National Aeronautics andSpace Administration

Goddard Space Flight CenterGreenbelt, Maryland

Gamma-Ray Astronomy in the Compton Era

Looking into the night sky, onesometimes feels that the visibleUniverse is a calm, comfortable place.NASA’s Compton Gamma RayObservatory looks at avery different cosmos.This is the Universeviewed through gamma-ray photons, the highestenergy portion of the elec-tromagnetic spectrum.This is a place of explo-sive energy, cosmic parti-cle accelerators, and exot-ic environments such ascollapsed stars and mys-terious bursters. A placeof nuclear decays, particlecollisions, extraordinarytemperatures, and violentexplosions. For the first time, theentire sky has been comprehensivelystudied using this high-energy radia-tion. From the Sun to the furthestgalaxies — from diffuse clouds ofinterstellar gas to the cores of power-ful, gigantic black holes — theCompton Observatory has reshapedthe way scientists view nature.

Each gamma-ray photon (whichis the basic “piece” of light, muchlike electrons are one of the basicpieces of normal matter), carries atleast 10,000 times the energy of a

common pho-ton of visiblelight. This ener-gy is usuallymeasured inelectron-volts(or eV). A typi-cal optical pho-ton has around2 or 3 eV’s ofenergy. Thegamma-rayspectrumbegins at ener-gies of around50,000 eV (or

50 keV) and extends up to 1 TeV(1,000,000,000,000 eV) or evenhigher! The window provided bygamma-ray astronomy givesresearchers valuable informationabout a wide variety of astrophysi-cal phenomena, information whichis impossible to get by other means.The high-energy part of the electro-magnetic spectrum reveals informa-

tion about nuclear interactions anddecays, about how elementary parti-cles are created and accelerated inastrophysical sources, and clues tothe origin of cosmic rays. Gammarays are not easily scattered ordestroyed. So once created, gammarays can provide unambiguousinformation about very importantastrophysical environments. Overthe years, astrophysicists have real-ized that in order to fully under-stand nature, one must observe inas many ways possible to provide aunified view. With the launch ofthe Compton mission, gamma-rayastronomy has, for the first time,become an integral part of thismulti-wavelength approach.

This success did not come quicklyor easily. Gamma-ray astronomy is avery difficult discipline to pursuefor several reasons. First, it cannotbe accomplished from the groundlike radio or optical astronomy. Theearth’s atmosphere is opaque atgamma-ray energies. Cosmicgamma rays are absorbed high up inthe atmosphere. Only at the highestenergies are gamma rays detectablefrom the ground, through the largeshowers of particles caused by inter-action with the atmosphere. Also,gamma rays are scarce. Even thebrightest sources emit relatively fewphotons at these energies as com-pared to other energy bands. TheCrab pulsar, a bright source, emitsabout one gamma-ray photon forevery 10,000 optical photons.Finally, the cosmic gamma-ray

Gamma-Ray Astronomy in the Compton Era: Introduction

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"Who is there, who has not asked himself, ‘What is this world around me?’Rocks, trees, people — what are the parts of which they are made, and how arethese parts put together? Light, radio waves, x-rays — what are these radiations?”

A.H. Compton

Observatory Science Highlights:Extragalactic Sources

✹ The emergence of blazar Active GalacticNuclei (AGN) — highly variable cores of distantgalaxies — as a primary source of cosmicgamma rays at the highest energies.

✹ Sensitive measurements of the energy distrib-ution for another type of active galaxy knownas Seyfert galaxies, which show that the pro-duction of gamma-ray photons dies out atmuch lower energies than previously thought.

✹ The detection of diffuse gamma-ray emissionfrom the Large Magellanic Cloud, a nearbygalaxy whose gamma-ray flux indicates thatcosmic rays are galactic in origin — helping tosolve a longstanding problem in high-energyastrophysics.

Compton Observatory Science Highlights:Galactic Sources

✹ An increase in the number of detectedgamma-ray pulsars from 2 to 7 — this hasresulted in much greater understanding of thebasic physics of these rapidly rotating neutronstars.

✹ The discovery of the “Bursting Pulsar”, anexotic object near the galactic center which isunlike any previously known gamma-ray emitter.

✹ A large increase in the number of mysterious,unidentified gamma-ray sources both in andout of the plane of the Milky Way galaxy.

✹ The discovery, with the help of x-ray obser-vations, that the previously unidentified sourceknown as Geminga is a gamma-ray pulsar. Thiscould be an important clue to the nature ofseveral other unidentified sources.

Compton Observatory Science Highlights:Diffuse Emission

✹ The mapping of the Milky Way galaxy usingthe 26Al isotope uncovered surprising localizedenhancements in emission. This has importantconsequences for how chemical elements arecreated in our galaxy.

✹ Extensive studies of electron/positron anni-hilation radiation from the center of thegalaxy.

✹ The discovery of gamma-ray line emissionfrom the supernova remnant Cas A. This is alsovery important for the synthesis of elements.

✹ The discovery of gamma-ray lines from theOrion complex of diffuse gas clouds. Theselines are probably caused by energetic cosmicrays interacting with local gas.

Compton Observatory Science Highlights:Gamma-ray Bursts

✹ The discovery that gamma-ray bursts comeuniformly from all directions in the sky indi-cates that the bursts are not confined to ourMilky Way galaxy, but are probably due tohuge explosions in the distant reaches of theuniverse.

✹ The careful measurements of gamma-rayburst brightness has revealed that there are sur-prisingly few dim bursts being measured.Assuming that dimmer bursts come from fur-ther away implies that the burst distributionhas an outer edge.

✹ The detection of high-energy gamma rayshours after the bursts indicates some kind oflingering activity must be occurring in thesources after the initial fireball.

TOP: Arthur Holly Compton, the NobelPrize winning American physicist forwhom the Gamma Ray Observatory isnamed.

LEFT: Gamma rays occupy the highestenergy portion of the electromagneticspectrum. Only at the highest energies doremnants of the cosmic gamma rays survive to ground level.

signal is partially masked by a simi-lar signal from cosmic rays – ele-mentary particles which have a uni-form distribution on the sky andoften affect detectors in ways simi-lar to gamma rays. Given these con-straints, the only way to get usefulinformation is to make very large,complex detectors and fly themabove the Earth’s atmosphere.

As a result of these difficulties, ithas taken several decades forgamma-ray astronomy to fulfill itspromise. Theoretical studies datingback to the 1950’s emphasized thevalue of gamma-ray astronomy.Pioneering instruments, whichpaved the way for later satellite-borne experiments, were flown onrockets and more frequently inhigh-altitude balloons throughoutthe 1960’s and 70’s, a practice thatsurvives today as a way to test newinstrumentation. These vehiclesprovide only short observations,insufficient to carefully observenature at gamma-ray wavelengths.The first real inroads came with thepioneering space-borne missions ofthe 1970’s like the SAS 2 and COS–Bmissions which provided truly fun-damental results in this field. Theseinstruments explored the nature ofdiffuse gamma-ray emission, provid-ed detailed observations of the firstknown gamma-ray pulsars, anduncovered a population of unidenti-fied gamma-ray emitters which stillpuzzle scientists today.

