Lecture: April 8, 2003

• Continuum Emission in AGN

• UV-Optical Continuum

• Infrared Continuum

• High Energy Continuum

• Radio Continuum - Jets and superluminal motion

Goal: The foundation of all astrophysical observations is the photon. All morphological and spectral information about astrophysical sources is derived from the emitted radiation. We learned about the power of line emission (spectroscopy) Continuum radiation is a natural consequence of the principle that accelerating charges radiate.

Can have : thermal or nonthermal emission

Spectral Energy Distribution

AGN show emission lines in all astrophysically relevant wavelength regimes

Power Law Continuum• Emission observed from 108 Hz to 1027Hz:

α=energy index now know to differ in different

bands

ανAνL )(

Actual SED is a function of the AGN Class

From last class:AGN Taxonomy

• Seyfert galaxies 1 and 2

• Quasars (QSOs and QSRs)

• Radio Galaxies

• LINERs

• Blazars

• Related phenomena

• Definition: radio-loud if

is larger than 10 (Kellermann et al. 1989)

• RL AGN have prominent radio features 10% of AGN population • RL: BLRGs, NLRGs, QSRs, Blazars RQ: Seyferts, most QSOs

• Deep radio surveys show intermediate sources

opticalradio LLR /

The Continuum

A phenomenological approach:

• Power law continuum

• Thermal features

• Spectral Energy Distributions of

Radio-loud and Radio-quiet AGN

Observing the SEDs 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)

Spectral Energy Distributions

IR bump

Big Blue BumpEUV gap

Sanders et al. 1989

The radio and IR bands

• Radio emission is two orders of magnitude or more larger in radio-loud than in radio-quiet

• Radio and IR are disconnected, implying different origins

The IR and Blue 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 lines

The highest energies• Typically α=0.7-0.9 in 2-10 keV • Radio-loud AGN (BLRGs, QSRs) have flatter X-ray

continua than radio-quiet• Soft X-ray excess is also observed, often smoothly

connected to UV bump• The only AGN emitting at gamma-rays

( MeV) are blazars

Blazars’ SEDs

Red blazars: 3C279 Blue blazars: PKS 2155-398

Wehrle et al. 1999 Bertone et al. 2001

Blazar SEDs main features

• Two main components:• Radio to UV/X-rays • X-rays to gamma-rays

• Component 1 is polarized and variable Synchrotron emission from jet• Component 2: possibly inverse Compton scattering

A fundamental question

How much of the AGN radiation is primary and how much is secondary?

• Primary: due to particles powered directly by the central engine (e.g., synchrotron, accretion disk)

• Secondary: due to gas illuminated by primary and re-radiating

An important issue

Isotropy of emitted radiation

• Thermal radiation is usually isotropic• Non-thermal radiation can be highly

directed (“beamed”). In this case: • We can not obtain the true luminosity of

the AGN• We will not have a true picture of various

AGN emission processes

Interpreting the BBBFrom accretion disk theory (last class),

And the maximum emission frequency is at

i.e., in the EUV/soft X-ray emission region.

Hz 106.3 16max ν

BBB=thermal disk emission?!

1. UV-Optical Continuum

Model Spectrum of an Accretion Disk

Spectrum from an accretion disk• Optically thick, geometrically thin accretion disk

radiates locally as a blackbody due to sheer viscosity

• Total integrated spectrum goes like ~ν2 at low frequencies, decays exponentially at high frequencies

• For intermediate frequencies spectrum goes as ~ ν1/3

• T=T(R) and T is max in the inner regions in correspondence of UV emission

• 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

Observations of optical-to-UV continuum

Disk interpretation is controversial!

Alternative interpretation

• Optical-UV could be due to Free-free (bremsstrahlung) emission from many small clouds Barvainis 1993

• Slope consistent with observed (α~0.3), low polarization and weak Lyman edge predicted

• Requires high T~106 K

Is an accretion disk really there?

Indirect evidence:

Fitting of SEDs Double-peaked line profiles

Direct evidence: Water maser in NGC 4258

Optical emission lines

Eracleous and Halpern 1984

Water Masers in NGC 4258

Within the innermost 0.7 ly, Doppler-shifted molecular clouds:

• Obey Kepler’s Law• Massive object at

center

2. The IR emission

• 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

Evidence for IR thermal emission

• Obscuration :

Many IR-bright AGN are obscured (UV and optical radiation is strongly attenuated)

IR excess is due to re-radiation by dust

Radial dependence of dust temperature

From the balance between emission and absorption:

With R in pc, Leff in erg/s, T in Kelvin

5/12

6 )(10R

LT eff

Hotter dust lies closer to the AGN

Evidence for IR thermal emission

• 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

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 from different distances because of the radial dependence of temperature

The 1μ minimum

• General feature of AGN

• Consistent with the above picture: hottest dust has T~2000 K (sublimation temperature) and is at 0.1 pc

• This temperature limit gives a natural explanation for constancy of the 1μ minimum flux