The Trails of Superluminal Jet Components in 3C111

1.2 MB·

by  M. Kadler

ASTROPHYSICALJOURNAL
PreprinttypesetusingLATEXstyleemulateapjv.03/07/07
THETRAILSOFSUPERLUMINALJETCOMPONENTSIN3C111
M.KADLER1,2E.ROS2,M.PERUCHO2,Y.Y.KOVALEV2,3,D.C.HOMAN4,I.AGUDO5,2,K.I.KELLERMANN6,M.F.ALLER7,H.D.
ALLER7,M.L.LISTER8,ANDJ.A.ZENSUS2
(Received2007September26;Accepted2007December19)
ABSTRACT
In1996,amajorradioflux-densityoutburstoccuredinthebroad-lineradiogalaxy3C111. Itwasfollowed
bya particularlybrightplasmaejection associatedwith a superluminaljetcomponent,whichhasshapedthe
parsec-scalestructureof3C111foralmosta decade. Here, wepresentresultsfrom18epochsof VeryLong
8 Baseline Array (VLBA) observationsconductedsince 1995as partof the VLBA 2cm SurveyandMOJAVE
0 monitoringprograms. Thismajoreventallowsustostudyavarietyofprocessesassociatedwithoutburstsof
0 radio-loudAGNinmuchgreaterdetailthanhasbeenpossibleinothercases:theprimaryperturbationgivesrise
2
totheformationofaleadingandafollowingcomponent,whichareinterpretedasaforwardandabackward-

n shock. Both componentsevolve in characteristically differentways and allow us to draw conclusionsabout
a theworkflowofjet-productionevents;theexpansion,accelerationandrecollimationoftheejectedjetplasma
J in an environmentwith steep pressure and density gradientsare revealed; trailing componentsare formedin

7 thewakeoftheprimaryperturbationpossiblyasaresultofcouplingtoKelvin-Helmholtzinstabilitypinching
modes from the interaction of the jet with the external medium. The interaction of the jet with its ambient

] mediumisfurtherdescribedbythelinear-polarizationsignatureofjetcomponentstravelingalongthejetand
h
passingaregionofsteeppressure/densitygradients.
p
- Subjectheadings:galaxies:individual:3C111–galaxies:active–galaxies:jets–galaxies:nuclei
o
r
t
s 1. INTRODUCTION peratures (Homanetal. 2006), or their Lorentz factor dis-
a tribution (Kellermannetal. 2004) and luminosity function
Direct evidence for the existence of bulk relativistic out-
[ (Cara&Lister2007).
flows along the jets in blazars and other radio-loud ac-
The relativistic-jet model(e.g., Blandford&Konigl1979)
2 tive galactic nuclei (AGN) comes from Very-Long-Baseline
v Interferometry (VLBI) observations. The first evidence has become the de-facto paradigm in multiwavelength re-
7 for apparently superluminal structural changes was found search on blazars and other AGN, but VLBI observations
1 from changes in the fringe visibility curves of 3C279 and have demonstrated that the basic concept of ballistically-
6 moving isolated jet knots is clearly oversimplified: jet cur-
3C273(Whitneyetal.1971;Cohenetal.1971). Subsequent
0 vature (e.g., Vermeulen&Cohen 1994), stationary compo-
higher-quality VLBI observations (see, e.g., compilation by
. nents (e.g., Jorstadetal. 2001), and non-radial and acceler-
1 Vermeulen&Cohen 1994, and references therein) have es-
ated motions (e.g., Kellermannetal. 2004), are found to be
0 tablished the “core-jet” type milliarcsecond-scale structure
8 of compact extragalactic jets: the core being a bright and common features of relativistic jets. Within individual jets,
0 unresolved flat-spectrum component at the end of a lin- there are often characteristic velocities suggesting the pres-
v: ear structure, and the jet being composed out of individ- ence of an underlying continuous jet flow, but the “com-
ponents” themselves most likely represent patterns moving
i ual steep-spectrum components or “knots”. The knots fre-
X at a different speed than the underlying flow, e.g., as hy-
quently move away from the core with apparent veloci-
drodynamicallypropagatingshocks(Marscher&Gear1985;
r ties exceeding the speed of light. Monitoring observa-
a Hughesetal.1985).
tions of large source samples (Vermeulen&Cohen 1994;
Recentyearshavebroughtmajorimprovementsinnumeri-
Jorstadetal.2001;Kellermannetal.2004;Pineretal.2007)
calsimulationsofrelativisticjets(see,e.g.,Gómez2005,fora
haveprovidedimportantstatisticaltoolsforprobingrelativis-
review). Itisnowpossibletosimulatethree-dimensionalrel-
tic beaming and the intrinsic properties of extragalactic ra-
ativistic jets(e.g.,Aloyetal. 2003) andto computethe rela-
dio jets (Cohenetal. 2007), their intrinsic brightness tem-
tivisticprocesses(e.g.,Gómezetal.1997)thattransferhydro-
1Astrophysics Science Division, NASA’sGoddardSpaceFlightCenter, dynamic results into observed brightness distributions (e.g.,
GreenbeltRoad,Greenbelt,MD20771,USA;[email protected] relativistic light abberation and light travel time delays). In
2Max-Planck-Institut für Radioastronomie, Auf dem Hügel69, 53121 particular,interactionsbetweenstrongperturbationsorshocks
Bonn,Germany;ros,perucho,ykovalev,[email protected] withtheunderlyingjetflowandthejet-ambientmediumcan
3AstroSpaceCenterofLebedevPhysicalInstitute,Profsoyuznaya84/32,
besimulated(Agudoetal.2001).Withthesenewtechniques,
117997Moscow,Russia
4AstronomyDepartment, DepartmentofPhysicsandAstronomy, Deni- itisnowpossibletocomparethegeneration,propagationand
sonUniversity,Granville,OH43023,U.S.A.;[email protected] evolutionofemissionfeaturesinsimulatedandobservedrel-
5InstitutodeAstrofísicadeAndalucía(CSIC),Apartado3004,E-18080 ativisticjets.
Granada,Spain;[email protected] The nearby (z=0.049)9 broad-line radio galaxy 3C111
6National Radio Astronomy Observatory, 520 Edgemont Road, Char-
(PKS B0415+379) shows a classical FRII morphology on
lott7eAsvsitlrloe,noVmAy2D29e0p3a,rtUm.eSn.At,;[email protected],AnnArbor,MI48109- kiloparsec-scalesspanningmorethan200′′withahighlycol-
1042,U.S.A.;mfa,[email protected] limated jet connecting the central core and the northeastern
8Department of Physics, Purdue University, 525Northwestern Avenue,
WestLafayette,IN47907,U.S.A.;[email protected] 9AssumingH0=71kms- 1Mpc- 1,ΩM=0.3,ΩΛ=0.7(1mas=1.0pc).
2 Kadleretal.
lobeinpositionangle63◦whilenocounterjetisobservedto-
wards the southwestern lobe (Linfield&Perley 1984). This
asymmetry is usually explained via relativistic boosting of
the jet and de-boosting of the counter-jet. 3C111 exhibits
the brightest compact radio core at cm/mm wavelengths of
all FRII radio galaxies, a blazar-like spectral energy distri-
bution (Sgueraetal. 2005), and it was one of the first (and
only) radio galaxies in which superluminal motion was de-
tected (Goetzetal. 1987; Preuss,Alef,&Kellermann1988).