Sometimes early progress came inmore unexpected ways. As far backas 1968 the Vela satellites — createdto monitor the Earth’s atmospherefor nuclear explosions — detectedbrief bursts of gamma rays of cos-mic origin coming from randomdirections on the sky. Networks ofsubsequent satellites carryinggamma-ray burst detectors estab-lished in the inner solar systemsince 1976 have produced sourcelocations for these transients usingwavefront arrival timing analysis (amethod analogous to triangulationof ships at sea). However, searchesof these source locations have pro-duced no obvious counterparts tothe gamma-ray bursts at any otherwavelength.

Whether the source of gammarays was expected or not, knowinghow gamma rays are produced isthe key to understanding what theyreveal about a source. Nature createsgamma rays in a variety of ways. Ifthe temperature of the environmentis high enough — around 100 mil-lion degrees Celsius — they can beproduced in a manner similar to lessenergetic x-ray radiation. These so-called thermal photons are oftenproduced as a result of an explosiveevent such as a supernova. They arealso produced through powerfulgravitational interactions such aswhen a black hole or neutron star iscapturing matter from a companionstar. The detection of gamma-ray

photons produced in this way yieldsvaluable information about howcompact stars interact with theirenvironments and the energy bud-get of these sources.

One of the original goals ofgamma-ray astronomy research wasto study cosmic sources of radiationcreated by atomic nuclei. Thesenuclear lines are the emission ofphotons at characteristic energiesand are typically produced as aresult of normal radioactive decayor through interactions of particleswith nuclei. Just like the morefamiliar optical lines caused by theelectron transitions in atoms,nuclear lines represent the muchmore energetic transitions andinteractions of protons and neu-trons inside the atomic nucleus.Gamma rays produced in this man-ner give scientists valuable informa-tion about how elements are createdand destroyed in our galaxy andbeyond.

Gamma rays are also produced byother “nonthermal” processes. Thisterm implies that the photons arethe results of fundamental interac-tions between elementary particlesor between these particles and otherphotons, rather than being emittedby a hot, thermal “stew” of plasma.These types of interactions occur invery special environments. Sites likethe incredibly high magnetic fieldsof young neutron stars or the powerful jets of radiation emanat-

ing from a gigantic black hole. Themain processes taking place havenames like pair annihilation (parti-cles and antiparticles destroyingeach other with the result beinggamma-ray emission), Comptonscattering (wherein a charged parti-cle collides with a photon — chang-ing the energy and direction of thephoton), and synchrotron emission(the photons emitted when relativis-tic charged particles travel in a mag-netic field).

The means by which gamma-rayphotons are created are also relatedto how these photons are detected.The nonthermal processes are thekey to how instruments detect high-energy photons. Unlike optical andx-ray photons, gamma rays cannotbe focussed by a mirror — they sim-ply pass through thin materials.Paradoxically, one of the reasonsgamma rays are valuable is that theyare less affected by their environ-ment than lower energy photons,thus providing a more pristine viewof the source. Interactions withingamma-ray detectors such as paircreation, Compton scattering andnuclear excitation — those processeswhich give rise to the photons atthe cosmic source — are used tomeasure the time of gamma-rayarrival and energy. Gamma-rayinstrumentation relies on detectiontechniques developed in the worldof high-energy physics. Phototubes . . . spark chambers . . . scintillators . . . collimators . . . these are someof the terms in the lexicon ofgamma-ray instrument builders.Putting all these devices togetherinto a coherent mission to investi-gate the unexplored gamma-ray uni-verse was the challenge of theCompton Gamma Ray ObservatoryProject.

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An accretion disk and jet associated withthe center of an active galaxy — a gamma-ray quasar. Such objects are an importantcomponent of the gamma-ray sky.

Some of the fundamental physical interactions whichresult in gamma-ray line emission. Other processes canproduce gamma rays over broad energy ranges.

Matter-antimatter annihilation

Particle-particle collisions

Radioactive nuclei

Cyclotron radiation

A 14-year effort of scientific vision,careful instrument design, spacecraftengineering, and mission developmentculminated in the 1991 space shuttlelaunch of Compton. Since launch, theCompton project has exceeded expec-tations, providing high-quality sciencedata to over 750 scientists from 23countries. The international scope ofthe Compton mission is also revealedthrough the instrument builders — theUnited States, the Federal Republic ofGermany, the Netherlands and theUnited Kingdom all contributed. Thefour onboard science instruments com-bine to provide complementary capa-bilities for mapping the gamma-raysky, probing the energy distribution ofindividual sources, and monitoring thesky for time variable phenomena.

These capabilities were intendedto solve some of the outstandingquestions earlier missions had posed— questions about the nature ofgamma-ray bursts, about the behav-ior and number of gamma-ray emit-ting pulsars and active galaxies —and perhaps most importantly, towatch for the unexpected. Theenhancements in sensitivity of theCompton instruments over previousexperiments hold the key to thisprogress. To detect more gammarays, one simply needs larger instru-ments. The size of the Comptoninstruments, the benefits of a longmission, and anticoincidence tech-niques which prevent cosmic raysfrom mimicking gamma-ray signalshave allowed Compton’s instrumentsto make significant contributions tohigh-energy astrophysics. Whilethere is some overlap in capabilities,each of the four instruments hasspecial design characteristics whichallow them to perform unique andvaluable science.

BATSE – the Burst and TransientSource Experiment – is the all-skymonitor for Compton. This experi-ment uses eight independent detec-tors, one at each corner of the satel-lite, resulting in a continuous viewof those parts of the sky notblocked by the Earth. The individ-ual modules use scintillators as theactive detection element.Scintillators are large crystals, in theBATSE case made of sodium iodide,which convert the energy of the

gamma ray into visible light.Photomultipliers viewing the crystalthen detect the visible photons. Thestrength of the resulting electronicsignal is the record of the gamma-ray energy. BATSE can detect pho-tons in the range of about 20 keV to1 MeV. This instrument has thecapability to detect many gammarays in small periods of time, allow-ing scientists to study the temporalstructure of gamma-ray bursts.While an individual BATSE modulecannot record the direction of theincident gamma rays, the relativestrengths of the burst signal in eachmodule result in an approximate tri-angulation of the burst position.

The origin of gamma-ray burstsremains the longest unsolved mys-tery in modern astronomy.Hundreds of gamma-ray bursts weredetected prior to the launch of theCompton Gamma Ray Observatory,but few clues to their nature havebeen forthcoming. One of BATSE’sprimary objectives is to study thesesources by mapping their positionson the sky and by measuring their

brightness, spectra and temporaldevelopment. Their time profiles areparticularly enigmatic, lasting a fewtens of milliseconds to hundreds ofseconds, each burst exhibiting aunique and complex evolution ofbrightness with time. SinceCompton’s launch, approximatelyone gamma-ray burst per day hasbeen detected by BATSE. In addi-tion, BATSE has the capability ofperforming long-term monitoring ofother types of gamma-ray sources.Many objects emit photons in thesame energy band as gamma-raybursts, including accreting neutronstars and black holes, some radiopulsars and active galaxies. BATSEprovides a valuable capability fordetecting changes in flux associatedwith these objects over a large rangeof timescales and helps scientistsunderstand the mechanisms respon-sible for their high-energy emission.

BATSE also uses its gamma-rayburst design capabilities to providevaluable data on the hard x-rayemission from solar flares.

