The innermost regions of relativistic jets and their magnetic fields in radio-loud Active Galactic Nuclei

1.6 MB·English

by  I. Donnarumma

The innermost regions of relativistic jets
and their magnetic fields in radio-loud
Active Galactic Nuclei
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White Paper in Support of the Mission Concept of the
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Large Observatory for X-ray Timing
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h Authors
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o I.Donnarumma1,I.Agudo2,L.Costamante3,F.D’Ammando1,4,G.Giovannini1,4,
tr P.Giommi5,M.Giroletti1,P.Grandi1,S.G.Jorstad7,8,A.P.Marscher7,
s
a M.Orienti1,L.Pacciani1,T.Savolainen6,9,A.Stamerra1,10,F.Tavecchio1,
[ E.Torresi1,A.Tramacere11,S.Turriziani12,S.Vercellone1,A.Zech13

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7 1 INAF,Italy 8 AstronomicalInstitute,St. PetersburgStateUniver-
7 2 InstitutodeAstrofísicadeAndalucía,Granada,Spain sity,UniversitetskijPr. 28,Petrodvorets,198504St.
2 3 UniversityofPerugia,Perugia,Italy Petersburg,Russia
0 4 UniversitádegliStudidiBologna,Bologna,Italy 9 MaxPlanckInstitutfürRadioastronomie,Bonn,Ger-
.
1 5 ASIScienceDataCenter(ASDC),Frascati,Italy many
0 6 AaltoUniversityMetsähoviRadioObservatory,Met- 10ScuolaNormaleSuperiore,PiazzadeiCavalieri,7,
5
sähovintie114,02540Kylmälä,Finland 56126Pisa,Italy
1
7 InstituteforAstrophysicalResearch,BostonUniver- 11ISDC,UniversityofGeneva,Switzerland
:
v sity,725CommonwealthAvenue,Boston,MA02215, 12UniversityofTorVergata,Rome,Italy
Xi USA 13LUTH,UniversitéParis7,Paris,France
r
a
Radio-loudAGNwithLOFT
Preamble
TheLargeObservatoryforX-rayTiming,LOFT,isdesignedtoperformfastX-raytimingandspectroscopy
with uniquely large throughput (Feroci et al., 2014). LOFT focuses on two fundamental questions of
ESA’s Cosmic Vision Theme “Matter under extreme conditions”: what is the equation of state of ultra-
dense matter in neutron stars? Does matter orbiting close to the event horizon follow the predictions of
generalrelativity? ThesegoalsareelaboratedinthemissionYellowBook(http://sci.esa.int/loft/
53447-loft-yellow-book/)describingtheLOFT missionasproposedinM3,whichcloselyresembles
theLOFT missionnowbeingproposedforM4.
TheextensiveassessmentstudyofLOFT asESA’sM3missioncandidatedemonstratesthehighlevel
ofmaturityandthetechnicalfeasibilityofthemission,aswellasthescientificimportanceofitsunique
core science goals. For this reason, the LOFT development has been continued, aiming at the new M4
launch opportunity, for which the M3 science goals have been confirmed. The unprecedentedly large
effectivearea,largegrasp,andspectroscopiccapabilitiesofLOFT’sinstrumentsmakethemissioncapable
of state-of-the-art science not only for its core science case, but also for many other open questions in
astrophysics.
LOFT’s primary instrument is the Large Area Detector (LAD), a 8.5m2 instrument operating in the
2–30keVenergyrange,whichwillrevolutionisestudiesofGalacticandextragalacticX-raysourcesdown
totheirfundamentaltimescales. ThemissionalsofeaturesaWideFieldMonitor(WFM),whichinthe
2–50keVrangesimultaneouslyobservesmorethanathirdoftheskyatanytime,detectingobjectsdownto
mCrabfluxesandprovidingdatawithexcellenttimingandspectralresolution. Additionally,themissionis
equippedwithanon-boardalertsystemforthedetectionandrapidbroadcastingtothegroundofcelestial
brightandfastoutburstsofX-rays(particularly,Gamma-rayBursts).
This paper is one of twelve White Papers that illustrate the unique potential of LOFT as an X-ray
observatoryinavarietyofastrophysicalfieldsinadditiontothecorescience.
