosion a spinning disk, jets - can still happen when the stellar collapse ends with the formation of a fast spinning neutron star rather than a black hole.

Attempts have been made to characterize the physics of an ordinary supernova type-II, such as SN1987a, in terms of a complicated process in which the gravitational energy of star collapse is converted to the outward energy of explosion sufficient to drive off the outer layers of the star. This is not a small feat. About 16 of an original 18 solar masses must be driven completely away from the remaining 1 1/2 solar mass neutron star using only the collapse energy of that portion of the star! The postulated driving mechanism requires collapse to an incompressible state, whereupon a shock wave then provides the outward explosive force. The shock wave doesn't have sufficient energy of its own to propagate to the outer layers, so it needs to be heated. The assumed energy transfer mechanism is neutrino heating, and a super dense state is required in order to produce an adequate rate of energy transfer. There is also a question about how fast a shock wave can propagate under these conditions. Despite huge computational expense, theoretical results are still not very satisfactory. In particular, in order to explain copious iron ejecta from SN1987a, a convective mechanism is needed to bring this material out from the center of the star. That explanation has now been superceded by postulation of oscillations in the gasses and production of sound waves that transfer the energy, but there is no proof as yet that this mechanism is much better, just that it seems to give asymmetric explosions in numerical simulations that match to some extent asymmetries observed in supernova explosions.

A corresponding shock wave mechanism for a hypernova is essentially impossible! How can anything bounce off of a totally absorbing surface? Nonetheless, in a January 2002 PBS NOVA show about gamma bursts, they concluded that these bursts come from hypernovas of giant stars collapsing directly to black holes, with rotation producing a strong electromagnetic field that causes focused beams of gamma rays to go out at the poles. Again, there is a problem of how such rotation can take place so quickly as to focus an almost instantaneous release of energy. They don't really understand the mechanism at work in an ordinary supernova that releases so much energy, and they clearly don't know what goes on in a black hole, and yet they've combined these two together into a hypernova model! Even if they were right, the fraction of such events having their poles oriented in our direction would be very small.

 

SN1987a Data

Neutrino reactions are used in two different types of neutrino detectors. Cross sections are in the range of 10-46 up to approximately 10-37 cm2/neutrino. Antineutrinos are detected when they interact with a proton, effectively turning an up quark to a down, producing a positron and a free neutron. Detection of the neutron capture and positron annihilation in coincidence confirms antineutrino detection. Neutrinos are detected when they interact with a neutron, effectively turning a down quark to an up quark and producing a proton, a negatron and a neutrino. Detection is based on the measurement of Cherenkov radiation.

During SN1987a, approximately 20 such events were detected in a 12 second interval in two different detectors, and these were taken as proof that these antineutrinos were produced by electron capture during iron core collapse and explosion of the star. The data are shown below, plotted as antineutrino energy per event against time.

The corresponding plot of antineutrino detection rate versus time is shown below, indicating that the arrival rate is peaked in the first two seconds, and drops rapidly to zero in about 12 seconds. If each neutrino represents the same energy release per event, then this plot is proportional to power released versus time, and the area under the curve is proportional to total energy released in the explosion!

 

New Energy Release Mechanism: Neutrino-Induced Proton Fission

Thus, we must search for a new mechanism, as suggested by Bly [1], that can explosively convert mass to energy, causing it to be released in an isotropic manner by the process E = mc2 in the appropriate amounts to explain these and other phenomena. This fission mechanism does not have to produce a symmetric explosion. If the collapse is slightly asymmetric, then the point of maximum energy production can be off center, and the resulting explosion will be asymmetric. Such an asymmetric explosion can indeed produce a net sidewise velocity to the remaining central core.

