Ch.3

AGNthe term Active Galactic Nuclei (AGN) refers to the existence of energetic phenomena in the central regions of galaxies, phenomena which cannot be attributed clearly and directly to stars

same phenomenon, but initially the two classes were selected with different criteria

1) galaxies with bright nucleus

2) high luminosity objects, apparently not in galaxiesrare in the nearby Universe, thus hardly referable to galaxies

2 main classes were initially found:

1) Seyfert galaxies luminosity ~1044 erg/s (~galaxy)

2) quasar luminosity > 1044 erg/seven 1046-1047 erg/s

Seyfert galaxies

1908, Fath, Lick Observatory: first optical spectrum of a Seyfert galaxy,

strong emission lines in NGC 1068

1917, Slipher, Lowell Observatory: confirmation, line width several hundreds km/s

1943, Seyfert: selects a sample of galaxies with high central surface brightness

(nucleus with stellar appearence) NGC 1068, NGC 1275, NGC 3516, NGC 4051, NGC 4151, NGC

7469 spectra contain high excitation emission lines

important characteristics of spectra:1) broad lines up to ~8500 km/s FWZI (Full Width at Zero

Intensity)2) Hydrogen lines sometimes larger than other lines

1955, NGC 1068 and NGC 1275 discovered radiosources

1959 Woltjer, first trial to understand physics of Seyfert galaxies:1) non resolved nuclei, r < 100 pc2) duration of emission > 108 yr, in fact Seyferts ~ 1/100 galaxies3) if matter in nucleus is gravitationally bound, then Virial

Theorem:

Seyfert galaxies: typical optical spectrum

Seyfert galaxies: typical UV spectrum

radio surveys

quasars originally discovered as radiosources, identified with optical objects with stellar appearence3C and 3CR third Cambridge catalog (Edge et al 1959) 158 MHzand revision (Bennett 1961) 178 MHzflux limit 9 Jy [ 1 Jy (Jansky)=10-26 W/m2/Hz=10-23 erg/s/cm2/Hz ]declination > -22o (3C), > -5o (3CR)but sample limited by confusion limit: many faint sources close in the sky, within ~1 degree resolution3C: 471 sources, 3CR: 328 sources, numbered sequentially in RA,e.g. 3C273, 3C390.3 (decimal digit: sources added in 3CR)

3C273 Merlin 408 MHz

3C390.3 VLA 1565 MHzWilkinson et al. (1991)

3C273 optical image

radio surveys

PKS survey of the southern sky (<+25o) performed at Parkes, Australia (Ekers 1969)

408 MHz (Slim=4 Jy), 1410 MHz (1 Jy), 2650 MHz (0.3 Jy)sources named by position (1950 coordinates),e.g. 3C273 = PKS 1226+023 : 12h 26m +02o.3

4C fourth Cambridge catalog (Pilkington & Scott 1965, Gower et al. 1967)

178 MHz, 2 Jynamed by declination and sequential number, e.g. 3C273=4C

02.32(source no. 32 between 02 and 03 declination degrees)AO Arecibo occultation survey (Hazard et al. 1967)

extremely accurate positions obtained through lunar occultation

named by positionOHIO (Ehman et al 1979), 1415 MHz

names Ox yyy, where x progressive letter indicating RA (except A, O), and yyy sequential number

e.g. 3C273=ON 044 [N: between 12h and 13h]

radio surveys

first powerful radiosource identified with a pointlike object was 3C48, Matthews & Sandage (1963): optical counterpart was a 16 mag star

3C48 Merlin 1666MHz

in some cases instead, position coincided with starlike objects, e.g. on Palomar Sky Survey plates

most radiosources were identified with galaxies

however, the spectrum was anomalous, with very broad emission lines, and at apparently unknown wavelengths

z=0.367

6566x1.158=7603

4861x1.158=5629

3C 273

z=0.158

3C273.Maarten Schmidt understood (1963) that emission lines were related to Balmer Hydrogen series, and to MgII transition (2798Å), at the quite large redshift z=0.158