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Radio-loudAGNwithLOFT
1 Summary
TheLargeAreaDetector(2–50keV,LAD)aboardtheLOFT missionthankstoitsunprecententedareapeaking
at8keV( 8.5m2)willhaveagreatpotentialfortheinvestigationofjetpropertiesinradio-loudActiveGalactic
∼
Nuclei(AGN).Activegalacticnuclei(AGN)offerideallaboratoriestoaddressseveralkeyproblemsofmodern
astrophysics. LOFT is planned in the same timeframe as other observatories that are opening up the AGN
timedomainonshorttimescales,e.g.,theCherenkovTelescopeArray(CTA)atTeVenergiesandtheSquare
Kilometer Array (SKA) and Atacama Large Millimeter Array (ALMA) in the radio and (sub-)mm bands,
respectively. Inthiscontext,LOFT/LADwillbeabletofollow-upalargenumberofAGNs(upto100sources,
seeTable1),whiletheLOFT/WFMcanprovideweeklymonitoring(afewdaysforthebrightest)thankstoits
skycoverage.
Blazarsarethemostvariablesourcesovertimescalesthatcanbeasshortasminutes. Thisisthecase
•
oftheHighEnergyPeakedBLLac(HBL)objectsshowingstrongvariabilityinX-rays,whichhighly
correlate with that of the TeV emission. The degree of this correlation is still debated, particularly
when the flaring activity is followed down to very short time scales. We argue that the LOFT LAD,
thanks to its timing capability, will allow us to detect the X-ray counterpart (2–50keV) of the very
fast variability observed at TeV energies, shedding light on the nature of the X-TeV connection. We
also discuss the potential of LOFT in measuring the change in spectral curvature of the synchrotron
spectra in HBLs which will make possible to directly study the mechanism of acceleration of highly
energeticelectrons. LOFT timingcapabilitywillbealsopromisinginthestudyofFlatSpectrumRadio
Quasars(FSRQs)with2–10keVflux& 2 10 11ergcm 2s 1. Forthiskindofobjects,LOFT willprobe
− − −
×
theinverseComptonemission,whereaslatestgenerationradioand(sub-)mminstrumentswillprovide
excellenttimingcapabilitiestostudyshorttime-scalecorrelationswiththesynchrotronspectraldomain.
Constraintstothelocationofthehighenergyemissionwillbeprovidedby: a)temporalinvestigationon
secondtimescales;b)spectraltrendinvestigationonminutetimescales. Thisrepresentsafurtherlinkwith
CTAbecauseoftherapid(unexpected)TeVemissionrecentlydetectedinsomeFSRQs. Inthisrespect,
theLOFT/WFMwillprovidenotonlyanexcellenttriggerforbothCTAand(sub-)mmobservatoriesbut
alsoalong-termmonitoringforalargesampleofblazars.
MisalignedAGNs: WFMlongtermmonitoring(monthlytimescale)ofradiogalaxieshostingefficient
•
accretion disks (mainly FRIIs) will provide an optimal tool to investigate the disc-jet connection by
triggeringbothradioandLADX-rayfollow-ups. LOFT willinvestigatethedisc-jetconnectioninatight
linkwithX-raybinaries,whichexhibitthesamebehavior.
Recent detections of radio galaxies hosting inefficient accretion flows at high and very high energies
(e.g.,IC310)willprovideafurthersampleofradio-loudsourceswellsuitedforfollow-upobservations
withLAD.CombiningLADandCTAsimultaneousobservationswillbecrucialinconstrainingtheTeV
emittingregion,likelyconnectedwiththeclosesurroundingsofthecentralengine.
Radio-LoudNarrow-LineSeyfert1LADobservationswillbeverypromisingnotonlyforstudyingthe
•
physical mechanisms responsible for the higher energy emission (in γ-rays or at TeV energies) and
thentheblazar-likebehavior,butalsofordeterminingtheroleofitsthermalcomponent. Thismeansto
investigatethepossiblelinkbetweenaccretionandjetproperties.
TheLADcharacteristicswillthereforematchverywellwiththoseofatmosphericCherenkovtelescopessuch
astheCTA,aswellasthemm/radioobservatoriessuchasSKAandALMAbecausealltheseobservatoriesare
timingexplorers,characterizedbyhugecollectingareasintherespectivebands. Withitshugecollectingarea
peakingat8–10keV,theLADwillallowustostudymoreindepththetemporalandspectralevolutioninthe
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Radio-loudAGNwithLOFT
2–50keVband,forsourceswithfluxesabove2 10 12ergcm 2s 1 (seeFig.1). Inthefollowingsections,we
− − −
×
willfocusonthetemporalandspectralinvestigationinalltheseradio-loudAGNs.