Moreover,the (sub-)parsecscale jetof3C111isintimately
relatedtoitshigh-energyemission: Marscher (2006)reports
a disk-jet connection, similar to the well-established one in
3C120(Marscheretal.2002),inthesensethatdipsintheX-
ray light curve indicate accretion events which are followed
by VLBI jet component ejections. Recently, R.C.Hartman
& M. Kadler (in prep.) showed that the gamma-ray source
3EGJ0416+3650 can be decomposed into multiple individ-
ual sources inside the EGRET full-band point-spread func- FIG. 1.— University of Michigan Radio Astronomy Observatory light
tion,revealinga significantsignalfromthenominalposition curvesof3C111at4.8GHz,8GHz,and14.5GHz.Theshadedareasindicate
of 3C111 in the higher-resolution, high-energy band above theejectionepochsoftheindividuallylabeledjetcomponentsasdiscussed
inSect.3.2. Thelightestshadingcorrespondstominorejectionsoftherel-
1GeV. This association of 3C111 with 3EGJ0416+3650,
ativelyweakcomponentsI,J,K,LwithfluxdensitiesSbelow0.2Jy,medium
whichhadoriginallybeensuggestedbyHartmanetal.(1999) shadingcorresponds tocomponents B,C,G,H,M,Nwith0.2Jy<S<0.6Jy
and Sgueraetal. (2005), makes 3C111 one of the very andthedarkestshadingtocomponentsEandFwithS>0.6Jy.
rare radio galaxiesdetected at gamma-rayenergiesand sup-
ports the view that this source may be considered a lower-
luminosityversionofpowerfulradio-loudquasars.
Here, we report the results from ten years of Very-Long-
Baseline Interferometry (VLBI) observations of 3C111
as part of the VLBA 2cm Survey10 (Kellermannetal.
1998; Zensusetal. 2002; Kellermannetal. 2004;
Kovalevetal. 2005) and its follow-up program MO-
JAVE11 (Lister&Homan 2005; Homan&Lister 2006). We
investigate the parsec-scale source structure during a major
flux-density outburst and during its aftermath. We find
that this outburst was associated with the formation of an
exceptionallybrightfeatureinthejetof3C111. Avarietyof
processes (beyondthe predictionsof simple ballistic motion
models) are observed and discussed in view of modern
relativistic-jet simulations. In Sect. 2, our observations and
the data reduction are described. A detailed report of the
observationalresultsisgiveninSect.3. InSect.4,wediscuss
thevariousprocessesobservedinthejetof3C111asaresult
FIG. 2.—Spectral-indexcurvesof3C111between14.5GHzand8GHz
of the outburst and during the propagation of the new jet (top),and8GHzand4.8GHz(bottom)fromtheUMRAOmonitoringpro-
featurealongthejet. InSect.5,weputtheseresultsintothe gram.TheshadedareasarethesameasinFig.1.
contextof future simulations and observationswith the goal
ofunderstandingtheproductionmechanismsofAGNjets. atthevariousepochsaregiveninTable2. Themodelswere
alignedbyassumingthewesternmostcomponent(namely,the
2. OBSERVATIONSANDDATAANALYSIS “core”)tobestationarysothatthepositionofjetcomponents
3C111hasbeenmonitoredaspartoftheVLBA2cmSur- can be measured relative to it. Because of the coupling of
vey program since April 1995. The observational details thefluxdensitiesofnearbymodelcomponents,theuncertain-
are given by Kellermannetal. (1998). Following the meth- tiesinthecomponentfluxdensitiesarelargerthantheformal
ods described there, the data from 17 epochs of VLBA ob- (statistical) errors unless the given model component is far
servations of 3C111 between 1995 and 2005 (see Table 1) enoughfromitsclosestneighbor. Throughoutthispaper,er-
were phase and amplitude self calibrated and the brightness rorsof 15% are assumed for the flux densities of individual
distribution was determined via hybrid mapping. An addi- model-fitcomponents. In mostcases, this shouldbeconsid-
tional epoch from June 2000 was made available to us by ered a conservative estimate that accounts for absolute cal-
G.Taylor. Thepolarizationcalibrationwasperformedasde- ibration uncertainties and formal model-fitting uncertainties
scribed in Lister&Homan (2005). Two-dimensional Gaus- (see,e.g.,Homanetal.2002). Positionuncertaintieswerede-
sian componentswere fitted in the (u,v)-domainto the fully terminedinternallyfromthedeviationsofthedatafromlinear
calibrated visibility data of each epoch using the program motion.
DIFMAP (Shepherd1997). Theparametersofeachmodelfit
3. RESULTS
10http://www.cv.nrao.edu/2cmsurvey/ 3.1. The1996RadioOutburstof3C111
11http://www.physics.purdue.edu/MOJAVE
TheTrailsofSuperluminalJetComponentsin3C111 3
FIG. 3.— Naturallyweightedimagesoftheparsec-scalejetof3C111fromthe2cmVLBAmonitoring. Acommonrestoringbeamof(0.5 1.0)masat
P.A.0◦wasused.Thetotalrecoveredfluxdensityineachimage,thermsnoise,andthelowestcontoursforeachimagearegiveninTable1.Cont×oursincrease
logarithmicallybyafactorof2.OnlycomponentsE,G,Handtheircorrespondingtrailingcomponentsareindicatedbycirclesenclosingacross.
Astrongfluxdensityoutburstof3C111occurredin1996, the two lower frequencies, the flux-density maximum was
which was first visible in the mm band and some months reachedatsubsequentlatertimes, inmid1997at8GHz and
later at lower radio frequencies. This outburst was first de- in late 1997 at 4.8GHz. The profile of the outburst in the
tected at 90GHz with the IRAM interferometer at Plateau flux-densityvs. timedomainshowsanarrow,high-amplitude
de Bure in January 1996 with flux densities greater than peakbetweenearly1996andlate1997,whichisalmostsym-
10Jy (Alefetal. 1998), at 37GHz in March 1996, and at metric.Afterlate1997,aslower-decreasingcomponentdom-
22GHzinAugust1996withtheMetsähoviradioobservatory inatesthelightcurves,mostclearlyvisibleat14.5GHz.
(Teräsrantaetal.2004). Figure1showsthesingle-dishradio Figure 1 shows that the flare propagated through the
lightcurvesof 3C111at4.8GHz, 8GHz, and14.5GHz ob- spectrum as qualitatively expected by standard jet theory
tained from the UMRAO radio-flux-densitymonitoring pro- (e.g.,Marscher&Gear1985):high-frequencyradioemission
gram (Aller,Aller,&Hughes 2003). These data show that comesfromthemostcompactregionsofthejet,theemission
from early 1996 on, the radio-flux density of 3C111 was peak shifts to lower frequenciesas a newly ejected jet com-
rising at 14.5GHz, reaching its maximum in late 1996. At ponenttravelsdownthe jet andbecomesoptically thin. The
4 Kadleretal.
peakfluxdensityshiftedwithfrequencyatabout10GHzyr- 1.