=> large distance:

distance modulus

=> high luminosity

Maarten Schmidt

~100x Milky Way (cf MB ~ -21)

quasars

in modern terms, one quasar characteristic is a very extended SED (spectral energy distribution)

AGNs cannot be described in terms of black-body emission, non-thermal processes are needed, e.g. synchrotron

Schmidt, quasar general properties:• objects with stellar appearence identified with

radiosources• variable continuum flux• large UV flux• broad emission lines• high redshifts

stars(~black body)

however, not all the objects now called AGNs possess all these properties

and other properties have later resulted important: e.g. one property which seems to be shared by all AGNs is a high X-ray luminosity (Elvis et al 1978)

SED

typically, for a given frequency interval, it is used a power-law (but sometimes the spectral index is defined with implicit - sign, )

if wavelength is used,

then

case corresponds to a “flat spectrum” in the plotand has equal energy per unity frequency interval

case has equal energy per unity interval of log frequency: , flat in the plot

in fact:

radio properties

generally 2 components:

1) extended (spatially resolved)usually double, 2 lobes located ~ symmetricallyextension < ~1 Mpcoptically thin in radio

2) compact (not resolved at ~1” resolution)usually coinciding with quasar’s optical imageoptically thick in radio

3C47 3C175

radio properties

compact component flat spectrum due to superposition of more non resolved sources with different turnover frequencies

extended component: steep spectrumbut at low frequencies source becomes optically thick and self-absorbed spectrum becomes

both emit through synchrotron mechanismelectrons in magnetic field, with power-law energy distribution,

produce a power-law spectrumwith

Fanaroff Riley classes

Fanaroff Riley 1974

FR I

FR II

low power

extended component brighter at the center, and gradually decreasing towards borders

high powerextended component brighter at borderquasars are FR II sources

variability

quasars vary in all electromagnetic bands, both in the continuum and in broad emission lines

variability of the optical continuum was established even before understanding of the spectral redshift(e.g. Matthews & Sandage 1963)

radioUV

X

emission lines

many quasars vary with amplitudes 0.3-0.5 mag over time scales of some months

some sources vary more strongly, up to ~1 mag in few days (blazars)

in X-rays time-scales are usually few days

in such cases, radiation must originate in regions of size ~light-days~1016 cm

UV fluxes

quasars have often quite blue colors. in particular, there is a UV-excess, U-B is small compared to MS stars

in a 2-color diagram (B-V, U-B), quasars are located in a different position compared to the locus of stars

this is caused by the differents SEDs, power-law for quasars, blackbody for stars

more UV

broad lines

one quasar carachteristic are the strong and broad emission linesstrongest are those of Balmer and Lyman series

and those of some abundant ions

these lines are virtually present in all quasar spectra, but,depending on redshift, they can be non-observable if theyfall outside the spectral range of the detector

the Equivalent Width

corresponds to the lambda interval over which a line of height unitymust be integrated to obtain the same flux of the emission line

e.g. at z~2, Ly falls in the U band, and contributes ~12% of the flux ~75/680,being W(U)=680Å the width of U bandpass

therefore, quasars with strong emission lines in a given bandpass are more easily detected, and this could lead to overestimate the quasar number at particular redshifts

relative EW flux (Å)---------------------Ly +NV 100 75CIV 40 35CIII] 20 20MgII 20 30H 4 30H 8 60

1

EW

redshift

first quasars discovered had redshifts comparable to farthest known clusters of galaxies (few tenths). later, with the refinement of selection techniques, highest redshifts continued to increase strongly. at the end of ‘70s, many quasars with z>3 were known

1993Hewitt

&Burbidge

2007, SDSS

high redshift sources like quasars are also useful as cosmological probes. but they must be used with some caution

redshiftcaution in cosmological studies: quasars are not standard candles, instead they have a large luminosity range (broad luminosity function) therefore Hubble diagram is unuseful

quasar density is maximum at z~2: high-redshift quasars are rare, and important, because they constrain formation of first cosmic structures

viceversa, for galaxies or other sources with L=constant,

Hubble diagram has a small dispersion

radio quiet quasarssoon it was realized that radio surveys were not the only method to find

quasars:each of the characteristic properties (variability, UV-excess, broad lines, high z) could be used as selection criterion