2 Blazars: the standard framework and its challenges
Blazars (Urry & Padovani, 1995) are among the most powerful persistent cosmic sources, able to release
(apparent)luminositiesexceeding1048ergs 1 overtheentireelectromagneticspectrum,fromtheradiotothe
−
veryhighenergyγ-rayband. Thisintensenon-thermalemissionisproducedwithinarelativistic(bulkLorentz
factorΓ = 10–20)jetpointingtowardtheEarth. Relativisticeffects,incorporatedintheso-calledrelativistic
Dopplerfactorδ,leadtothebeamingoftheradiationwithinanarrowconeofsemiaperture 1/Γwithstrong
∼
amplificationoftheapparentluminosityandshorteningofthevariabilitytimescalesinthecasesinwhich(asin
blazars)thejetiscloselyalignedtothelineofsight.
The spectral energy distribution (SED) of blazars displays two characteristic broad humps, whose peak
frequenciesappeartobeanti-correlatedwiththeluminosity(Fossatietal.,2011). Thefirstcomponentresults
from synchrotron emission of leptons in the jet. The origin of the second component, peaking in the γ-ray
band,ismoredebated. InthemostpopularscenarioitisattributedtotheinverseComptonscatteringbetween
thesynchrotron-emittingelectronsandsofttargetphotons,eitherthesynchrotronphotonsthemselvesorthose
comingfromtheexternalenvironment. Alternatively,thehigh-energybumpcouldbetheresultofapopulation
ofrelativistichadronsinthejet,loosingenergyeitherthroughsynchrotronemissionorphoto-mesonreactions
withsofttargetphotonfields(Böettcheretal.,2012).
Blazarsoccurintwoflavors. Thoseshowingopticalbroademissionlinesastypicallyobservedinquasarsare
calledFlatSpectrumRadioQuasars(FSRQs). Theweakestsourcesingeneraldisplayratherweakorevenabsent
emissionlinesandarecollectivelygroupedintheBLLacobjectclass. Thelattersourcesarethoseshowingthe
mostextremevariability(withvariationsasfastasfewminutes)andthemostenergeticphotons,withsubstantial
emissionabove100GeV.
While the standard leptonic scenarios provide in general good descriptions of the blazar SEDs, they face
difficultieswhenconfrontedwiththemostextremeHBLs(Cerrutietal.,2014),characterizedbyhigh-energy
bumpspeakingabove 1TeV,andwithintermediateobjects(Böettcheretal.,2012),i.e. low-orintermediate-
∼
frequencypeakedBLLacobjects(LBLs/IBLs).
Theconventionalscenario(e.g.,Ghisellini&Tavecchio,2009)foreseesasingleportionofthejetdominating
theoverallemission,atleastduringhighactivitystates. However,thissimpleideahasbeenrecentlychallenged
by the observation of very fast (minutes) variability, which requires the existence of very compact emission
sites. ThemostextremeexampleisPKS2155 304,whichexperiencedtwoexceptionalflaresin2006,ontop
−
of which – with a recorded luminosity of the order of several 1047ergs 1 – events with rise times of 100s
−
∼
havebeendetected. Evenassumingthatthesestrongeventsoriginatedinverycompactemissionregions,the
physicalconditionsshouldberatherextremeandLorentzfactorsofΓ 50–100seemunavoidable(Begelmanet
∼
al.,2008). Physicalmechanismspossiblytriggeringtheformationofthesecompactemissionregionsinclude
magneticreconnectionevents(Giannios,2013)orrelativisticturbulence(Narayan&Piran,2012;Marscher,
2014).
Alternatively,narrowbeamsofelectronscanattainultra-relativisticenergiesγ 106 intheblackholevicinity
∼
through processes involving the BH magnetosphere (Rieger & Aharonian, 2008; Ghisellini et al., 2009). In
thiscase,itisexpectedthattheemissionshowsuponlyinhighenergyγ-rayregion,givingnosignalsatlower
frequencies(thusresemblingthecaseoftheso-called“orphan”flares;differentfromthesimultaneousX-rayand
TeVflaringobservedfromPKS2155 304in2006).
−
Untilnow,ultra-fastvariabilityhasbeenrecordedonlyintheγ-rayband,especiallyatTeVenergies,atwhich
the Cherenkov arrays are characterized by gigantic collection areas, required to probe such short timescales.
Current instruments do not provide an analogous sensitivity in the key X-ray band, at which the low energy
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Radio-loudAGNwithLOFT
Figure1: LADsensitivitylimitsatdifferentsignaltonoise
ratiocorrespondingtoasystematicsof0.25%ontheback-
groundknowledge. Frombottomtotop: S/N 3,5,10,150,
∼
250.
componentofTeVemittingBLLacs(theso-calledHBLs)peaksandthusitisnotpossibletostudypotential
ultra-fastvariabilityeventsatthesefrequencies. Moreover,sucheventsappeartobequiterare,withadutycycle
lessthan1%. Thereforeamonitoringoftheskyisrequiredtocatchtheseevents.
