Lecture 7 Continuum Emission in AGN UVOptical Continuum

Lecture 7 • 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 1027 Hz: α=energy index now know to differ in different bands 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

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 Sanders et al. 1989 EUV gap Big Blue Bump

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 Fe. II lines •

The highest energies • Typically α=0. 7 -0. 9 in 2 -10 ke. V • 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 ( Me. V) are blazars

Blazars’ SEDs Red blazars: 3 C 279 Wehrle et al. 1999 Blue blazars: PKS 2155 -398 Bertone et al. 2001

Blazar SEDs main features • • • Two main components: Radio to UV/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

1. UV-Optical Continuum Interpreting the BBB From accretion disk theory (last class), And the maximum emission frequency is at i. e. , in the EUV/soft X-ray emission region. BBB=thermal disk emission? !

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 ~ν 2 at low frequencies, decays exponentially at high frequencies • For intermediate frequencies spectrum goes as ~ ν 1/3 • T=T(R) and T is max in the inner regions in correspondence of UV emission

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!

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: ü Fitting of SEDs ü Double-peaked line profiles Direct evidence: ü 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 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-1 mm 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

3. Radio properties of AGN I) Basic features of radio morphology II) Observed phenomena • Superluminal motion • Beaming

Radio features Lobes Hotspot Core Jet

Speed of Jets What is the speed of radio jets in AGN? Since this is non-thermal plasma where no spectral lines are seen, the Doppler-shift cannot be used to derive a jet velocity for the nucleus!

Radio Telescopes: VLA, VLBI • The Very Large Array has angular resolution • At z=0. 5 this is ~2 kpc • For the Very Long Baseline Interferometry, R~1 m. a. s. • At z=0. 5 this is ~2 pc

The power of resolution Energy is transported by jets from the cores to the outer regions

Superluminal Motion • VLBI observations of the inner jet of 3 C 273 shows ejected blobs moving at v~3 -4 c • This is called superluminal motion How is this possible? ?

Historgram of observed v/c in 33 jets

Explanation of apparent superluminal motion Explain apparent superluminal motion as an optical illusion caused by the finite speed of light. Consider a knot in the jet moving almost directly towards us at high speed: The blobs are moving towards us at an angle measured from the line of sight. Photon emitted along the line of sight at time t=0, travels a distance d to us, taking a time t 1 to arrive: t 1 = d/c A second photon is emitted at a time te later, when The blob is a distance d – vte cos away from us. The second photon arrives at t 2 = te + (d - vte cos )/c The observed difference in the time of arrival from photon 1 & 2 is: tobs = t 2 - t 1 = te (1 – vcos /c) < te

The apparent transverse velocity is v. T = vte sin / t = v sin / (1 – v cos /c) As v approaches c, v. T can appear > than c! Superluminal motion, typically 5 -10 c! Let = 1/(1 - v 2/c 2)1/2, this is the Lorentz factor. Then: v. T v (the maximum observed velocity) which occurs when cos = v/c. We will only observe superluminal motion when the jets are pointed within an angle of 1/ towards the line of sight, but this light will be beamed and brightened.

Relativistic motion of plasma • 1. 2. 3. 4. Relativistic bulk motion in radio sources has important consequences on the following observed quantities: Frequency Length and time Intensity Direction light is emitted

Relativistic Doppler Effect Assume an emitting source moving at a speed v with respect to the observer. c at an angle q Time-dilation tells us that dt in the observers rest frame for a periodic signal with frequency n’ in the co-moving (primed) frame is: However, since the emitting source is moving almost as fast as the emitted photon, the source will be catching up on the photon, and travel a distance s = v tcos q. The time difference in the arrival time of the two photons will therefore be reduced by s/c, i. e

and the observed frequency is This is the relativistic Doppler effect which defines the Doppler factor One can show (i. e. Rybicki & Lightman, chap. 4. 9) that the ratio of the flux density Sn and the frequency cubed is invariant under Lorentz transormation: Since the observed frequency is n=Dn’, = we find that also the observed flux has to be (S’n= flux density in co-moving frame)

Even for relatively modest relativistic velocities of v=0. 97 c, for example, the flux in the forward direction can be boosted by a factor 1000, while it is reduced by a factor 1000 in the backward direction! The transformation from a spherical to an elliptical polar diagram shows that angles are also transformed by relativistic effects. The so-called relativistic aberration (see Rybicki & Lightman, chap. 4. 1) is given by: In the rest frame of the source, half of the radiation will be emitted from – p/2 to p/2, hence setting q’ = p/2 will give thus for >> 1 half of the radiation will be emitted in a cone with half-opening angle

