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Introduction Active Galactic Nuclei Lecture -6- Continuum Emission

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Introduction Active Galactic Nuclei Lecture -6- Continuum Emission Introduction Active Galactic Nuclei Lecture -6- Continuum Emission This Lecture Give a general overview of the continuum emission from AGN. Read Chapt.4 of Peterson Observing the SED of AGN Types of Continuum Spectra • Blazars: Non-thermal emission from radio to gamma-rays (2 components) • Seyferts, QSOs, BLRGs: IR and UV bumps (thermal) radio, X-rays (non-thermal) Spectral Energy Distributions (SEDs): plots of power per decade versus frequency (log-log) Continuum Emission in AGN • UV-Optical Continuum • Infrared Continuum • High Energy Continuum • Radio Continuum - Jets and superluminal motion Spectral Energy Distribution of Seyferts, QSOs, BLRGs Radio Quiet Quasars Radio-Loud Quasars IR bump Big Blue Bump Sub-mm break Radio 1μ minimum Soft X-ray Excess Spectral Energy Distribution of Blazars Red blazars: 3C279 Blue blazars: PKS 2155-398 Wehrle et al. 1999 Bertone et al. 2001 AGN: Spectral Energy Distribution Many different types of AGN SEDs The 3000Å Bump Low-energy Comptonized disk tail disk Balmer cont, high-energy ) ν FeII lines disk tail F ν ( g o L optical UV EUV soft X-rays X-rays 14 15 16 17 18 Log ν The Blue and IR bumps • LIR contains up to 1/3 of Lbol LBBB contains a significant fraction of Lbol • IR bump due to dust reradiation, BBB due to blackbody from an accretion disk •The 3000 A bump in 4000-1800 A: • Balmer Continuum • Blended Balmer lines • Forest of FeII line s AGN: (Non-)Thermal Emission Fundamental Questions: (1) How much and which part of AGN SED is thermal and non-thermal? Thermal: Particles have Maxwellian velocity distribution due to collisions Non-Thermal: e.g. Synchrotron radiation with power-law energy distribution of particles (2) How much emission is primary and secondary ? From Central Engine Re-radiation UV-Optical Continuum “Big Blue Bump” is assumed to be thermal emission from the accretion disk with T=105±1 K (100Å) What emission is expected from an accretion disk? Assumptions: ● Locally the disk emits like a Black Body ● Geometrically/optically thin/thick disk UV-Optical Continuum Gravitational potential energy is released half (virial theorem) into kinetic energy and half in to radiation: ˙ G M M 2 4 L= =2 r T 2r From which is follows: G M M˙ 1/4 T= 4 r3 (this is an approximation, averaged over the disk) UV-Optical Continuum In reality, energy is dissipated locally in the disk through viscosity. This yields: ˙ 1/4 3G M M 1/2 T r= {1−Ri /r } [ 8 r3 ] or R 1/ 4 −3/4 3G M M˙ r T r= [ 3 ] R 8 Rs s when r>>R and R ≈ R i i s UV-Optical Continuum Inserting R = 2GM/c2, we find: s 1/ 4 −3/ 4 5 M˙ −1/4 r T r=6×10 M K 8 M E RS This peaks at ~100Å for this typical temperature of a million K. Hence the acretion disk continuum spectrum is a superposition of many BB's with different temperatures. UV-Optical Continuum When we assume that the disk is optically thick (hence the luminosity does not depend on the surface mass density of the disk), then: dLr=2 r cosi Bdr is the luminosity from a bin of width dr. 2 3 R 4 h cosi o r dr L = ∫ 2 R c i exph/k T r−1 This does not have a simple so lution, but ... UV-Optical Continuum At low frequency all BBs emit like a Wien spectrum, hence: 2 L ∝ (long wavelengths) At high frequencies, the spectrum has an expontial cutoff, determined by the highest temperature (at R ) i 3 L ∝ exp−h/k T Ri (short wav elengths) UV-Optical Continuum In the intermediate regime, if we define: h h 3/4 x= = r /RS k T r k T S Substituting this back into the previous equation, we find: 1/3 L ∝ (intermediate wavelengths) UV-Optical Continuum The superposition of these BB spectra will thus look like: Total disk spectrum ν F ν g o L Annular BB emission Log ν Observations of Optical-to-UV Continuum • After removing the small blue bump, the observed continuum goes as ν-0.3 • Removing the extrapolation of the IR power law gives ν-1/3 - but is the IR really described by a power law? • More complex models predict Polarization and Lyman edge – neither convincingly observed Disk interpretatio n is controversial! Variability More observational clues come from variability: ● UV & Optical vary in phase ● Variation are larger at higher frequencies ● Small variation in UV are smoothed out at lower freq. ● Most variability at longer time-scales [P(f)∝ 1/f2-3] Simultaneous UV & Optical variability is a major problem for models with the T-gradient outward! Alternative interpretation? ● Optical-UV could be due to Free-free (bremsstrahlung) emission from many small clouds in a optically thin disk (Barvainis 1993) ● Slope consistent with observed (α~0.3), low polarization and weak Lyman edge predicted ● Requires high T~106 