FromthisbriefsketchitshouldbeclearhowthefeaturesofLOFT areidealtoaddressthesekeyproblems. In
thefollowingwediscussindetailsomecasestudies.
2.1 High energy peaked BL Lac objects
2.1.1 X-ray/TeVconnection
HBLsaregoodtargetsfortheLADasitsenergyband(2–50keV)probesthesynchrotronemissionintheregion
aboveorclosetothepeakwherethevariabilityisexpectedtobehigher.
ThemeasurementoftheTeV/X-raylagalsoconstrainstheemissionprocess: e.g.,synchrotronselfCompton
(SSC)timedependentnon-homogenousmodelingpredictsaTeVlagequal,approximately,tothelighttravel
time across the emission region, whereas external Compton (EC), under the assumption the main source of
variabilityisintherelativisticelectronpopulationofthejet,predictsalmostsimultaneity(Sokolovetal.,2004).
Present data do not provide measurements of lags less than a few hundred seconds (Aharonian et al., 2009),
whilewithLADthistimescalewillbepushedtoafew/tensofseconds. Thismeansthatfora2–10keVfluxof
theorderof10 10ergcm 2s 1,i.e.,thetypicalflaringstateformanyHBLsandaveragestateforsomeofthem,
− − −
theblazarflaringactivitycouldbefolloweddowntothedecayingtailofthelightcurvewithcomparablebin
timeinX-rayandTeVenergybands(Soletal.,2013). WeshowinFig.2howLADobservations(redpoints)
willdetectaflarelasting 60s(whichisonehalfoftheshortesttimebinexploredsofarinX-rays), shaped
∼
withafluxriseof20%withrespectto 10 10ergcm 2s 1 over120s. Wedidthesamesimulationsforboth
− − −
∼
Chandra and XMM-Newton, adopting a time resolution of 2s. It is worth noticing that the same statistical
accuracy of the LAD can be achieved with XMM if a time bin an order of magnitude larger is considered.
However,sucharebinningmakeshardtodefinetheflarerisetime. Therefore,LADobservationswillprovidea
realstep-forwardintheexplorationoftheX-rayandTeVcorrelation,possiblyunveilingX-raycounterpartsof
TeVflaringstructures.
Thesetimingcapabilitieswillprovideauniquediagnosticstoconstrainthenewemergingscenarioofmulti-
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Radio-loudAGNwithLOFT
Figure2: LAD (red), XMM (black,
pn+MOS1,2) and Chandra (blue, ACIS-I)
simulations of an X-ray enhancement of a
20% over a 10 10ergcm 2s 1 in a rise
− − −
∼
timeofoneminute(thisisoneoftheshortest
timescaleexploredsofarasinthecaseofPKS
2155 304). Eachbinis2swide.
−
zone(multipleelectronpopulations)SSCmodeling,assuggestedbythedegreeofcorrelationobservedbetween
X-raysandTeVemissionsduringtheflaringactivity(Aharonianetal.,2009). Thedetectionofflaringactivityon
veryshorttimescaleinX-rayandTeVwillbecrucialintheunderstandingofjetphysicsanditsconnectionwith
thecentralengine: extremevariabilityimpliesverycompactregionsallowingustoinvestigatethepropertiesof
blackholesandtheirsurroundings.
Whilemulti-wavebanddataonflaresprovideverysignificantconstraintsonourcurrentblazarmodels,itisalso
importanttoinvestigatethetemporalbehaviourofblazarsduringquiescentstates. Inmulti-zonemodels,where
differentemissionregionscouldberesponsibleforthepermanenthigh-energyemissionontheonehandandfor
flaresontheother,thoseregionsmightbeidentifiedbydifferentvariabilitypatternsonshorttime-scales. Rapid
variabilityinlowstatescouldbeprobedwithLOFT/LADforthemostluminousHBLs,suchasPKS2155 304,
−
Mrk421andMrk501,whichwillalsofigureamongthemostprominentsourcesforCTA.
ForthemostextremeHBLswiththehighestpeakenergies, suchase.g.1ES0229+200or1ES0347 121,
−
although not the most luminous sources in X-rays, spectral data in the hard X-ray range, and especially
informationontemporalbehaviour,willbeaccessiblewithLOFT/LADandwillhelptobettercharacterizethe
stillpoorlyunderstooddistributionofrelativisticparticlesinthosesources.
Moreover,theWFMwillprovideanexcellenttriggerforCTA(andHESS,MAGIC,VERITAS)observations
asmostofthetargetssuitableforTeVobservationarerelativelybrightX-raysources.