The evolution of the spectral index, α (S ∼ να), for 15 AB
(14.5/8.0)GHzand(8.0/4.8)GHzisshowninFig.2. Before C
D
1996,thesamplingwastoosparsetoderivethechangeofthe E
E1
spectralindexinthe(14.5/8.0)GHzband. Between,8.0GHz E2
i1wann9igdt9h6t4ha.ce8omGrparrHexesi-zpm1,o9utnh9mde6essdpppeteerocciottarrdaas.lluiibTnnsddheeeeqxxur,eawαndati∼soflaafl0pt,utpeixrnno-idxtnheigmenos(aif1ttye4thl.yo5eu/-s8tp0b.0e.u7)crGtsdrtuHuimrnz- Distance [mas]10 EEFGHH341
band reached in mid 1996. In the post-outburst period be- Core HH23
tween 1998 and 2004, α was typically in the range - 0.5 5 H4
I
to - 0.7 between 14.5GHz and 8.0GHz and slightly steeper J
(- 0.7to- 0.9)inthe(8.0/4.8)GHzband. Theoverallsteeper KL
spectralindexat lowerfrequenciescan be understoodas the M
N
contribution of optically thin large-scale emission from the
0
radiolobesof3C111tothesesingle-dishlightcurves. 1995 2000 2005
Epoch [yr]
FIG. 4.— Core separation ofmodel-fit components vs. time. Crosses
3.2. VLBAMonitoringResults
representcomponents whichcouldnotbecrossidentified overtheepochs.
Figure 3 shows the variable parsec-scale structure of Positionuncertaintieshavebeenestimatedfromtheinternaldeviationsofthe
datafromlinearmotionsforeachcomponent.Theuncertaintiesfortheposi-
3C111at18differentepochsofVLBAobservationsbetween
tionofE1havebeendeterminedseparatelyforthepre-2004andpost-2004
1995.26 and 2005.73. The variable source structure can be epochsbecauseofthepartialresolutionofthiscomponentafter2004.Uncer-
describedbyaclassicalone-sidedcore-jetmorphologyinthe taintiesforcomponentswithlessthanthreeepochswereestimatedfromother
firsttwoepochswithtypicalvelocitiesoftheoutwardmoving componentsatsimilarpositionsinthejetandwithsimilarfluxdensities.
jetcomponentsofabout1.4to1.7masyr- 1 correspondingto
about5c. In1996.82anewjetcomponent,evenbrighterthan
the core, dominated the source structure. By 1997.19, this C
new componentwas even brighter (∼3.4Jy) and in the fol- 1 C CCAore C C C C C C C C C CC C C C CC C
lowingepochsittraveledalongthejetwhileitbecamegradu- B
allymorestretchedoutalongthejet-ridgeline. C
D
y] E
mMododeellfiFticttoinmg:p—oneInntsFfirgo.m4,tthheecroardeiaisldshisotwannceasoaftfhuencvtaiorinouosf nsity [J 0.1 EEE123
time. Thecomponentidentificationwasbasedonacompari- ux De EFG4
sonofthepositionsandfluxdensities,andalinearregression Fl H
H1
ofthedistancesfromthecoreasafunctionoftimewasused 0.01 H2
H3
todeterminethekinematics. Thederivedcomponentveloci- H4
tiesaretabulatedinTable3. Theearlyouterjetcomponents
(A,B,C, D)ofthe1995.26epochcanbetracedovertwoto
0.001
fourepochsbeforetheirfluxdensitiesfallbelowthedetection
1995 2000 2005
threshold(compareFig.5). Inlate1996andearly1997,the Epoch [yr]
source structure was dominated by the emission of the core FIG. 5.—Flux-densityevolutionofthecoreandthejetcomponentswith
andthenewlyformedjetcomponentsE andF, with E being time. Forclarity, onlycomponentsejectedbefore2001.50areshown. The
the leading component. The two components traveled out- fluxdensitiesofE1,E2,E3,andE4wereaddedforthepost-1999epochs
wardswithameanapparentvelocityof(1.00±0.02)masyr- 1 andaflux-densityweightedeffectivepositionwascalculated todisplaythe
flux-densityevolutionoftheblendedfeaturethatwouldbevisibleatlower
and (0.64±0.07)masyr- 1, respectively. Before mid 1997, resolution. NotethatcomponentsE4andGareblendedinepoch2004.27
component F was substantially brighter than component E andthatthefluxdensityofE4maybeoverestimatedforthisepoch.
butafterthat,itsfluxdensitydroppedsteeply. Fwasnotde-
tectedatanyepochlaterthan1998.18,whileEwasstillabout wasseen from2004.80on. In thefollowing,we referto the
800mJy atthattime. Thelightcurvesof E andFreproduce componentsE1,andH1asthe“leadingcomponents”andto
qualitativelythetwo-componentshapeoftheflux-densityout- E2, E3, E4, H2, H3, andH4 as the “trailing components”
burst in Fig. 1 with component F being responsible for the ofEandH,respectively.
narrowerandhigher-amplitudepeakbetweenearly1996and For the pc-scale jet of 3C111, the ejection epochs of the
late1997andcomponentEdominatingtheslower-decreasing individualjetcomponentscanbedeterminedfromthelinear
tail of the outburst after late 1997 (compare Fig. 5 and dis- regression by back-extrapolating the component trajectories
cussionbelow).Inthefollowingepochs,Esplitintofourdis- to the core. In Fig. 1, these ejection epochs and the associ-
tinct components(E1, E2, E3, E4) at distances of 3.5mas ateduncertaintiesareindicatedasshadedareas. Itisapparent
to4.5masfromthecore. E2,E3,andE4allmovedatsub- that the ejection of the components E and F coincides with
sequently slower speeds than E1, resulting in an elongated theonsetofthemajorflux-densityoutburstin1996described
morphologicalstructureoftheassociatedemissioncomplex. above. Thefollowingmajorcomponentejections(G,H,and
In later epochs, new components have been ejected from thecombinedM/Nevent)allhavedirectcounterpartsinlocal
the core into the jet. The two strongest components (G,H) maximaoftheradiolightcurve,especiallyat14.5GHz. Fig-
canbetracedthroughthefollowingeightandninemonitoring ure2showsthatalltheseejectionepochscoincidedwithlocal
epochs,respectively.ComponentHsplitintothreeindividual maximaof the spectralindexin the 14.5/8.0GHz band. Be-
components in 2004.27 and a fourth associated component tween2002and2004,anumberofminorcomponentejections
TheTrailsofSuperluminalJetComponentsin3C111 5
1e+12
E (pre 1999)
E (post 1999)
F 1e+11
1 E1
E2
Flux Density [Jy] 0.1 EE34 ghtness Temperature [K]111eee+++100098
0.01 Bri
1e+07
0.001 1e+06
0 5 10 15 1 10
Core Distance [mas] Core Distance [mas]
FIG. 6.—Flux-densityevolutionofcomponentEanditstrailingcompo- FIG. 8.—Brightnesstemperaturesofmodel-fitcomponentsasafunction
nentsvs. thedistance traveledfromthecore. NotethatcomponentE4is of their distance to the core. The brightness temperatures of components
blendedwithcomponentGinepoch2004.27andthatitsfluxdensitymaybe belongingtotheE–,F–andH–componentsareindicatedbyfilledblackcir-
overestimatedforthisepoch. cles. Thesolidlinerepresents aleast-squares fittoallbuttheE–,F–and
H–components.Theslopeoftheregressioncurveis- 2.4 0.2.
