arXiv:astro-ph/9902185v1 12 Feb 1999FrontiersinHigh-Energy
AstroparticlePhysics
KarlMannheim
Universit¨ats-Sternwarte
Geismarlandstraße11
D-37083G¨ottingen
Germany
Abstract
Withthediscoveryofevidenceforneutrinomass,avividgammaraysky
atmulti-TeVenergies,andcosmicrayparticleswithunexpectedlyhighenergies,
astroparticlephysicscurrentlyrunsthroughaneraofrapidprogressandmoving
frontiers.Thenon-vanishingneutrinomassestablishesonesmoothcomponent
ofdarkmatterwhichdoesnot,however,supplyacriticalmasstotheUniverse.
Otherdarkmatterparticlesarelikelytobeverymassiveandshouldproducehigh-
energygammarays,neutrinos,andprotonsinannihilationsordecays.Thesearch
forexoticrelicswithnewgammaraytelescopes,extensiveairshowerarrays,and
underwater/-iceneutrinotelescopesisafascinatingchallenge,butrequirestoun-
derstandtheastrophysicalbackgroundradiationsathighenergies.Amongthe
high-energysourcesintheUniverse,radio-loudactivegalacticnucleiseemtobe
themostpowerfulaccountingforatleastasizablefractionoftheextragalactic
gammarayflux.Theycouldalsosupplythebulkoftheobservedcosmicraysat
ultrahighenergiesandproduceinterestingeventratesinneutrinotelescopesaim-
ingatthekubickilometerscalesuchasAMANDAandANTARES.Itisproposed
thattheextragalacticneutrinobeamcanbeusedtosearchfortauleptonappear-
ancethusallowingforaproofoftheneutrinooscillationhypothesis.Furthermore,
anewmethodforprobingtheeraofstarformationathighredshiftsusinggamma
raysispresentedwhichrequiresnew-generationgammaraytelescopesoperating
inthe10-100GeVregimesuchasMAGICandGLAST.
1Introductionandpracticaldefinitionofhigh-energy
astroparticlephysics
Centraltomodernastronomyisthedarkmatterproblemanditiscommonly
believedthatitssolutionwilltriggermajoradvancesinparticlephysicsandcos-
mology[1].Sofardarkmatterisonlyknownthroughitsgravitationaleffects,
buttheunderstandingofthenatureandoriginofdarkmatterrequirestoobtain
moredirectinformationaboutitsmassandinteractions.Cosmologyandparticle
physicsqualifyweaklyinteractingmassiveparticles(WIMPs)withmassesbetween
100GeVandafewTeVaslikelycandidates.TheWIMPsviolentlyannihilatewith
theiranti-particlesinrarecollisionsortheycouldbeunstable.Thesemodeslead
tosecondarygammaraysandneutrinoswhichcanbedetectedonEarth[2].The
1suspectedlargeWIMPmassthencorrespondstogammarayandneutrinoenergies
inexcessof100GeV.Inadditiontothedarkmatterparticles,theremaybeother
relicsfromtheearlyUniverse,suchasquintessence,vacuumenergy,ortopolog-
icaldefects.Quintessenceisaslowlyrollingscalarfieldconnectedwithmassive
bosons,andthiscouldalsoleadhigh-energyphenomena,althoughnoworked-out
modelexiststomyknowledge.TherecentdiscoveryofacceleratingexpansionusingSNIaasatracerofcosmicgeometryseemstomakeastrongcaseforquintessence
[3,4].Similarly,ifthescalarfieldhassettledtosomefalse(meta-stable)vacuum,
theenergydensityofthisvacuumcoulddrivethedecelerationparameteraway
fromΩ/2.Topologicaldefectspreservefalsevacuuminpointlike(monopoles),
one-dimensional(strings),two-dimensional(domainwalls),orhigherdimensional
space-timestructures.Theyaretopologicallystable,buthaveavarietyofwaysto
communicatetoourworldinadditiontotheirgravitation.E.g.,theycandissipate
intoGUT-scalebosons(∼1016GeV)whichareunstablethemselvesandfragment
intojetsconsistingofgammarays,neutrinos,andprotonsatultra-highenergies
[5].Aftertheirpropagationthroughintergalacticspace,electromagneticcascading
andsecondaryparticleproductionshiftmostenergyinjectedbytheseexoticpro-
cessestomuchlowerenergieswheretheenergyreleasecompeteswiththatdueto
ordinaryastrophysicalsources.
Inordertoidentifynewphysicsphenomena,itisthereforeofcrucialimpor-
tancetoobtainacompleteinventoryoftheastrophysicalhigh-energysourceswhich
actasabackgroundforthesesearches.Thisdefinesthehigh-energyastroparticle
physicsprogramfromapracticalpointofviewandfollowsthelogicinherenttothe
generalastronomicalexplorationoftheskytocovertheentirerangeofwavelengths
withcomparablesensitivity.
Amongthenon-thermalsourcesintheUniverse,radio-loudactivegalacticnu-
clei(AGN)seemtobethemostimportantenergetically.Thereareotherinterest-
ingsources,suchasgammaraybursts(GRB)andclustersofgalaxies,buttheir
non-thermalenergyreleasedoesnotcomeclosethatofAGN.Therearesomein-
triguingcomplicationsarisingthroughcalorimetriceffects,sincetheintracluster
mediuminclustersofgalaxiessurroundingAGNconfinesescapingrelativisticpar-
ticlesforsometimeandtherebygivesrisetoasecondaryluminositytiedtothe
energyreleaseoftheAGNandthecoolingtimescaleoftheintraclustermedium.
Theradio-loudAGNcomeinvariousdisguises,dependingontheorientationof
theirradiojetaxesandthepropertiesofthecircum-nuclearmatterintheirhost
galaxies.Themostextremeversionsareradiogalaxieswiththeradiojetaxes
almostintheplaneoftheskyandtheblazarswiththeradiojetspointingcloseto
thelineofsighttotheobserverwhichleadstoadramaticfluxincreaseowingto
specialrelativisticeffects(theso-calledDopplerboosting).InSect.2itisargued
thatradio-loudAGNcanbeexpectedtoproducetheentireextragalacticgamma
raybackgroundfromanenergeticalpointofview.Owingtobeamingstatistics,
thenumberofunresolvedsourcesresponsibleformostofthisbackgroundshould
notbetoolarge.Indeedthefluxfromthe∼50resolvedsourcesintheflux-limited
EGRETsamplealreadyequalsasizablefraction(oftheorderof15%)oftheextra-
galacticbackgroundflux.Withthenext-generationgammaraytelescopesMAGIC
2
