Jets and Radio Loud AGN

33 Pages·2010·11.57 MB·English

ACMAVDLELIMGIAISEFERAIDNEIRRIDCONAXALE 10–1
Jets and Radio Loud AGN
M87:ImageCredit&Copyright:AdamBlock,Mt.LemmonSkyCenter,U.Arizona
ACMAVDLELIMGIAISEFERAIDNEIRRIDCONAXALE 10–7
Outline
Jetsarebroadbandemitters! Firstconsidertheradioband. Thenmoveonto
higherenergies.
Mostimportantjetemissionprocessintheradioband: synchrotronradiation.
Synchrotron-Radiation(=Magnetobremsstrahlung): Radiationemittedby
relativisticelectronsinamagneticfield.
Outlineforthefollowingdiscussionoftheoryofsynchrotron-radiation: Shortand
qualitativedescription. SeeRybicki&Lightman(1979,Chapters3,6,and7).
1.Motionofelectronsinmagneticfields,
2.Lookatemissionfromasingleelectron,
3.Considerelectrondistributionandopacityeffectstoobtainthefinalspectrum.
4.Considerprocessestotransfertheprimarysynchrotronemissiontothehigh-
Credit:X-ray:NASA/CXC/MIT/H.Marshalletal.Radio:F.Zhou,F.Owen(NRAO),J.Biretta(STScI) estenergies(ifneeded).
Optical:NASA/STScI/UMBC/E.Perlmanetal.
SynchrotronRadiation 1
ACMAVDLELIMGIAISEFERAIDNEIRRIDCONAXALE 10–6 ACMAVDLELIMGIAISEFERAIDNEIRRIDCONAXALE 10–8
AGN Jets Relativistic Motion
Movingelectroninmagneticfield(E=0):InGaussianunits,theLorentz-Forceis
dp e m v
= v B where p= e =γm v (10.1)
dt c × 1 β2 e
−
where p
1 v
γ= andwhere β= (10.2)
1 β2 c
−
Thereforetheaccelerationis
p dv e
= v B (10.3)
dt cγm ×
e
Sincev BisalwaysperpendiculartovandB,thecomponentofvalongtheB-fielddoesnot
×
change.ThisconstantperpendicularforceresultstoahelicalmotionaroundtheB-fieldlinewith
thefrequency
eB ω
ωB=γm c= γL (10.4)
e
wheretheLarmorfrequency(alsoCyclotronfrequency,gyrofrequency)
(M87;Perlmanetal.,2002) eB
ω =2πν = (10.5)
L L m c
Spectralshapeofjetemissionisapowerlaw= synchrotronradiation e
⇒
Typicalpowerlawindex:α 0.65betweenradioandoptical.
∼
Introduction 5 SynchrotronRadiation 2
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Numerical values Radiated Energy, II
Numerically,theLarmorfrequencyis
νL =2.8B1GMHz (10.6) ∆θ
r
Theradiusoftheorbit(Larmorradius)is
1/γ
1
γv E B −
RL = ω⊥ ∼2AU·1GeV· 10 6G (10.7) Observer
L (cid:18) − (cid:19) 1 2
1
E B −
300km (10.8)
∼ ·1GeV· 1G
(cid:18) (cid:19)
i.e.,smalloncosmicalscales
(afterFig.6.2ofRybicki&Lightman,1979)
Unitsandordersofmagnitude:
Relativisticelectrons:radiationisforwardbeamedintoconewithopeningangle∆θ 1/γ.Inthe
••1thGet=yp1ic0a−l4BT-,fieldintheinterstellarmediumis∼10−6G, Electronframeofrest:beampassesobserverduringtime ∼
•closetothecentersofAGNB∼1G. ∆t=∆θ =mecγ 2 = 2 (10.12)
ω eB γ ω
B L
SynchrotronRadiation 3 SynchrotronRadiation 5
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Radiated Energy, I Radiated Energy, III
MotionaroundB-fieldlines:acceleration.
