color study of blazars robert filgas supervisor: rndr. ren é hudec, csc., a Ú av Čr
DESCRIPTION
COLOR STUDY OF BLAZARS Robert Filgas Supervisor: RNDr. Ren é Hudec, CSc., A Ú 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. - PowerPoint PPT PresentationTRANSCRIPT
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COLOR STUDY OF BLAZARS
Robert Filgas
Supervisor: RNDr. René Hudec, CSc., AÚ AV ČR
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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.
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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.
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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
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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
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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!
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Spectra of blazars
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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
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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
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Spectra of blazars
• Synchrotron emission
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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
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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
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Variability of blazars
• Microvariation– minutes to hours
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Causes of the variability
• Extrinsic causes
– microlensing
– interstellar scintillation
• Intrinsic causes
– accretion disk models
– geometrical effects
– shocks in jet
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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!
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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
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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
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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
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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
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Causes of the variability• Shock-in-jet model
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Data analysis• Dataset
– 33 blazars – 11 LBLs, 19 HBLs and 2 FSRQs
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Data analysis• Finding power-law spectra
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Data analysis• Bluer-when-brighter tendency
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Data analysis
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Data analysis
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Data analysis• not corresponding, SED deformation (thermal contribution from host
galaxy or non-thermal contribution from different regions ?)
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Data analysis
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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
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Data analysis• Color analysis
– color-color diagrams project dispersion of light on its way to the Earth
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Data analysis
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Data analysis
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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
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Data analysis
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Data analysis
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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
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Data analysis
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Data analysis
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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
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Data analysis• Color-color diagram positions
– possibility to distinguish between various types of objects like it is with stars
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Data analysis
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Data analysis
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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:
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THE END