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BLAZARS of Our Times. The nature and remarkable variability of the blazars. AAVSO-HEA3, Las Cruces March 2005. Dr. Gordon G. Spear NASA E/PO Group Department of Physics and Astronomy Sonoma State University. BLAZARS of our Times What will we cover?. What’s in a name?

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BLAZARS of Our Times

The nature and remarkable variability of the blazars

AAVSO-HEA3, Las Cruces

March 2005

Dr. Gordon G. Spear


Department of Physics and Astronomy

Sonoma State University

BLAZARS of our TimesWhat will we cover?

  • What’s in a name?

  • AGNs (active galaxies)

  • Blazars

  • Variability -- the observations

  • Origins of the variability

  • Recommendations for observers

  • Why observe blazars?

What is a Blazar?… the origin of the name …

  • A blazar is a type of active galaxy (AGN)

  • The first blazar identified was BL Lac

  • Blazars are similar to quasars

  • The name is a combination of “BL Lac” and “quasar”

  • The term was first used in a title for a paper in 1984

  • Blazars are the most variable of the AGNs

  • Blazars are point sources of gamma rays

A Significant CGRO Discovery

  • There are some jewels in the EGRET catalog.

  • Nearly all point sources of gamma rays outside the Milky Way are AGNs.

  • These are the blazars!

The EGRET Gamma-Ray Sources

What is an AGN?the standard model

  • An extragalactic object

  • More luminous than normal galaxies

  • Generally compact, point sources (stellar appearance) at the center of a galaxy

  • Active Galactic Nucleus

  • The point source can be 10-100 times the brightness of the underlying galaxy

  • Some exhibit jets

  • All exhibit variability at some level!

Seyfert Galaxies

Sy 1, Sy 2

Radio Galaxies

Narrow Line Radio Galaxies (NLRG... FR I, FR II)

Broad Line Radio Galaxies (BLRG)


Broad Absorption Line Quasars (BALQ)

Steep Spectrum Radio Quasars (SSRQ)

Flat Spectrum Radio Quasars (FSRQ)

Optically Violent Variables (OVV)


Some Categories of AGNs

Properties of Blazars A blazar is similar to a quasar

  • Compact radio source

  • Generally a point source in visible light

  • Compact X-ray source

  • Point source of gamma rays

  • Highly polarized in both radio and optical

  • Variable polarization

Properties of Blazars (continued) A blazar is similar to a quasar

  • Generally a flat spectrum radio source (FSRS)

    • Not a thermal source (like stars)

    • Not a classical synchrotron source (like many radio sources)

  • Optical spectrum virtually featureless

    • If present, any emission or absorption lines are very weak

    • Equivalent width of any lines less than 4 Angstroms

  • Large irregular variability in brightness at all time scales

    • Variable over decades, years, days, and even hours

    • Over long time scales can vary up to 4 magnitudes

    • Day-to-day variations of 0.4 magnitude are possible

Quasars and blazars usually look like stars

PKS 1117-248

But their spectra do not look like the spectra of stars…or galaxies…or even quasars…

Their broad band spectral energy distributions (SEDs) do not look like normal active galaxies…Blazar SEDs have two broad peaks!

AGN UnificationOne model ---> All AGNs

  • Super massive black hole within the nucleus of the galaxy

  • Accretion disks produced as matter falls toward the black hole

  • Frictional heating within the disk produces very high temperatures and X-rays

  • Jets produced (not always!)

  • Orientation of the disk (and jet) to our line of sight determines what type of AGN we will see

Components of the AGN Unification Model

  • Supermassive black hole (energy source)

  • Accretion disk (UV, X-rays)

  • Dusty torus (IR)

  • Gas clouds near the black hole and disk traveling at high speeds (broad line region)

  • Gas clouds far from the black hole traveling at low speeds (narrow line region)

  • Jets (sometimes -- highly relativistic particles, synchrotron radiation, gamma rays)

An AGN Cartoon

Artistic Impression of an AGN

AGN -- The Movie


  • Blazars are AGNs with jets that are pointing directly at us.

  • We are looking down the throat of the dragon!

  • Radiation from the jet swamps out all other sources of radiation.

  • Highly relativistic beams of particles in the jets produce the observed gamma rays.

Types of Blazarsblazar classification

Slope of the energy distribution in the visible

  • Brighter toward longer wavelengths (lower energy) -- Red Blazar

  • Synchrotron peak at lower energies -- LBL

  • Brighter toward shorter wavelengths (higher energy) -- Blue Blazar

  • Synchrotron peak at higher energies -- HBL

The Grand Blazar Sequence


The Global Telescope Network (GTN) was originally created to observe blazars.

The GTN Observing Program

  • EGRET sources that have optical identifications

  • Optical sources bright enough to be observed using small to moderate size telescopes

  • 26 blazars

    • 13 bright (magnitude 12 to 16), 13 faint (magnitude 17)

    • But they are all variable! (up to 4 magnitudes)

  • Accumulate and archive as much photometric data as possible

  • CCD (V and I bands), plus visual surveillance

  • Support for the GLAST mission (“The Optical Eyes of GLAST”)

  • Learn about these perplexing energetic objects!