Butacceleratedchargesemitradiation(Larmor’sformula):
dW q2v˙2 2q2v˙2 ∆θ
P = dt =4πc3 sin2θdΩ= 3c3 (10.9) r
Z
Assumptionofisotropicvelocitydistribution,relativisticelectrons(β 1),andamessyderiva-
−→ 1/γ
tion(seeRybicki&Lightman)yieldsfortheaverageemittedpowerofanelectroninaB-field
4
hPemi=3β2γ2cσTUB (10.10) 1 2 Observer
withUB = B2/8π,themagneticfieldenergydensity,andσT = 8πe2/(3m2ec4),theThomson
crosssection.
Note:SinceE=γmec2=⇒P ∝E2UB.
Note:Pem ∝ σT ∝ m−e2=⇒Synchrotronradiationfromchargedparticleswithlargermass(pro- (afterFig.6.2ofRybicki&Lightman,1979)
tons,...)isnegligible. Observerframe:Dopplereffect!(electronisclosertousatendoftimeinterval)
Note:Life-timeofparticlesofenergyEis
= observedpulseduration:
⇒
t1/2∼EP ∝B12E =5s(cid:18)1BT(cid:19)−2γ−1=1.6×107years(cid:18)10B−7T(cid:19)−2γ−1 (10.11) τ =(cid:18)1−vc(cid:19)∆t=(1−β)∆t (10.13)
SynchrotronRadiation 4 SynchrotronRadiation 6
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Radiated Energy, IV Nonthermal Synchrotron Radiation, II
Forγ 1,i.e.,β =v/c 1
≫ ∼
1 v2
=1 =(1+β)(1 β) 2(1 β) (10.14) power−law superposition Assumethatphotonsareonly
γ2 − c2 − ≈ −
emittedatthecharacteristicfre-
suchthat τ =(1−β)∆t= 12 1−vc22 ∆t= γ21ω (10.15) log flux iesnpldeeicvctirtdroaunal qthuiseinscaygoγo2dνLap(pEroqx.im10at.i1on6)s.incethe
L
(cid:18) (cid:19) spectrumemittedbyanelectronhasa
strongpeakatthatfrequency
Thusthecharacteristicfrequencyoftheradiationisgivenby Therefore
log frequency
ω =γ2ω = eB E 2 (10.16) after(Shu,1991,Fig.18.4) φν(γ)∼δ(ν−γ2νL) (10.21)
c L m c m c2
e (cid:18) e (cid:19)
Thereforetheemittedpoweratfrequencyν (=spectrum)is
Shortgyrationpulses= broadspectrum(Heisenberg: ∆ω∆t > 1)withthe
⇒
highestfrequencyintheregimeofν =ω /2π. ∞
c c P = P (γ) n(γ)dγ (10.22)
ν ν
h i
Z1
SynchrotronRadiation 7 SynchrotronRadiation 9
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Nonthermal Synchrotron Radiation, I Nonthermal Synchrotron Radiation, III
Foranelectrondistribution,n(γ),theemittedspectrumisfoundbyproperlyweightingcontribu- Therefore,fortheelectronpower-lawdistributionEq.10.18
tionsofelectronswithdifferentenergies: 4
Pν= ∞Pν(γ)n(γ)dγ (10.17) Pν=Z1∞3β2γ2cσTUBδ(ν−γ2νL)n0γ−pdγ (10.23)
Z1 sinceγ≫1:β≈1
Mostimportantcase:nonthermalsynchrotronradiation,whereelectronshaveapower-lawdistri-
bution n(γ)dγ=n0γ−pdγ . (10.18) substitutingν′=γ2νL,i.e.,dν′==AνLZ21γ∞dγγ2−pδ(ν−γ2νL)dγ (10.24)
ThespectralenergydistributionPνofanelectronwithtotalenergyE=γmec2canbewrittenas =BZνL∞γ1−pδ(ν−ν′)dν′ (10.25)
4
Pν(γ)=3β2γ2cσTUBφν(γ) (10.19) sinceγ=(ν′/νL)1/2,wefind
wherethespectralshapeisdescribedbyafunctionφν(γ)with P =2cσ n UB ν −p−21 (10.26)
ν 3 T 0ν ν
φν(γ)dγ=1 . (10.20) L (cid:18) L(cid:19)
Z
Thespectrumofanelectronpower-lawdistributionisapower-law!
