In order to understand the nature of gamma ray astronomy it is first important to know some background information. Gamma rays are photons with energies above .1 mega-electron volts (Mev). Radiation of this energy can be created through a variety of physical processes, including inverse-compton radiation, Bremsstrahlung radiation, matter-antimatter annihilation, and nuclear decay, to name a few.

An attractive feature of gamma rays is that the universe is largely transparent to them. A high-energy gamma ray passing through the diameter of the central pane of the Milky Way, for example, has about a one percent chance of being absorbed. Conversely, a visible light photon can only penetrate about one-tenth the diameter of the disk. Analogously, a high-energy gamma ray can travel from the earth to the edge of the observable universe with less than a one percent chance of being absorbed. In this case, of course, there will be a redshift.

Unlike visible light, however, gamma ray photons cannot penetrate Earth’s atmosphere. The reason for this is that gamma ray interaction is a function of the amount of matter in a given path, while visible light absorption depends on the form of this matter. When gamma rays enter the atmosphere they create extensive showers of secondary particles. For this reason gamma ray observations are most easily done either from high altitude balloons, or in space.

It must be noted that it is possible to obtain information from the extensive particle showers that are created when gamma rays interact with the atmosphere. The "Atmospheric Cherenkov Technique," for example, is a method in which several specially aligned telescopes simultaneously view a particle shower resulting from such an interaction. Although rather complicated, this method actually has some advantages over space observations. Suppression of background noise due to the stereoscopic arrangement, for example, makes these observatories much more sensitive to weak gamma ray sources. Several observatories around the world are currently using this technique including the F.L. Whipple observatory in Arizona, and the "Cangaroo" observatory in Australia.

Brief History

Gamma Ray astronomy has its roots in the height of the cold world. During this time the United States was worried that the Soviet Union or China might break the nuclear test ban treaty by exploding nuclear weapons in space. In order to guard against this, a fleet of satellites, nicknamed the "Vela" series, was deployed in 1963. These spacecraft operated in pairs at opposite ends of a 250,000 kilometer radius orbit. They contained a multitude of sensors including gamma ray detectors designed and built by a team at the Los Alamos Scientific Laboratory. Interestingly a major motivation for the gamma ray sensors was the fear that a shield or even the moon could hide an initial blast. The radioactive cloud resulting from the explosion, which would emit copious gamma rays, however, could not so easily be hidden.

The vela satellites never detected any nuclear explosions, but they did detect gamma ray radiation. Specifically they detected bursts of hard gamma rays coming from random directions in the sky about three or four time a year (the detectors were not highly sensitive thirty years ago). Due to relatively imprecise sensors and limited resources, however, it was not until 1973 that scientists were able to confirm that these bursts were of cosmic origin. Since the Vela series, a large number of Gamma ray observing satellites have been sent into orbit.

The most significant of these satellites is the Compton Gamma Ray Observatory (CGO) that was launched in 1991. This observatory (which is still operational) is equipped with four instruments, which combined cover a range of energies from 15 Kev to 30Gev. The CGO has been paramount to advancing our understanding of many high-energy phenomena; the most noteworthy of which being Gamma Ray Bursts.

The future of gamma ray observing is bright, as the United States has two major projects in preparation. The Gamma Ray Large Area Space Telescope, scheduled for launch in a few years will observe the entire gamma ray sky. The High Energy Transient Experiment (Hete), designed for the study of gamma ray bursts, is scheduled for launch in October of this year. The technology on both of these satellites will enable much greater precision and sensitivity than previous missions. As an illustration, the CGO is currently able to transmit gamma ray burst locations within 4-degree error boxes in nearly real time. The Italian-Dutch X-ray satellite Beppo-Sax, another satellite currently operational, can transmit error boxes of about 3 arc minutes in a couple of hours. The Hete telescope, however, will be able to locate gamma ray bursts within 10 arc seconds in nearly real time.