2.1.2 Theaccelerationmechanism
InthecaseofHBLs,adetailedmeasurementoftheshapeofthesynchrotronspectralcomponentfromOptical/UV
tohardX-rayfrequenciesprovidesphysicalinformationabouttheparticleaccelerationprocessinthejetsinceit
directlytracestheshapeoftheunderlyingparticledistribution.
ThebroadbandspectraldistributionsofseveralHBLsiswelldescribedbyalog-parabolicfit,withthesecond-
degreetermmeasuringthecurvatureinthespectrum. Thisisthoughttobeafingerprintofstochasticacceleration
(Tramacere et al., 2007). In this scenario, the discrimination between a log-parabolic and power-law cut-off
shapeprovidesapowerfultooltodisentangleaccelerationdominatedstates,fromstatesattheequilibrium,or
verycloseto,givingsolidconstraintsonthecompetitionbetweenaccelerationandcoolingtimes,andonthe
magneticfieldintensity. Wehavesimulatedapurelog-parabolicspectrumpeakinga1.5keV,resemblingthe
typical spectrum of a HBL object in a quiescent state. We assumed a 2–10keV flux of 10 10ergcm 2s 1
− − −
∼
over an integration time of 10ks. In Fig. 3 we show the capability of the LAD to discriminate between a
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Radio-loudAGNwithLOFT
100 100
V−1 V−1
malized counts s ke−1 101 malized counts s ke−1 101
nor nor
4 4
2 2
χ 0 χ 0
−2 −2
−4 −4
2 5 10 20 2 5 10 20
Energy (keV) Energy (keV)
Figure3: Left: LAD:log-parabolicspectrumfittedwithapower-lawcutoff(top)andthemodelfitresiduals(bottom).
Right: LAD:samemodelfittedwithalog-parabola.
log-parabola(rightpanel)andapower-lawcut-offmodel(leftpanel)withahighstatisticalsignificance. Thanks
totheunprecedentedLOFT largeareapeakingat8keVanditsbroadenergyrangethisresultprovidesarelevant
improvementcomparedtotheperformanceoffocusedX-rayinstrumentsoperatingintheusual0.2–10.0keV
range.
TheLADwillthereforeexplorethecurvatureoftheX-rayspectrumaround10keVforHBLswithsyncrotron
peakat1–2keV.Moreover,thecapabilitytoextractdetailedspectrawithasub-kstemporalintegrationduring
thehigherstatestogetherwiththelowenergythresholdof2keVwillcomplementthecurrentunderstanding
providedbyX-rayobservatoriessuchasNuSTAR(Harrisonetal.,2013).
2.2 FSRQs: X-γ connection and bulk-Comptonization
ContrarytoHBLs,FSRQsareblazarswhoseemissionpeaksatlowenergies,withthesynchrotroncomponent
peaking in the radio/IR bands and the inverse Compton (IC) one in the Fermi and AGILE bands (50MeV–
50GeV). As such, they were not expected to significantly emit at VHE (> 0.2TeV). The strong and highly
variableemissionrecentlydiscoveredbyTeVtelescopes(e.g.,3C279,4C21.35,PKS1510 089)camethusas
−
asurprise. Theoriginisstillunclear,buttheLADcanprovidetheanswer.
Shorttime-scaletimingstudiestocomparethesynchrotronandICvariabilitypropertiesasthoseoutlinedabove
forHBLswillbekeyhere. IftheemissionintheLADspectralrangeisdominatedbyICemission,simultaneous
measurementsofthesynchrotronemissionbyultra-sensitiveradioand(sub-)mminstrumentssuchasSKAand
ALMA will provide relevant comparisons of the two emission mechanisms. Alternatively, if the strong and
highlyvariableTeVemissionisduetoatailofhigh-energyelectrons,theirsynchrotronradiationshouldappear
inthehardX-raybandaswell(likeinIntermediateBLLacorHBLobjects),dominatingovertheIConeand
completelychangingthespectralslope(fromβ 1.5toβ > 2). Timingiscritical: giventhehighlyvariableTeV
∼
emission(minutes-hours),suchX-rayfeaturescandisappearquicklyandnotbemeasurablebyinstrumentsof
smallercollectingareaorpoorpointingflexibility. Inthisrespect,LOFT/LADlargerareawouldprovide1-s
binnedlightcurvesforflux 1mCrab( 2 10 11ergcm 2s 1 in2–10keV)and,inaddition,spectraltrend
− − −
≥ ∼ ×
investigationduringtheflaringactivitywillbeperformedonminutetimescale,i.e.,thenon-thermalcontinuum
power-lawwillbeconstrainedwithinafewpercent.