Fredriksson [2] has proposed diquark fragmentation reactions for neutrinos and antineutrinos on protons, as shown below. For sufficiently high-energy excitation, the neutrino turns the single down quark to an up quark by formation of a virtual W+ particle, producing an unstable configuration that disintegrates and subsequently converts the proton almost entirely into energy. Antineutrinos produce a similar reaction via formation of a virtual W- particle. The former produces two new neutrinos and an antineutrino, while the latter produces two antineutrinos and a neutrino. We calculate that sufficient gravitational excitation is indeed available for this purpose [3] !

The ratio of the two reactions, experimentally, is slightly more than one-to-one, as shown in the figure below. The fact that the reaction produces essentially 3 additional neutrinos and antineutrinos for each one consumed is the necessary condition for a sustainable supercritical chain reaction! The potential energy release per proton disintegration in an iron atom is about 5 times the energy release produced by the fission of a uranium atom by a neutron! In addition, all the gravitational excitation energy is transferred to the fragmentation products. This might explain the lower energy neutrinos seen at the end of the burst, when the iron core collapse is almost finished.

We explain SN1987a as being analogous to an implosion-type atomic bomb. As the pressure in the iron core of the star exceeds the Chandrasekhar limit, the two inner-shell electrons in the iron atoms strike the nuclear surface and convert two surface protons to neutrons. The result is the collapse of the next electron shell towards the nucleus, which rapidly decreases the volume of the atom and highly excites the two now-unpaired surface protons. The sum of all of the individual atom collapses is equal to the collapse of the star core, which is said to take place in a fraction of a second and is reported to have two different time components.

Conventional theory seems to consider that each nucleus would first decompose into alpha particles, and then into individual protons and neutrons intermixed with electrons before electron capture would be completed to form neutrons. In such a high pressure confined situation, it is hard to believe that the original nucleons would be able to move freely at all, and it is much easier to think of each nucleus as being separate and in a highly excited state that preserves Pauli rules locally.

Collapse proceeds from the star center outward, so that 2nd shell collapse would neutralize these surface protons in each nucleus and make them non-participating. Hence, we can have a central partially "neutronized" star core surrounded by a vulnerable outer annulus that is susceptible to the diquark reaction. If supercriticality occurs early while all surface protons participate, the entire core can explode, whereas if supercriticality occurs late, a neutron star ash is left in the center while the annulus explodes away from it. Hence, sometimes a pulsar forms and sometimes it doesn't! This would explain why pulsars are not always observed after a type-II supernova has occurred.

Once a core goes supercritical, the disintegration rate, and hence the power production, increases exponentially with an extremely short period. This would burn out the excess proton fuel, and bring the core subcritical again. Hence, the reaction would actually proceed at the rate that new fuel is added by the core collapse, which would decrease with time as the iron core is used up. We argue here that the antineutrino detection curve, shown above, represents this process, and the core collapse for supernova 1987a actually took place during a 12 second interval. The Supernova of 2/18/06 appears to have taken about 30 minutes, and could have a similar explanation.

With a basic reproduction factor of 3 new neutrinos per neutrino consumed, we only need a ratio of fission cross section to non-productive capture plus fission of somewhat greater than 1/3 to make a chain reaction possible. We then need sufficient density such that product of the macroscopic cross section times thickness approximates an optical thickness of unity, thus minimizing leakage. Using neutrino-proton reaction cross sections in the experimental range of 10-44 to 10-42 cm2 per iron nucleus, we have calculated that SN1987a would easily go supercritical somewhere during core collapse, as shown in the figure below, where Keff = 1.00 is the boundary where the reaction is supercritical.

  Data computed by Bly, 1995.

Considering that at most 2 protons are available to be converted to energy in an iron atom with 56 nucleons, and only one will react, we have at best a 2% conversion of matter to energy plus a return of the gravitational excitation energy. Since not all atoms will take part in this reaction, less than 1% of the star's mass would be converted to kinetic energy. However, this would be sufficient to blow some of the core material and the outer layers of the star away without resorting to shock waves and neutrino-heating. If more protons survive to contribute, then the yield would be even higher.