SynchrotronRadiation 8 SynchrotronRadiation 10
ACMAVDLELIMGIAISEFERAIDNEIRRIDCONAXALE 10–16 ACMAVDLELIMGIAISEFERAIDNEIRRIDCONAXALE 10–18
Summary of Synchrotron Radiation Process Polarization of Synchrotron Radiation
Whatwehavedonesofar:
1.Motionoftheelectron
2.Radiationcharacteristicfromrelativisticmotion
3.Doppler-effect
4.Integrationoverelectrondistribution
Itispossibletodothesameanalyticallywithoutanyapproximations. Thisistoo
complicatedtobedonehere. Seethereferencesfordetails.
(Rybicki&Lightman,1979,Fig.6.7)
Exactcalculationneedstotakeintoaccountpolarizationofsynchrotronradia-
tion.
SynchrotronRadiation 11 SynchrotronRadiation 13
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Synchrotron Self-Absorption Polarization of Synchrotron Radiation
Resultofexactcalculationforbothpolarizationdirections:
afterShu(1991,Fig.18.6) Atlowν:synchrotronemitting P √3e3B F(ν/ν ) G(ν/ν )
electronscanabsorbsynchrotron k = c − c (10.27)
P 2 mc2 F(ν/ν )+G(ν/ν )
photons: c c
ν−(p−1)/2 (cid:18) ⊥(cid:19) (cid:18) (cid:19)
x synchrotronself-absorption. where
u
log fl ν5/2 F(x)=x ∞K5/3(y)dy (10.28)
Zx
G(x)=xK (x) (10.29)
2/3
andK aremodifiedBessel-functionsofi-thorder
log frequency i
Forapowerlawelectrondistribution∝E−p,totalspectralshapeis: Polarizationallowstomeasurethemagneticfielddirection
Forlowfrequencies:Pν∝B−1/2ν5/2 (independentofp!)
Forlargefrequencies:Pν∝ν−(p−1)/2
Oneoftenusesthetermsopticallythick/thintodescribetheabsorbed/unabsorbedpartofasynchrotron
spectrum.Theturnoverdescribestheτ = 1surface,e.g.,ofajet.Ingeneral:τ R(R:sizeoftheemit-
∝
tingregion).Morecompactregionsareopticallythick,moreextendedregionsareopticallythin.
SynchrotronRadiation 12 SynchrotronRadiation 14
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10–19 Polarization of Synchrotron Radiation
Thedegreeofpolarizationisdefinedby
P P
degreeofpolarization:= ⊥− k (10.34)
P +P
⊥ k
Forapowerlawelectrondistribution:
P P J p+1
⊥− k = G = (10.35)
P +P J p+7/3
F
⊥ k
Forp=2.5thedegreeofpolarizationis 70%. Thisisverylarge!!
∼
Caveat:Faraday-rotationandB-fieldinhomogeneitiescandecreasethedegreeofpolarization.
SynchrotronRadiation 16
ACMAVDLELIMGIAISEFERAIDNEIRRIDCONAXALE 10–20 ACMAVDLELIMGIAISEFERAIDNEIRRIDCONAXALE 10–22
Polarization of Synchrotron Radiation Introduction, I
Thetotalemittedpowerformonoenergeticelectronsis ThepowerfulradiogalaxyCygnusAatz =0.057(d=230Mpc).