K Disadvantage: low efficiency! Infrared Continuum ● In most radio-quiet AGN, there is evidence that the IR emission is thermal and due to heated dust ● However, in some radio-loud AGN and blazars the IR emission is non-thermal and due to synchrotron emission from a jet. Infrared Continuum: Evidence Obscuration : ● Many IR-bright AGN are obscured (UV and optical radiation is strongly attenuated) ● IR excess is due to re-radiation by dust Infrared Continuum: Evidence IR continuum variability : • IR continuum shows same variations as UV/optical but with significant delay • Variations arise as dust emissivity changes in response to changes of UV/optical that heats it Dust Reverberation • Optical varied by factor ~20 • IR variations follow by ~1 year • IR time delays increased with increasing wavelength Evidence for dust(torus) a light year from the AGN nucleus, with decreasing T as function of radius Emerging picture ● The 2μ-1mm region is dominated Radio Quiet Quasars by thermal emission from dust (except in blazars and some IR bump Big Blue Bump other radio-loud AGN) ● Different regions of the IR come Sub-mm break from different distances because 1μ minimum of the radial dependence of temperature Soft X-ray Excess ● 1μ minimum: hottest dust has T~2000 K (sublimation T) and is at ~0.1 pc (generic feature of AGN) X-ray emission ● AGN are easy to find in X-rays. Away from the Galactic plane most X-ray sources are AGN. Many X-ray selected AGN show weak or no optical signatures. ● X-rays come from very close to the SMBH. The most rapid variability is seen in X-rays. ● The only spectral lines observed that come from close to the MBH are in the X-ray band. The strongest line is from Fe at ~6.4 keV but other lines have been observed. ● All types of AGN are strong X-ray sources. ● We can “X-ray” the material around AGN using the emission from close to the MBH as a background source. X-ray emission: Origin ● Accretion flow surrounded by dusty torus ● BB radiation from disk -> ‘big blue bump’ ● B-field loops -> optically thin corona ● Isotropic X-rays from Comptonization of disk photons in hot corona ● Power law spectrum Reflection and Fluorescence The MBH is surrounded by an accretion disk. Suppose that X-rays are generated above the disk: ● We observe some photons directly. ● Others hit the accretion disk. Some are reflected. Some eject an inner shell electron from an atom to give fluorescent line emission. NGC 4945 direct fluorescence reflected Madejski et al. 2000 Reflected X-ray Spectra Astrophysical Jets in Radio- Loud AGN Chandra commonly resolves kpc-scale X-ray jet emission in nearby RL AGN: • FRIs -> kpc X-ray emission synchrotron in nature (e.g. Hardcastle et al. 2001, 2003, 2005) • FRIIs -> X-ray emission tends to be inverse-Compton • What about (unresolved) parsec-scale X-ray jets? Radio-Galaxy Nuclei – Two Competing Models ● Is nuclear X-ray emission dominated by: ● The parsec-scale jet? -or- ● The accretion flow? Evidence for jet-dominated nuclear X-ray emission ● Correlations between the ROSAT soft X-ray and VLA radio core fluxes ● Parsec-scale radio emission is jet-generated and strongly affected by beaming ●Tight correlations suggest X-ray emission NGC 6251 - 5 GHz VLBI affected by beaming in same manner as radio ● Soft X-ray emission originates in a jet ● Double-peaked SED (modeled with syn+SSC) 10 pc Jones et al. (1986) Evidence for accretion-dominated nuclear X-ray emission ● Short (~ks) timescale variability in broad-line FRII 3C 390.3 (Gliozzi et al. 2005) ● Broadened Fe K -> line emission in narrow-line FRI NGC 6251 (Gliozzi et al. 2004) ● Implies Fe K -> origin in inner NGC 6251 - XMM Gliozzi et al. (2004) regions of accretion flow Radio-Galaxy Nuclei – Two Competing Models ● X-ray continuum emission in the nuclei of RL AGN consists of: – “Radio-quiet” accretion-related component – “Radio-loud” jet-related component ROSAT Chandra/XMM X-ray emission: some conclusions ● X-ray emission of FRI radio-galaxy nuclei is unabsorbed and dominated by a parsec-scale jet ● X-ray emission of FRII radio-galaxy nuclei is heavily absorbed and accretion-related ● Each FRII also has an unabsorbed component of X- ray emission -> jet origin ● Data do not exclude the presence of a heavily obscured, accretion-related emission in FRI-type source Summary ● Blazar emision is mostly dominated by relativistic jet emission, where the other AGN-types show a mixture of accretion-disk/torus/ hot-corona & jet emission ● Big Blue Bump (UV/Optical) comes (most likely) from thermal emission from the accretion disk ● The IR bump is probably re-radiated emission from the dust torus ● X-ray emission is a mixture of upscattered photons (inverse Compton) from the accretion disk through hot-corona electrons (also direct emission and fluorescence).
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