±
G
H
H1 E
H2 1e+11 E1
1 H3 E2
H4 E3
E4
K]1e+10 F
Flux Density [Jy] 0.1 ness Temperature [1e+09 I
ht1e+08
0.01 g
Bri
1e+07 II
0.001
0 5 10 15 1e+06
Core Distance [mas] 1 10
Core Distance [mas]
FIG. 7.—Flux-density evolution ofcomponentGandHanditstrailing
componentsvs.distancetraveledfromthecore. FIG.9.—BrightnesstemperaturesofcomponentE,itsleadingandtrailing
components, and component F as a function oftheir distance to the core.
Thetworegimesofbrightness-temperaturegradientdiscussedinthetextare
took place but the regression-fit quality (due to the neareby indicatedwithdashedlines.
components, the low flux densities and the small time base-
line)onlymoderatlyconstrainstheejectionepochs. Inaddi- the factthatitdoesnotappearto have splitinto leadingand
tion,thetimesamplingofUMRAOobservationsinthistime trailing components like E and H, did exhibit a pronounced
range is relatively poor, in particular from mid 2001 to mid fluxdensitymaximumafter2002,aswell, about6masfrom
2003. thecore.
FluxDensityEvolution:— Figure5showsthebrightnessevo- The Tb gradient along the jet:— Following Kadleretal.
lution of the core and the jet components that have been (2004); Kadler (2005), the power-law index s, which de-
ejected priorto 2001.5. Apparently,the trailing components scribes the brightness temperature gradient via Tb ∝rs, can
E4 and H4 appearedfirst in a rising state, i.e., theyfirst in- beparametrizedas
creased in flux density before they became fainter in later s = l+n+b(1- α) (1)
epochs. Component F showed an extraordinary steep de-
crease in brightnessin 1997–1998. In Fig. 6 and Fig. 7, the where l, n and b are the power law indicesthat describe the
flux-densityevolutionofthecomponentsE,G,andHandthe gradientsofjetdiameterd∝rl,particledensityn ∝rn,and
e
associated leading and trailing components are shown with magneticfield B∝rb with distance r from the core, respec-
distancetraveledfromthecore,respectively. Theejectafirst tively. Therefore,measuringthebrightnesstemperaturegra-
roseinfluxdensitywithintheinner1masfromthecore,then dientprovidesamethodtoconstrainthecriticalphysicalprop-
they showed a decline about almost three orders of magni- ertiesalongthejetandabruptchangesintheT -gradientcan
b
tudeinthefollowingdecade,exhibitingaplateauorbroadlo- highlightregionsinthejetwherethedensity,magneticfield,
calmaximumin1998–2000atadistancefrom2–4masfrom orjetdiameterchangerapidly.
the core. ComponentH and its leading and trailing compo- Figure8showsthebrightnesstemperaturesofalljetcom-
nentsexhibitedasimilarbehavioralthoughonaboutanorder ponentsintheparsec-scalejetof3C111at2cmwavelength
ofmagnitudelowerflux-densitylevelsandatslightlyfurther between 1995 and 2005as a functionof their core distance.
downstream,4–6masfromthecore.ComponentG,inspiteof Ingeneral, thebrightnesstemperatureofallcomponentsde-
6 Kadleretal.
creased as the componentstraveledoutwardsbut an approx-
imationwithasimplepowerlawdoesnotyielda goodfitto H
thefulldataset(χ2 =1.8,115degreesoffreedom[d.o.f.]).
red Oct. 2002
Visual inspection of Fig. 8 shows that this is due to the E–,
F–andH–componentsandtheirleadingandtrailingcompo-
nents, respectively. This behavior is different than expected
for a straight and stable jet geometry in which the power-
H
law dependences of the particle density, the magnetic field
strengthandthejetdiameteronthecoredistancepredictsthat Aug. 2003
thebrightnesstemperaturealongthejetcanbedescribedwith
awell-definedpower-lawindexs. Mostextragalacticparsec- H
scale jets which do not show pronounced curvature, show a
power-law decrease with increasing distance from the core
and power-law indices typically around - 2.5 (Kadler 2005). April 2004
Infact,excludingtheE–,F–andH–componentsfromthefit
yields a statistically better result (χ2 =1.3, 52 d.o.f.) and H
red
a gradient of - 2.4±0.2. In Sect. 4.1 and Sect. 4.5, we dis-
cusspossible physicalreasonsforthe differentbehaviorand I
Oct. 2004
natureof thesecomponents. Themeasuredrelationbetween
component sizes and distance along the jet is affected by a
large degreeof scatter and doesnot provideindependentin- H
formation from the flux density and brightness temperature
I
plots described above. Therefore we do not show plots of
Jan. 2005
componentsizeversusjetdistance.
The brightness-temperature gradient of component E was H
firstflatorinvertedimmediatelyafterthecreationofthisnew
component within approximately 1mas from the core and
then reached steep values of - 2.5 to - 2.8 (regime I; com- Sept. 2005
pareFig.9)through1997whenthecomponenttraveledfrom
1masto2mas. Between2masand4mas,thedetermination
ofthebrightness-temperaturegradientrequiresanidentifica-
tion of component E with either component E1 or E3 (see
below). Independently of this identification, the brightness-
temperaturegradienteventuallychangedtoverysteepvalues FIG. 10.—Naturallyweightedimagesofthelinear-polarization structure
(s<- 5)beyond5masfromthecore(regimeII).Component of3C111between2002and2005. Therestoring-beamdimensionsandori-
entationsforeachepochareindicatedbyacrosstotheleftofeachStokes-I
F began its very rapid decline in brightness temperature at
image. StokesIcontoursstartat1mJy/beamandincreasebyfactorsof2.
a very small distance from the core (<0.7mas) with an ex-
Fractionalpolarizationisover-plottedontheStokes-Icontoursincolor. To
tremelysteepTb-gradient(s<- 8). therightoftheStokes-Iimagesarethepolarizationintensitycontoursstarting
at1mJy/beamandincreasingbyfactors√2. Thepolarizationcontoursare
LinearPolarization:— From1995to2002,2cm-Surveyob- over-plottedwithtick-marksrepresentingtheelectricvectorpositionangle.
Asingle Stokes-I1mJycontour surroundsthepolarization imagetoshow
servations were done in left circular polarization only, so
registration. The dotted line marks the distance of 3.3mas from the core
nolinear-polarizationinformationcan bederivedfromthese wherethemostpronouncedchangesofthepolarizationpropertiestakeplace
data. MOJAVEobservations(from2002on)aredoneinfull- (seetext).
polarimetric mode. Figure 10 shows our polarization data
ofHwasapproximatelyperpendiculartothejet.
throughSeptember2005.