AfurtherdiagnosticwillbeprovidedbytheLADonthejetcomposition,inconnectionwithGeVflares. If
thejetismatter-dominatedandejectedinblobs,thecoldelectronsareexpectedtoCompton-upscatterBroad
LineRegion(BLR)photonsyieldingaspectralsignatureasanexcessemissionat (Γ/10)2 keV,whereΓisthe
∼
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Radio-loudAGNwithLOFT
bulk-motionLorentzfactorofthejet(Sikora&Madejski,2000). Thisfeature,referredasbulk-Comptonization,
hasnotbeenseenyet,but1)withvaluesofΓ > 20,thisfeaturecouldactuallypeakaround10keV,andthus
couldnotberecognizedsofar;2)ifthejetacceleratesslowly,itcouldbevisibleonlyforfewhours,fromthe
timetheblobbecomesrelativistictothetimeitgoesoutsidetheBLR(atadistanceof 1018cmfromthecentral
∼
engine)asreportedbyCelottietal.(2007)(seetheirFig. 1). TheLADwillbeabletodiscoverthisfeaturefor
thefirsttime,orputverystrongupperlimitsontheparticlecontentofthejetatitsbase.
Acomplementaryandindependentassessmentofthejetcompositionwillbepossiblewithfutureultra-sensitive
radioinstrumentssuchasSKAthroughmodelingoffull-polarimetric(linearandcircularpolarization)spectra
(Homanetal.,2009;Agudoetal.,2015).
Concerningintermediateorquiescentfluxstates,itisgenerallyassumedthatFSRQs,andprobablyalsoLBLs
and IBLs, require target photons from external photon fields (such as BLR, accretion disk, dust torus, host
galaxy,...) foranECcomponentthatcanaccountforthehigh-energypartoftheirSEDs(e.g.,Böettcheretal.,
2012). For the brightest of these objects, LOFT/LAD could help to distinguish such components from SSC
emission by their different temporal behaviour and by comparison with the temporal and spectral properties
ofthesynchrotronspectrumasobservedbyradiotoIRfacilities,whichwouldbeanimportantsteptowardsa
unifiedviewofdifferentblazarclasses.
CombiningLADrepointingwithradioand(sub-)mmobservationswithSKA,EVLA,andALMAcanhave
alsoastrongimpactontheSEDmodelingandthusonthenatureoftheγ-rayemittingregion,beingthehigher
radiofrequencies(30–950GHz)properlyrelatedtotheflaringactivity. Further,longtermmonitoringofFSRQ
and LBL-IBLs in all available spectral ranges, including X- and γ-ray timing on weekly/daily time scales,
photo-polarimetryifpossible,andmillimeterverylongbaselineinterferometric(VLBI)imaginghasprovento
beapowerfultooltoestablishtherelativelocationofthedifferentemittingregionsfromtheradiodomainupto
γ-rays(Marscheretal.,2010;Agudoetal.,2011). Inthatregard,WFMwillnotonlyprovidethetriggeringof
dedicatedcoordinatedobservationsofparticularlyinterestingevents,butalsoexcellentlongtermX-raylight
curvesforbrightblazarssampledontimescalesofuptoonedayorbetter.
3 The LOFT perspective on misaligned AGN
TheLOFT capabilitiescanbeexploitedalsotoinvestigateanotherclassofradio-loudsources,theMisaligned
AGN(MAGN).WiththetermMAGNwerefertoradiogalaxies(radiogalaxieshavebeenhistoricallyclassified
asedge-darkenedFRIandedge-brightenedFRII,onthebasisoftheirextendedmorphologythatchangesover,
orunder, acriticalradiopowerat178MHz, P 1025WHz 1s 1;Fanaroff&Riley,1974)andsteep
178MHz − −
∼
spectrumradioquasars,i.e.,radiosourceswiththejetpointedawayfromtheobserver’sline-of-sightoffering
a unique opportunity to study the jet phenomenon from a perspective different, but complementary, to that
of blazars. MAGN are generally characterized by steep radio spectra (α > 0.5) and resolved and possibly
r
symmetricalradiostructures. Moreover,giventhelargejetinclinationanglethenon-thermalradiationproduced
byMAGNjetsisweaklyDopplerboosted.
Radiogalaxieshostingefficientaccretiondisks(mainlyFRIIs),areuniqueextragalacticlaboratorieswherea
simultaneousviewoftheaccretionflowandejectionofrelativisticplasmaispossiblethroughmulti-wavelength
observations. High-resolutionradioimages,combinedwithhigh-energystudies,allowtofollowthetemporal
andspectralevolutionofthejet,whileX-raymonitoringcanprovideinformationontheaccretionprocesses.
