COLOR STUDY OF BLAZARS

Robert Filgas

Supervisor: RNDr. René Hudec, CSc., AÚ AV ČR

Goals of the thesis

• To summarize and discuss known facts about blazar spectra, luminosity in optical band and its time evolution.

• To build and analyze my own database of optical photometry of blazars.

• To interpret and discuss acquired results; conclusions, comparison with theoretical models, physics of the environment.

Active galaxies

1943 Carl Seyfert – fundamentals of AGN class (Seyfert galaxies)

Great luminosity (1042 to 1047 erg s-1) in a very compact region => supermassive black hole (107 to 109 MS) in the center of the galaxy with accretion disk, dust torus and sometimes a jet.

Rarities of AGN

• Strong emission linesemission lines much broader than absorption lines

broadness caused by Doppler effect, gravity, velocity, temperatures

• Variability of the luminosityvariability of such giant objects unexpected

brightness can increase 1000x on scale of days

gives us the upper limit for size of the radiating area

• Differences in spectradominates non-thermal emission due to presence of active nuclei

Types of AGN

• Blazars– name given by Ed Spiegel, 1978– these days blazar rather a phenomenon then a category of

objects– common is a relativistic jet pointed almost at us– radiation of the jet is non-thermal, relativistically boosted and

polarized

Physical processes in jets of AGN

• Superluminal motion

• Relativistic beaming

– increasing or decreasing of the radiation intensity from the source moving with relativistic speed

where

for homogenous sphere P = 3 + α

Lorentz factor ~ 10 => boosting 100x – 1000x

!can explain why we see only one jet!

Spectra of blazars

Spectra of blazars

• Synchrotron emission– radiation of relativistic electron accelerated in a magnetic

field

gyrofrequency Larmor radius

energy radiated by the electron increases as

– spectrum broad and centered on the critical frequency

Spectra of blazars

• Synchrotron emission– nature of spectrum depends on the speed of electrons

– electron energy distribution has a power-law nature =>

power-law synchrotron spectrum

– some of radio sources so compact => under certain frequency electrons optically thick for their own radiation

» synchrotron self-absorption

Spectra of blazars

• Synchrotron emission

Variability of blazars

• Long-term variation– variations on time scales of months to years– many models: binary BH, precession of the jet,

perturbations to the disk,..– periodicity found

Variability of blazars• Short-term variation

– days to months– orbital motion of jet from less massive BH?

• Intraday variation– night to night variations– can give upper limit to the mass of central BH and the size

of emitting region

Variability of blazars

• Microvariation– minutes to hours

Causes of the variability

• Extrinsic causes

– microlensing

– interstellar scintillation

• Intrinsic causes

– accretion disk models

– geometrical effects

– shocks in jet

Causes of the variability

• Interstellar scintillation

– result of wavefronts from a distant radio source being perturbed by refractive index fluctuations in the turbulent, ionized interstellar medium of our Galaxy

– observed only in the most compact radio sources

– principal cause of the rapid radio IDV in BL Lac objects

– !affects only radio band of the spectrum!

Causes of the variability

• Microlensing

– one of Einstein’s general relativity predictions

– light from distant source bent around massive object

– microlensing: no distortion in shape

amount of light received increases

– brightness variations:» Symmetric outburst» Frequency independence across spectrum» Duration related to the lens speed

Causes of the variability

• Accretion disk models

– bright spots on disks» modulation of variability by radiating flare

– vortices forming within a disk

– plasma dominated events just above disk

– spiral shocks produced in disk by passing massive stars, molecular clouds or companion BH

Causes of the variability

• Geometrical effects• based on changing of Doppler factor

– binary BH» precession of system» gravitational lensing from the secondary BH

– helical jet models» knots or blobs of plasma spiraling in the jet

Causes of the variability

• Shock-in-jet model– major increase in bulk velocity or internal energy of the jet

flow will cause shock waves to form and propagate down the jet

» decelerates supersonic flows to subsonic speeds

– compression of plasma and enhancement of parallel component of magnetic field

– flares results from increased density behind the shock front and increased magnetic field

– frequency dependent effect

Causes of the variability• Shock-in-jet model

Data analysis• Dataset

– 33 blazars – 11 LBLs, 19 HBLs and 2 FSRQs

Data analysis• Finding power-law spectra

Data analysis• Bluer-when-brighter tendency

Data analysis

Data analysis

Data analysis• not corresponding, SED deformation (thermal contribution from host

galaxy or non-thermal contribution from different regions ?)

Data analysis

Data analysis• Inconsistent models:

– Interstellar scintillation – affects radio and only

– Gravitational lenses – frequency independent

• Acceptable models:

– Accretion disk models – thermal contribution from disk during quiescent state of HBLs and FSRQs

– Geometrical effects – extreme dependency on slight changes of Doppler factor, binary holes commonly accepted

– Shock-in-jet model – consistent with spectral hardening, need for data in all spectral bands

Data analysis• Color analysis

– color-color diagrams project dispersion of light on its way to the Earth

Data analysis

Data analysis

Data analysis• Color analysis

– color-color diagrams project dispersion of light on its way to the Earth

– compared with OA of GRBs we see similarities

Data analysis

Data analysis

Data analysis• Color analysis

– color-color diagrams project dispersion of light on its way to the Earth

– compared with OA of GRBs we see similarities

– comparison with AGN shows large differences

Data analysis

Data analysis

Data analysis• Color analysis

– color-color diagrams project dispersion of light on its way to the Earth

– compared with OA of GRBs we see similarities

– comparison with AGN shows large differences

– GRB and blazars have either similar or no environment

» possibility that dust and other environment is destructed along the line of sight by high-energy photons

» for blazars the origin for destruction might be in the jet

Data analysis• Color-color diagram positions

– possibility to distinguish between various types of objects like it is with stars

Data analysis

Data analysis

Data analysis• Conclusions

– synchrotron emission confirmed due to power-law spectra

– spectral index ~1.5 for LBLs well corresponding with theory

– bluer-when-brighter tendency observed – models

– small scatter in color-color diagrams – dust destruction

– mean values:

THE END