AGN Models High-Energy Emission

Astrophysical models for the high-energy emission of blazars are reviewed. Blazars ejecting relativistic radio jets at small angles to the line-of-sight are the only type of active galactic nuclei (AGN) discovered above 100 MeV. The gamma-rays apparently o

AGNModels:High-EnergyEmission

KarlMannheim

Universit¨ats-Sternwarte,Geismarlandstraße11,D-37083,Germany

arXiv:astro-ph/9703184v1 27 Mar 1997Astrophysicalmodelsforthehigh-energyemissionofblazarsarereviewed.Blazarsejectingrelativisticradiojetsatsmallanglestotheline-of-sightaretheonlytypeofactivegalacticnuclei(AGN)discoveredabove100MeV.Theγ-raysapparentlyoriginateinthejetswhichexplainstheabsenceofγ-raypaircreationattenuation.ThebulkLorentzfactoroftheradiojetsismuchsmallerthantheLorentzfactorsoftheemittingparticlesrequiringinsituparticleaccelerationsuchasFermiaccelerationatshocks.Themountingevidenceforacorrelationbetweentheopticalandγ-rayemissionarguesforthesameacceleratedelectronsemittingthepolarizedopticalsynchrotronphotonstoberesponsibleforthehigh-energyemission.Alternatively,theγ-rayscouldbeduetoelectronsofasecondaryoriginrelatedtotheenergylossesofprotonsacceleratedatthesameshocksasthesynchrotronemittingelectrons.Inthiscaseblazarswouldproduceanobservable uxofhigh-energyneutrinos.Uni edschemesforAGNpredictacircum-nuclearwarmdusttorusattenuatingγ-raysabove～300GeVemittedfromwithinacentralsphereofradius～2×104rSwhichrulesoutexternalComptonscatteringmodelsastheoriginoftheTeVγ-raysfromMrk421andMrk501.1IntroductionTheEGRETdiscoveryofγ-raysourcesathighgalacticlatitudesandtheiridenti cationwithblazars(～ at-spectrumradio-loudAGN)haspromptedalargenumberofauthorstothinkaboutpossibleexplanationsforthephenomenon.Asof1996,Ihavecountedalmostonemodeloronemodelrami cationpersourcescanningthroughNASA’sAstrophysicsDataSystem.The

considerabletheoreticalinterestdocumentstheimportanceofthediscoveryandatthesametimedemonstratestheon-goingstruggleforaviableexplanation.Ofcourse,notallofthepaperscanbediscussedinthisreview,andIhavedecidedtocriticallydiscussonlythebasicmodelassumptionsandtheirconsequences.Theassumptionsthat(i)blazarsrepresentaccretingsupermassiveblackholesejectingbipolarjetsalongtheangularmomentumaxisoftheinneraccretiondiskandthat(ii)theγ-raysoriginateinthejetsarecommontoallthemodels.ThisconsensusisbasedmainlyonthefactsthatEGREThasobservedγ-raysonlyfromAGNwithjetsandthatjetsprovideanaturalresolutionoftheso-calledcompactnessproblem:Forγ-raysofenergymec2thepaircreationopticaldepthγ+γ→e++e ofanEddingtonaccretingblackholeistheuniversalconstant

τγγ Ledd

16=3

me 200(1)

Eddingtonaccretionontoasupermassiveblackholeistheonlymechanisme cientenoughtoexplainthehighluminositiesandshortvariabilitytimescalesofblazarsassumingisotropicemission.However,largeopticaldepthEq.(1)isinconsistentwiththeobservedopticallythinpowerlawspectra17andthereforetheemissionmustbeanisotropic.

Astrophysical models for the high-energy emission of blazars are reviewed. Blazars ejecting relativistic radio jets at small angles to the line-of-sight are the only type of active galactic nuclei (AGN) discovered above 100 MeV. The gamma-rays apparently o

Figure1:Coarseoverviewofmodels.SeeSect.1.1fordetails.

Theaccretingblackholepictureforblazarsisbasedonuni edschemesforAGNwhicharediscussedextensivelyinthecontributionbyPadovani34.AsageneralcautiousremarkIwanttoemphasizethatthepredictionsoftheuni edschemesforthedensityofmatterandradiationinthemediumsurroundingthejetsaresubjecttochangeinthecourseoffurtherastronomicalstudiesandshouldthereforeberegardedaspreliminary.Furthermore,theuni edschemesleaveitessentiallyopenwhetherthejetsformasahydromagnetice pwindfromtherelativisticinneraccretiondisk11orasaPoynting uxdrivene±windextractingrotationalenergyfromthecentralblackhole8.Themerginghistoryofellipticalsasthetypicalhostgalaxiesofradio-loudAGNlendssomesupportfortherotatingblackholepicture45,althoughjetsarealsoassociatedwithyoungstarswhichdonotcontainblackholes.Itishopedthattheγ-raypropertiesofblazarsprobingthephysicalconditionsinthejetsamelioratetheoryinthisimportantissue.However,baryonpollution30couldalsowashoutthedi erencebetweenpairwindsande pdiskwinds.

1.1Relativisticparticles,coldandhotbeams,internalandexternaltargetphotons

Byanalogywiththelocalcosmicrays,therelativisticparticlesresponsiblefortheγ-raysareexpectedtobeelectronsandprotons.Inthetenuousplasmaofthejets,electrons(positrons)produceγ-raysmainlybyinverse-Comptonscatteringorsynchrotronemission.Forprotonstheseprocessesaregenerallyunimportantowingtotheirlargermass.Thepionproductiononmatter,whichisimportantasaγ-rayproductionprocessintheGalaxy,canbeneglectedduetothelowplasmadensityinradiojets.However,thephoto-productionofsecondarypionsandpairsincollisionswithambientlow-energyphotonscanbeimportant.Inspiteofthehadroniccross-sectionσpγ=510 28cm2beingmuchsmallerthantheThomsoncross-sectionσT=6.710 25cm2,theprotoncoolingtimescalecanbeasshortastheelectronThomsoncoolingtimescaleowingtothemuchlargerinelasticityofhadronicprocesses.Theprotonandelectronenergylosses(i=e,p)aregivenby

dEi

3σT me

8πγi2fic(2)

Astrophysical models for the high-energy emission of blazars are reviewed. Blazars ejecting relativistic radio jets at small angles to the line-of-sight are the only type of active galactic nuclei (AGN) discovered above 100 MeV. The gamma-rays apparently o

wherethefactorsfe=1+uγ/uBandfp=1+500uγ/uBtakeintoaccounttherelativecontri-butionofsynchrotron,inverseCompton,Bethe-Heitlerpairproduction,andpionproduction.BalancingwithenergygainsduetoFermiacceleration

dE

2B 1

p2(4)

forprotonsand

γe,max=410K71

2f 1

tp=1dt 1dt 1= me

γefp

te

ueue meγefp

ueLe(8)

Photoproductionbyultra-relativisticprotonsinarelativisticjethasbeenconsideredbyanum-berofauthors

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