Ryle & Sandage (1964) noted that a search of UV-excess objects could be very simple: e.g. 2 photographs of the same sky area, in B and U, with exposure times chosen such that A stars had same intensity in the two pictures. comparison of the two photographs in fast sequence (blink) could allow identification of quasars as the only sources brighter in U than in B, differently from cold stars. there could be contamination by hot O and B stars, but this is negligibile, if an area at high galactic latitudes is chosen

this technique was used by Sandage at Mt Wilson and by Lynds at Kitt Peak and was so efficient that much more quasars than expected were found. many blue stellar objects were found, up to ~3/deg2 at 18.5 mag. most of them were white dwarfs and RR Lyrae, however there was also a substantial population of objects with quasar properties, which could be selected optically: they were called quasi-stellar objects or QSOs

today, terms quasars and QSOs are used almost as synonyms

radio-quiet quasars are 10-20 times more numerous thanradio-selected quasars, therefore optical selection techniqueswere used with increasing frequency. in any case, in optically-selected samples also radio-loud objects are present, e,g. 3C273

5-10%

90-95%

radio-loud/radio-quiet threshold:

radio-loud

radio-quiet

taxonomy

Seyfert galaxiesthey are AGNs of relatively low luminosity MB > -23 (Schmidt & Green 1983).

usually found in spiral galaxiesSeyfert 1 display 2 line systems:

narrow lines ~hundreds km/s,mainly characteristic of low density ionized gas, ne~103-106 cm-3 (forbidden lines)

broad lines ~thousands km/s, characteristic of higher density gas,ne > ~109 cm-3 (permitted lines)

[ actually, permitted lines can also be narrow ]

Seyfert 2 have only narrow lines

in fact, forbidden linescan be produced onlyin NLR, but permittedlines can be producedboth in BLR and NLR

forbidden lines*e,g. [OIII] 5007 Å*they are produced by transitions which are forbidden by the parity selection rule of quantum mechanics: parity must change (even or odd sum of orbital angular momenta)*the parity selection rule applies to electric dipole transitions which are generally dominant, but it can happen that the electric dipole term be zero for particular simmetries*then, magnetic dipole and electric quadrupole terms can become significant*for them, different selection rules apply (parity doesn’t change)*in standard conditions, the probability of magnetic dipole and electric quadrupole transitions is however very low, and in practice the transition doesn’t occur, because the atom de-excites through a collision with another particle*instead, in very low density conditions, collisions are rare, and atoms can remain for a long time in meta-stable states, and then de-excite through forbidden transitions*e.g., favorable conditions are found in ISM, NLR

semiforbidden lines*e.g. CIII] 1909 Å*for these transitions a different selection rule is violated (either ∆S=0, or ∆L=0,+1,-1)

BLR NLRBH

high density: only permitted lines

low density: both forbidden and permitted lines

quasars usually non resolved in PSS plates: < ~7”

however, sometimes they show some nebulosity (fuzz)

spectra are similar to Seyferts, but narrow lines are relatively weaker

average spectrum obtained by 700 QSOs of the Large Bright Quasar Sample (Francis et al 1991)

MB < -23

spectra of various classes of AGNs

powerful radio sources (FRII) are identified, besides quasars, with giant elliptical galaxies

radiogalaxies

Broad Line Radio Galaxies

(BLRGs)

2 subclasses

Narrow Line Radio Galaxies (NLRGs)

(analogous of Seyfert 1 and Seyfert 2)

low power radio sources (FRI) have weaker emission lines, and it is difficult to determine whether also broad lines are present

LINERs

essentially, diagnostic diagram measures the SED of the ionizing flux needed to account for the line intensity rations

for the case of LINERs, the SED is compatible with a very diluted Seyfert continuum