Jet-sidedness Since we expect jets to be two-sided, we always have two angles under which the emission is seen by an observer: q and q+p. We can now calculate the flux ratio R between jet and counter-jet under the assumption of intrinsically symmetric jets: Even for mildly relativistic jets one side will always be significantly brighter than the other

• Most of the strong, compact radio cores seem to come from sources where the angle to the line of sight is small, these jets are always one-sided. • Even most of the large scale jets appear to be one- sided, even though 2 extended lobes are seen indicating that really two jets are present. Nearby FRI radio galaxy and LINER galaxy M 87 - no counter. Jet observed

Summary: evidence for relativistic motion in AGN • Superluminal motion • One-sided jets (pc and kpc scales) Caveats • • • None of the above evidence proves that relativistic motion exists Alternative explanation exist for each observed property (e. g. , one-side jets) But relativistic motion=beaming is the only and the simplest explanation for all of them at once

Physics of AGN The Emission-Line Regions (BLR, NRL) Reprocessed Radiation An AGN produces a lot of ionizing radiation, most likely from the accretion disk. This emission is intercepted by gas and dust in the host galaxy. Correspondingly an AGN spectrum shows reprocessed radiation from this gas and dust. The respective features are: • Broad-Line Region (BLR) • Narrow-Line Region (NLR) • IR-bump from a molecular (dusty) “torus” (which we talked about last class)

BLR: Properties Broad, permitted emission lines (e. g. , Ha) in the optical spectrum: • FWHM several thousand km/sec up to 30000 km/sec FWZI (zero intensity) • derived gas temperatures are several 104 K • Doppler broadening through bulk motion of gas in gravitational field • with velocities as high as 0. 1 c, the distance from the Black Hole can be as close as 100 Rs • Comparison of continuum and BLR fluxes indicate that only 10% of the continuum radiation is absorbed by BLR clouds • The volume filling factor is very low - a few millionth of the central region is occupied by BLR 'clouds' • The necessary mass in the BLR to produce the observed luminosity is only a few solar masses • Broad-lines are very smooth - they are either made up of a huge number of small clouds or represent a coherent structure

Reverberation Mapping Lines from highly ionized gas (He II 1640, C IV 1549) respond faster than lines from lower ionization levels (e. g. Balmer lines) ionization structure in BLR more highly excited lines are further in

Reverberation Mapping Size of BLR For Keplerian rotation, the FWHM of the lines should correspond to the typical velocity dispersion at the radius where the line is produced. More highly ionized lines, which are closer in, should have larger FWHM and shorter time-lags.

AGN Unification Schemes Luminous AGN are classified as: • Seyfert galaxies (Type I and II) • Quasars, QSOs • BL Lacs • Radio galaxies (in `Broad line’ and `Narrow line’ variants) • LINERs All powered by accretion onto supermassive black holes. But why so many classes - are these all physically distinct objects?

AGN: Basic Ingredients An AGN consists of the following basic ingredients: • Black Hole (power source) • Accretion Disk (UV/x-rays) • Jet (radio) • -Core (compact, flat-spectrum, radio-to-gamma emission) • -Jet • -Lobes & Hotspots (extended, steep spectrum) • • Broad-Line Region (BLR) Narrow-Line Region (NLR) molecular (dusty) “torus" (feeding and obscuration) host galaxy (feeding)

Seyferts 1 and 2: Unification Scheme In the broad-line region (BLR) ·The Keplerian orbital speeds of the clouds around the central massive body will be large => lines are Doppler broadened. ·Density is high => no forbidden lines are emitted In the narrow-line region (NLR) ·The Keplerian orbital speeds of the clouds will be much smaller => lines are narrow ·Density is low => forbidden lines are emitted

Seyferts 1 and 2: Unification Scheme So, if the above Seyfert were viewed from direction (1), you would see: ·Broad permitted lines ·Narrow Forbidden Lines ·Bright continuum from the central engine ·i. e. a Seyfert 1 If, on the other hand, it were viewed from direction (2), you would see: ·No broad permitted lines (obscured by dust torus) ·Narrow Forbidden Lines ·No bright continuum from the obscured central engine ·except in the infrared and X-ray region, which gets through the dust ·i. e. a Seyfert 2

Seyferts 1 and 2: Unification Scheme: Evidence for Torus