Gamma-Ray Astronomy in the Compton Era: The Instruments

BATSE

OSSE

COMPTEL

EGRET

Compton Instruments-Energy Ranges

10keV 100keV 1MeV 10MeV 100MeV 1GeV 10GeV

BATSE

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The STS 37 space shuttle launch whichcarried the Compton satellite into orbit.Compton, at 17 tons, is the largest scien-tific payload carried by the shuttle.

The Compton instruments cover more thansix orders of magnitude in energy in acomplementary manner.

BATSE - the all-sky monitor for Comptonis designed to detect gamma-ray burstsand search the sky for other transientemission.

Unlike BATSE and OSSE, theImaging Compton Telescope –COMPTEL – is an imaging detector.This experiment allows scientists tomake images of the gamma-ray sky,

albeit at a reduced resolution ascompared to optical astronomy,over the energy range from about 1to 30 MeV. This experimentemploys a novel design wheregamma rays are Compton scatteredin the top detector and absorbed inthe lower detector. By tracing back-wards the path of the scatteredgamma ray from top to bottom andmeasuring the total energy deposit-ed in the detectors, the location ofthe cosmic gamma ray on the sky isfound to lie on an circle and not ata point as seen by a traditionaldetector. The intersection of “eventcircles” from many photons deter-mines the location of the gamma-ray source in the sky.

The COMPTEL energy range isimportant to nuclear astrophysics.Nuclear reactions occur during cata-

clysmic events such as novae, whichare nuclear explosions on the sur-face of a white dwarf or other com-pact star, or supernovae, which aretotally catastrophic stellar explo-sions. During these events, nuclearreactions fuse together the light ele-ments of helium, carbon, nitrogen,and oxygen into the heavier ele-ments of aluminum, nickel, andiron while releasing gamma rays inthe 0.5 - 5 MeV energy range. Manyof these heavier elements are thenejected into interstellar space asradioactive isotopes, which thendecay into stable isotopes releasingneutrons and gamma rays. Thisenergy range has historically been avery difficult one to work in.COMPTEL does a good job of bridg-ing the gap between more tradition-al detectors like OSSE and EGRET.

COMPTEL

The Energetic Gamma-RayExperiment Telescope (EGRET) issensitive to photons in the energyrange from 20 MeV to 30 GeV, thehighest energies accessible by theCompton instruments. Like COMP-TEL, EGRET is an imaging instru-ment. The basic instrument designis that of a spark chamber, technol-ogy which is borrowed directly fromhigh-energy physics. The cosmicgamma ray enters the instrumentand interacts with absorber materi-al. At these energies, pair produc-tion is the dominant process. Thegamma ray is converted into anelectron/ positron pair. As this pairtravels through the gas-filled cham-ber, sparks are detected in electrifiedwires which criss-cross the chamber.The spark chamber essentially takesa picture of the trail of the pair asthey pass through the instrument.These tracks lead back to the inci-dent direction of the cosmic pho-ton, allowing EGRET to image thesky. Once the pair exit the chamber,they are absorbed by a scintillatorwhich gives a good estimate of thetotal pair energy, and hence theenergy of the incident photon.

EGRET is in the tradition of previ-ous spark chamber experiments

which have detected pulsars andactive galaxies as high-energygamma-ray sources. The improvedsensitivity of EGRET, however,allows for much more detailedstudy of such objects. EGRET is alsointended to perform detailed studiesof diffuse gamma-ray emission. Thisemission is the result of interactionsof cosmic rays with interstellarmaterial, the study of which pro-vides important information aboutthe origin of cosmic rays. There arealso many gamma-ray sourcesdetected at EGRET energies whichare unidentified at any other ener-gy. No known counterpart exists forthese sources and uncovering theiridentity is another important aspectof EGRET science.

Beyond their individual capabili-ties, the CGRO instruments worktogether to provide an even morecomprehensive study of nature. Forinstance, BATSE can provide signalsto the other three instrumentswhich allow them to respond, withtailor-made instrument configura-tions, to solar flares or gamma-raybursts. EGRET, COMPTEL, and OSSEoften observe the same objectsimultaneously, affording tremen-dous potential for further under-standing of detected sources. TheCGRO instruments often observesources simultaneously with othersatellite- or ground-based observato-ries to provide a truly comprehen-sive view. This approach, calledmulti-wavelength astronomy, givesscientists the most complete infor-mation about our universe.Ultimately, however, the gamma rayview by itself has proven to be pretty spectacular!

EGRETOSSE

OSSE – Complementing BATSE isthe Oriented ScintillatorSpectrometer Experiment (OSSE)which provides the capability ofdetecting sources in the band from50 keV to about 10 MeV. OSSE iscomposed of four independentmodules, each a collimated scintilla-tor. Collimation refers to the prac-tice of placing a baffle on the detec-

tor to reduce the field-of-view. Thisallows the detector to concentrateon a single source. The independentmodules of OSSE can also view abackground region simultaneouslyfor comparison with the sourceregion. This comparison allowsOSSE to detect sources which aremuch weaker than those routinelyobservable with BATSE. In addition,OSSE provides very good spectralcapabilities over its entire energyrange. This allows for sensitivesearches for features which areimportant diagnostics of sourceenvironments.

These spectral features are the pri-mary science objectives for OSSE.There are many types of featuresOSSE can look for. Gamma-ray lines,which are enhancements in thenumbers of photons detectedaround some characteristic energy,

are the main type. An example ofsuch a line is the 511 keV line asso-ciated with the pair annihilation ofelectrons and positrons (the elec-tron’s antiparticle). Other typesinclude lines from nuclear decays orinteractions, and cyclotron lineswhich are caused by the effect ofvery strong magnetic fields ongamma-ray emitting particles. OSSEis sensitive to other spectral featuressuch as changes in the continuumemission from gamma-ray sources.Most gamma-ray sources have asmooth distribution of strength ofemission (flux) versus energy. Byfinding variations in this distribu-tion, a great deal is learned aboutthe source environment. OSSE hasthe capability to identify such fea-tures which in turn are crucial tounderstanding the basic physicsinvolved.

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OSSE provides sensitive spectral measure-ments of individual gamma-ray pointsources.

The COMPTEL instrument has providedthe first comprehensive study of the uni-verse in the 1 - 30 MeV energy range.

EGRET is the highest energy experimentonboard Compton with sensitivity up to30 GeV.

PulsarsThe Crab pulsar,

which isone ofthe mostimportantobjects inthe sky atany wave-length, is astandard candle ofgamma-rayastronomy —strongly emit-ting photons atall energies. Theneutron star sit-ting in the rem-nant of the super-nova which domi-nated the sky in 1054 A.D. is rotat-ing 30 times per second. With eachrotation a beam of gamma rayssweeps past the Earth creating theeffect of pulsations. Pulsars havebeen likened to spinning magnets.This description, although some-what accurate, hides the complexitythat exists in the incredibly strongmagnetic fields associated with neu-tron stars and the unknown struc-ture of the collapsed star. Carefulstudy of the gamma-ray energy dis-tribution, and pulse shape of theCrab pulsar have supplied neededinformation about how gamma-raypulsars work.