Seyfert 2

HII regions

LINERs

Low Ionization Nuclear Emission-line Regions

low luminosity objects with low ionization lines, e.g. [OI] 6300Å and [NII] 6548Å, 6583Å are relatively strong

they are very frequent sources, may be half of all the spiral galaxies

spectra can be distinguished from those of Seyfert 2 and HII regions by a diagnostic diagram which uses 2 line ratios

blazars defining properties:* large continuum variations ~0.5 mag in few days* large polarization (some %)* always radio-loudBL Lacertae objects

OVV(optically violently variable)

BL Lac objects do not have strong emission lines

blazars are AGNs with a relativistically amplified component toward the observer

blazar

type 1 AGN

type 2 AGN

UNIFIED SCHEME

relation between Seyferts and quasarsthere is much overlap of properties: most luminous Seyferts are similar to

quasars.inizially it was not clear that the classes were related

main differences between the two classes were:

•variability: for Seyferts it was discovered only in 1967, it wasn’t searched before, because there was not suspect that it could be present; instead for quasars it was discovered soon

•luminosity: first quasars were very luminous, first Seyferts were very weak, there was no overlap

•emission lines: Seyferts seemed to have lines with larger equivalent width compared to quasars; but comparison was done using lines in different spectral regions, Balmer lines and strong forbidden lines for Seyferts, UV lines for high redshift quasars, and the different continuum intensity affects the ratio

Weedman noted (1976) that the first discovered objects in the two classes were extreme in both cases: first Seyferts were nearby galaxies with peculiar nuclei, first quasars were 3C, radio-loud, objects, and many of them were OVV

there is continuity between Seyfert e quasar properties

HST observations show that quasars are hosted in galaxies, like Seyferts

BH paradigm

Eddington limit:accreting matter is braked by radiation pressure from central source

it is needed:

[ ]

work model: accretion disk surrounding a supermassive BH

inverting the disequality, a limit on mass is found:

masses measured more directly by other methods, e.g. Virial Theorem ( ),

e.g. echo mapping, are ~ 1 order of magnitude greater than Eddington limit

however, Eddington limit is not absolute, it holds under the assumption of isotropic emission, and for systems in equilibrium

Fgrav Frad

accretion luminosity

in Active Galactic Nuclei, emitted power derives from accretion of matter onto the BH.energy associated with mass is radiated with efficiency :therefore, luminosity is proportional to accretion rate:

gravitational potential energy is converted into radiation with the rate:

consider a mass m falling from (recalling that is Schwarzschild radius)

this distance is slightly more than minimum stable orbit (3RS) and corresponds to inner regions of the accretion disk, where most of the optical/UV radiation originates.then:

(much higher than the efficiency of nuclear

energy production in stars )

we can derive an Eddington limit for the accretion rate:

loss of angular momentum

falling gas must lose angular momentum before reaching accretion disk.consider a particle in circular orbit at galactocentric radius r=10 kpc.mass rotation velocity angular momentum per mass unit:

if such particle must reach rd~0.01 pcwith

then J/m must decrease by a factor

angular momentum must be transferred outwards, e.g. in interactions with other galaxies

r

tidal disruptions

gas which fuels the accretion disk can come from stars, but these must be tidally dirupted, otherwise, if swallowed whole by BH, do not emit e.m. radiation (however they emit GW).condition for the tidal disruption is that the star approaches BH closer than the Roche limit:

simplified model:

rR

2rR

using it is found

i.e., again substituting Rs,

stars with can be tidally disrupted outside RS by SMBHs with

m mMBH

rR

RS

accretion disk

it is assumed that power radiated by the AGN derives from accretion, and that the energy of a particle at distance r is dissipated locally, and that the medium is optically thick.then we can approximate the local emission as a black body:

Virial: half energy goes in heating the gas:

2 disk sides

more precisely, taking into account dissipation by viscosity:

for a disk accreting at Eddington rate onto a BH with , emission from inner regions is maximum for

at r>>Rin (inner border) it is found:

i.e.:

~100Å (EUV)

accretion disk

~100Å (EUV)

for

indeed, AGNs have typically a peak in the UV region, the Big Blue Bump (BBB):

as T scales like M -1/4, for a stellar mass BH it is instead found (in the X-ray band)

accretion disk

structure of the accretion disk is determined by the actual value of the accretion rate compared to and by the opacity of the accreting material

at low accretion rates and high opacities, accretion disk is thin (physical height is small compared to diameter) and disk radiates efficiently

therefore, the rate at which energy is transported inwards is negligible compared to the rate at which energy is radiated vertically

the emitted spectum is a composite of optically thick thermal spectra over the range of temperatures persisting through the disk

at high accretion rates radiation is partially trapped by the accreting material and the disk expands vertically into a “radiation torus” or thick disk, which radiates inefficiently approximately asstructure is similar to an early-type star, with a spectrum approximately given by a black-body at a single temperature ~104 K

at very low accretion rates the disk becomes optically thin, inner regions are not able to cool efficiently and a 2-temperature structure is formed, an “ion torus”, with decoupled electrons and ions. ions reach a temperature .the ionized torus keeps a strong anchored magnetic field which can be able to collimate the outflow of charged particles along rotation axis (jet)

continuum

SED

one possibility is SSC, synchrotron self-Compton: same electrons producing synchrotron interact again with sychrotron photons and produce inverse Compton

synchrotron

synchrotron

inverse Compton

but: polarization not observed,except blazars

synchrotron ? cf SNR, radiosources

non thermal ?

continuum

R IR O UV X gamma

holes in the SED: submm, lack of detectors (but ALMA is beginning operation, 0.3-3mm); EUV, opacity ISM Milky Way below 912 Åin detail: various structures, valleys, bumps -> more components

Big Blue Bump (BBB), 4000Å-1000Å, + soft X-ray excess (perhaps continuation)(+ small BB structure due to many emission lines by FeII, 2000Å-4000Å)BBB: ~thermal, optically thick (~ black body, accretion disk), or optically thin (free-free)

minimum + IR bump, due to dust reprocessing

complex SED, depending on many parametersgeneral questions: thermal vs non-thermal, primary vs secondary, isotropic vs anisotropic

UV-optical

main characteristic is the BBB, attributed to some kind of thermal emission between 104 and 106 K

assume local black body:

we can approximate in the different regimes:

Rayleigh-Jeans

Wien

(large range of T)

UV-optical

SED measurements, after correcting for emission lines (including small blue bump) and for contribution by host galaxy, indicate rather than

acceptable fits can be obtained adding a power-law ~ from IR to X-ray, or an IR bump of thermal origin

however, it is a simplified explanation that BBB derives from black body emission: continuum emission is not yet fully understood, but important progress comes from variability studies, ideal for separating different components

BBB

UV-optical

where do greatest contributions in different parts of the spectrum come from?

suppose

i.e.

1500Å: 2x1015 Hz, T~3x104 K, r~ 50 Rs

~1.5x1015 cm ~0.5 light-days

5000Å: 6x1014 Hz, T~104 K, r~250 Rs ~7.5x1015 cm ~2.5 light-days

optical-UV variability

1. UV and optical vary in phasethey come from different regions of the disk and are nearly simultaneous==> communication between different regions cannot occur at sound velocity, but at least at v=0.1 c

2. harder when brighterspectral variations are correlated with flux variations==> variability increases with frequency

3. variations are irregular, aperiodic, on time scales from days to yearson time scale of day, var= few %, on time scale of some days var= tens % ==> r ~1016 cm NGC

5548

variability increases with delay between two observations and is described by Structure Function (SF)

or(factor normalizes S to mean square value for a Gaussian distribution)

SF has been modeled, e.g., as

Bonoli et al 1979

Sirola et al 1999

in this case is a characteristic time at which variability saturates

however, deVries et al 2005, comparing SDSS data with historical POSS data, have shown that SF still increases even at times as long as 40 years. this suggests that variability is due to superposition of flares of all temporal scales

optical-UV variability

variability increses with emission frequency (Di Clemente et al 1996)

moreover, variability decreases with source luminosity and this favors models in which variability is due to the superposition of many subunits: flux increases like , number of subunits, total flux variation increases like (christmas tree model)

low z

high z

and increases with redshift

and both trends are explained by variability of the spectral slope

among the proposed models for the origin of variability, there are: local instabilities in the accretion disk, starburts, microlensing, stellar collisions