After component H passed through this region (epochs
InOctoberof2002,componentHwasabout2.5masfrom
April2004throughSeptember2005),itsplitintoanumberof
thebaseofthejetandshowedafractionalpolarizationof5%
subcomponentsasdescribedearlier,anditspolarizationgrad-
to10%increasingtowardsthedownstreamsideofthecompo-
ually became more uniform. Consistent fractional polariza-
nent. Theelectricvectorpositionangle(EVPA)displayedby
tion of 5% to 10% was approachedwith the electric vectors
thecomponentwasapproximatelyalignedwiththejet. Inthis
approximatelyperpendiculartothelocaljetdirection.
epoch, the jet material just downstream of component H at
∼3.3masfromthebaseofthejetwasmorehighlypolarized, The much weaker component, I, developed polarization
verysimilartoHasitpassedthroughthesameregion,about
exceeding 20% fractional polarization on the jet’s southern
side,andtheEVPAofthepolarizationturnedtobeabout45◦ 3.3masfromthecore,withfractionalpolarizationexceeding
20%towardthesouthernsideofthejetwithanEVPAatap-
tothemainjetdirection.
proximately 45◦ to the main jet axis. This is also the same
ByAugustof2003,componentHhadenteredaregionap-
regionofthejetinwhichcomponentEhadbrokenupintoa
proximately3.3masfromthecoreanditsobservedpolariza-
number of sub-components. In future epochs, we will have
tionwasnowsimilartotheemissionin thissameregionob-
the opportunity to follow component K as it passes through
servedinthepreviousepoch. Theobservedfractionalpolar-
thissameregion.
izationofHnowclimbedsharplytovaluesinexcessof20%
toward the jet’s southern side while there was no detectable
4. DISCUSSION
polarizationfromthenorthernsideofH.TheobservedEVPA
ofHhadrotatedfurthertobeapproximately60◦ tothemain In this section, we discuss the aftermath of the major out-
jetdirection. However,the EVPA onthe southern-mostside burst in 3C111 in 1996 and the following component ejec-
TheTrailsofSuperluminalJetComponentsin3C111 7
tionsthrough2005. We organizethe subsectionsof our dis- requiresan accelerationof this component(see Fig. 4) from
cussionaccordingtothedownstreamdistancefromtheVLBI β =3.26±0.07toβ =5.5±0.1between1998.18and
app,E app,E1
corewhereweobservetheeffectofinterest. 1999.38.Thismaybeinterpretedintermsofanexpansionof
the jet in a rarefied medium. Taking an angle to the line of
4.1. Within1pc: ForwardandReverseStructures sightof 19◦ (see AppendixA), the componentwouldbe ac-
Numerical simulations (Aloyetal. 2003) show that an celeratedfrom β =0.956(γ∼3.4) to β =0.995(γ ∼10.3).
abrupt perturbation of the fluid density at the jet injection Theincreaseofvelocityislessatsmallerviewingangles.
point during a short time propagates downstream, evolves Analternativemodelfortheaccelerationandbrighteningof
spreadingasymmetricallyalongthejetandfinallysplitsinto componentE would be a changeof the jet inclinationto the
twodistinctregions.Bothofthesetworegionshaveenhanced lineofsightfromabout24◦toabout11◦atthislocationinthe
energy density with respect to the underlying jet, and they jetasobservedinthecaseofthequasar3C279(Homanetal.
emitsynchrotronradiation. Theleading(forwardshock)and 2003). However,weseenosignificantchangeinjetposition
the following region (reverse shock) have higher and lower angleintheskywhichwouldbeexpectedtoaccompanysuch
Lorentz factors, respectively, than the underlying jet. Thus, a large change in jet direction. Moreover, subsequent com-
theyshouldseparatewithtimeastheypropagatedownstream ponents, particularly G, do not show the same kind of large
inthejet. accelerationinthisregion.
ComponentFmatchesthedescriptionofabackwardmov- Direct identification of component E with component E1
ing wave associated with the major injection into the jet of is not straightforward in the frame of expansion, as compo-
3C111 after the flux-density outburst of 1996. It follows nentE1 in epoch 1999.38was smaller than componentE in
the trail of component E but at a lower speed. If compo- 1998.18 (see Table 2). However, component E3 in epoch
nent F is identified with a reverse shock and component E 1999.38is larger than componentE in 1998.18. We can in-
with a forwardshock,it ispossiblethen tocomputethe size terpretthisascomponentEincludingcomponentsE1andE3
of the shocked region (Peruchoetal. 2007). In 1996.82and (andmaybeE4). Thesecomponentswouldbeindistinguish-
1997.19,EandFwerebothverybrightandseparatedbyonly ableinourobservationsbefore1999.38.Infact,Jorstadetal.
∼0.3pc in projected distance. During these two epochs, F (2005) monitored 3C111 between 1998 and 2001 with the
was 300mJy to 500mJy brighter than the leading compo- VLBA at 43GHz. They find an emission complex, that can
nent E. Following Aloyetal. (2003), a backward shock can be identified with our componentE, that graduallystretches
be brighter than a forward shock if the latter is beamed in a out as it travels from ∼ 2 mas from the core in 1998 to
cone smaller than the viewing angle due to its larger speed. roughly between 5mas and 8mas from the core in 2001.
WehaveexaminedtheDopplerfactorsofcomponentsEand TheirleadingcomponentC1canbeidentifiedwithourcom-
Ffortherangeofpossibleviewingangles(seeAppendixA) ponent E1, their component c2 with E2 and their c1 with
andthemeasuredvelocitiesandconcludethatthisalonecan- E3. At their higher angular resolution, Jorstad et al. can
not explain the brightness difference between component E separate components C1 and c1 already in early 1998. In
and F because the difference in apparent speed is not large agreement with our analysis at 15GHz, they detect c2 (E2)
enough. Jorstadetal.(2005)pointoutthatbackwardshocks about a year after they detect c1 (E3). They do not detect
can be brighter than forward shocks as long as the distur- a component corresponding to E4 but this may be an effect
bance is prolonged and there is a continuous supply of par- of partially resolvingout the jet structure at their higherob-
ticles enteringfrom the underlyingjet throughthe shock re- serving frequency, particularly in later epochs. It is further
gion.Withinhalfayear,between1997.19and1997.66,Flost interesting to note that the observed speeds at both frequen-
abouthalfofitsbrightness.Thisextraordinarilyfastdimming cies agree well. For E1(C1), µ =1.69±0.04masyr- 1
app,2cm
ofthebackwardshockcanbecausedbythelackofinputof at 15 GHz and µ = 1.77±0.06masyr- 1 at 43 GHz;
app,7mm
particlesfrombehind,i.e., a lowerplasmaejectionrateafter for E2(c2), µ = 1.29±0.06masyr- 1 at 15 GHz and
app,2cm
theprimaryinjectionpossiblyduetoadepletionoftheinner µ =1.23±0.04masyr- 1 at 43 GHz; and for E3(c1),
accretiondisk (Marscheretal. 2002;Marscher 2006) which app,7mm
µ = 1.22±0.05masyr- 1 at 15 GHz and µ =
feedstheplasmainjection. app,2cm app,7mm
ComponentFcanalsobeinterpretedasararefactionpropa- 1.07±0.02masyr- 1at43GHz.Thediscrepancyinthespeeds
gatingbackwardsinthereferenceframeoftheejectedblobof measuredforE3andc1seemstobeduetoaslightaccelera-
gas.Ararefactionisproducedwhentheblobisoverpressured tionofE3after2002.Afittothe15GHzdataofE3between
with respect to the jet, as this overpressure causes the front 1999and2002aloneyieldsaslowerspeedof∼1.0masyr- 1
toaccelerateinthejet,thusleavingararefiedregionbetween similartothespeedofc1inthesametimeperiodat43GHz.