Before Compton, only twogamma-ray pulsars were known, theCrab and Vela pulsars. There arenow seven: PSR 1509-58, PSR 1706 44, PSR 1055-52, PSR 1951+32 andGeminga all being added to the list.The names of most of these arebased on coordinates from radioobservations. In fact, there arearound 550 radio pulsars known atthis time. The fact that there are

only sevengamma-ray pulsars shows how

special the environment must be tocreate gamma rays. The gamma-raypulsars tend to be of the young,rapidly-spinning variety. Each pulsarhas unique characteristics and thepulse shapes can be highly depen-dent on the energy range. Knowingwhy some pulsars emit gamma raysand some don’t, how the pulseshapes in each energy range arise,and what the gamma-ray photonssay about the structure and environ-ment of the neutron star, are at theheart of gamma-ray pulsar research.

The whimsically named Gemingais of a somewhat different nature.The name, which means “is notthere” in a certain Italian dialect,refers to the fact that there was noapparent counterpart at any otherwavelength to the very stronggamma-ray source first detected inthe 1970’s. Geminga was one of thelarge class of unidentified gamma-ray sources, the nature of which isstill largely unknown. In Geminga’scase, however, EGRET observationsverified the x-ray detection of pulsa-tions in the flux from Geminga,

positively iden-tifying it as agamma-raypulsar. Oddly,

Geminga still has notbeen detected at radio wavelengths.Whether the Geminga story is ahint at the nature of the otherunidentified sources or simply ananomaly is an important area ofcurrent research.

Active GalaxiesBack to our anticenter image, just

slightly away from the Crab, thegamma-ray emitting active galaxyknown as PKS 0528+134 is also

The Compton instruments combine

to provide many different views of the

high-energy universe. Perhaps the

most interesting, and one representing

a primary mission goal for Compton,

is the all-sky survey. The first two and

one half years were devoted to viewing

the entire gamma-ray sky with the

two imaging detectors, EGRET and

COMPTEL. The remarkable images

which resulted from this survey show

for the first time what the sky looks

like over the medium (1-30 MeV) and

high (>100 MeV) energy ranges.

The dominant feature is the widestrip across the middle of the imagewhich is the gamma-ray glow of theMilky Way. This is shown particular-ly well by the EGRET map on thefront cover. This diffuse emissiongives astronomers insight into theinteractions of cosmic rays andinterstellar material. Unfortunately,this emission also makes it moredifficult to detect point sources of

gamma rays which lie in the galac-tic plane. The other main feature ofthese maps is the occasional islandof enhanced emission seen over thediffuse background. These are thegamma-ray emitting pulsars andgalaxies which are of considerableinterest to astronomers. Many ofthese point sources are not obviousfrom simply inspecting the map.For instance, the EGRET point

source catalog con-tains 157 individualsources. Advancedstatistical methodsare required to dis-tinguish the weakersources from merefluctuations and tomake the best esti-mates of the sourcecharacteristics suchas true location,total flux, and variability.

For a more detailed look, we canfocus in on a smaller region of sky.The anticenter region (so-calledbecause it is 180 degrees away fromthe direction to the galactic center)is a good place to start. The COMP-TEL image of the anticenter showsfour different types of objects whichrepresent the variety of the gamma-ray sky. The Crab pulsar, the activegalaxy PKS 0528+134, the transientgamma-ray source GRO J0422+32,and the gamma-ray burst GRB910503 were all detected by COMP-TEL in this region. Much of the skyis similar, look closer and you willfind several sources, each with dis-tinct characteristics. It is worthspending a little time discussing allthese types of sources in more detailto understand the contributions ofthe Compton instruments.

Gamma-Ray Astronomy in the Compton Era: The Universe in Gamma Rays

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This COMPTELall-sky map is thefirst ever at MeV energies.This image complements the EGRET all-sky map which is shown on the front cover.

The COMPTEL view of the galactic anti-center. The diversity of the gamma-ray skyis apparent even from this limited region.

The folded light-curves ofvarious gamma-ray pul-sars at different ener-gies. A straight linemeans no known puls-es. Explaining thevariety of pulse shapesand energy dependen-cies is a challenge totheorists.

One of the first Compton surprises was theEGRET detection of the gamma-ray blazar3C 279. The only previously detectedgamma-ray blazar, 3C 273 can be seen tobe weakly emitting.

H665.001

INT

EN

SIT

Y A

S A

FU

NC

TIO

N O

F T

IME

TIME IN FRACTIONS OF A PULSE PERIODP ~ 89 mSEC

PSR B1706-44

P ~ 102 mSEC

VELA

CRAB

RADIO

OPTICAL

X-RAY

GAMMA RAY

P ~ 33 mSEC

GEMINGA

P ~ 237 mSEC

GAMMA-RAY PULSARS

PSR B1509-58

P ~ 150 mSEC

PSR B1951+32

P ~ 39 mSEC

PSR B1055-52

P ~ 197 mSEC

3C 273

3C 279

technique shows that these low-energy gamma-ray emitters arenumerous. OSSE and BATSE obser-vations continue to provide infor-mation on such topics as the defini-tion of the binary orbit, thestrength of the neutron star mag-netic field, and the basic physics ofaccretion.

Gamma-Ray BurstsThe final source in the anticenter

image is that of a gamma-ray burst.Normally, BATSE provides the onlyinformation on bursts. Once in awhile, however, one of the imaginginstruments can see the high-energytail of a gamma-ray burst and actu-ally image the burst emission. Thisinformation is quite valuable, allow-ing for a better localization of theburst, which is importantwhen searching forcounterparts.

Before the launch of Compton, theconsensus of the astronomical com-munity was that gamma-raybursters must be a class of neutronstar, born in the supernova explo-sions of dying massive stars thatinhabit regions near the plane ofour galaxy. BATSE was expected toreveal a clustering of burst direc-tions near the galactic plane.Furthermore, the neutron stars wereexpected by some investigators toproduce repeated burst perfor-mances.

BATSE, which detects about oneburst per day, has not confirmedthis galactic disk hypothesis.Instead, the celestial distribution ofmore than 1400 BATSE burst local-izations is completely uniform on

the sky, and no convincing evi-dence for repetitions has beenfound. The burst directions appearto be unassociated with luminousmatter as far away as the Virgosupercluster of galaxies, with whichour Milky Way galaxy is peripheral-ly connected.

These are just a few of the imagesof our universe that Compton hasprovided astronomers. There ismuch more to the gamma-ray skythan this. Enhanced emission fromdiffuse clouds of gas, supernovaremnants, and nearby normal galax-ies are also detectable. Such staticimages give a somewhat misleadingimpression — perhaps the mostimportant characteristic of the uni-verse seen by Compton is that it isdynamic and unpredictable.

1429 Gamma-Ray Bursts

clearly detected in the COMPTELenergy range. Active galaxies repre-sent the largest class of gamma-rayemitters. These sources are incredi-bly powerful but are also highlyvariable. Look once and it is there,look again and there is no emission.This variability is because of thephysics of the region surroundingthe gigantic black hole presumed topower the active galactic nuclei.Emission from these “Blazar” AGNare believed to come from the jetsof relativistic particles emanatingout from “a central engine”. Suchjets are seen in radio maps of thesesources. Compton studies of activegalaxies have been one of the great-est successes of the mission. Over 50Blazar-type AGN have been detectedby the EGRET instrument alone.