In the same way that a diffuse optical spectrum can be observed, a diffuse Gamma ray spectrum exists. A diffuse spectrum is a flux of photons that does not emanate from a particular source. Usually this flux is plotted as photons per given intensity. The Egret instrument aboard the CGO took an all sky survey for photons with energy greater that 100 Mev, and found such a flux of gamma rays (Fig. 1). As figure 1 illustrates more gamma ray emission emanates from the disk of the galaxy than at high galactic latitudes.

Analysis of this diffuse spectrum reveals that it has a steeper slope at high galactic latitudes than in the plane of the galaxy. This fact is interpreted as evidence that a diffuse spectrum is generated both within the galaxy and extragalacticly.

Currently, we believe that most of the diffuse spectrum emanating from within the Milky Way is caused by cosmic ray interactions with interstellar matter. Cosmic rays (high-energy electrons and nuclei that pervade space) interact with ISM in three main ways: Nucleon ISM interactions, Bremsstrahlung Radiation, and inverse-compton radiation. In nucleon interactions, high-energy protons collide producing large amounts of neutral pions which decay in 10^-16 seconds. This decay produces two 68 Mev gamma rays. Bremsstrahlung radiation involves the interaction of high-energy electrons with protons. In inverse-compton radiation high-energy electrons interact with the three-degree blackbody background and ambient starlight. The relative contribution of each of these types of radiation depends on the nature of the ISM and the number and direction of cosmic rays.

suggests that their contribution to the diffuse background is minimal in comparison to that of cosmic rays.

As previously stated a diffuse gamma ray background emanating from outside the galaxy has also been identified. The variability of this extragalactic background, however, has made it exceedingly difficult to quantify. The most accurate results yet observed have been obtained using the Comptel telescope aboard the CGO. Although the exact shape of this spectrum is up for debate, it can be said that it is similar to the galactic background with a steeper slope. It is currently accepted that this spectrum arises from unresolved point sources such as syfert galaxies or blazars.

One interesting theory postulates that the source for the higher end of this spectrum could be truly diffuse–arising from matter-antimatter annihilation at the boundaries of superclusters of galaxies. This theory has serious implications for cosmology, for if it is true, it requires that there be symmetry between matter and antimatter in the galaxy. Recent calculations, however, indicate that the observed gamma ray flux is a bit too low for this to be the case.

As previously stated, unresolved point sources, radioactive decay, and matter antimatter annihilation all add to the diffuse galactic spectrum. Each of these sources has been the subject of much scientific research.

The radioactive decay of some heavy elements created in supernova explosions result in observable gamma ray lines of specific energies. Observations done at these wavelengths, then, are accurate measures of the concentrations of these elements, and therefore provide an image of past supernova activity in the Milky Way. One such element is aluminum 26, which decays in approximately 1 million years by emitting one 1.8 Mev photon (Fig. 3).

Positron-electron annihilation also produces gamma rays of specific energies that can be observed. Positrons are created through a number of processes including radioactive beta decay, and pair-production. Pair production is process by which high-energy protons in the vicinity of a black hole can spontaneously create an electron and a positron. The positron and the electron then stream away from the black hole in two anti-parallel jets.

The collision of positrons and electrons produces a roughly triangular spectrum that peaks around 0.511 Mev (the rest mass of an electron). Observations done at 0.511 Mev, then, can accurately map the abundances of positrons in the galaxy. Such observations have been carried out by the Oriented Scintillation Spectrometer Experiment (OSSE) aboard the CGO, and have yielded interesting results. These observations show a diffuse glow emanating from the galactic disk, with a small increase near the galactic center. Recently a cloud of positrons was discovered above the galactic center. (Fig. 4) Although the origin of this cloud is unknown, one theory proposes

that it originates from an extremely high number of supernova explosions that have occurred near the center of the galaxy over the past million years. Galaxies with such activity have been observed, and are generally called "star-burst" galaxies.

