TEMA 6. Continuum Emission AGN Dr. Juan Pablo Torres-Papaqui Departamento de Astronomía Universidad de Guanajuato DA-UG (México) papaqui@astro.ugto.mx División de Ciencias Naturales y Exactas, Campus Guanajuato, Sede Valenciana Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 1 / 48
Give a general overview of the continuum emission from AGN. Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 2 / 48
Observing the SED of AGN Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 3 / 48
Types of Continuum Spectra 1) Blazars: Non-thermal emission from radio to gamma-rays (2 components) 2) Seyferts, QSOs, BLRGs: IR and UV bumps (thermal) radio, X-rays (non-thermal) Spectral Energy Distributions (SEDs): plots of power versus frequency (log-log) Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 4 / 48
Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum Jets superluminal motion Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 5 / 48
Spectral Energy Distribution of Seyferts, QSOs, BLRGs Radio Quiet Quasars Radio Loud Quasars Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 6 / 48
Spectral Energy Distribution of Seyferts, QSOs, BLRGs Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 7 / 48
Spectral Energy Distribution of Seyferts, QSOs, BLRGs Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 8 / 48
Spectral Energy Distribution of Blazars Red blazars: 3C279 Blue blazars: PKS 2155-398 Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 9 / 48
AGN: Spectral Energy Distribution Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 10 / 48
AGN: Spectral Energy Distribution Many different types of AGN SEDs Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 11 / 48
AGN: Spectral Energy Distribution Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 12 / 48
The Blue and IR bumps L IR contains up to 1/3 of L bol L BBB contains a significant fraction of L bol (Big Blue Bump) IR bump due to dust reradiation, BBB due to blackbody from an accretion disk The 3000Å bump in 1800-4000Å: Balmer Continuum Blended Balmer lines Forest of FeII lines Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 13 / 48
The 3000Å Bump Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 14 / 48
AGN: (Non-)Thermal Emission Fundamental Questions: (1) How much and which part of AGN SED is thermal and non-thermal? a) Thermal: Particles have Maxwellian velocity distribution due to collisions b) Non-Thermal: e.g. Synchrotron radiation with power-law energy distribution of particles (2) How much emission is primary and secondary? From Central Engine (primary) Re-radiation (secondary) Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 15 / 48
UV-Optical Continuum Big Blue Bump is assumed to be thermal emission from the accretion disk with T = 10 5±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 Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 16 / 48
UV-Optical Continuum Gravitational potential energy is released half (virial theorem) into kinetic energy and half in to radiation: From which is follows: L = G M Ṁ 2r T = ( = 2πr 2 σt 4 ) 1/4 G M Ṁ 4πσr 3 (this is an approximation, averaged over the disk) Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 17 / 48
UV-Optical Continuum In reality, energy is dissipated locally in the disk through viscosity. This yields: T (r) = or T (r) = [ ] 1/4 3GMṀ 8πσr 3 {1 (R i/r) 1/2 } [ 3GMṀ 8πσR 3 s ] 1/4 ( r R s ) 3/4 when r >> R i and R i R s Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 18 / 48
UV-Optical Continuum Inserting R s = 2GM/c 2, we find: ( ) 1/4 ( ) Ṁ T (r) = 6 10 5 M 1/4 r 3/4 8 K M E R s This peaks at 100Å for this typical temperature of a million K. Hence the accretion disk continuum spectrum is a superposition of many BB s with different temperatures. Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 19 / 48
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: dl ν (r) = 2π r cos(i)(πb ν )dr is the luminosity from a bin of width dr. L ν = 4π2 hν 3 cos(i) c 2 Ro This does not have a simple solution, but... Ri r dr exp(hν/kt (r)) 1 Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 20 / 48
UV-Optical Continuum At low frequency all BBs emit like a Wien spectrum, hence: (long wavelengths) L ν ν 2 At high frequencies, the spectrum has an exponential cutoff, determined by the highest temperature (at R i ) (short wavelengths) L ν ν 3 exp( hν/kt (R i )) Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 21 / 48
UV-Optical Continuum In the intermediate regime, if we define: x = hν kt (r) = hν (r/r s ) 3/4 kt s Substituting this back into the previous equation, we find: (intermediate wavelengths) L ν ν 1/3 Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 22 / 48
UV-Optical Continuum The superposition of these BB spectra will thus look like: Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 23 / 48
UV-Optical Continuum Homework #5: Compute the four velocity of the Black Bodies showed in the last figure. Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 24 / 48
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 interpretation is controversial! Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 25 / 48
Variability More observational clues come from variability: UV & Optical vary in phase Variation are larger at higher frequencies Small variations in UV are smoothed out at lower freq. Most variability at longer time-scales [P(f ) 1/f 2 3 ] Simultaneous UV & Optical variability is a major problem for models with the T-gradient outward! Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 26 / 48
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 10 6 K Disadvantage: low efficiency! Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 27 / 48
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. Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 28 / 48
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 Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 29 / 48
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 Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 30 / 48
Dust Reverberation Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 31 / 48
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 Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 32 / 48
Emerging picture The 2µ-1mm region is dominated by thermal emission from dust (except in blazars and some other radio-loud AGN) Different regions of the IR come Sub-mm break from different distances because of the radial dependence of temperature 1µ minimum: hottest dust has T 2000 K (sublimation T ) and is at 0.1 pc (generic feature of AGN) Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 33 / 48
Emerging picture Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 34 / 48
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. Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 35 / 48
X-ray emission: Origin Accretion flow surrounded by dusty torus BBB 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 Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 36 / 48
X-ray emission: Origin Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 37 / 48
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. Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 38 / 48
NGC 4945 Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 39 / 48
Reflected X-ray Spectra Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 40 / 48
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? Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 41 / 48
Radio-Galaxy Nuclei - Two Competing Models Is nuclear X-ray emission dominated by: The parsec-scale jet? -or- The accretion flow? Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 42 / 48
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 affected by beaming in same manner as radio NGC 6251-5 GHz VLBI Soft X-ray emission originates in a jet Double-peaked SED (modeled with syn+ssc) Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 43 / 48
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 regions of accretion flow Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 44 / 48
Evidence for accretion-dominated nuclear X-ray emission Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 45 / 48
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 Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 46 / 48
Radio-Galaxy Nuclei - Two Competing Models Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 47 / 48
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 Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 48 / 48