A second type of AGN, known asSeyfert galaxies, also comprise animportant class of gamma-ray emit-ting AGN. About 17 of these galax-ies, which are much closer to theMilky Way than Blazars, have beendetected mainly by the OSSE instru-ment. In the theoretical unifiedview of gamma-ray emission fromactive galaxies, Seyferts differ fromBlazars in part because we are view-ing them from a different angle, sothat we are looking through thegalaxy rather than looking face onas in the Blazars. In this way, the jetemission is less important thanemission more closely associatedwith the inner regions of the galaxyitself. The OSSE instrument hasfound that the average spectrum ofthe Seyfert galaxies is well-described

by this sort of picture. Unexpectedly,the Seyfert spectrum falls off muchmore quickly than previouslythought. Previous observations sug-gested that Seyfert emission extend-ed up to the MeV range. The aver-age Seyfert spectrum indicates oth-erwise. This is an important clue tothe inner workings of AGN.

Accreting Galactic Sources Back in our own galaxy, the

COMPTEL source named GROJ0422+32 is a system known as a x-ray nova. This is an example of athird type of gamma-ray source —studied mostly with OSSE andBATSE — accreting galactic binarysystems. The name describes thesystem well: a pair of stars, one nor-mal and the other collapsed into aneutron star or black hole, are inorbit around one another.Depending on the mass of the com-panion star and the evolutionaryhistory of the system, there can berelatively steady emission of low-energy gamma rays, or episodicemission whenever the pair are attheir closest orbital approach.Studies of such objects have beenaided by the sensitivity and longmission of Compton.

Another example of these systemscan be found by looking at the cen-ter of our own galaxy. While not animaging instrument in the classicalsense, BATSE does monitor theentire sky. As the Compton satelliteorbits, sources are alternately hid-den and revealed by the Earth.Techniques can be used to calculatea coarse image of the sky using thisinformation. The image of thegalactic center calculated using this

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The center of our galaxy is populated by many low-energy, rapidly changing gamma-ray sources. BATSE monitoring helps to unravel the mystery of these sources.

A remarkably uniform distribution ofgamma-ray bursts on the sky has meanttrouble for models of bursts based upongalactic objects. A galactic populationwould preferentially show a clusteringalong the galactic plane.

We have known since the 1970’s

that the gamma-ray sky is variable.

Only since Compton have we realized

the tremendous richness of the high-

energy sky. Supernovae, x-ray novae,

active galaxies, the Sun, bursts — the

sky is constantly changing. The flux

from an individual source can often

change by an order of magnitude or

more.

No other sources, however, matchgamma-ray bursts for their extraor-dinary behavior. For example, whilemillions of TV viewers watched theSuperbowl on January 31, 1993, thewavefront of a very intense gamma-ray burst passed across the Earthand was detected by all four instru-ments on Compton. This burst wasbriefly more than 1000 timesbrighter than the rest of thegamma-ray sky. As seen by BATSE atlow gamma-ray energies, 10 keV toa few MeV, the intense part of theburst consisted of a few short, close-ly spaced pulses lasting only aboutone second. EGRET also detectedthis initial phase of the burst, butquite unexpectedly, emission wasseen to persist almost one minutelater at high gamma-ray energies,

100 MeV to severalGeV, where EGRET ismost sensitive. This“Superbowl Burst”was also seen by adetector on theUlysses spacecraft,allowing the locationto be refined wellbeyond the positionaluncertainty of mostgamma-ray bursts.Unfortunately, theephemeral characterof gamma-ray burstsmakes it difficult toidentify the source ofthe burst. There aremany typical celestial objects withineven the smallest burst locationerror circle. Unless a flash of light atother energies, say radio or opticalenergies, is seen simultaneouslywith the gamma-ray burst, it isextremely difficult to argue for anyof the candidates. This is whatmakes the problem of gamma-raybursts so difficult. At this time, wemust rely almost exclusively on thegamma-ray information alone.

For example, the burster distanceor emission pattern is constrainedby the presence of high energygamma-rays such as those detected

in the Superbowl Burst. The moredistant the gamma-ray burst sourceis, the higher its intrinsic luminosi-ty, and correspondingly the higherthe gamma-radiation density withinthe source itself. In a very denseradiation field these high-energyphotons can collide and annihilate.The survival of GeV photons againstgamma-gamma annihilation implieseither that the sources must be veryclose by — less than tens of light-years distant — so that the gamma-radiation field is not too dense, orthat the radiation is highly collimat-ed or “beamed”, thereby avoidingcollisions between the gamma raysand allowing their exit from thesource. Thus the gamma-raybursters may effectively be enor-mously powerful cosmic beammachines.

While not as mysterious asgamma-ray bursts, solar flares canbe just as exciting. Most normalstars are not candidates for study byCompton instruments, but the Sun isso close by that even weak emissioncan be detected. In the case of solarflares, which are highly energeticbursts of radiation from the Sun dueto acceleration of particles in loopsformed in the outer solar atmos-phere, the same capabilities BATSEuses to detect cosmic gamma-raybursts are used for flare studies.Flare activity is related to the so-called solar cycle, an approximately11-year period during which solaractivity (and hence flares andsunspots) go through relative maxi-ma and minima. The launch ofCompton came shortly after such asolar maximum. Luckily, solar activ-ity was still at a very high level andvaluable observations were made.Compton will still be in orbit duringthe next solar maximum.

Much has been learned about thefundamentals of particle accelera-tion in solar flares from Comptonobservations. Gamma-ray observa-tions can also provide informationon which elements are present inthe ambient coronal gas. By study-ing the gamma-ray time history offlares, and correlating the gamma-ray emission with observations atother wavelengths, it has been dis-covered that particles are acceleratedfor much longer periods of timethan just the impulsive beginningof the flare. An example of such aflare was detected on June 4, 1991.EGRET, COMPTEL, and OSSE alldetected evidence for particle accel-eration lasting for several hours.

This flare was one of the fewenergetic enough to be detected byall the Compton experiments. Thesebroadband observations are very

important. Perhaps one of the mostintriguing observations yet made byCompton is the COMPTEL detectionof neutron flux from a solar flare.Neutrons interact in the COMPTELinstrument in a manner similar tothat of gamma rays. Usually, neu-trons are removed from the data ascontaminants. During a flare, how-ever, solar neutrons are actually avalid signal considered worthy ofstudy. As a result, not only canCOMPTEL construct an image ofthe Sun in the light of gamma rays,but an actual image of the Sunusing neutron flux can be formed!This is a remarkable achievement.Neutrons decay with a half life of 5minutes, so it is unlikely that anyother astrophysical systems couldever be detected via neutron flux.

GRO J1655-40: A GalacticTransient

Most sources which exhibit timevariability are a little less spectacularthan bursts, but just as important.For example, in late July of 1994,BATSE began to detect a strong, pre-viously unknown source of low-energy gamma radiation from thedirection of the southern constella-tion Scorpius. The source, dubbedGRO J1655-40, rapidly increased inbrightness, becoming, over thecourse of about 10 days, among thebrightest sources in the sky inBATSE’s sensitivity range. But whatwas it? Its “light curve”, which is aterm astronomers use in referring tothe source brightness as a functionof time, vaguely resembled that ofpreviously known transient objectswhich consist of a neutron starorbiting a very luminous, hot,young star. On the other hand itsspectrum, which is simply the num-ber of photons emitted versus pho-ton energy, resembled that of

Gamma-Ray Astronomy in the Compton Era: The Transient Sky

12 13

Lightcurve of the "Superbowl Burst" show-ing complex structure at differenttimescales. Understanding the wide varietyof burst time profiles and durations arepart of the mystery of gamma-ray burstastrophysics.

