• Introduction to High-Energy Astrophysics
• What are gamma-rays, compared to other radiation that we know of?
• What makes gamma-rays so special that we need space observatories?
• What are the processes that make gamma-rays?
• How does a gamma-ray interact with other matter?
• How can we detect gamma-rays?
• Which cosmic sites feature gamma-ray source processes?
• The gamma-rays' fate on their way from source to astronomical image
• What do we learn by studying gamma-ray sources?

Introduction to High-Energy Astrophysics

The research of deep space with gamma-rays has been developed as a novel discipline only in the recent three decades. After the discovery of X-rays from cosmic objects, first successful search for cosmic gamma-rays with the Apollo spacecraft confirmed expectations by theorists, namely that violent processes in the universe can be seen in gamma-rays. Subsequent more detailed studies of theory and pioneering experiments had put this on more solid grounds, such that NASA decided in the late 70's to devote a "Great Observatory" to an exploration of the gamma-ray sky.

Here we describe the basic physical and astrophysical considerations for an understanding of cosmic gamma-ray astronomy, with subsections:

• What are gamma-rays, compared to other radiation that we know of? What makes them so special that we need space observatories?
• What are the processes that make gamma-rays? How does a gamma-ray then interact with normal matter? How can we detect them?
• Which cosmic sites feature gamma-ray source processes? What do we learn studying them?

What are gamma-rays, compared to other radiation that we know of?

"Radiation" in common language means mostly something that travels on straight paths with high penetration power, such that it practically fills the space, sometimes in beams. And we think it is mostly "dangerous" to be exposed to radiation, be it "cosmic radiation", solar UV radiation, or mobile phone radio waves. Let us first clarify two basic distinctions, namely corpuscular and electromagnetic radiation. "Cosmic rays" are very energetic small particles pervading the Galaxy in the empty space between stars; "electromagnetic radiation" are waves of travelling fluctuations in electrical and magnetic fields. Gamma-rays are electromagnetic radiation. The electromagnetic spectrum spans more than 20 decades of frequency range, from the low-frequency / large wavelength regime of radio waves through the infrared, optical and ultraviolet regimes up to the high frequency / short wavelength regime of X and gamma-rays. This means that the wavelength of a gamma-ray is only a very tiny fraction 0.00000000000000000001 of the wavelength of a radio wave! (Note that we may translate between wavelength and frequency through c=l n with c=speed of light, 300000 km / sec, and l being wavelength in these km units, and n frequency in cycles per second)

The subject gets more complicated in gamma-rays than it is at radio or optical wavelenght due to the principle of wave/particle dualism: X-ray or gamma-ray "waves" can be usefully viewed at as photon particles to understand their behaviour. This dualism was discovered early this century by de Broglie and others, and relates to the development of quantum theories for classical mechanics or electromagnetism, lately even gravity. When we want to study the interaction of material bodies with electromagnetic radiation, most of what matters is the relative dimensions of the two interacting partners: For an electromagnetic wave, the characteristic dimension is its wavelength; for material bodies, it may be its thickness. Thin films of oil have thicknesses comparable to the wavelength of visible light, hence small variations in thickness of the oil layer strongly prefer reflection or absorption of light with correspondingly small wavelength differences, or slightly different colors. Therefore we can see nice colors of the oil film, caused by the differences in interactions of light of different wavelength with this oil film. In much the same way we can understand the penetration power of X-rays or gamma-rays: here the wavelength of the radiation is short compared to the spacing of the atoms in the material, hence the radiation mainly sees the atom's components, the compact nucleus and the electrons orbiting the nucleus far out, like planets orbit the sun. The atom thus mostly is "empty space" for an X or gamma-ray, and it slips through between the atoms. In this case, it becomes more appropriate to view X and gamma-rays as energetic particles, their size being their wavelength. So we can understand (see below) that materials acting as mirrors for X and gamma-rays need very special characteristics and geometries to operate at all, and do not function at all below a certain wavelength regime. In the domain of gamma-rays, we thus speak of gamma-ray photons that hit sub-atomic particles, get bounced off, and so on. In such a mechanical view, it becomes more appropriate to use the energy of the photon as its characteristic quantity, rather than its wavelength or frequency (reasons will become clear below, from the interaction of photons with matter and fields). Energy is measured in voltage units for electromagnetic particles (photons), one million electron volts (1 MeV) being the energy an electron would have if accelerated by a voltage of a million volts. The gamma-ray domain spans the energy regime from about 0.1 MeV to 10000 MeV (or wavelength 0.00001 to 0.0000000001 micrometers), five orders of magnitude in frequency / wavelength / energy. (For comparison, our eyes see the optical regime from about 0.4-0.7 micrometers wavelength, from colors blue to red - a fairly modest dynamical range.)