There are also many galactic and extragalactic point sources of gamma rays. The Egret instrument aboard the CGO identified several hundred such point sources emitting energies greater that 100 Mev. Within the galaxy most of the point sources are attributed to pulsars. Although pulsars are well know as strong radio sources, most emit a greater amount of energy in gamma rays. A typical radio pulsar emits 10^31 ergs/s in the radio region and 10^34 to 10^35 ergs/s in gamma rays. These gamma rays are produced through synchrotron, and possibly inverse-compton radiation. Some of the brighter pulsars in our galaxy are the Crab and the Vela pulsars. These objects show up as bright knots slightly below the galactic plane on the right side of the Egret all sky map (fig. 1).

It is believed that most extragalactic point gamma ray sources are active galactic nuclei (AGN’s) such as blazars or syfert galaxies. The brightest knot above the galactic plane in the Egret all sky map is the Blazar 3C279. The exact mechanisms by which these AGNs produce gamma ray radiation, however, are still unknown. Most models incorporate pair-production, inverse Compton radiation, and possibly synchrotron radiation occurring in jets of relativistic plasma. Almost all observed AGN display more rapid time variability in gamma rays than in radio waves suggesting that the source of gamma rays lie between the central engine and the radio jet. The physics behind this portion of the jets is little understood. This puzzle is further complicated by the fact that only a portion of AGN's produce gamma rays.

Although many point gamma ray sources are attributable to known phenomena, it must be noted that over 55% have not been identified with sources at other wavelengths. These unidentified point sources present an interesting enigma for astrophysicists to unravel. At high galactic latitudes many of these point sources show spectral and time variations similar to those of known gamma ray emitting blazars. It is quite possible that further observations will reveal them as such.¹¹ Unidentified sources in the plain of the galaxy, however, present more of a problem. Two main possibilities for these sources are radio quite pulsars, or supernova remnants. More sensitive observations of these sources are needed, however, in order to confirm or refute current theories.

The question of gamma ray bursts has been one of the biggest mysteries in astronomy for the past 20 years. As previously stated these bursts were discovered in the late sixties, and their origin was identified as cosmic in 1973. Since then debate has raged as to both their distance, and consequently their origin. In recent years the distance debate has been more or less quelled by the discovery of afterglows at high redshifts. The cause of the bursts is still undetermined. In other words, Although recent advances have brought us great strides closer to understanding the nature of these transient phenomena, a comprehensive, all-inclusive model has yet to be developed.

Before discussing GBRs in detail it should be noted that Soft Gamma ray Repeaters (SGRs), a small sub class of GRBs, are rather well understood. These sources produce lower energy bursts with a spectrum that is usually singly peaked at about 30 Mev. SGR vary from GBRs in that their spectra are almost thermal, and they are known to repeat. Four such SGRs have been identified to date. Three of these are in our own galaxy, and one has been identified in the Large Megellaninc cloud. All four of these sources are identified with supernova remnants which strongly links them to neutron stars and suggests an upper limit top their age of about 10^4 years.

One of these sources has gamma ray emission that varies slightly in comparison to the other three. This source is known to be a neutron star in a binary system, and its bursts are attributed to accretion. The source of the other three bursts is still unknown, however a promising theory has been developed. Robert Duncan of the University of Texas at Austin and Christopher Thompson of the University of North Carolina have developed a theory that appears to agree relatively well with the data. These scientists propose that SGRs originate in neutron stars with exceptionally high magnetic fields (10^14 to 10^15 gauss as opposed to that of regular pulsars with 10^12 gauss). The huge magnetic fields surrounding these stars (termed "Magnet Stars") arise from a process called dynamo action, by which magnetic field lines are twisted around by hot moving liquid within the star’s center. As these field lines twist with the star they create more magnetic flux which in turn steadily builds up the field. As these field lines move through the star’s crust they should greatly heat it, consequently producing a lot of strain. Eventually this strain should cause the crust to crack, releasing a surge of magnetic energy which would interact with the tenuous particle atmosphere and produce copious soft gamma rays.