LEFT: Localization of the Superbowl Burst.Since it was detected by all four Comptoninstruments as well as another spacecraft,this location is particularly good. Even thebest burst locations, however, have not yet resulted in the identification of an objectresponsible for a burst.

BELOW: Details in the OSSE solar spec-trum help identify the nature of particleacceleration during solar flares. TheCOMPTEL image is a remarkable pictureof the Sun in the "light" of neutrons.

1 10Energy (MeV)

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GRB 930131 - COMPTEL COMPTEL + EGRET

another class of tran-sient objects whichconsist of a muchdimmer, cooler, oldstar orbiting a blackhole.

In either case it isaccretion onto the com-pact object which isresponsible for thegamma-radiation. A moreaccurate spectral measure-ment using OSSE gave sup-port to the black-hole sce-nario, as did the fact thatperiodic pulsations — a signthat the radiation is associat-ed with the rotation of a neu-tron star — were not evidentin the data. The true nature ofGRO J1655-40 could not beunambiguously determined byCompton alone — observationsat radio, optical and x-ray arerequired to distinguish betweenthe likely possibilities, or perhapsreveal some unexpected alternative.

To gamma-rayastronomers, or for that matter

to scientists in general, the prospectof finding the unexpected isoften the most excitingendeavor!

For GRO J1655-40, theteam of gamma-rayastronomers using CGRO,radio astronomers using theVery Large Array (VLA – anarray of radio telescopesmounted in an 22-mile long“Y” shaped configuration onthe desert of New Mexico),and astronomers working inthe visible domain usingtelescopes in the ChileanAndes were not disappoint-ed! What they discoveredwas that GRO J1655-40 did

in fact contain a black hole.Optical studies showed that

its orbital period was 2.6 days;8-12 hours is more typical.However, the real surprisewas in the radio. GRO J1655-40 was found to be ejectingmatter in highly collimatedstreams in a direction near-ly perpendicular to theplane of its binary orbit.Astronomers call suchstreams of matter, “jets”,in analogy to the streamof water emitted fromthe nozzle of a high-pressure water hose.

Jets had been seenbefore, and are in fact quite com-mon, in extremely distant, energeticobjects known as “quasars”.However, in our own galaxy, thereare only a few examples of jet-pro-ducing sources. What made GROJ1655-40 all the more interestingwas that time-lapsed sequences ofobservations revealed that the jet-ejection velocities approached thespeed of light; 92% of the speed oflight to be precise! Einstein’s theoryof relativity dictates that no materi-al object move with a velocity thatexceeds the speed of light, so GROJ1655-40 appeared to be hurlinglarge quantities of material intospace at 92% of the “cosmic speedlimit”. Actually not quite: scientistsstudying quasars had noted in the1970’s that a combination of projec-tion effects and relativity can pro-duce the illusion of motion at, oreven exceeding the speed of light,but it still remains a hard and fasttruth that physical matter neverreally exceeds the speed of light.

In any case, an enormousamount of energy is required togenerate and sustain the jets seen inGRO J1655-40. Since it is muchcloser to home than the distantquasars — about 10,000 light yearsas opposed to about 1,000,000,000light years — scientists infer that itis far less luminous and fills a muchsmaller volume of space than aquasar. This means, among otherthings, that it evolves more rapidly— on time scales of days ratherthan decades or longer! As such,GRO J1655-40 affords scientists aunique opportunity to study howjets are formed and how they evolveover time. With the combination ofnearly continuous monitoring ofthe gamma-ray emission providedby BATSE, and frequent observa-tions in the radio, evidence linkingthe accretion process and jet forma-tion has been revealed for the firsttime! It was found that jet-ejectionepisodes followed gamma-ray“flares” by 5-10 days. Because of thelarger distances to quasars, the anal-ogous effect could take decades toobserve.

Combined radio, optical, ultravio-let and x-ray studies of GRO J1655-40, and about a half-dozen othertransients believed to contain blackholes, have provided an unprece-dented wealth of information.Additional transients are likely tooccur within the lifetime ofCompton, and undoubtedly, moresurprises will be forthcoming.

Quasars: Cosmic EnginesSurprises have been business as

usual for Compton. In the first fewweeks of the mission, the instru-ments pointed to the Virgo regionto observe the only active galaxyever seen to shine at gamma-rayenergies greater than 100 MeV — aquasar called 3C 273. An ordinary

galaxy mainly shines with the visi-ble light emitted by its billions ofstars. But many galaxies are brightat other wavelengths: radio,infrared, x-ray, or even gamma-ray.The energy from such an activegalaxy may arise from bursts of starformation, a collision with anothergalaxy, or perhaps even a massivecentral black hole. When activegalaxies lie at extreme distances,their apparent size is so small theyappear like a star. In this case, theymay be known as a quasi-stellarobject, or quasar. Despite the factthat quasars lie in the outer reachesof the universe, they are stilldetectable by our telescopes. Thisimplies, given their great distances,that they are among the most ener-getic phenomena in the universe.

The observation of 3C 273 sur-prised scientists because the brightgamma-ray source they expected tosee was several degrees away fromthe position of 3C 273! It turnedout that they were in fact detecting3C 279, a similar type of quasar. 3C273 was weakly detectable in thesame image but thebright emission froman unanticipatedsource set the tone forthe Compton mission.The variability of thegamma-ray quasars areshown by the lightcurve of 3C 279 duringthis period. The rise inintensity over a fewdays and then the fluxplummeting backdown is not atypical ofgamma-ray quasars.Starting with the unex-pected detection ofgamma rays from theobject known as 3C279, EGRET has detect-ed over 50 additional

gamma-ray quasars, some at dis-tances of a billion light years fromEarth. Many of them are highlyvariable, extremely powerful cosmicaccelerators. For example, theobserved flux from 3C 279 implies atotal luminosity over 10,000 timesthat of the total luminous output ofour entire galaxy, assuming theemission is isotropic, that is, uni-form in all directions. Most scien-tists however, now consider thisvery unlikely and believe that theemission is “beamed”, or directedinto a narrow cone. The quasarsthat EGRET detects are the onesthat are fortuitously directedtowards us. Like the galactic objectGRO J1655-40, the variability of thegamma-ray quasars provide scien-tists the opportunity to carefullymonitor emission at many wave-lengths, providing a unified pictureof the gamma-ray source. The ever-changing gamma-ray sky makes thisapproach necessary.

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The radio images of GRO J1655-40.The ejection of “blobs” of materialwhich stream away from the centralsource can be discerned.

The BATSE time history ofgamma-ray emission from thetransient source GRO J1655-40as compared to the radiolightcurve.

GRO J1655-40 X-ray and Radio Light Curves

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Lightcurve for the EGRET observation of3C 279. This bright, variable quasar wasthe first of over 50 to be detected.

stage of evolution which will not beseen for a very long time. How cantheories created to separatelyexplain bursters and pulsars bemodified to explain this object?What will other wavelength bandstell us?

Aside from surprises like GROJ1744-28, there are many other pos-sible discoveries which await thefinal years of the Compton mission.In the next few years, a new super-nova in a nearby galaxy is a possi-bility. Observations of gamma-raylines will constrain models of explo-sive nucleosynthesis — the theoryof how heavy elements are createdin our galaxy. Additional observa-tions of our own galaxy may revealmore areas of line emission whichcan probe the supernova rates inour own galaxy. Further mapping ofthe diffuse emission from ourgalaxy will provide crucialinformation regardingthe transport and pro-duction of cosmicrays.