Uptonowthereareatleasttwobroadlineradiogalaxies,i.e.,3C120and3C111,forwhichsignaturesofa
possiblelinkbetweeneventsoccurringinthejetandintheaccretiondiskhavebeenfound(Marscheretal.,2002;
Chatterjeeetal.,2009,2011)similarlytothoseobservedinX-raybinarysystems(Mirabel&Rodriguez,1998).
Thedisk/jetconnectionisrevealedbyasuddendecreaseintheX-rayfluxesshortlyfollowedbytheappearance
of superluminal radio emitting features propagating along the jet. This phenomenon has been interpreted as
resulting from an instability in the accretion disk with matter in the inner region partially ejected down the
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Radio-loudAGNwithLOFT
jet(Marscheretal.,2002). Fastdiskwindsrevealedasanabsorptionblue-shiftedfeatureataround7keV,if
confirmed, could be also a signature of the ongoing disk/jet perturbation. As this is a random and transient
phenomenon, the WFM on-board LOFT is the ideal instrument to perform an accurate X-ray monitoring in
ordertocatchtheperturbationexactlyduringitsoccurrence. Thesignofthiscatastrophiceventisadipinthe
X-ray light curve. The flux decreases by about a factor of two and the source remains in a low state for few
months. TheWFMcantracethetemporalevolutionofpowerfulradiogalaxieswithfluxesinthe2–10keVband
above10 11ergcm 2s 1 allowingtodetectadecreaseintheX-rayfluxbyatleastafactoroftwoontime-scale
− − −
ofmonths. TheWFMcanthentriggeraLADpointingofthetargettostudyindetailthespectralevolutionof
thesourceduringtheaccretionmodification(forexample,asuppressionoftheironlineproducedintheinner
regionofthediskisexpected). TheWFMdipdetectioncanbeusedtotriggerVLBIobservationsthatcantrace
theexpectedejectionofanewemissionfeaturefromtheradiocore. Sincetheejectionsofnewemissionfeatures
areconnectedtoflaresatmm-wavelengths(Savolainenetal.,2002),itisalsopossibletocombinetheWFMdip
monitoringwithexistinglong-termmm-wavelengthmonitoringprogramslikeMetsähovi37GHzmonitoring
(e.g.,Teräsrantaetal.,2004)forstudyingthedisk-jetconnection. In3C120X-rayfluxand37GHzfluxare
indeedanticorrelatedwithX-raysleadingasexpected(Chatterjeeetal.,2009). Thisstudycanbeappliedatleast
tootherthreenearbybroadlineradiogalaxies,i.e. 3C390.3,3C382,3C445.
Radio galaxies hosting inefficient accretion flows (mainly FRIs) are more easily detected at high and very
highenergies. FRIsaremorenumerousthanFRIIsintheGeVskyandfourofthemhavebeenalsodetectedat
TeVenergieswithcurrentimagingatmosphericCherenkovtelescopes(M87,NGC1275,CentaurusA,IC310).
UnlikeFRIIs,theradio-to-TeVspectrumoflowpowerradiogalaxiesisdominatedbynon-thermalemission.
Considering that in these sources the Doppler boosting is less important than in blazars, the study of their
SED can carry out important information on the jet structure. Indeed, the assumption of a pure, one-zone
homogeneous,SynchrotronSelf-Compton(SSC)emissionregionisinadequate(Abdoetal.,2009a,b;Migliori
etal.,2011)inthesesource,whileastratifiedjetwithdifferentregionsinteractingeachothercouldbeapossible
solution(Georganopoulos&Kazanas,2003;Ghisellinietal.,2005;Böttcheretal.,2010). Hadronicmodels
couldbeanotherpossibility. Thesehavebecomeverypopularinthelastyearsbecauseoftheirdirectconnection
to Ultra High Energy Cosmic Rays (UHECR) and to the recent discovery of high-energy neutrinos by the
IceCube detector (Aartsen et al., 2013). Cosmic rays, accelerated at different sites in AGN, could produce
γ-raysandneutrinos(Beckeretal.,2009). AmongAGNs, radiogalaxiesarethemostprominentcandidates
(Beckeretal.,2014). EstimatesoftheneutrinofluxinthetworadiogalaxiesCentaurusAandM87havebeen
recentlyattempted. InordertoaddressthisissuethestudyandmodelingoftheSEDofFRIradiosources(in
differentluminositystates)ismandatory. LOFT isthesatellitethatcanefficientlycovertheX-rayregion. FRIs
aregenerallylessbrightthanFRIIsbetween2–10keV.However,withanexposureof10ksormoretheLAD
guaranteesa5σdetectionforsourcesasweakasafew10 12ergcm 2s 1. Notethat,threeoutoffourTeVradio
− − −
galaxieshavefluxesgreaterthan10 12ergcm 2s 1. LOFT canbethenthereferenceX-raysatelliteforfuture
− − −
multi-wavelengthcampaignsonFRIsledbytheCTAobservatorymonitoringpreferentiallyquiescentstates. At
the same time, if a CTA alert is issued, the LOFT capability to rapidly point the flaring target, will offer the
uniqueopportunitytofollowthespectral/fluxevolutionofaradiogalaxyinahighluminositystate.
