The Temporal Characteristics of the TeV Gamma-Radi

arXiv:astro-ph/9808296v126Aug1998A&Amanuscriptno.(willbeinsertedbyhandlater)Yourthesauruscodesare:3(11.02.2,13.07.2)ASTRONOMYANDASTROPHYSICSTheTemporalCharacteristicsoftheTeVGamma-RadiationfromMkn501in1997,I:DatafromtheStereoscopicImagingAtmosphericCherenkovTelescopeSystemofHEGRAF.Aharonian1,A.G.Akhperjanian7,J.A.Barrio2,3,K.Bernl¨ohr1,8,H.Bojahr6,J.L.Contreras3,J.Cortina3,A.Daum1,T.Deckers5,V.Fonseca3,J.C.Gonzalez3,G.Heinzelmann4,M.Hemberger1,G.Hermann1,M.Hess1,A.Heusler1,W.Hofmann1,H.Hohl6,D.Horns4,A.Ibarra3,R.Kankanyan1,O.Kirstein5,C.K¨ohler1,A.Konopelko1,H.Kornmeyer2,D.Kranich2,H.Krawczynski1,4,H.Lampeitl1,A.Lindner4,E.Lorenz2,N.Magnussen6,H.Meyer6,R.Mirzoyan2,A.Moralejo3,L.Padilla3,M.Panter1,D.Petry2,6,9,R.Plaga2,A.Plyasheshnikov1,J.Prahl4,G.P¨uhlhofer1,G.Rauterberg5,C.Renault1,W.Rhode6,V.Sahakian7,M.Samorski5,D.Schmele4,F.Schr¨oder6,W.Stamm5,H.V¨olk1,B.Wiebel-Sooth6,C.Wiedner1,M.Willmer5,H.Wirth11MaxPlanckInstitutf¨urKernphysik,Postfach103980,D-69029Heidelberg,Germany2MaxPlanckInstitutf¨urPhysik,F¨ohringerRing6,D-80805M¨unchen,Germany3UniversidadComplutense,FacultaddeCienciasF´ısicas,CiudadUniversitaria,E-28040Madrid,Spain4Universit¨atHamburg,II.InstitutfuerExperimentalphysik,LuruperChausse149,D-22761Hamburg,Germany5Universit¨atKiel,Institutf¨urKernphysik,Olshausenstr.40,D-24118Kiel,Germany6Universit¨atWuppertal,FachbereichPhysik,Gaußstr.20,D-42097Wuppertal,Germany7YerevanPhysicsInstitute,AlikhanianBr.2,375036Yerevan,Armenia8NowatForschungszentrumKarlsruhe,P.O.Box3640,D-76021Karlsruhe9NowatUniversidadAut´onomadeBarcelona,InstitutdeF´isicad’AltesEnergies,E-08193Bellaterra,SpainReceivedAugust13,1998;accepted—Abstract.During1997,theBLLacObjectMkn501wasthebrightestknownobjectintheTeVγ-raysky.Theemis-sionwascharacterizedbydramaticvariationsinintensitywithameanfluxexceedingbyafactorofthreethesteadyγ-rayfluxoftheCrabNebula.ThestereoscopicHEGRAsystemof4ImagingAtmosphericCherenkovTelescopes,withanenergythresholdofabout500GeV,anangularresolutionof0.1◦,anenergyresolutionof20%,andafluxsensitivityνFνat1TeVof10−11ergs/cm2sec≃1/4Crabfor1hourofobservationtime(S/√B=5σ),hasbeenusedin1997foracomprehensivestudyofthespectralandtemporalcharacteristicsoftheTeVγ-rayemissionfromMkn501ontimescalesofseveralhoursorless.Inthispaper(PartI)theγ-rayfluxesandspectraonadiurnalbasisduringtheperiodMarchtoOctober1997arepre-sented.Furthermore,thecorrelationoftheTeVemissionwiththefluxmeasuredbytheRXTEAllSkyMonitorintheenergyrangefrom2to12keVarestudied.FinallytheimplicationsoftheseresultsonthephysicsofrelativisticjetsinBLLacobjectsarebrieflydiscussed.Thecompan-ionpaper(PartII)describestheresultsfromthestandalonetelescopesCT1andCT2.Sendoffprintrequeststo:HenricKrawczynski,emailadress:[e-mail]:BLLacertaeobjects:individual(Mkn501)-gammarays:observations1.IntroductionMrk501wasdiscoveredasasourceofTeV-γ-radiationin1995bytheWhipplegroup(Quinnetal.1996).TheobservationwasconfirmedlaterbytheHEGRAcol-laboration(Bradburyetal.1997).TogetherwithtwootherextragalacticTeVγ-raysourcesdetectedsofar,Mrk421(Punchetal.1992;Petryetal.1996)and1ES2344+514(Cataneseetal.1998),Mrk501belongstoasub-populationofActiveGalacticNuclei(AGNs),theso-calledBLLacobjects.Fluxvariabilityonvarioustimescales,rangingfromdramaticflaresofMrk421inMay1996withdurationsofabout1h(Gaidosetal.1996)toastateofhighflaringactivityofMrk501whichlastedseveralmonths(e.g.Protheroeetal.1997),isacharacter-isticfeatureoftheTeVemissionobservedfromBLLacobjects.ThisagreeswellwiththegeneralpropertiesofBLLacobjects–highlyvariableAGNswithoutsignifi-cantopticallineemission,butshowingastrongnonther-mal(synchrotron)componentofradiationfromradiotoX-raywavelengths(e.g.Urry&Padovani1995).ThecorrelatedflaresofBLLacobjectsinthekeVen-ergybandandintheTeVenergyband,discoveredforthe2F.Aharonianetal.:TeVCharacteristicsofMkn501firsttimeduringsimultaneousobservationsofMrk421bytheWhippleandASCAdetectors(Takahashietal.1996;Buckleyetal.1996),stronglysupportthecommonlyac-ceptedviewthatbothcomponentsoriginateinarelativis-ticjet,withDopplerfactorsδj≥5,duetosynchrotronandinverseCompton(IC)radiationofthesamepopulationofultrarelativisticelectrons(forareviewseee.g.Ulrichetal.1997).SinceintheThomsonregimetheICcoolingtimetIC=−Ee/(dEe/dt)isproportionalto1/Ee,andsincetheComptonscatteringboostsambientphotonswithenergiesǫ0uptoEγ∝ǫ0·E2e,thecharacteristictimeofγ-rayemis-siondecreaseswithenergyas∝E−1/2γ.ThisexplainsinanaturalwaythelessdramaticvariationsoftheMeV/GeVγ-rayfluxesduringthekeV/TeVflares;therelativelylowenergyelectrons,responsiblefortheGeVICphotonsaswellasfortheoptical/UVsynchrotronradiationdonotrespondasrapidlytochangesofthephysicalconditionsinthejetsasthehighenergyelectronsdo.Inaddition,theexpectedhardspectraofICradiationbelow100GeVexplainthelowfluxesofGeVγ-raysfromMrk421,andtheirnon-detectionbyEGRETinthecaseofMkn501and1ES2344+514.ThisimpliesthattheVHEγ-rayregion,combinedwithX-rayobservations,islikelytobethemostimportantwindowoftheelectromagneticspectrumtoin-ferthehighlynon-stationaryprocessesofparticleacceler-atio