BLAZARS of Our TimesThe nature and remarkable variability

of the blazars

Dr. Gordon G. SpearNASA E/PO Group

Department of Physics and AstronomySonoma State University

AAVSO-HEA3, Las CrucesMarch 2005

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

Publications About Blazars

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180

1975 1980 1985 1990 1995 2000 2005

Year of Publication

Number

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!

Some Categories of AGNs

• Seyfert Galaxies– Sy 1, Sy 2

• Radio Galaxies– Narrow Line Radio

Galaxies (NLRG... FR I, FR II)

– Broad Line Radio Galaxies (BLRG)

• Quasars– Broad Absorption Line

Quasars (BALQ)– Steep Spectrum Radio

Quasars (SSRQ)– Flat Spectrum Radio

Quasars (FSRQ)

• Optically Violent Variables (OVV)

• Blazars

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

BLAZARSBLAZARS• 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 GTN

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

GLAST

•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…

Variability!

Variability!

• 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 VariabilityBL Lac Campaign - 10 days

13.5

14.0

14.5

2453317 2453322 2453327

JD

CCDV mag

BL Lac - AAVSONovember 2004

PKS 0537-441Romero et al. (2000)

Microvariability

BL Lac Campaign - Nov. 15 UT

13.8

13.9

14.0

14.1

14.2

14.3

2453325.4 2453325.6 2453325.8

JD

CCDV mag

Mrk 501 Carini (1982)

Mrk 501 - AAVSO

BL Lac - 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 - variableC - comparisonK - check

Differential LightcurvesV-CK-C

Multiple Comparison StarsEnsemble Photometry

GTN sequence for PKS 0537-441

Some simple statistics…Standard Deviation Variability Confidence Criterion

€

σ =(1/n) (xi−x )2

i∑

⎡

⎣

⎢ ⎢ ⎢

⎤

⎦

⎥ ⎥ ⎥

1/2

x =(1/n) xii∑

€

ΔmV

=V −C

ΔmK =K−C

σ V =σ (ΔmV )

σ K =σ (ΔmK )

CV

=σ Vσ K

>2.576 indicates variability at a confidence level of 99%

€

CV

How variable is it?

• Use the amplitude of the light curve.

• Use the variability confidence index.

Nightly Variability Measures

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0.10

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C-index

Deltamag (V)

variable

non-variable

What is going on here?

Variability Measures for Selected Objects

0.000.050.100.150.200.250.300.350.400.450.50

0 5 10 15 20

C-index

Deltamag (V)

0235+164

0537-441

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

Visual Inspection

- time between maximum and minimum brightness

z is the redshift

€

ΔtV

€

tV =(1+z)−1ΔtV

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)

€

DC=100Ni(1/Δti )

i∑

(1/Δtii

∑

⎡

⎣

⎢ ⎢ ⎢ ⎢

⎤

⎦

⎥ ⎥ ⎥ ⎥

€

Δti

€

Δti=(1+z)−1Δtobs

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

Radio Loud

RL 23

X-ray Loud

XL 3

Gamma Loud

GL* 20

Radio Quiet

RQ 40

RL 74%

XL 30%

GL* 50%

RQ 11%

Incidence of microvariability

Sample of 86 AGNs

RL 68%

XL 28%

GL* 49%

RQ 7%

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 observationsΔt > 20 days, whole note+restΔt < 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 - http://gtn.sonoma.edu• 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 Campaign

November-December 2004

BL Lac - campaign1

All data - 1445 points -

BL Lac Campaign

13.5

14.0

14.5

2453315 2453335 2453355

JD

CCDV mag

10 days - 1077 points

BL Lac Campaign - 10 days

13.5

14.0

14.5

2453317 2453322 2453327

JD

CCDV mag

5 days - 950 points

BL Lac Campaign - 5 days

13.5

14.0

14.5

2453322 2453323 2453324 2453325 2453326 2453327

JD

CCDV mag

Nov. 14 - 361 points

BL Lac Campaign - Nov. 14 UT

13.6

13.7

13.8

13.9

14.0

2453324.2 2453324.4 2453324.6

JD

CCDV mag

Nov. 15 - 197 points - 0.4 mag decline

BL Lac Campaign - Nov. 15 UT

13.8

13.9

14.0

14.1

14.2

14.3

2453325.4 2453325.6 2453325.8

JD

CCDV mag

AAVSO-GTN Mrk 501 Observations

March 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