optical-UV variability

another function used to describe variability behavior is the Power Spectral Density (PSD)

n=0 white noisen=-1 flicker noisen=-1.5 red noisen=-2 shot noise

observations indicate for normal (non blazar) quasars:

variability is higher at long time scales, but there is not enough sampling at small f (long time scales) to determine a break in the P(f)break is needed for convergence reasons, in fact for it must be n > -1and for it must be n < -1

determination of the break would imply a characteristic time scale

if C(t) is the light-curve, one can derive the Fourier transform

and the PSD

it is usually assumed a power-law form:

optical-UV variability

X-ray emission

it comes from most central AGN regions, indeed X-ray variability time scales are the shortest, down to ~100 s

generally poor spectral resolution , sometimes e.g. ASCA, XMM-Newton, Chandra, emission/absorption lines not directly detected, but determined through parametric fits

conventional terminology in the X-ray band, partially artificial:

soft X-rays: 0.1-2 keVhard X-rays: 2-100 keVgamma-rays: >100 keV

cold and hot gas:for E=1 keV, equivalent temperaturefor comparison,104 K: cold, 108 K: hot

SED, units of measure:photons/s/keV rather than erg/s/Hz

in detail, a description with two slopes works better: at low energy , then increases more steeply

photon index

energy index

2-20 keV: in average increases as

UV/X-ray ratio

often expressed in terms of a spectral index for a power-law connecting the two bands UV

X[but in the book it is defined with the opposite sign]

depends on luminosity, e.g.:

(Gibson et al. 2008, ApJ 685, 773)

provides information on the relation between the disk (emitting optical-UV) and the corona (emitting X-rays)

large dispersion:

variabilityabsorptionBAL QSOsRL AGNs

x-ray-weak AGN

i.e.:

X-ray spectral components

soft excessin some models it is due to comptonization of the Wien tail of the BBB

inverse Comptonon O/UV continuumby electrons in hot corona + exponential cut off

Compton reflected component

warm absorber: ionized material along the line of sight; this material might be an ionized wind

Fe 6.4 keVwidth 104-105 km/s

relativistic effects in the Fe Kα line

in some cases, the line appears distorted and broadened: this is due to combined effects of special relativity (relativistic Doppler effect) and general relativity (gravitational redshift)

MCG-6-30-15 (Tanaka 1995)

important: it allows to determine BH mass and disk geometry, which is also related to BH spin:

Schwarzschild

Kerr

X-ray variability

the short time scales (~100 s) are important to probe internal regions.at longer time scales (days and more) X-ray variations are apparently correlated with optical/UV.data suggest that variations are simultaneous within some days

there is no evidence of periodicity

power spectral density:there is relatively more variability at high temporal frequencies compared to UV/opticalthere is no evidence of steepening at short time scales

the fact that X-ray and UV/optical variations appear correlated and the steeper UV/optical PSD suggest that UV/optical is a reprocessed version of X-ray spectum

only in a few well sampled cases there is evidence of a flattening toward small f’s (long time scales). the break characterizes a typical time scale, which appears correlated with BH’s mass, including also galactic X-ray sources and the associated stellar mass BHs

Uttley & McHardy 2004

gamma-ray emission

detection:100 keV - 30 GeV only by satellite (CGRO, Integral, Agile, Fermi)> 30 GeV also from ground with Cherenkov telescopes50-150 keV: steeper than X-ray estrapolation

Seyferts: cut-off at ~ hundreds keV, SED consistent withgamma-ray background (probably produced by these sources)

at higher energies (TeV) only blazars are detected (exception: NGC 4151)

IR

once it was believed that IR was an extension of the power-law from Radio, possibly forSSC, now there is increasing agreement that most IR emission is thermal, and due to dust

3 pieces of evidence:

* minimum at is a general AGN characteristic, and is explained by sublimation radius (for graphite).within such radius, dust grains sublimate, and spectrum presents a cut-off in correspondence to the Wien tail of a blackbody at ~2000 K