What makes gamma-rays so special that we need space observatories?

In spite of its penetrating power, there are limits to the range of gamma-ray photons in matter. In a thick layer of material you can imagine a gamma-ray must eventually hit an atomic nucleus in spite of their relatively large spacing, much like you would hit a tree when entering a forest on straight path in spite of a relatively large spacing between trees. The penetration depth for gamma-rays corresponds to a few grams of material per cm². Now how thick is the earth atmosphere in these terms? For a characteristic atmosphere thickness of 10 km, and a specific weight of air being one milligram per cm³, this amounts to 1000 g cm^-2 - the earth atmosphere is a thick shield! Therfore we must go to high altitudes above 40 km to be able to see cosmic gamma-rays directly (remember: the stratification of the atmosphere is logarithmic, the density of the air falls off rapidly near earth, but more shallow at large altitudes; at 40 km, the residual atmosphere above comprises about 3 g cm²). Additionally, the cosmic ray irradiation of atmospheric gas results in an enormously bright glow of the upper gas layers of the atmosphere in gamma-rays, much brighter than any cosmic sources. Instruments for cosmci gamma-rays have to be brought above this luminous layer, and be sufficiently shielded to not confuse gamma-rays from the atmosphere below with cosmic gamma-rays.

How does this compare to other frequencies? Above description of the interaction between electromagnetic radiation and matter tells us that different effects will characterize the absorbing power of the atmosphere as we go up in frequency from radio waves to gamma rays. At radio waves (l ~1m) the atmosphere does not present obstacles, the atoms are tiny compared to radio waves, similar to a swimmer not really affecting the wave propagation from a boat passing by. As we approach the optical frequencies, different atom and molecule species have dimensions that resonate with radiation and hence absorb its energy - the complex structure of the atmospheric absorption reflects the species characteristics, molecules like water vapour absorbing in the infrared regime, atoms like oxygen and nitrogen absorbing strongly in the ultraviolet regime. A window remains in what we call the visible light, this window determined the way how life was shaped on earth. Towards X rays, the inner electrons of atoms have characteristical orbital energies similar to the photon energies, and excitation of these electrons absorbs this radiation. Towards the gamma-ray regime, scattering off electrons is the prime absorbing effect, falling off as the gamma-rays become more energetic. (Figure 1) The different windows from the surface of the earth into deep space determine which kind of astronomy can be made from earth: optical, radio, and very high gamma-ray; spaceborn instrumentation is required for ultraviolet, X- and gamma-ray as well as infrared telescopes.
 
 

What are the processes that make gamma-rays?

Electromagnetic radiation can be made through two principal types of processes: through "thermal" processes like a fireball, or through other special processes that are summarized as "non-thermal" among astrophysicists.

Thermal radiation is characterized by a temperature, and the spectrum of radiation intensity (or relative number of photons) versus frequency follows the "black-body" distribution. This distribution is a result of the population of states in the radiation field in equilibrium, as derived by Max Planck early this century: the interaction of the radiating material and the radiation is so intense that the energy density of both are identical. When the fire becomes hotter, the black body distribution of light shifts its median towards more energetic light, so that each of the photons carries more energy, again balancing the increased energy of a hotter radiator. This shift is called Wien's law, the product of temperature and the wavelength of the peak of the radiation spectrum is a constant: 0.3 [cm K] = l _max * T. From this relation, l _max is in the visible regime for 6000K, approximately the temperature of the sun's surface. The big bang residual radiation at 3K shows its peak emission at millimeter wavelength. For thermal gamma-rays of 1 MeV, the corresponding temperature of the fireball would be above two billion degrees! If you may remember that nuclear fusion inside the sun occurs at 15 million degrees, then gamma-ray fireballs must be even much hotter; clearly we cannot expect such energetic fireballs to withstand explosion, and we may expect to study violent explosions of fireballs through gamma-rays.

Less extreme are non-thermal processes to make gamma-rays, from specific interaction processes of matter and radiation: we focus our attention to the exeptional process that generates a gamma-ray, and do not require that the entire source environment obtains so much energy that the gamma-ray emission dominates all radiation processes.