The Magnet star theory has a few implications that make it extremely appealing. For one it predicts a soft x-ray glow emanating from the neutron star, (resulting from the frictional heat of the moving field lines) and this has been observed in SGRs. Calculations also suggest that in order to sufficiently increase the magnetic field, the period of the neutron star must slow down to approximately one rotation every few seconds. This fact has also been confirmed through an approximately 8 second repetitious cycle recorded in a major bursts that occurred on March 5^th 1979. Finally, calculations indicate that the magnetic field of these stars should decay in about 10,000 years. This fact is consistent with the observation that SGRs are associated with young supernova remnants.

If the phenomenon of soft gamma repeaters appears to be relatively well understood, the subject of Gamma ray bursts (GBR) could not be more different. GBR ‘s are bursts of gamma radiation that are detected coming from random directions about once a day. Their energy ranges from .1 Mev to 100 Mev, with the majority falling around 1Mev. GRBs last anywhere from 4 ms to 1000 seconds, with most lasting about 2 seconds. Almost all bursts have very hard (high-energy) non-thermal spectrums that are multiply peaked and complex.

GRB have very diverse time structures. Temporal structure is basically a plot of the number of photon counts received verses time. Such plots are usually, as previously stated, multiply peaked and complex, however some display rather simple structure (Fig 5). Some trends have been identified within this relative disorder. Higher energy bursts, for example, tend to be shorter and their sub-pulses tend to have shorter rise times. Some suggest that this fact is evidence of time dilination cause by high redshifts. This, however, has not been confirmed. Although redshifts do occur, it is also possible, this pattern is due to the nature of bursts themselves. The duration of a burst, for example, could be inversely proportional to its energy.

A separate class of bursts with Fast Rises and Exponential Decays (FREDS) has also been identified. The shape of such bursts resembles their title. Furthermore, the energy range of these bursts falls between 10^-7 ergs/s and 10^-3 ergs/s, with a peak flux between .1 and 100 photons per cm^2 per second. This category of bursts does not seem to have any theoretical importance as of yet, and so is simply a method of classification.

Some researchers have suggested that line features can be found in GRB. Before the launch of the CGO some teams detected absorption lines at 10 and 100 Mev, which they attributed to cyclotron resonant scattering.¹³ This effect would imply a high magnetic field (approximately 10^12 gauss) which would, in turn imply a neutron star. It has not, however, been confirm. The Batse spectroscopy team has been searching for line features for a few years, and has yet to come up with any conclusive evidence.

Another defining characteristic of Gamma ray bursts is their spatial distribution. GRB occur uniformly around the sky (Fig 6). It can also be stated that there exists an edge to this distribution. This calculation can be done if one assumes that all bursts have the same energy and are uniformly distributed in space. From this assumption it follows that, assuming space is Euclidean, the number of bursts with energy greater than some

given energy E, should be proportional to the energy of this source raised to the —3/2 power. This proportionality is related to the fact that while the number of bursts increase by the radius of the volume cubed, the value of this radius is proportional to the initial burst energy E squared. Such analysis of bursts energies show that this proportionality is violated for weaker bursts, suggesting that there is an edge to the distribution. Essentially this data suggests that we are at or near the center of a spherically symmetric distribution of gamma ray bursts.

These two characteristics alone lent themselves to two primary interpretations of GRB. One was that GRBs originate from neutron stars that form some sort of "halo" around the milky. This theory was supported by the recent discovery of high velocity neutron stars that could escape galactic gravitation. The other theory held that GRBs originate at cosmological distances. In 1995 a major debate, commemorating the famous Shapley-Curtis debate on the scale of the universe, was held in an attempt to shed light on this distance problem.