P(ν)=P (ν)+P (ν) F(ν) (10.30)
k ⊥ ∝
Asbefore,thetotalemittedspectrumisfoundbyintegratingovertheelectron
energydistribution. Forapower-law:
P (ν) √3 e3B JF JG 2ν −(p−1)/2
Pk(ν) = 2 n0m c2 J −+J 3ν (10.31)
(cid:18) ⊥ (cid:19) (cid:18) (cid:19) e (cid:18) F G(cid:19)(cid:18) L(cid:19)
where
2(p+1)/2 p 19 p 19
J = Γ + Γ (10.32)
F
p+1 4 12 4−12
(cid:16)p 7 (cid:17) (cid:16)p 1 (cid:17)
JG =2(p−3)/2Γ 4+12 Γ 4−12 (10.33) Radio(VLA:6cm);Credit:NRAO/AUI
(cid:16) (cid:17) (cid:16) (cid:17) Size: 2.2arcmin 600000ly,abouteighttimesthesizeofthemilkyway!
Γ(x)= 0∞t−xe−tdtistheGamma-function. ∼ Radiom∼orphology: Core–Jets–Hotspots–Lobes
R
SynchrotronRadiation 15 Radio-LoudAGN:Classification 1
ACMAVDLELIMGIAISEFERAIDNEIRRIDCONAXALE 10–22 ACMAVDLELIMGIAISEFERAIDNEIRRIDCONAXALE 10–24
Introduction, II Classification, I
ThepowerfulradiogalaxyCygnusAatz =0.057(d=230Mpc). Atarcsecresolution,mostradio-loudAGNare
unresolved! Butgreatvarietyinspectralshape.
Condonetal.(1998);Kuehretal.(1981)
X-Ray(Chandra:0.2keV–8keV);Credit:NASA/UMD/A.Wilsonetal.
Size: 2.2arcmin 600000ly,abouteighttimesthesizeofthemilkyway!
∼ ∼
Radiomorphology: Core–Jets–Hotspots–Lobes
X-Raymorphology: Nucleus–Cavity–Hotspots
Radio-LoudAGN:Classification 2 Radio-LoudAGN:Classification 4
ACMAVDLELIMGIAISEFERAIDNEIRRIDCONAXALE 10–25
Classification, II
NGC1275:extendedsteep-spectrumemissionpluscompactself-absorbednucleus
•
3C123:3C123:opticallythinatallplottedfrequencies
•
3C48:self-absorbedbelow100MHz
•
3C454.3:superpositionofmanyjetregionswhichbecomeopaqueatdifferentfrequencies
•
(flat-spectrumradioquasar)
FornaxA:Radio(VLA)overlaidonoptical(STScI/POSS-II);Credit:NRAO/AUIandJ.M.Uson Radio-LoudAGN:Classification 5
ACMAVDLELIMGIAISEFERAIDNEIRRIDCONAXALE 10–26
Classification, III
Classificationbasedonmorphologyandradiospectrum:
1.Powerfuldouble-lobedradiogalaxieswithhotspotsandasteepradiospec-
trumfallingtowardhigherfrequencies(Fanaroff-RileyclassII,FRII)
2.Weakersteep-spectrum,double-lobedradiogalaxieswithoutleading
hotspots(FRItypes)
3.Core-dominatedflat-spectrumsources(Blazars: quasarsandBLLacobjects)
4.Compactsteep-spectrumsources(CSSsources)andgigahertz-peaked
spectrumsources(GPSsources);nolarge-scaleradiostructure;morpholog-
icalclassificationterm: compactsymmetricobjects(CSOs)orcompactdou-
bles
Observingtechniqueandfrequencystronglyaffectssamplecomposition(e.g.,low-frequencyflux-
densitylimitedsurveystendtoselectsteep-spectrumsources.Flat-spectrumsourcesareclas-
sicaltargetsforVery-Long-BaselineInterferometry(VLBI)observations,whicharesensitiveto
compactemission.