Other important stud-ies that can be accom-plished include deepobservations at highgalactic latitudes todetect ever fainterquasars, multiwavelengthobservations withground-based observato-ries which will be used tofurther refine our under-standing of gamma-rayemitting galaxies, thesearch for more hiddenpulsars similar toGeminga, and collectingmore and more gamma-ray bursts in the hopethat we can constrainmodels of gamma-rayburst emission.

We have only touched on a few of

the scientific investigations Compton

continues to perform. A large number

of exciting and important investiga-

tions remain to be done during the

coming years of the Compton mission.

As with many good experiments, the

Compton instruments have revealed

new questions for every old question

they have answered. Only by exploring

the universe, can we truly find how

much we have to learn.

Solving the gamma-ray burstmystery is a prime example. The useof real-time telemetry data fromBATSE and COMPTEL may help tolocate gamma-ray bursts rapidly andcommunicate localizations to opti-cal and radio observatories whichcan then search for burst counter-parts. This means that the signalcoming down from the spacecraftitself is immediately checked for thepresence of a gamma-ray burst. Asystem to support such efforts calledBACODINE, for BATSE COordinates

DIstribution NEtwork, has beenimplemented by a team of scien-tists. This real-time alert system cur-rently notifies 25 observatoriesoperating 31 instruments coveringradio, optical, and TeV energies(1012 electron volts; ground-basedgamma-ray observatories detectshowers of particles created by theabsorption of the initial gamma rayin the Earth's atmosphere, a tech-nique available only at the highestenergies). Several of these instru-ments are fully-automated withrapid-slewing capabilities. The burstpositions are computed withinabout 5 seconds following burstdetection by BATSE and distributedto the participating observatorieswithin seconds. Hopefully, suchreal-time observations in longerwavelength bands will detect the"Holy Grail" of gamma-ray burstastrophysics — a counterpart to thegamma-ray burst emission — andhelp solve the mystery.

The dynamic evolution of thegamma-ray sky makes scientists real-ize that any day could bring some-

thing new. On December 2, 1995,BATSE began to detect bursts of low-energy gamma rays from a directionwithin a few degrees of the center ofour galaxy. These bright flashes ofradiation repeat every few minutes.This train of bursts continued to bedetected, although at a slower rateof about one per hour for the nexttwo months. This was a new type ofbursting galactic source, namedGRO J1744-28, which would turnout to be even more remarkablethan first thought. While someaccreting galactic sources haveshown bursting behavior, particular-ly at x-ray energies, and otheraccreting galactic sources are x-raypulsars — emitting x-rays at therotational period of the neutronstar, it was soon discovered that thissource was not only bursting, but itwas emitting regular pulses as well!About twice per second. This"Bursting Pulsar" is unlike any otherknown astrophysical object. Is this acompletely unique object or simplythe first of a new class? Maybe it isan example of a source in a brief

Gamma-Ray Astronomy in the Compton Era: The Future

16 17

Early time history of the "Bursting Pulsar"showing some of the thousands of burstsseen from this unique source.

ABOVE: Determining the source of enhancements, obvious in this map ofthe 1.8 MeV gamma-ray line from 26Al, is a continuing project for theCOMPTEL and OSSE experiments.

The simulated all-sky view as seen bythe proposedGLAST experiment.The detail in a one-year mission isgreatly enhancedover the EGRETresults.

BELOW: A simulated comparison of the Virgoregion which contains 3C 279 as seen above 1 GeV by EGRET (top) as compared to GLAST(bottom). The development of better telescopes isdriven by the need to explain current results.

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Beyond ComptonIt is a paradox that successful mis-

sions can create new questions asefficently as they provide answersfor the old questions. Such is thenature of the scientific enterprise— you often don't realize how littleyou know! Science is in a constantsearch for better techniques to ana-lyze and interpret old data. Newexperiments which take advantageof advanced technology or betterunderstanding of the space envi-ronment are always in develop-ment.

Proposed instruments such asGLAST (which stands for Gamma-ray Large Area Space Telescope)would provide an order ofmagnitude improvement insensitivity over the alreadysuccessful EGRET instrument.GLAST is an example of thepossible improvements indetector technology whichcan provide a much moredetailed view of the gamma-ray sky. The GLAST conceptutilizes advanced silicon-stripparticle detector technologyand would operate in the 10MeV to 300 GeV energy range.The goals of an instrumentlike GLAST are to collect morephotons using larger area and

thereby provide higher spatial reso-lution (to better estimate sourcelocations), and to have better ener-gy resolution (to better estimate thesource spectrum), enabling evenmore detailed study of the high-energy emission of active galacticnuclei, neutron stars, and diffuseradiations which comprise much ofthe gamma-ray sky. Other missionssuch as the European-based INTE-GRAL (for INTErnational Gamma-Ray Astrophysics Laboratory) mis-sion, which is scheduled to launchin the year 2001, will provideimages of the universe in the impor-tant energy range from 20 keV toabout 10 MeV. INTEGRAL will pro-

vide very sensitive energy resolutionover this range of nuclear transi-tions, where many interestinggamma-ray lines may be seen incosmic sources.

No matter the nature of theinstruments to follow, the ComptonGamma Ray Observatory has givenscientists an incredible view of theuniverse. The breadth of scientificresults that continue to come fromthe Compton instruments are reveal-ing the wonder and power of themost energetic processes occurringin nature. The journey of explo-ration which began with the firstancient astronomers has now cometo the point where we are using the

tools of elementary particlephysics to probe astrophysicalsites so exotic that they wouldhave been unimaginable a fewdecades ago. Where we go fromhere is limited only by ourimaginations and our determi-nation to understand the uni-verse and our place in it.

accretion - the process whereby matter from a normalstar or diffuse cloud is captured by a compact com-panion such as a black hole or neutron star

Active Galactic Nuclei (AGN) - a term used to describethe central region of a distant galaxy which canappear to be a pointlike source of strong gamma-rayemission. AGN are generally thought to be due tosupermassive central black holes accreting nearbymatter

BATSE - Burst and Transient Source Experiment, theCompton instrument operating from 15 keV to 1MeV; the instrument primarily intended to detectbursts, flares, and to monitor long-term emissionfrom other sources

black hole - a star which has evolved to the pointwhere the self-gravity of the star cannot be balancedby any other nuclear or electromagnetic forces. Theresult is the complete collapse of the star to a pointor singularity

blazar - a type of distant AGN which often appears tobe a pointlike source of bright, highly variable radia-tion

collimator - a structure which is used to narrow thefield-of-view of a gamma-ray detector

COMPTEL - the Imaging Compton Telescope, whichoperates in the 1 - 30 MeV energy range and is usefulfor imaging and the detection of nuclear lines

Compton scattering - The dominant particle interac-tion process at MeV energies wherein a photon scat-ters off of an electron, energizing the electron andchanging the trajectory of the photon

cosmic ray - refers to high-velocity elementary parti-cles such as electrons or protons or atomic nucleiwhich fill much of interstellar space

COS–B - a European gamma-ray experiment operatedfrom 1975-1982. Sensitive over much the same ener-gy range as the EGRET experiment

cyclotron emission - the gamma-ray emission fromelectrons in a strong magnetic field. Quantummechanical effects make this type of emission quan-tized at discrete energies

diffuse emission - gamma-ray emission which is notconfined to a point source. Typically due to theinteractions of cosmic rays with interstellar material.Sometimes refers to emission from extended sourcessuch as supernova remnants