Several processes are distinguished:

• Charged-particle acceleration in strong electrical or magnetic fields:

• Release of quanta of energy that fall into the gamma-ray regime:

• Atomic nuclei: By construction, the nuclear (or strong) force holds together protons and neutrons in a tight system called the atomic nucleus. The strong force outweighs the electric repulsion of all those positively charged protons at small distance, yet it requires very specific, quantized, states of energy for this compact assembly of nucleons. Those states have typical energy spacings of MeV, hence any transition in states of atomic nuclei may involve absorption or emission of MeV gamma-rays! Now we just need something that brings disorder into the state of a nucleus, and what can it be? An energetic collision, two nuclei bouncing at each other with energies of MeV or more will just do that, as is happening when cosmic rays hit interstellar gas in our Galaxy. Another possibility is radioactive decay of an atomic nucleus. In such events, one of the particles of the nucleus transforms into one of another kind as a consequence of the "weak force"; neutrons decay into protons ("b ^--decay"), or protons can transform into neutrons ("b ⁺-decay"). This causes disorder in the tight arrangement of nucleons under the influence of the strong and electric force, and new, stable arrangements have to be sought, through emission of the energy difference, often as a gamma-ray in the MeV regime. Observation of characeristics gamma-ray lines thus may tell us about nuclear transitions in a source region.

• Pairs of particle and anti-particle: According to the equivalence of mass and energy discovered by Einstein early this century, energy may be converted into pairs of particle and anti-particle in the presence of a strong field. (Actually, one does not need an additional field for this conversion to occur, but the inverse process occurs so rapidly that there is no observable net particle production in the absence of fields). The lightest particle-antiparticle pair that can be generated is the electron and the anit-electron, called "positron": it requires an energy of 1.022 MeV being available. The inverse process, when a particle encounters its anti-particle, is called "annihilation": the mass of both particles is radiated into sheer energy. Momentum conservations demands that an electron-positron encounter also will produce two photons, and each of those photons obtains an energy of 0.511 MeV in the rest mass system of the annihilation process, again because of momentum conservation. Annihilation photons thus are common products of environments where anti-electrons are generated. These can be regions of high energy density and strong fields, as described above, but also radioactive decay processes involving "beta-(b )-decays" caused by the weak interaction: In a beta-decay, for example, a proton can decay into an anti-electron and a neutron (plus an electron-neutrino, we ignore this highly penetrating particle here). Annihilation photons of typical energy 0.511 keV (a "line", like a single color emitted by yellow street lights containing sodium) thus will tell us about radioactive decay regions, and very violent regions.
• Charged particles caught in strong magnetic fields: Similar to the electrical (or Coulomb) force holding electrons close to a nucleus in the quantized system called an atom, strong magnetic fields can force electrons into orbits around their field lines, which have quantized energy levels. Magnetic fields close to neutron stars can be sufficiently strong such that the steps between such allowed electron orbit levels are in the regime of tens of keV, the low gamma-ray regime (or high X-ray regime). Electrons jumping from one allowed state to another one will eject or absorb photons of this characteristic step energy, so-called "cyclotron lines".
• Up-scattering of photons of lower energy through collisions with energetic particles: As we speak of light as 'particles', we can easily imagine them collide with other particles such as electrons. The collision, or scattering, of a photon and an electron is called Compton scattering, and can be pictured like billard ball bounces. In 'normal' Compton scattering a gamma-ray photon will hit an electron within some material, and be deflected by the collision, passing some of its energy to the electron. The inverse energetics may apply also, however, and result in a gamma-ray production process: If energetic electrons collide with photons of lower energy, those photons may gain energy in such collisions, thus being promoted in energy, or color, from X-rays to gamma-rays, for example! This 'inverse Compton scattering' is important when high densities of photons, like in the light from all the stars of our Galaxy, encounter energetic electrons like those in cosmic rays. Other examples are compact stars where a hot accretion disk shines in X-rays, and the compact object generates beams of charged particles in its vicinity (see below).

How does a gamma-ray interact with other matter?

From above description, we learn that gamma-rays do not interact with material surfaces (like optical light is deflected from mirror surfaces). Rather, gamma-rays scatter off electrons within materials, thus randomly penetrate some variable depth of material before their interaction; the scattering itself is a random process with a wide spectrum of possible results, averaging out to the distributions presented above when many interactions are summed. Individual gamma-ray fate in material is not predictable.