theheadoftheblob(forwardshock)anditsrearpart,whichis Intheirwork,Jorstadetal. donotreportaccelerationofcom-
stillslower(itmoveswiththeinjectionvelocity).Inthiscase, ponentsfrom2masto4mas.However,thisislikelyduetothe
theemissionincomponentFcouldbeassociatedtothedenser factthattheirobservationsstartedinearly1998,thusmissing
andoverpressuredgasintheblobwhichhasstillnotbeenrar- thefirstobservationsofcomponentEpresentedinthispaper,
efied. Thisgaswouldceasetoemitassoonasitreachesthe whenitsspeedhasbeenmeasuredtobesmaller.
rarefiedregion,which mayalso explainthe suddendecrease
4.3. Between2pcand6pc: RecollimationoftheJet
in brightnessof this component. An extendeddiscussion on
thenatureofcomponentFandtheevolutionofitsbrightness Inspection of Fig. 9 shows that the back-extrapolation of
willbegiveninPeruchoetal.(2007). thebrightnesstemperatureofcomponentEfromregimeIIto
regime I is at least two orders of magnitude too high if this
4.2. Between2pcand4pc: ExpansionandAcceleration
extrapolationis basedon the gradientgivenby E1. Thelow
Itisnota-prioriclearwithwhichpost-split-upcomponent brightnesstemperatureofcomponentEinregimeIcannotbe
theoriginalfeatureEshouldbeidentifiedafter1999.Anatu- explainedby opacityeffectsbecausethe radio-lightcurvein
ralidentificationwouldbetheleadingcomponentE1butthat Fig.1showsthatthesourcewasopticallythinfrom1997on.
8 Kadleretal.
Moreover,ifweidentifycomponentEwithE1,itisDoppler- ering an inhomogeneous external Faraday screen. Such a
deboostedfromepoch1998.18to1999.38duetotheaccelera- screen could produce the observed differential rotation of
tionandarelativelylargeviewingangle;thus,wearenotable the EVPA while a componenttravelsthrougha givenregion
to explain the increase in brightnesstemperaturein terms of alongthe jet. Zavala&Taylor (2002) observed3C111 with
Doppler boosting. However, compact sub-components may theVLBAandproducedaFaradayrotation-measuremapbe-
havelargerbrightnesstemperatures,sothattheT valuesplot- tween8GHzand15GHz. TheyfindstrongFaradayrotation,
b
ted in Fig. 9 for E in regime I inward of about 3mas may ∼730radm- 2, at the same distance from the core (3.3mas)
represent lower limits for compact components already em- where our observations show the swing of the EVPA of the
beddedintheunresolvedstructure. componentHandsteeplydecreasingFaradayrotationfurther
Not only E/E1 but also components G and H show downstream. However,wenotethat730radm- 2 translatesto
an increase in total flux density several milliarcseconds 17◦ of rotation at 15GHz which alone is not enough to ex-
downstream. Compared to E/E1, these somewhat weaker plainthe changein EVPA thatwe observewhile component
components exhibit their flux-density maxima at somewhat Htravelsthroughthisregion.Ontheotherhand,thesteepde-
larger distances from the core (compare Fig. 6 and Fig. 7). creaseoftheFaradayrotationmeasuredup-anddownstream
This can be explained if the gas in the components trav- ofthisregionbyZavala&Taylor(2002)againagreeswitha
els through a mild standing shock in a recollimation re- changeoftheexternalgasdensityatthispoint,whichinturn
gion. This effect has been observed in numerical simula- may be identified with the pressure gradient responsible for
tionsof parsec (Gómezetal. 1997) and kiloparsecscale jets thecomponentexpansionandaccelleration.
(Perucho&Martí 2007). The material in the componentsis AcombinationofinhomogeneousFaradayrotationandan
expectedtobeoverpressuredwithrespecttoitsenvironment, interaction between the jet plasma and its ambient medium
thus expandinginto it. After the initial expansion, the com- appears most likely to explain our observations of the vary-
ponentsbecomeunderpressuredwith respect to the underly- ing linear polarizationstructure; however, both explanations
ing flow. The resulting recollimation leads to the formation pointtotheroleoftheexternalmedium,eitherthroughadis-
of a shock, whose strength depends on the initial degree of creteinteractionorarapiddecreaseinexternalgaspressure,
overpressureof the materialin the component. Thisprocess inshapingthejetflowdownstreamofthislocation.
explainstheincreaseinfluxdensityandbrightnesstempera-
4.5. Between3pcand5pc: FormationofTrailing
tureasduetocompressionofthegasintherecollimation. In
Components
Figs.6and7,weseethatthefluxdensityofcomponentEin-
creasesclosertothecorethanforcomponentGandH,which ThecomponentsE2,E3,andE4canbeinterpretedastrail-
isconsistentwiththeformerbeingslowerthanthelatter,thus ingcomponentsforminginthewakeoftheleadingE1which
recollimatingearlier(seePerucho&Martí2007). Italsoex- is identical with the original component E. This scenario is
plainswhyweseeasignificantaccelerationonlyinthefaster attractive because the basic concept of trailing components
expanding,brightercomponentE/E1. as introduced by Agudoetal. (2001) predicts the formation
Finally, after this mild recollimation, the fluid becomes of trailing features in the wake of the initial perturbation in
overpressuredwithrespecttoitsenvironment,thusfurtherex- the jet flow. Such a behaviour has first been found both as-
pandingandacceleratingdownstream. sociatedwithbrightsub-andsuperluminaljetcomponentsin
Centaurus A and 3C 120 (Tingay,Preston,&Jauncey 2001;
4.4. Near3pc: TheRoleoftheExternalMedium
Gómezetal.2001). Jorstadetal.(2005)reporttrailingcom-
The polarization behavior of components H and I can be ponentsinfouradditionalsources(3C273,3C345,CTA102
understoodintermsofaninteractionbetweenthejetandthe and3C454.3)andin3C111(seebelow).
