Lecture: April 8, 2003

1
(No Transcript)
2
Lecture April 8, 2003

• Continuum Emission in AGN

• UV-Optical Continuum

• Infrared Continuum

• High Energy Continuum

• Radio Continuum - Jets and superluminal motion

3
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
4
Spectral Energy Distribution
AGN show emission lines in all astrophysically
relevant wavelength regimes
5
Power Law Continuum

• Emission observed from 108 Hz to 1027Hz
• aenergy index now know to differ in different
  bands

Actual SED is a function of the AGN Class
6
From last classAGN Taxonomy

• Seyfert galaxies 1 and 2
• Quasars (QSOs and QSRs)
• Radio Galaxies
• LINERs
• Blazars
• Related phenomena

7

• 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

8
The Continuum

• A phenomenological approach
• Power law continuum
• Thermal features
• Spectral Energy Distributions of
• Radio-loud and Radio-quiet AGN

9
Observing the SEDs of AGN
10
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)

11
Spectral Energy Distributions
Big Blue Bump
EUV gap
IR bump
Sanders et al. 1989
12
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

13
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

14
The highest energies

• Typically a0.7-0.9 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

15
Blazars SEDs
Blue blazars PKS 2155-398
Red blazars 3C279
Wehrle et al. 1999
Bertone et al. 2001
16
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

17
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

18
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

19
Interpreting the BBB
1. UV-Optical Continuum

• From accretion disk theory (last class),
• And the maximum emission frequency is at
• i.e., in the EUV/soft X-ray emission region.

BBBthermal disk emission?!
20
Model Spectrum of an Accretion Disk
21
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 ?2 at low
  frequencies, decays exponentially at high
  frequencies
• For intermediate frequencies spectrum goes as
  ?1/3
• TT(R) and T is max in the inner regions in
  correspondence of UV emission

22
Observations of optical-to-UV continuum

• After removing the small blue bump, the observed
  continuum goes as ?-0.3
• Removing the extrapolation of the IR power law
  gives ?-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!
23
Alternative interpretation

• Optical-UV could be due to Free-free
  (bremsstrahlung) emission from many small clouds
  Barvainis 1993
• Slope consistent with observed (a0.3), low
  polarization and weak Lyman edge predicted
• Requires high T106 K

24
Is an accretion disk really there?

• Indirect evidence
• Fitting of SEDs
• Double-peaked line profiles
• Direct evidence
• Water maser in NGC 4258

25
Optical emission lines
Eracleous and Halpern 1984
26
Water Masers in NGC 4258

• Within the innermost 0.7 ly, Doppler-shifted
  molecular clouds
• Obey Keplers Law
• Massive object at center

27
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

28
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

29
Radial dependence of dust temperature

• From the balance between emission and
  absorption
• With R in pc, Leff in erg/s, T in Kelvin

Hotter dust lies closer to the AGN
30
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

31
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

32
The 1µ minimum

• General feature of AGN
• Consistent with the above picture hottest dust
  has T2000 K (sublimation temperature) and is at
  0.1 pc
• This temperature limit gives a natural
  explanation for constancy of the 1µ minimum flux