Radio-LoudAGN:Classification 6
Fanaroff-RileyType1: asymmetric
jetswithwideopeningangleendingin
plumes
M84(3C272.1)(Laing&Bridle,1987):
VLA4885MHz,134′′ × 170′′;seealso
www.jb.man.ac.uk/atlas/other/3C272P1.html A.Bridle,www.cv.nrao.edu/~abridle/images.htm
Fanaroff-RileyType2: powerfullobedominateddoubles;jetsoftenone-sided
ACMAVDLELIMGIAISEFERAIDNEIRRIDCONAXALE 10–30 RadioInterferometry: Longerbaselinesandhigherfrequenciesyieldhigherresolution
Classification, VII
polarizationintwo-sidedjet
sources(FR1): upto40%
SynchrotronRadiation
⇒
B-fieldorientation:
closetocore: B jetaxis
• k
awayfromcore( 10%jet
• ∼
length): B jetaxis
⊥
B-fieldcanchangeorientationagainin
knots
(B-fieldconfigurationinIC4296;
Killeen,Bicknell&Ekers,1986,
Fig.25b)
Radio-LoudAGN:Classification 10 ImagecourtesyofMPIfR,NRAO/AUIandEarthimagecourtesyoftheSeaWiFSProjectNASA/GSFCandORBIMAGE
ACMAVDLELIMGIAISEFERAIDNEIRRIDCONAXALE 10–33
Multifrequency VLBI Observations, I
Athigherfrequencies
1.theangularresolutionimproves
2.thestructurechanges: differentparts
ofthejetdominatetheemissionat
differentfrequencies(superposition
toaflatspectrum)
polarizationinone-sidedjetsources(FR2): similar 3.emissionshowsupinthecentral
toFR1,i.e.,40%andhigher emissiongap;spectralindexα >
2.5 noselfabsorption
⇒
B-fieldorientationinFR2: paralleltojetaxis
4.theabsorptioniscausedbyfree-
throughoutthejet
freeabsorptioninthecircumnuclear
torus;athighfrequencies,thetorus
TheTwin-JetinNGC1052observedwiththeVLBA
becomestransparent
(E-fieldconfigurationinNGC6251,note:B-fieldisperpendicularto
E-field!;Perley,Bridle&Willis,1984,Fig.17) at4frequencies;Image:M.Kadler Kamenoetal.(2001);Kadleretal.(2004)
Radio-LoudAGN:Classification 13
ACMAVDLELIMGIAISEFERAIDNEIRRIDCONAXALE 10–34 ACMAVDLELIMGIAISEFERAIDNEIRRIDCONAXALE 10–36
Multifrequency VLBI Observations, II Flat-Spectrum Radio Sources: Blazars
Movie: SpinningthedialonNGC1052
Almostallthefluxdensityisconcentrated
withinafewmilliarcseconds-sizecompact
jet!
ImageCourtesy:MOJAVE
Imagecourtesy:U.Bach,MPIfR
“Roughlyequalnumbersofsteep-spectrumex-
0716+714,BLLacobjectatredshiftz = 0.3
tendeddouble-lobedsourcesandflat-spectrum
(Nilssonetal.,2008)
Highlyvariable,coredominatedobject objectsthatareunresolvedonarcsecscales.”
“Fried-egg”morphology–reallytheend-on (Zensus,1997)
http://www.sternwarte.uni-erlangen.de/~kadler/movies/zoom_1052.avi viewofaradiolobe?
Radio-LoudAGN:Classification 14 Radio-LoudAGN:Classification 16
ACMAVDLELIMGIAISEFERAIDNEIRRIDCONAXALE 10–37
Superluminal Motion, I
VLBIresolution1mas
3C111: Apparentspeedofjet: 5c
∼
Superluminalmotion: Theappar-
entvelocitiesofjetfeatures(“blobs”)
measuredinmanyAGNjetsoften
exceedthespeedoflight.
Firstdiscoveredin1971in3C279
(Cohenetal.,1971;Whitneyetal.,1971).
Kadleretal.(2008) VLAresolution 1arcsec
∼
JetPropagation 1