The interaction processes vary with energy of the gamma-ray, and are (from low to high energy; see figure):

• Photoelectric absorption: atomic electrons are removed from their nuclei, thus removing the electron's binding energy from the gamma-ray photon. This is the dominant process at energies below ~ 100 keV.
• Compton scattering: electrons are hit by the gamma-ray photons, and obtain a fraction of the photon's kinetic energy in this collision. This is the dominating interaction process in the 0.1 MeV to a few MeV regime.
• Pair creation: In the electric field of the atomic nucleus, the gamma-ray energy is converted into a pair of electron-anti-electron. In the rest frame of the gamma-ray, those two particles are generated moving into opposite direction; transformation into the laboratory system makes them appear as forking out of the gamma-rays incidence direction with a small opening angle, becoming even smaller with energy. This interaction process cannot occur below 1.022 MeV energy, and increases in importance until outweighing the Compton scatter process above several MeV. Ionization tracks can be usefully recorded only above several tens of MeV.

How can we detect gamma-rays?

We can measure the effects of a gamma-ray interaction in special materials, by measuring the characteristics of secondaries after the event: an electron obtains substantial energy through a Compton scatter, hence travels fast through its surrounding material. Depending on the electron's energy, it may preferentially leave behind an ionization track, or excite light centers in scintillation material, or produce an electron-antielectron pair in the strong Coulomb field of an atomic nucleus (see above: particle-antiparticle generation). Obviously, we need to construct and optimize detectors which can measure precisely the most likely outcome of a gamma-ray interaction, with best achievable precision. This immediately explains why astronomical gamma-ray telescopes actually appear like high-energy physics laboratory equipment we know from accelerator laboratories.

For the Compton observatory, four instruments have been constructed, each following a different path of optimization, for complementary equipment to cover the five decades of different gamma-ray energies:

The BATSE, OSSE, and COMPTEL telescopes feature scintillation detector units as their key elements. Optimized for different purposes and energy regimes, those detect the light flashes that fast electrons from cascades of Compton scatterings leave behind in those detectors. Glass blocks made of sodium-iodide are the basic material, impurities of thallium generate energy levels between the valence and conduction bands of the basic material; electrons falling through those levels produce scintillation light at colors that match the characteristics of the most sensitive light detectors available, the photo-multipliers. In the case of the imaging COMPTEL telescope, two layers of scintillation detectors of different kind have been arranged to enable detection of a single Compton scatter of the incoming gamma-ray, through simultaneous detection of the Compton-scattered electron and its downward-scattered secondary photon. This more complex setup reduces the efficiency of detecting gamma-rays. But if detected, precious information about the incidence direction can be calculated from the measured quantities, thus enabling imaging of gamma-rays in this energy regime for the first time. The EGRET telescope is optimized for higher energy, where the generation of electron-positron pairs dominates the interaction of gamma-rays with matter. Incident gamma-rays thus produce pairs in the upper layer of the instrument, and the main detector consists of a spark chamber detecting the ionzation tracks of those charged particles. The electron and positron travel almost parallel, yet the weak magnetc field of the earth bends their trajectories sufficiently that they do not annihilate with each other immediately. Back-projection of the electron and positron ionization tracks tells the direction of the incoming gamma-ray, hence EGRET also is an imaging telescope. BATSE derives directional information from shadowing considerations, comparing among their eight detectors at the extreme corners of the spacecraft. The OSSE scintillators have been equipped with collimators - sets of long tungston pipes - which allow gamma-rays to hit the scintillators only from a narrow range of incidence directions; rocking the viewing angle of the collimator between two directions enables comparison of the gamma-ray brightness of such two directions in the sky.
 
 

Which cosmic sites feature gamma-ray source processes?