Something for everyone…

  • Observing

  • Analyzing data

  • Monitoring your favorite blazar

  • Making discoveries

  • Mentoring others

GTN Blazars and Polars


  • The gamma ray observatory!

  • Long term multi-wavelength observations

  • Follow-up optical observations of gamma ray events

  • Optical activity can trigger pointed observations

  • New blazars will be discovered… some will be bright!

But, what are we all really here for…

But, what are we all really here for…



  • Yes, blazars are variable.

  • Blazars are actually highly variable.

  • In their own way, blazars are among the most highly variable objects known.

  • Blazars do not appear to be periodic at any level. There are no periods or cycles.

  • Blazars are irregular variables.

Irregular Variability

  • Long-term (decades, years, months)

  • Intraday (night to night)

  • Microvariability (within a night, hours)

  • Blazars are non-periodic seemingly over all time scales.

  • There are three time scales that are normally considered.

A blazar with a long history

More OJ 287

Double peaks?

Outbursts - 1982,1994

Are the jets precessing or wobbling?

Intraday Variability

PKS 0537-441

Romero et al. (2000)


November 2004


Mrk 501 Carini (1982)

Mrk 501 - AAVSO


How do we know if an object is variable?

  • Look at its lightcurve.

  • Compute some statistics.

  • No matter how you do it you need a local comparison star, and you need to check on the comparison.

  • The more comparisons the better!

Standard variable star procedures…

V - variable

C - comparison

K - check

Differential Lightcurves



Multiple Comparison StarsEnsemble Photometry

GTN sequence for PKS 0537-441

Some simple statistics…

Standard Deviation

Variability Confidence Criterion

>2.576 indicates variability at a confidence level of 99%

How variable is it?

  • Use the amplitude of the light curve.

  • Use the variability confidence index.

What is going on here?

What is the time scale of the variability?

  • Time scales for observation of significant variability

  • Presumable related to physical sizes of processes causing variability

  • Limited by variability amplitude considered

  • Limited by observational interval considered

  • Must be corrected for cosmological effect

Variability Timescales

  • Duty Cycle

  • Fraction of time an object is active or variable

  • One object with a number of observing sessions, or a collection of objects

  • - corrected length of observing session i

  • N=1 (variable)

  • N=0 (non-variable)

Visual Inspection

- time between maximum and minimum brightness

z is the redshift

Some Timescale ResultsTypical minimum variability time scale --> several hours (1to3 h)

Sample of 86 AGNs

Incidence of microvariability

Duty Cycle

* --> blazars

What is the source of the variability?

  • Precession or wobbling of the jets (long term)

  • Shock fronts in the jets

  • Instabilities or hot spots in the accretion disk

  • Gravitational microlensing (extrinsic)

Blazar music

Does your head hurt yet from trying to understand the blazars?

Let’s try the other side of our brains.

Blazar music

Could we interpret a blazar lightcurve as a piece of music?

Could we “hear” the blazars?

Musical interpretation by Jim Webb (FIU, SARAH)

  • Looks sort-of like musical notation?

  • Determine max and min in lightcurve and assign musical notes.

  • From A below middle C to two octaves above middle C

  • Timing… depends on telescope availability and weather, but…

time between observations

Dt > 20 days, whole note+rest

Dt < 5 days, sixteenth note

PKS 1156+295 (long term)

The GTN Observing Program…some recommendations for blazar observers

  • Always use comparison and check stars.

  • Strive for SNR ~100 or magnitude errors of +/- 0.01 or better.

  • Always obtain several data points.

    • 3 to 5 minimum

    • Internal consistency checks, statistics

  • Compute standard deviation for program object lightcurve and check starlight curve, compute variability confidence index.

  • Consider using more than one comparison star.

  • For non-stellar targets use analysis aperture radius at least 2X FWHM for stars.

The GTN Observing Program…some tools for blazar observers

  • The GTN web site -

  • Targets for everyone (observing list)

  • Finding charts and sequences (AAVSO)

  • Observing list uploadable to TheSky

  • Tool to generate scripts for ACP

  • Sample images

  • Image archive

  • Current lightcurves (AAVSO)

Why observe blazars?

Why observe blazars?

  • We can learn about jets and the processes that produce jets.

  • For a blazar, the jet swamps out everything else.

  • With enough data we can begin to understand jets and the central engine.

  • When you observe blazars, you are seeing as close as humans may ever see to the central engine of an AGN. ( “down the throat of the dragon” )

  • By observing blazar jets we get to see the most direct effects of the central supermassive black hole.

  • When you observe a blazar, you are looking right at a naked supermassive black hole.

The future is bright…

The future is bright…

Especially if you are observing blazars

AAVSO-GTN BL Lac CampaignNovember-December 2004

BL Lac - campaign1

All data - 1445 points -

10 days - 1077 points

5 days - 950 points

Nov. 14 - 361 points

Nov. 15 - 197 points - 0.4 mag decline

AAVSO-GTN Mrk 501 ObservationsMarch 2005

Microvariability for PKS 0308-612

Microvariability for PKS 0537-441

0.1 mag in a night (or less)

Microvariability for PKS 0537-441

Some nights you just lose!

Variability Time Scales and Duty Cycles

The Synchrotron ProcessProbably produces the synchrotron peak

Inverse Compton ScatteringProbably produces the compton peak

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