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Ultra-Energetic Cosmic Rays and Gamma Ray Bursts

by John G. Cramer

Alternate View Column AV-76
Keywords: ultra high energy cosmic rays gamma reay bursts correlation Fly's Eye BATSE
Published in the January-1996 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 7/2/95 and is copyrighted ©1995 by John G. Cramer.
All rights reserved. No part may be reproduced in any form without
the explicit permission of the author.


    Perhaps the two most important problems in astrophysics today are the origins of ultra-high energy cosmic rays and the phenomenon of gamma ray bursts, strange blasts of gamma of radiation from deep space [see my AV column in Analog - October-95]. Now there is some evidence that the two phenomena may be related. This column is about this possible cosmic connection.

    Cosmic rays have been a standard if mysterious phenomenon in astrophysics since the 1930s when experimental physicists first began to detect charged particles with Wilson cloud chambers and with Geiger counters and other electronic detectors. They found that energetic particles were detected even when no radioactive sources were nearby and inferred from the angles of the tracks in the cloud chambers that these particles were coming from the sky.

    Those detected particles are now called muons, a more massive lepton cousin of the electron. They were produced when a primary cosmic ray, probably a very energetic proton, hit the atmosphere and produced a shower of thousands of secondary particles, which in turn showered to produce more particles, and so on. At the end of this upper atmosphere production chain are the muons.

    The muon decay half life is only about 2 microseconds, only enough time to travel 600 meters at the speed of light and certainly not enough travel distance to reach the ground. However, the cosmic ray muons receive a "life extension" from the relativistic time dilation of their near-lightspeed velocities, allowing them to travel all the way from the upper atmosphere to ground level before they decay. These were the atmosphere-penetrating messengers that first carried the news to physicists on the Earth that cosmic rays exist. These muons, continually bombarding the planet from space, make a significant contribution to the bath of ground-level radiation in which we all live.

    Over the intervening decades since the discovery of cosmic rays we have come to understand that space is a radiation rich environment. Solar flares, supernovae, and binary star systems with a black hole as one binary member all produce floods of X-rays, gamma rays and energetic charged particles that, with space-based imaging detectors we are able to detect, localize at the source, and study . The source and production of the highest energy cosmic rays, however, remains a mystery.

    Until recently, the detection of ultra-energetic cosmic rays was limited in maximum energy sensitivity because of a basic fact of detector physics: the higher the particle energy, the larger must be the detector that measures that energy. The most energetic cosmic rays have far more energy than particles from our largest particle accelerators, and their detectors must be of heroic proportions involving large sensitive volumes and many tons of detector material.

    In principle, the best place to study primary cosmic rays is in space, avoiding atmospheric showering and measuring the primary particles directly. However, the requirement of very large detectors has so far forced space-based measurements to concentrate on relatively low energy cosmic rays. During the past decade, however, ground based experimenters have devised another strategy to deal with the detection problem. They have begun to use the Earth's atmosphere itself as a detector.

    An example of this trick is the Fly's Eye detector, which has been constructed by University of Utah astrophysicists and co-workers at the Army's Dugway Proving Grounds. The Fly's Eye is a very large array of searchlight-like mirrors and photomultiplier tubes that literally watches the sky looking for light flashes. When a primary cosmic ray hits the atmosphere and showers, the flood of very energetic charged particles produced in the upper atmosphere make Cerenkov radiation, a bright and highly directed flash of visible light. The Fly's Eye, so named because it operates like the compound eye of an insect, collects light from the Cerenkov flash and uses it to deduce the energy and direction of the primary cosmic ray that produced it.

    On October 15, 1991, the Fly's Eye observed a primary cosmic ray that would win the gold medal as the most energetic cosmic ray event ever recorded. The measured energy of this event was about 3 x 1020 electron volts, and its direction in space was localized to an angular "box" about 1o by 10o in the sky.

    The energy of the primary cosmic ray particle, presumably a proton, that produced this event is amazingly large. Converted to SI units, it hit the atmosphere with a total kinetic energy of about 5 joules. This is a truly macroscopic amount of energy, enough to lift a one kilogram mass (2.2 lb) by half a meter (20 in) against the force of gravity!

    The second most energetic cosmic ray ever recorded, the runner up and silver medal winner so to speak, had an energy of about 2 x 1020 electron volts and was detected by the Japanese AGASA experiment in December 3, 1993 and localized to a 1o error circle in the sky. There have also been earlier reports of a few cosmic ray events with energies that approach the 1020 electron volt range, and we can expect more as detector technology improves and larger detector arrays are constructed.