The thermal radiation from a "gamma-ray fireball" constitutes probaly the most extreme violent site one can imagine, from the above. Explosions of stars in supernovae in principle come close to this extreme. In those events, gigantic amounts of energy are released within short times (fractions of seconds), and thus provide such extreme heat. For core collapse supernovae, the energy originates from the collapse of a star when the fusion fire in its core starved from fuel exhaustion, and no internal energy source can counterbalance the gravitational pressure of the overlying masses. In the case of thermonuclear supernovae, accumulated nuclear fuel ignites on the surface of a perfectly heat-conducting compact remnant of a star, and causes the entire remnant to catch nuclear fire, thus burning all the fuel in an instant. Temperatures in those supernova conditions easily reach several billion degrees, and cause atomic nuclei to dissolve and rearrange upon cooling down, with radioactivities as by products. Less extreme nuclear burning occurs in nova events, when the igniton of accumulated hydrogen fuel proceeds more slowly into a nuclear surface fire on the compact remnant. Temperatures in this case are below billion degrees, yet sufficient to generate radioactivities among light elements. These explosive events cannot be studied in gamma-rays however, because the explosion itself is in the inner regions of the event, hidden behind large amounts of overlying envelopes. This occultation is the reason why the nuclear burning inside stars like our sun proceeds without associated gamma-rays escaping; yet, for very massive stars 20 or more times as massive as the sun this is not necessarily so, their atmosphere is more violently mixed. Radioactive products generated in those events, however, in some cases have sufficiently long decay times to produce their characteristic gamma-rays only after the explosion of the event has sufficiently diluted the material. Direct gamma-ray observations of sufficiently hot fireballs may be possible for events where the energy release is not covered by stellar envelopes, such as neutron star collisions and similarly extreme and rare events, which have been discussed as possible explanations of gamma-ray bursts.

Yet, gamma-rays may provide unique insights to such processes, even though originating from secondary, non-thermal processes, like in the case of radioactive decay. The very compact neutron stars are known to be common sources of X-rays, mostly caused by release of gravitational energy when matter falls onto these 10km-sized stars as massive as the sun. The complex path of matter when falling onto a netron star with strong gravitational and magnetic field is subject to a broad astrophysical study, involving radiation from radio frequencies to gamma-rays. The extreme plasma motions near the neutron stars causes complicated beams of particles, and these in turn produce the fascinating pulsing phenomena of these objects in the X-ray regime; close to the compact star, nuclear excitation of infalling matter from close to the neutron star's surface could be expected to result in characteristic line emission. Instrument sensitivities were inadequate so far to detect such nuclear lines from accreting neutron stars. - Further out in the magnetosphere, gamma-rays within a broad frequency range are known to be produced in isolated neutron stars whose magnetosphere is relatively undisturbed by accreting matter. The gamma-ray emission is attributed to curvature radiation of particles accelerated by large electrical fields. Observed pulsing behaviour varies strongly with frequency of the radiation, and can be explored to diagnose the plasma acceleration / magnetic field configurations in great detail. - their patterns are yet to be understood, with gamma-ray pulses observed only from half a dozen such objects due to the narrow beaming of this radiation. - On the other hand, charged particle accelerators of even more gigantic dimensions than in neutron star magnetospheres are observed in the extremely luminous nuclei of a sub-type of galaxies, the "active galactic nuclei". Here the particle energies must reach the highest energies one can imagine from physics considerations, gamma-rays up to 10²⁰eV have been seen from one such object recently! Even more spectacular are the jets of plasma that are ejected from these active galaxies, extending many thousand lightyears into space. Most intense gamma-rays have been observed recently from such galaxies when our viewing angle diretly peaks into such jets! We still have no idea what the source of this enormous energy could be, which makes these inner core of galaxies more luminous than thousand times the entire luminosity of the billions of stars of normal galaxies. It is possible that we can learn from detailed studies of nearby accreting compact stars how gravitational energy can be converted into jet-like plasma beams, while magnetosphere physics near such objects reveals itself in studies of gamma-ray pulsars. Combining these lessons, gamma-rays can be unique tools to study the extremely violent active galaxies far out in the universe. It is suspected that the combined gamma radiation from those active galactic nuclei comprises the major contribution to the general glow of the sky in gamma-rays, which has been termed the "cosmic diffuse gamma-ray background".

The brightest gamma-ray emission observed from our Galaxy is caused by interaction of cosmic rays with interstellar gas. Predominantly bremsstrahlung processes cause a diffuse glow of the Galaxy in gamma-rays from MeV energies up to 10000 MeV, supplemented by pion decay contributions and inverse Compton gamma-rays from starlight boosted by cosmic rays. This gamma-ray glow provides our unique trace to study cosmic rays indirectly throughout the Galaxy. The sources of cosmic rays, the bath of charged particles with energies up to 10²⁰ eV, are still unknown; acceleration by the violently expanding remnants of supernova explosions is one of the explanations that have been proposed.

The gamma-rays' fate on their way from source to astronomical image:

Travelling from the source region to our detectors, gamma-rays may traverse environments with substantial interaction probabilities, both near the extreme source region as well as close to our bulky telescope equipment. Both will add side effects of our measurement, and we must account for them in our studies.