EGRET - the Energetic Gamma-Ray ExperimentTelescope which operates from 30 MeV to 30 GeV

electron-volt - a unit of electromagnetic energy, suffi-cient to excite atoms to emit visible light, keV =1,000 eV, MeV = 1,000 keV, GeV = 1,000 MeV

flux - a detector-independent measure of the brightnessof a gamma-ray source

gamma ray - a photon carrying more energy than anx-ray (more than about 50 electron volts)

gamma-ray burst - brief, intense, random gamma-rayemission from an unknown source

jet - typically a stream of relativistic particles whichflows out from a central source

lightcurve - a time history of emission from a gamma-ray source

light-year - a measure of distance travelled by light inone year’s time = 9.7x1015 meters

neutron star - An end point of stellar evolution where-in the star is supported by the force of repulsionbetween neutrons; very compact with a radius ofabout 10 km and mass about 1.4 times the mass ofthe Sun

nuclear lines - gamma-ray emission centered on acharacteristic energy; caused by interactions of parti-cles with atomic nuclei or by transitions within anucleus

occultation - the technique of measuring the flux of asource by comparing a detector signal while thesource is in the field of view to the signal when thesource is hidden or occulted by the Earth

OSSE - Oriented Scintillation Spectrometer Experiment,the low-energy gamma-ray instrument on theCompton satellite used for detailed study of gamma-ray spectra

pair annihilation - A fundamental interaction betweena particle such as an electron and its antiparticle (i.e.a positron) which results in the emission of two pho-tons with the initial pair of particles destroyed. Thisprocess dominates other gamma-ray interactionswith matter at energies above a few tens of MeV

pair production - the inverse process to pair annihila-tion where a particle/antiparticle pair are createdfrom two gamma rays

Glossary

18 19

A conception of the INTEGRAL spacecraft. Future mis-sions would build on the Compton legacy.

"The progress of

the human race in

understanding the

universe has

established a small

corner of order in

an increasingly

disordered

universe."

S.W. Hawking

photon - the fundamental particle of electromagneticradiation (light). Photons carry energy proportionalto their frequency

pulsar - a subclass of neutron stars characterized by abeam of emission which sweeps around as the neu-tron star rotates, alternately coming into and goingout of the field of view of an observer, thus givingrise to a pulsing effect

phototube - an electronic device which converts a pho-tonic signal into an electronic signal. Used ingamma-ray astronomy instrumentation to detect thearrival and energy of an incident photon

positron - anti-particle of the electron. Capable ofmutually annihilating with an electron to emit char-acteristic radiation around 511 keV

SAS 2 - a pioneering spark-chamber instrument likeEGRET launched in 1972

scintillator - a crystal which detects gamma rays byconverting the energy into detectable light

Seyfert - a type of AGN which is dominated by abright, compact blue nucleus

solar flare - a burst-like emission of radiation from pro-turbances in the Sun’s outer atmosphere

spark chamber - a gamma-ray detection device utiliz-ing pair production of the initial photon followed bythe detection of a trail of sparks in a grid which fol-low the trajectory of the resultant electron/positronpair

spectrum - the number of photons a source emits as afunction of energy

supernova - the endpoint of stellar evolution wherebythe star, having exhausted its nuclear fuel explodes,usually leaving behind a compact object such as ablack hole or neutron star

synchrotron emission - radiation emitted by chargedparticles when accelerated by a magnetic field

thermal Brehmstrahlung - the x- and gamma-rayemission process caused by the collisions of particlesin a hot plasma (“braking” radiation)

white dwarf - an evolved, compact star which is sup-ported against self-gravity by the repulsive interac-tion of electrons

x-ray nova - a binary system where episodic accretionfrom the companion star to the black-hole compan-ion results in explosive gamma-ray emission whichgradually declines over a period of months

Acknowledgments

Credits

This brochure was designed and prepared at NASA’s Goddard Space Flight Center. Major contributions were made by

the Compton Observatory Science Support Center (Daryl Macomb, Chris Shrader, Paul Barrett, and Jerry Bonnell) and the

Laboratory for High Energy Astrophysics (Neil Gehrels, Jay Norris) with the help of the Technical Information Services

(Carol Ladd and Terri Randall) and NASA Headquarters (Sethanne Howard).

The Program Scientist for the Compton Gamma Ray Observatory is Alan Bunner and the Project Scientist Neil Gehrels.

The Principal Investigators of the four Compton instruments are Jerry Fishman (BATSE, Marshall Space Flight Center),

Jim Kurfess (OSSE, Naval Research Laboratory), Volker Schönfelder (COMPTEL, Max Planck Institute), and Carl Fichtel

(EGRET, Goddard Space Flight Center). The Compton Gamma Ray Observatory was built by TRW. Additional funding is

provided by the German Space Agency (DARA) and the Space Research Organization Netherlands (SRON).

Cover - EGRET all-sky gamma-ray image courtesy of Goddard SpaceFlight Center

Cover - images of Compton Observatory deployment courtesy of NASA

Page 1 - picture of A. H. Compton courtesy of the University of ChicagoArchives.

Page 1 - artist’s illustration of the electromagnetic spectrum.

Page 2 - artist’s conception of accretion flow and jet surrounding a com-pact star

Page 3 - artist’s illustration of gamma-ray line emission processes cour-tesy of the INTEGRAL Science Team

Page 4 - launch of the Compton Observatory courtesy of NASA

Page 4 - artist’s illustration of the Compton instruments energy ranges

Page 5 - artist’s illustration of the BATSE instruments

Page 6 - artist’s illustrations of the OSSE and COMPTEL instruments

Page 7 - artist’s illustration of EGRET instrument

Page 8 - COMPTEL all-sky map courtesy of Max Planck Institute,Germany

Page 8 - COMPTEL anticenter image courtesy of Max Planck Institute,Germany

Page 9 - pulsar lightcurves courtesy of Goddard Space Flight Center

Page 9 - 3C 279 image courtesy of Goddard Space Flight Center

Page 10 - BATSE galactic center image courtesy of the Marshall SpaceFlight Center

Page 11 - BATSE burst distribution courtesy of Marshall Space FlightCenter

Page 12 - Superbowl burst lightcurves courtesy of Goddard Space FlightCenter

Page 12 - Superbowl burst localization courtesy University of NewHampshire

Page 13 - COMPTEL image courtesy of the University of New Hampshire

Page 13 - Solar flare spectrum courtesy of the Naval Research Laboratory

Page 14 - GRO J1655-40 lightcurve courtesy Marshall Space Flight Center

Page 14 - radio image of GRO J1655-40 courtesy of National RadioAstronomy Observatory, R. Hjellming and M. Rupen

Page 15 - lightcurve of 3C 279 courtesy of Goddard Space Flight Center

Page 16 - Bursting Pulsar lightcurve courtesy of Marshall Space FlightCenter

Page 17 - Comptel 26Al map courtesy of the Max Planck Institute,Germany

Page 17 - GLAST simulations of Virgo and all sky courtesy of StanfordUniversity, P. Michelson

Page 18 - artist’s conception of the INTEGRAL mission courtesy of theINTEGRAL Science Team

20