In this debate Donald Lamb of the University of Chicago argued for the neutron star model. In one of his man points he suggested that more sensitive observations would reveal a slight bias in the number GRB towards the Andromeda galaxy. Thus suggesting that such a halo exist around it as well. Paczynski, on the other hand, argued that gamma ray bursts originated at cosmological distance. Neither provided a model by which the bursts were created.

In recent years, however, this debate has been, for the most part resolved. Satellite networks have been established that send information on burst location to ground based telescopes. As previously stated, these networks can send very accurate locations in a few hours, or less accurate information in nearly real time. Ground based telescopes were able to use this information to search for afterglows, presumably arising from some sort of explosion. In 1997 the first afterglow was found, and since then a dozen or so have been identified. All of these afterglows have been at very large redshifts. These redshifts were obtained by analyzing the emission spectrum of the optical portion of the afterglow. Furthermore most of these afterglows were found superimposed on a galaxy of very high redshift–presumably the host galaxy. Because of the time delay, however, none of these afterglows were observed simultaneously with GRB. On January 23 of this year, however, scientists found what they needed. An afterglow of optical and x-ray photons was detected concurrently with a GRB. This afterglow was found to be at a redshift of 1.6, which places it at more or less 10 billion light years away. For most this puts the GRB distance debate to rest. Gamma ray bursts are believed to originate at cosmological distances.

Placing GRBs at cosmological distances puts constraints on, but does not solve the issue of a source. Given the amount of energy received per cm^2, and given their distance, there are two, possibly three major possibilities for GRB sources. For one, the energy could be beamed in some way. Some scientists have also suggested that the radiation is partially beamed through some sort of gravitational lensing. Most models, however, assume isotropic radiation, such as occurs in a supernova. For this, calculations indicate that over 10^51 ergs of energy would need to be released–far more than a supernova.

Currently there are over a hundred theories for sources to GRBs. This number is largely due to the fact that GRB have non-thermal spectra. As Bohdan Paczynski writes,

Most of these models, however, involve some sort of relativistic shock wave or "fireball" that originates either from a neutron star binary merger, black hole mergers, or a combination of the two. In a recent paper titled "The external Shock Model of Gamma-Ray bursts: Three predictions and a paradox resolved," for instance, Charles Dermer, Markus Bottcher and James Chiange, propose a model by which a relativistic shock-wave decelerates and radiates through interaction with the circumburst medium.¹⁶ This theory is promising for the variable nature of the circumburst medium could easily account for the variability in GRB time structures. No source for this shock wave was proposed in the paper.

As observation sensitivity, as well as our understanding of High energy phenomena increase, there is no limit to what the study of gamma ray astronomy can teach us. As previously mentioned, the future of observatories is promising as the GLAST and Hete telescopes will recently be put in orbit. These missions promise to shed light on some of the major questions in astronomy today.

Many scientists hope to answer specific questions using information from gamma ray astronomy. The idea of whether there existed symmetry between matter and antimatter in the early universe, for example, could solved with data from highly sensitive gamma ray telescopes. The nature of dark matter also has implication with respect to gamma ray observing. Some theorists suggest that that dark matter may be weakly interacting particles (WIMPS). If this is the case, a sensitive telescope should be able to detect faint gamma ray lines resulting from the rare annihilation of wimps with each other. Finally some theorists propose that primordial mini-black holes that were created soon after the big bang should be exploding at high redshifts. This would result from the fact that black holes radiate energy. When their mass reaches a critical minimum, the increased heat due to radiation should cause them to explode. ¹⁷ These explosions would have specific gamma ray spectra.

Of course gamma ray astronomy will also shed light on some of the less exotic processes know to occur in the universe. The abundences and sources of cosmic ray protons and electrons, for example, are still largely a mystery. The source of the positron cloud near the center of the galaxy is also unknown. And only time will tell what other questions and solutions Gamma ray astronomy will help us discover.