* IR variability is correlated with UV/optical variability, but with delay, e.g. Fairall 9, delay ~400 days < ~1 pc, this suggests that IR continuum is generated at <~1pcsublimation radius is also ~1 pc, taking e.g. T=1800 K and L~1.8 1046 erg/sdust mass needed to explain the emission is onlyin the far IR only radio-loud AGNs are variable, this also suggests that far IR from radio-quiet AGNs is thermal

* at submm wavelengths there is a sharp SED decline, this can be due to the fact that, at these wavelengths, the emitting efficiency of grains is strongly dependent on frequency

radio

radio spectrum gives a small contribution to bolometric luminosity: is ~ 10-3 compared to UV/optical in radio-loud AGNs, 10-5-10-6 in radio-quiet ones

compact radio sourcesspectra are flat, radiation is considered non-thermal (synchrotron)

evidences:

* spectral shape, flat at low frequencies, then steeper, characteristic of optically thick sources with continuous injection of high energy electrons flat part is due to sum of regions with different self-absorption cutoff, caused by gradients in density and magnetic field

* brightness temperature, temperature that the source should have, if radiating as a blackbody, to emit the observed specific flux at a given frequency

is the angular radius of the source, and we are in the Rayleigh-Jeans limit.measurements always give TB~1011-1012 K, which is evidence of non-thermal emission

superluminal motion

3C 273

compact radio sources have flat spectrum and rapid variability: hints of structure on small scales. this promoted the development of very high resolution (~mas) instruments: VLBI, very long baseline interferometry

in many cases it is possible to extrapolate backwards to the epoch of virtually zero separation. often, such epoch corresponds to a large and rapid flux variation (flare or outburst) detected on a broad range of frequencies (optical/UV, X-ray, gamma-ray)

VLBI repeated observations show structures with multiple components, with relative proper motions, always outwards, with apparent velocities of the order of c, and often higher

various explanations have been invoked: non-cosmological redshifts, really tachyonic motions... the simplest and more effective is based on the relavistic motion of the source in a direction close to the line of sight

superluminal motion

Blandford McKee Rees 1977:

observer

moving source

arrival times:

AB’~D [ ]

the measured interval between observations is shorter than the interval between emission times, due to the large approaching radial velocity

observed interval:

the apparent transverse velocity is increased due to shortening of the observed interval:

[ ]

superluminal motion

at what angle βT is maximum?

[ , Lorentz

factor]

observer

moving source

relativistic Doppler effect

Doppler factor:

relativistic boosting:

one factor comes from time compression (Doppler effect)one factor comes from solid angle transformationone factor comes from the intrinsic flux ratio between emitted and observed frequencyif source is a continuous jet, emitting volume decreases by a factor because of relativistic length contraction, and the overall factor in such case is

rest reference frame

observer reference framebeaming

boosting

note that in the relativistic case there is also the so called transversal Doppler effect, which occurs for vr=0

p={for it is obtained:

blazar spectra

blazars

BL Lacertae objects:

broad lines like in normal quasarsbut sometimes disappear if continuum is high

some blazars are superluminal

OVV (FSRQs):

blazars: always radio-loud (subsample of RL AGNs) few % of whole AGN population

peculiar characteristics:* often smooth featureless continuum, without lines* rapid optical variability (~days)* high (some %) and variable polarization* strong and variable radio emission

continuum without lines or or with very weak lines

blazar SEDs SED maxima occur at different frequencies for different objects, some peak in IR, other in O/UV, other extend to high energiesgenerally, AGNs above MeV are blazars, some arrive even at ~TeVthere are no the other components tipical of the rest of AGNs, spectrum is non-thermal

classified LBL and HBL depending on synchrotron peak position

blazar variability

variations are in more wavelengths, correlated reciprocally and with polarization, evidence of non-thermal nature

variations of 1 mag or more on time scales of months-years

+ microvariability in some objects, ~0.1 mag on time scales of hours

in some cases, e.g. PKS 2155-304, it is found:UV time lag on X-ray ~2-3 hoursagainst SSC model, in that case X-ray follows after UV

PKS 2155-304