4 The LOFT perspective on radio-loud narrow line Seyfert 1 galaxies
ThediscoverybyFermi-LATofvariableγ-rayemissionfromafewradio-loudnarrow-lineSeyfert1s(NLSy1s)
revealedthepresenceofapossiblethirdclassofAGNwithrelativisticjets(Abdoetal.,2009;D’Ammandoet
al.,2012). ByconsideringthatNLSy1sareusuallythoughttobehostedinspiralgalaxies(e.g.,Deoetal.,2006),
thepresenceofarelativisticjetinthesesourcesseemstobeincontrasttotheparadigmthattheformationof
thosestructurescouldhappenonlyinellipticalgalaxies(e.g.,Marscher,2009). Moreover,smallermassesofthe
centralBHandhigheraccretionratesthanthoseobservedinblazarsandradiogalaxiesareinferredforNLSy1s.
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Radio-loudAGNwithLOFT
Figure4: Chandra0.3–7keVimageoftheFRIIradiogalaxy3C111with3.6cmradiocontoursoverimposed. Inthe
insetthemulti-wavelengthlightcurvefrommmtoγ-raysisshown. Thesimultaneityoftheflareisimpressive: thecore
luminositywasincreasingfrommillimetertoX-rayfrequenciesexactlywhenthegreatestfluxofγ-rayphotonsoccurred.
Thisisaclearindicationofthecospatialityoftheevent. Theoutburstofphotonsisdirectlyconnectedtotheejectionofa
newradioknot(thetimeoftheejectionisindicatedbytheblackarrow).
InthiscontextthedetectionofNLSy1sinγ-raysposesintriguingquestionsaboutthenatureoftheseobjects,the
onsetofproductionofrelativisticjets,themechanismsofhigh-energyproduction,andthejet-discconnection.
Strongγ-rayflareswereobservedfromPMNJ0948+0022(Foschinietal.,2011;D’Ammandoetal.,2015),
SBS0846+513(D’Ammandoetal.,2013),and1H0323+342(Carpenteretal.,2013),withanisotropicγ-ray
luminosityof1048ergs 1,comparabletothatofthebrightFSRQs,andfluxvariabilityonadailytimescaleor
−
shorter. Follow-upobservationsofNLSy1sduringflaringactivitywiththeLOFT/LADwillbeimportantfor
investigatingrapidfluxandspectralchangesoccurringinX-rays. Thiswillbefundamentalforconstrainingsize
andlocationofthehigh-energyemittingregion(alsoinconjunctionwithCTA)andthepossiblepresenceduring
thosehighactivitystatesoftwodifferentemittingcomponents(e.g. SSC,externalComptonfromBLRand/or
dusttorus)inthebroadenergyrangecoveredbyLAD,characterizingindetailtheSEDofthesesources.
Furthermore,1H0323+342isthebrightestNLSy1(1–2 10 11ergcm 2s 1 in2–10keV)inX-raysamong
− − −
×
the5sourcesdetectedbyFermi-LATsofar,andtheonlyoneincludedinthe70-monthSwift-BATcatalogueas
wellassignificantlydetectedbyINTEGRAL(Baumgartneretal.,2013;Panessaetal.,2011). Thereforeitis
theidealtargetforinvestigatingthedisc-jetconnectionintheγ-rayNLSy1. ThesumoftheX-rayspectraof
1H0323+342collectedbySwift-XRTshowedthehintforthepresenceofanironKαline(Paliyaetal.,2014).
Thankstothehigh-qualityspectrumcollectedbytheLADwewillbeabletoconfirmthedetectionoftheiron
Kαline,andtestwithhighaccuracythereflectionmodelappliedtotheX-rayspectrumthankstoitsbroader
energyrange,thusremovingthedegeneracyoftheparameters. Thiswillallowustoconstrainthepropertiesof
theaccretionflow. Inparticular,wewillbeabletoobtainverygoodconstraintsonthespinoftheblackhole.
Thisinformationcanbeusedforprobingtheinneraccretiondisc,aregionstrictlyrelatedtothejetformation
andlaunching. IfthehighspinoftheblackholeclaimedbyPaliyaetal.(2014)isconfirmedbythehigh-quality
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