externalmediumatadistanceof 3.3mas(=3.3pc)inthejet. Theinteractionoftheexternalmediumwithastrongshock
Assuming no Faraday rotation, the EVPA of component H pinches the surface of the jet, leading to the production of
withinapproximately3.3masfromthecoreindicatesatrans- pinch-bodymodeKelvin-Helmholtzinstabilities:thetrailing
verse magnetic field order as might be expected for a trans- features. Hence,a singlestrongsuperluminalshockejection
verse shock propagating down the jet. The change in the fromthe jet nozzlemay lead to the productionof a multiple
fractional polarization, its north-south gradient, and the ro- setofemissionfeaturesthroughthismechanism.Thetrailing
tation of the EVPA suggest that a contactsurface persists at features have a characteristic set of properties, which make
the southern boundary of the jet beam at a distance of ap- them recognizable with high resolution VLBI: they form in
proximately3.3mas downstreamthe jet core. If the bulk jet thewakeofstrongcomponentsinsteadofbeingejectedfrom
materialflows faster than the flow at the southernboundary, thecoreofVLBIjets,theyarerelatedtoobliqueshocks,they
themagneticfieldisstretchedthroughshear. Ouroverallpic- arealwaysslowerthantheleadingfeature,and(iftheunder-
turethenisofanoriginallytransverseshockinteractingwith lyingjethasacertainopeningangle)theyshouldbegenerated
the jet on the southern side of the jet at 3.3mas from the withawiderangeofapparentspeeds(fromalmoststationary
core.Theinteractionchangesthecomponent’smagneticfield nearthecoretosuperluminalfurtherdownstream).Moreover,
throughsome combinationof oblique shock and differential Agudoetal. (2001) showed that the separation between the
flow resulting in a magnetic field approximately parallel to trailing components increases downstream due to their mo-
thejetaxisinthelaterepochs. Nostrongshockisneededat tiondownapressuregradient.
thislocation in the jetbut thisregionmaybe identifiedwith Allthisis in agreementwith whatwe observein thetrail-
therecollimationregion(seeSect.4.3)ataboutthesamepo- ing componentsof E1 and with our interpretation of an ex-
sition in the jet). In this picture, the jet-medium interaction pansion of the jet in a density decreasing ambient medium.
mayformaneffectivenozzlewhichacceleratesthejetonone Forthetimerangecoveredbytheirobservations(1998.23to
edgerelativetotheother. 2001.28),Jorstadetal.(2005)alsoidentifiedthetrailingphe-
An alternative explanation for the observed polarization nomenologyinthissource.
structure and dynamics of 3C111 can be found by consid- The north-south gradients detected in the linear-
TheTrailsofSuperluminalJetComponentsin3C111 9
polarization emission in the region where the trailing onthenatureandcharacteristicsoftheseinitialcomponents.
features are formed, is in agreement with an oblique shock Severalparsecsdownstream,thejetplasmaentersaregionof
structure. The steep brightness-temperaturegradients of the rapidlydecreasingexternalpressure,expandsintothejetam-
trailing components indicate that the particle and magnetic bient medium and accelerates. In the following, the plasma
field density associated with these components evolve in getsrecollimatedandtrailingfeaturesareformedinthewake
a different way compared to the “normal" jet flow. These oftheleadingcomponent.
shocked regions may be more overpressured with respect A particularly interesting aspect of the source 3C111
to their environment, making them expand rapidly. This in the light of this and other recent works is that it is
fast expansion implies a larger positive value of l, which, one of the very rare non-blazar gamma-ray bright AGN.
however, is compensated by an even larger (negative) value Besides CentaurusA (Sreekumaretal. 1999) and the pos-
of n and b(1- α) in equation 1, resulting in a very steep sible identification of NGC6251 with the EGRET source
brightnesstemperaturegradient(regimeII). 3EGJ1621+8203(Mukherjeeetal.2002),3C111istheonly
Pinching modes of the Kelvin-Helmholtz instability were AGN whose jet-system is inclinedat a relativelylargeangle
shown to couple to the trailing components observed in the tothelineofsightandthathasa reliableEGRETidentifica-
simulations in Agudoetal. (2001). In the case of compo- tion: Sgueraetal. (2005) reconsidered the possible identifi-
nentsE2-E4,the distancebetweenthemrangesfrom0.7-0.8 cation of the EGRET source 3EGJ0416+3650with 3C111,
masatthefirstepochsinwhichtheyareobserved,toalmost which was first suggested by Hartmanetal. (1999) but con-
2.0 mas in the latest epochs. Taking into account that: a) sidered unlikely because of the poor positional coincidence.
theirFWHMisofthesameorder(seeTable2);b)thatthese Very recently, R.C. Hartman & M. Kadler (in prep.) found
wavelengths have to be corrected for geometrical and rela- that 3EGJ0416+3650 is composed out of at least two dis-
tivistic effects, resulting in a maximum intrinsic wavelength tinctcomponents. Oneofthemisthedominantsourceabove
of∼0.7mas,andc)thatthesizeofthecomponentscanbeof 1GeVandisinexcellentpositionalagreementwiththeloca-
the order or smaller than the jet radius (Perucho&Lobanov tionof3C111.Comparedtoblazars,thelargeinclinationan-
2007),thisimpliescouplingofthepinchingtowavelengthsof gleandtherelativelysmalldistanceof3C111allowustore-
theorderorsmallerthanthejetradius. Peruchoetal.(2007) solvestructuresalongthejetthatareassmallasparsecsinde-
have shown that resonant Kelvin-Helmholtz instabilities as- projectionandwhichwouldbeheavilyblendedwithadjacent
sociated to high-order body modes appear in sheared jets at featuresinblazarjets. Asdemonstratedinthispaper,VLBA
thesewavelengths.Thesemodeshavelargergrowthratesthan observations of 3C111 probe a variety of physically differ-
low-order body modes or surface modes, and their growth ent regions in a relativistic extragalactic jet such as a com-
bringsthejettoafinalquasi-steadystateinwhichitremains pactcore,superluminaljetcomponents,recollimationshocks
well-collimatedandgeneratesahotshear-layerwhichshields andregionsofinteractionbetweenthejetanditssurrounding
thecoreofthejetfromtheambientmedium.Interestingly,the medium, which are all possible sites of gamma-ray produc-
jetin3C111isknowntobewell-collimateduptokiloparsec tion. From early 2008 on, the gamma-ray satellite GLAST
scales. Further research in this direction is needed in order (Lottetal. 2007) is goingto monitorthe sky. If detected by
tochecktheinfluenceoftheresonantmodesinthelongterm GLAST,3C111maybecomeakeysourceinthequestforan
evolutionofthisjet. understandingof the origin of gamma-raysfrom extragalac-
A by-productof the interpretationof these componentsas tic jets. In addition, the combinationof GLAST and VLBA
Kelvin-Helmholtzinstabilities is the fact that it allows us to data with spectral data at intermediate wavelengths(optical,
put constraints to the velocity of the jet. We can regard IR,X-ray)mayallowabetterdeterminationofjetparameters
the wave speed as the minimum speed of the jet flow, as andrelativisticbeamingeffectsthanin mostblazarsbecause
KH modes have an upper limit in their wave speeds that is ofthehigherlinearresolutionofferedbythisnearbyandonly
precisely the velocity of the flow in which they propagate weaklyprojectedjetsystem.
