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?
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 …
Publications About Blazars
180
160
140
120
Number
• 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
100
80
60
40
20
0
1975
1980
1985
1990
1995
Year of Publication
2000
2005
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 Unification
One 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
• 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 Blazars
blazar 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
•Blazars are nonperiodic seemingly
over all time scales.
•There are three
time scales that are
normally
considered.
• Long-term
(decades, years,
months)
• Intraday (night to
night)
• Microvariability
(within a night,
hours)
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)
BL Lac Campaign - 10 days
CCDV mag
13.5
14.0
14.5
2453317
2453322
JD
BL Lac - AAVSO
November 2004
2453327
Microvariability
Mrk 501 Carini (1982)
Mrk 501 - AAVSO
BL Lac Campaign - Nov. 15 UT
13.8
CCDV mag
13.9
14.0
BL Lac - AAVSO
14.1
14.2
14.3
2453325.4
2453325.6
2453325.8
JD
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
V-C
K-C
Multiple Comparison Stars
Ensemble Photometry
GTN sequence for
PKS 0537-441
Some simple statistics…
Standard Deviation

Variability Confidence Criterion

1/2
 (1/n) (x x )2 
i




i
m V C
V
mK KC
x (1/n) x
i
i
 V  (mV )
 K  (mK )
V
C 
V 


K
CV >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.
Nightly Variability Measures
Deltamag (V)
0.50
0.40
0.30
variable
non-variable
0.20
0.10
0.00
0
5
10
C-index
15
20
What is going on here?
Deltamag (V)
Variability Measures for Selected Objects
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0235+164
0537-441
0
5
10
C-index
15
20
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
Visual Inspection
•Fraction of time an object is active or
variable
•One object with a number of observing
sessions, or a collection of objects
tV - time between
maximum and minimum
brightness



 Ni (1/ti ) 

DC100 i

  (1/ti 
 i

tV (1z)1tV

z is the redshift



ti - corrected length of observing
session i
t (1z)1t
i
obs
N=1 (variable)
N=0 (non-variable)
Some Timescale Results
Typical minimum variability time scale --> several hours (1to3 h)
Sample of 86 AGNs
Radio
Loud
RL
23
X-ray
Loud
XL
3
Gamma
Loud
GL*
20
Radio
Quiet
RQ
40
* --> blazars
RL
74%
XL
30%
GL*
50%
RQ
11%
RL
68%
XL
28%
GL*
49%
RQ
7%
Incidence of
microvariability
Duty Cycle
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
t > 20 days, whole note+rest
t < 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
CCDV mag
13.5
14.0
14.5
2453315
2453335
2453355
JD
10 days - 1077 points
BL Lac Campaign - 10 days
CCDV mag
13.5
14.0
14.5
2453317
2453322
JD
2453327
5 days - 950 points
BL Lac Campaign - 5 days
CCDV mag
13.5
14.0
14.5
2453322
2453323
2453324
2453325
JD
2453326
2453327
Nov. 14 - 361 points
BL Lac Campaign - Nov. 14 UT
13.6
CCDV mag
13.7
13.8
13.9
14.0
2453324.2
2453324.4
JD
2453324.6
Nov. 15 - 197 points - 0.4 mag decline
BL Lac Campaign - Nov. 15 UT
13.8
CCDV mag
13.9
14.0
14.1
14.2
14.3
2453325.4
2453325.6
2453325.8
JD
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 Process
Probably produces the synchrotron peak
Inverse Compton Scattering
Probably produces the compton peak