In dense environments, gamma-rays will scatter off electrons mostly, and loose part of their energy to the plasma in this way. This Compton scattering will therefore modify the shape of the spectrum of gamma-rays, and we must account for the plasma conditions in order to reveal the original source gamma-ray spectrum through proper calculation. Note that photons may also obtain energy in such plasma collisions, if the plasma is more energetic than the photons; this results in observation of "Comptonized" gamma-radiation (see above). Line features in spectra can be significantly distorted from those effects, and scattering, too.

We can use such distortions to diagnose the plasma conditions itself, from the measured profile of gamma-ray lines. Even if unaffected by additional scattering, the line profile will tell us about the relative motion of the gamma-ray source and our observing telescope: Doppler shift of the original frequency may be familiar to you from listening to a whistling train passing by: if the radiating object moves towards you, the frequency appears higher, if it moves away, the shift is towards lower frequency. This same phenomenon modifies the colors of gamma-rays, and tells us about the kinetic energies of the source, like e.g., the expanding motion of radioactive matter after a supernova explosion, translating velocities of ~thousands of km per second into a line width of ~0.1 MeV for a 1 MeV line energy (normally, this line would be narrower than the instrumental resolution of ~ 0.001 MeV). Note that the gravitational field of compact sources such as neutron star surfaces can also resul in substantial changes of line energies: photons have a hard time to leave the star against this strong gravity, and can loose ~ 20% of their energy, with a corresponding frequency shift to the lower.

On their way after having left the source region and its environment, gamma-rays traverse long ranges of interstellar space without distortion: the interstellar space which absorbs optical radiation readily through its gas and dust at equivalent column densities of 10²³ hydrogen atomes per cm² beyond useful measurements still amount to a material coverage of less than one tenths g per cm², practically transparent to gamma-rays. The effective material thickness corresponds roughly to a sheet of paper: optically, we cannot look through easily, but gamma-rays hardly notice this material.

Getting close to the detector, gamma-rays again encounter the upper atmosphere of the earth, and a generally massive spacecraft. Gamma-rays also interact in the heavy structures of the spacecraft. This, and even more so the enormous bombardement of the spacecraft by charged particles from the radiation belts and from cosmic radiation, results in a glow of gamma-rays from spacecraft and instrument. Our telescopes have been built with complex triggering and measurement equipment to discriminate the cosmic primary gamma-rays from this background. Powerful analysis algorithms then are employed to pull out an image of the gamma-ray sky from such complex raw measurements.

What do we learn by studying gamma-ray sources?

The subjects which can be studied by gamma-rays most fruitfully clearly comprise most violent and energetic sites within our universe. The theories of nucleosynthesis can be tested with observations of radioactivity gamma-rays, such as the COMPTEL ²⁶Al mapping of the entire sky. Gamma-ray astronomy attempts to get access to astrophysical pocesses directly through observations of gamma-ray lines, following the example of optical astronomy: the relation of a line to a unique species allows more specific interpretations. Study of electrons and protons at energies and densities much above what can be achieved in terrestial laboratories can be done with gamma-rays in a variety of sites: cosmic rays and their origin is studied through diffuse Galactic gamma-rays, neutron star magnetospheres with the pulsar phenomenon are our nearby cosmic laboratory, only thousands of lightyears away, while the active nuclei in peculiar galaxies at distances of millions of light years certainly illustrate the other extreme. In between, acceleration of cosmic rays throughout our Galaxy is expected to leave trace gamma-rays, both in continuous spectra covering the full range of the Compton observatory instruments, as well as in nuclear excitation lines in the MeV region. Matter accretion onto compact objects such as neutron stars and black holes is expected to release sufficient gravitational energy to power gamma-ray source processes of different kinds - binary systems in our Galaxy may share physical processes with active nuclei of galaxies, only at different scales. And finally, the phenomenon of gamma-ray bursts, where one place in the sky outshines the rest of the universe for seconds to minutes, is completely mysterious in spite of more than ten years of intense study - most likely again extremely violent events deep in space cause this phenomenon.
 
 
 
 

Illustrations: [partial list of what had been used in the Lecture]

• electromagnetic spectrum
• earth atmosphere transmission versus frequency
• sketch of gamma-ray source processes
• gamma-ray interaction cross section versus energy: Photo/Compton/pair
• BATSE / OSSE / COMPTEL / EGRET
• gamma-ray source scenarios / object types