In fact, both neutrinos and antineutrinos would be released as leakage radiation during the explosion, comparable to the prompt neutron and capture gamma ray pulse from an atomic bomb. The detection of this pulse on Earth could have actually confirmed that neutrino-induced fission took place in SN1987a, rather than electron capture, and that the fission actually took place over a 12 second interval!

 

Hypernova Energy Production

In principle, a Fredricksson-type reaction should also work with anti-neutrinos and neutrons. However, the mere existence of neutron stars indicates that the cross sections for this reaction are at least an order of magnitude smaller than for the neutrino-proton reaction. The main question is whether or not sufficient neutron mass could come together quickly enough to go supercritical before the process of collapse to a black hole took place. The fission cross section for this reaction can be estimated. If we postulate that 2 just-touching 2+ solar mass neutron stars are just critical, then the cross section needs to be about 10-44 cm2 per anti-neutrino, which is indeed an order of magnitude or more smaller than the corresponding neutrino fission cross section. A single stable neutron star of 2+ solar masses is clearly subcritical at a keff of about 0.8, whereas if the stars were completely merged they would be highly supercritical, depending upon how the merger takes place.

We contend that the sudden collision of two large neutron stars is analogous to a gun-type atomic bomb where critical mass is achieved by the addition of two subcritical pieces, and that a supercritical mass could be achieved under this circumstance. The excitation energy would be supplied from the kinetic energy of collision! For such a reaction, almost all of the neutrons would be susceptible, and a mass-to-energy conversion approaching 100% in some regions would be possible. A vast amount of energy would be quickly released, but this would be followed by a slower process of energy transfer away from the collision site. There would be a prompt leakage pulse of antineutrinos and neutrinos that could in principle be detected if it were strong enough. Eventually, a gigantic slow gamma ray pulse would be produced that would be detected for a considerable period of time as the debris de-excites.

 

Gamma Bursters

The corresponding collision of a neutron star with an ordinary star, which is less dense but can be much more massive, provides the antineutrino-neutron fission mechanism for a gamma burster. The collision would supply both the critical mass and the excitation. Such collisions would be much more common than neutron star/neutron star collisions. The time rate of positive reactivity insertion would be smaller, due to the lesser density of the ordinary star, thus producing a smaller power pulse.

 

Eta Carinae

The logical extension of the neutrino-proton fission chain reaction is to the collision of two ordinary stars such as the Eta Carinae pair. Consider that the interface collision zone reaches high density and excitation, and attains Chandrasekhar collapse at some point. The achievement of supercriticality at the asymmetric interface location could release enough energy to blow the two stars apart with more velocity than they originally came together with.

Eta Carinae was first observed by Halley in 1677 as an estimated magnitude +4 eruption which was visible to the naked eye. 166 years later, in 1843, it was again observed as a much brighter magnitude ~+1 eruption, leading to what was called the "keyhole nebula". This artifact could have been formed by the asymmetric explosion pattern about the collision interface.

If the cycle holds, then Eta Carinae should flare up again in approximately the years 2009-2015, since it is now increasing in brightness, to an even brighter magnitude, ~ -1, and possibly survive for yet another cycle. Experimental evidence for our postulated neutrino fission mechanism would be the detection of a neutrino and antineutrino burst coincident with the eruption!

RETURN

References

1) C.A. Bly, "Neutrino-Driven Nucleon Fission Reactors: Supernovas, Quasars, and the Big Bang", Transactions American Nuclear Society, Vol. 66, pp 529-532, 1992.

2) S. Fredriksson, "The Stockholm Diquark Model", Proc. Workshop on Diquarks, World Publishing Company (1989).

3) C.A. Bly, private communication, University of Virginia, November 2000.

4) W. Zhang and S. Woosley, "Bang, the Cataclysmic Death of Stars", National Geographic, pp 78-95, March 2007.