(Peruchoetal.2006).Theupperlimitisgivenbythespeedof Ourobservationsof3C111arequalitativelyinremarkable
E1,interpretedasashockwave,thathastobethusfasterthan agreement with numerical relativistic hydrodynamic struc-
theunderlyingflow. Inthispicture,wewouldhavethestruc- tural and emission simulations of jets such as the ones pre-
tureE1movingwithLorentzfactorγ∼8.3throughajetwith sented by Agudoetal. (2001) and Aloyetal. (2003). Fur-
Lorentzfactor8.3>γ ≥4.6in theacceleratedregion(post ther progress is being made in the transition from two-
j
1999.38),wherethelowerlimitisgivenbytheLorentzfactor dimensional to three-dimensional simulations of relativistic
ofcomponentE2,thefastestofthethreetrailingcomponents jets and in the development of new methods considering
identifiedhere. magnetic fields (Leismannetal. 2005; Mizunoetal. 2007;
Roca-Sogorbetal.2008,e.g.,),theequationofstateforrela-
5. SUMMARYANDCONCLUSIONS tivisticgases(Perucho&Martí2007),andradiativeprocesses
Inthispaper,wehaveinvestigatedtheparsec-scalejetkine- (e.g., Mimicaetal. 2004, 2007, and Mimica et al. in prepa-
maticsandtheinteractionofthejetwithitsambientmedium ration). Butsofarneitherobservationaldatanorsimulations
in the broad-line radio galaxy 3C111. Our analysis has havereachedanadequatelevelofdetailandcompletenessin
demonstratedthatavarietyofprocessesinfluencethejetdy- orderto allow usa quantitativedirectcomparisonof numer-
namics in this source: a plasma injection into the jet beam icalmodelsand observedrelativistic jet structure andevolu-
associatedwithamajorflux-densityoutburstleadstothefor- tion. Inparticular,itisnotfeasibletodaytofititerativelythe
mationofmultipleshocksthattravelatdifferentspeedsdown- parametersofrelativisticmagneto-hydro-dynamical(RMHD)
stream and interact with each other and with the ambient jetsimulationsto match the brightnessdistributionobserved
medium. The primary perturbation causes the formation of for any individual source. The main reasons for this are a)
a forward and a backward shock (or rarefaction). The latter the immense computationalpower required to conducta re-
fadesawaysofastthatislikelytoremainundetectedinminor alistic (i.e., sufficiently detailed) modern 3D jet simulation
ejections. A separate work by Peruchoetal. (2007) focuses and b) the highly non-linear nature of RMHD plasmas and
10 Kadleretal.
their evolution. Simulation results depend critically on the ported under National Science Foundation grant 0406923-
starting conditions like the exact velocity, composition, and AST. The Very Long Baseline Array is operated by the Na-
transversalstructureoftheflow,thestructureandstrengthof tional Radio Astronomy Observatory, a facility of the Na-
the magnetic field and the jet environment. Future develop- tionalScienceFoundationoperatedundercooperativeagree-
mentofcomputationalpowerwillallowustouselargerres- ment by Associated Universities, Inc. UMRAO is partially
olutionsto decrease the numericalviscosities, and to imple- supportedby a series of grants from the NSF, most recently
mentnonlinearandmicrophysicsprocessesintosimulations. AST-0607523, and by funds from the University of Michi-
VLBA observations are capable of putting hard quantitative gan. MK has been supported in part by a Fellowship of the
constraintsontheinputparametersforRMHDjetsimulations InternationalMaxPlanckResearchSchoolforRadioandIn-
if they are densely sampled over several years. Polarimet- fraredAstronomyandinpartbyanappointmenttotheNASA
ric observationsat multiple radio frequenciesmay allow the PostdoctoralProgramattheGoddardSpaceFlightCenter,ad-
effects of jet-intrinsic magnetic-field variations and external ministered by Oak Ridge Associated Universities through a
Faraday-screen inhomogenities or temporal variations to be contract with NASA. MP acknowledges support in part by
disentangled.Suchdataat15GHzareontheway,e.g.,aspart theSpanishDirecciónGeneraldeEnseñanzaSuperiorunder
ofthenextphaseoftheMOJAVEprogram,inwhichrapidly grantAYA2004-08067-C03-01and in partby a postdoctoral
evolving sources like 3C111 are being observed every two fellowshipof the GeneralitatValenciana(BecaPostdoctoral
months. d’Excel·lència). YYK is a Research Fellow of the Alexan-
der von HumboldtFoundationand was supported in part by
the Russian Foundation for Basic Research (project 05-02-
We would like to acknowledge the support of the rest of
17377).DCHwassupportedbygrantsfromResearchCorpo-
the MOJAVE Team, who have contributed to the data used
ration and the National Science Foundation (AST-0707693)
in this paper, in particular we would like to thank Christian
IA has been supported in part by an I3P contract with the
Frommforhishelpwiththeproductionofthefiguresforthe
SpanishConsejoSuperiordeInvestigacionesCientíficasand
paper. We thankDharamVir Lal and Silke Britzen for their
in part by a contract with the German Max-Planck-Institut
careful reading of the manuscript and their comments. We
für Radioastronomie (through the ENIGMA network, con-
also thank the referee for his very constructive suggestions,
tractHPRN-CT-2002-00321),whichwerepartiallyfundedby
which have helped to improve this paper. We are grateful
theEU.
to Greg Taylor, who provided complementary 2cm VLBA
data for an additional epoch. The MOJAVE project is sup-
APPENDIX
A. THE JET INCLINATIONANGLE
The1996radiooutburstof3C111putsstrongconstraintsontheangletothelineofsightforthissource,ifoneassumesthata
similarlybrightcomponentasEhasbeenejectedinthecounterjet,aswell. DuetodifferentialDopplerboosting,thefluxdensity
ratiobetweenthejet-andcounter-jetemissionis
S 1+βcosθ 2- α
J = . (A1)
S (cid:18)1- βcosθ(cid:19)
CJ
Thus,foragivenjettocounter-jetratiox= SJ
SCJ x- 1
βcosθ= . (A2)
x+1
Withα=0.3,S =3.4Jy(componentsEandFin1997.19),S <10mJyandβ<1,θ<21◦. Forarealisticjetspeedof,e.g.,
J CJ
β=0.956(γ=3.4),theangletothelineofsightis: θ=19◦. Anestimateclosetothisvaluecanbederivedfromthevariability
Doppler factor measured by Lähteenmäki&Valtaoja (1999) and the apparentsuperluminaljet speed. As outlined in detail in
Cohenetal.(2007),thisleadstoavalueofθ∼15◦.
Itisimportanttonotethatthiscalculationimplicitlyassumessymmetrybetweenthejetandcounter-jet,whichinprojection
does not have to be the case if the counter-jet is covered by an obscuring torus as it is well-established for systems at larger
inclinationangles(e.g.,NGC1052:see Kadleretal.2004). Indeed,Faradayrotationmeasurementstowardsthe3C111pc-scale
jet (Zavala&Taylor 2002; see also Sect. 4.4) and X-ray spectral observations(Lewisetal. 2005) suggest substantialamounts
ofobscuringmaterial. Free-freeabsorptioncouldalsosubstantiallylowerthecounter-jetradioemissionandallowforlargerjet
anglestothelineofsight.
Anindependentlowerlimitontheinclinationangleofθ>21◦wasgivenbyLewisetal.(2005)assumingthatthedeprojected
size of the largescale 3C111 double-lobestructure is smaller than 500h- 1kpc. This discrepancy implies that either 3C111 is
unusuallylargeorthereisamisalignementbetweenthelarge-scalejet-axisandtheparsec-scalejetaxisinclinationtothelineof
sight,althoughtheprojectedpositionanglesofthelarge-scalejet(63◦)andtheparsec-scalejet(∼65◦)arealmostthesame.