    The enormous energy of these cosmic ray events is perplexing because the universe has a built in energy barrier, the 2.7 K black body photons from the Big Bang, which suppresses such ultra-energetic particles. These microwave background photons are the most abundant particles in the universe, and fill all of space. When an energetic proton moves with an energy of 1020 electron volts, its collisions with these photons can be violent enough to produce pi mesons. This collision process will slow down ultra-energetic cosmic rays until their energy drops below 1019 electron volts. From this, we know that the rare 1020 electron volt cosmic rays that are being detected must have been produced somewhere in our galactic neighborhood, no more than 100 million parsecs (330 million light years) away.

    Part of the puzzle of these super energetic cosmic ray is that there are no prominent astrophysical objects in the right directions in space and within this distance that might have produced them. Another problem is that it is extremely difficult to conceive of any natural mechanism (or any artificial mechanism, for that matter) that could accelerate a particle and give it this much energy.

    Recently Waxman at Princeton University and Milgrom and Usov at the Weizmann Institute of Science in Israel have hypothesized a possible connection between these particles and another astrophysical mystery, gamma ray bursts. As described in a previous column, gamma ray bursts were discovered accidentally by military satellites that were looking for nuclear weapons testing in space. They occur at the rate of about one per day, they probably originate a cosmological distances well outside our galaxy, and if the gamma rays from them are emitted uniformly in all directions, they represent a net energy release of something like the mass of the planet Jupiter converted completely into energy.

    Waxman suggested that whatever produced the primary cosmic ray might have made a burst of gamma rays at the same time, and demonstrated that the detection statistics are consistent with this idea. Milgrom and Usov studied the extensive catalog of gamma ray bursts that has been provided by the BATSE experiment and others, attempting to match them to cosmic ray events. They reasoned that photons (i.e., gamma rays) from a given burst source will arrive earlier than charged particles (i.e., primary cosmic rays) because small magnetic deflections of the charged particle in the intergalactic magnetic field will slightly increase its path length. They calculated that over the several hundred million years the cosmic ray was in flight, this effect could have produced a net time delay of a few months to a few years. Therefore, they searched the data on gamma ray bursts (GRB) for events coming from the same region of sky that arrived some time before the Fly's Eye and AGASA events.

    And they found what they were looking for. They discovered that on May 3, 1991, 5.5 months before the Fly's Eye event, the BATSE experiment and several other GRB instruments recorded the brightest gamma ray burst event in the BATSE catalog, and that this event matched the Fly's Eye event very well in angular position. The event included one gamma ray with a measured energy of 10 GeV.

    For the AGASA event, the correspondence of cosmic ray with GRB is less compelling, but Milgrom and Usov found a strong burst detected by BATSE on December 30, 1992, about 11 months before the AGASA event. The sky position of this GRB agrees with the AGASA event to within the 4o systematic angle-uncertainty of the BATSE experiment. This GRB is only 1/7 as strong as the one correlated with the Fly's Eye event, but its energy is in the top 10% of the GRB in the BATSE catalog.

    Because there are only two ultra-high energy cosmic ray events available to be treated with the Milgrom and Usov technique, these results are very suggestive but not conclusive. Nevertheless, the work provides a strong suggestion that the two most mysterious phenomena in astrophysics may have the same source. Something somewhere in our universe is pumping many joules of energy into single particles and loosing them on the universe. The leading mechanism for accomplishing this feat, the so-called Fermi mirror mechanism, is not plausible because it would require that a particle with an energy of around 1020 eV was repeatedly reversed in direction during the acceleration process. But if we reject Fermi's mechanism, we are left with no others.

    How could anything in our universe produce such energetic particles? And if something did, why (with the possible exception of gamma rays) do we see no evidence of the acceleration process in the form of radio waves, infra red, visible light, or X-rays ? Milgrom and Usov may have combined two mysteries to make one, but that one remains a very deep mystery indeed.


Ultra-Energetic Cosmic Rays:
D. J. Bird, et al., Phys. Rev. Lett. 71, 3401 (1993); (Fly's Eye event);
S. Yoshida, et al., Astropart. Phys. 3, 105 (1995); (AGASA event).

Gamma Ray Bursts:
G. J. Fishman, et al., Ap. J. Supplement. 92, 229 (1994).

Cosmic-Ray/GRB Correlations:
E. Waxman, Phys. Rev. Lett. (1995, in press);
M. Milgrom and V. Usov, Ap. J. Letters (1995, submitted).

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