Multiwavelength astronomy is
extreme astronomy!
OUTLINE
Importance of Multiwavelength Astronomy
o Some Basics
o A Picture of our Universe
o How Images are Made
o How Photons are Made
o
• In particular, Gamma Rays
o
Image isn’t Everything
• Spectral and Time domain information
o
The Blazar Example
Lucy Fortson - Extreme Astronomy Short Course
September 26, 2004
Importance of Multiwavelength astronomy
 No astrophysical object emits photons at a single wavelength.
 Some objects have many photon producing and changing
mechanisms going on at once
o
Need multiwavelength astronomy to piece together whole picture
 Because we can’t always see all wavelengths, we use other
wavelengths to detect objects.
Lucy Fortson Extreme Astronomy Short Course
September 26, 2004
What does the em-spectrum tell us?





Transports energy
Electric and magnetic fields oscillate: that’s the “wave”
Moves at speed of light, 3 x 108 m/s
Wavelength, frequency, energy all related
Type of radiation (usually) depends on energy/temperature of object
Lucy Fortson Extreme Astronomy Short Course
September 26, 2004
Putting it into perspective
Atmospheric effects
 Only visible, most radio and some infra-red gets through air!
 To see Gamma-ray, X-ray, UV and some IR, need to get above atmosphere.
o
Can indirectly “see” gamma-rays from ground through airshowers.
Lucy Fortson Extreme Astronomy Short Course
September 26, 2004
A Picture of our universe
 There’s a lot happening in the photon universe - in
spite of Dark Matter and Dark Energy
 From the objects in our Solar System to the furthest
quasar and even the Big Bang itself, photons are
emitted, scattered, absorbed and otherwise mangled
on their way to us.
 Let’s take a brief tour…
Lucy Fortson Extreme Astronomy Short Course
September 26, 2004
Moon
Galaxy
Heliosphere
IGM
Mag
Field
GCBH
GRB
Stellar
BH
Earth
Planets
ISM
IPM
Nebula
AGN
Supernova
Sun
Solar
wind
Photons
Moon
Visible
Infra-red
X-ray
UV
Radio
Sun
Visible
X-ray
Radio
EUV
IR
X-ray
Visible
Jupiter
Infrared
Radio
Saturn
Interplanetary Medium
• Dust
• Gas
• Magnetic Fields
• Cosmic Rays
Open cluster:
Pleiades - M45
at 380 ly
radio
x-ray
visible
near IR
ultra violet
Planetary Nebula
Dumbbell - M27
at 1250 ly
x-ray
Radio
Far IR
visible
near IR
Emission Nebula
(M17 - Omega Nebula)
5000 light years away
in Sagittarius
x-ray
radio
mid-IR
far IR
near IR
UV
Crab Nebula
M1 - 6300 ly
in Taurus
X-ray
Gamma-ray
Radio
Visible
Radio, Visible, X-ray
visible
short radio
infrared
x-ray
Multi-wavelength
Milkyway
long radio
gamma sources
low x-ray
ultraviolet
gamma-ray
Andromeda Galaxy
M31 - 2.9 mil ly
ultraviolet
mid-IR
visible
radio
x-ray
X-Ray
Centaurus A
10 mil ly
UV
Vis: Hubble
Vis: Ground
Mid IR
Near IR
Radio
Gamma Ray
More than just a pretty picture
 An “image” is made up of pixels containing number of
photons received by a detector
o
o
o
depends on sensitive range of detector
may be combinations of two or three “filters”
color is usually artificially determined
Hubble site example
 Comparing images of an object in different
wavelengths can tell us about the many processes
going on.
Lucy Fortson Extreme Astronomy Short Course
September 26, 2004
The peculiar Centaurus A
 This peculiar galaxy resulted from merging an elliptical and spiral
galaxy
 Colors tell us:
o blue - new stars
o red - old stars
o black - dust lanes
Lucy Fortson Extreme Astronomy Short Course
September 26, 2004
Timelapse images of Supernova 1987a
 Comparing visible, x-ray and radio
shows radical changes.
 Supernova blast wave reaches
surrounding material
 X-ray, radio images show where real
hotspots are:
o
o
radio confirms high energy electrons
in mag field
x-ray indicates temperatures of blast
millions of degrees
Lucy Fortson Extreme Astronomy Short Course
September 26, 2004
A little isn’t enough
 In some cases, images are
made because that is what is
expected.
 Each “new” wavelength goes
thru phase of low statistics
and/or low resolution.
 TeV gamma ray astronomy
has come of age with new
detectors - useful images!
HESS images
Crab Nebula
SNR RXJ1713
Lucy Fortson Extreme Astronomy Short Course
September 26, 2004
Image isn’t everything
 Images only tell part of the story
o
After all, x-ray and gamma-ray astronomy has told us lots before we got
to the point where we could make an “image”.
 Plot parameters of photons to understand the information
behind the image:
o
o
intensity versus energy (spectrum)
intensity versus time (light curve)
 But to understand all this - we need to know how photons are
made!
Lucy Fortson Extreme Astronomy Short Course
September 26, 2004
How photons are made or modified
 Thermal Radiation
 Nuclear, atomic or molecular excitations
o
(absorption, emission lines)
 Acceleration or de-acceleration of charged particles
 Elementary particle decay
 Scattering (gain or lose energy)
Lucy Fortson Extreme Astronomy Short Course
September 26, 2004
Thermal radiation
 Anything above absolute
zero emits EM radiation
o
Stars, gas, planets, YOU!
 “Blackbody Radiation”
o
o
The hotter an object the
higher the intensity
The hotter an object the
higher frequency the peak
emission.
Lucy Fortson Extreme Astronomy Short Course
September 26, 2004
Emission and absorption effects
 A spectrum may be modified
by medium it passes through
 Thermal spectrum of Sun from
photosphere is modified:
o
o
by its chemical elements to
produce absorption lines
by the corona (hot plasma) to
produce emission lines
Lucy Fortson Extreme Astronomy Short Course
September 26, 2004
Extreme effects from extreme astronomy
 The high energy world invades the
imagination
o
o
The Hulk created by gamma rays
Fantastic Four gain powers by exposure
to cosmic rays
Lucy Fortson Extreme Astronomy Short Course
September 26, 2004
How Gamma-rays are made
 Gamma-rays are emitted through four basic processes:
o
o
o
Transitions between nuclear energy levels (line emission)
Annihilation of particles with antiparticles (line emission)
Decays of elementary particles (broad band emission)
• neutral pion decay is major player in gamma ray astronomy
o
Acceleration of charged particles
• Bremsstrahlung - field around nucleus
• Synchrotron - static magnetic field
• Compton scattering - EM field of photon
Lucy Fortson Extreme Astronomy Short Course
September 26, 2004
High Energy emission mechanisms (1)
 Bremsstrahlung - “breaking radiation”
o
Radiation is emitted when charged particles accelerate in the field of an
ion
Lucy Fortson Extreme Astronomy Short Course
September 26, 2004
high energy emission mechanisms (2)
 Synchrotron - “ magnetic spin radiation”
o
o
Caused by a relativistic electron as it spirals around a magnetic field line
Non-relativistic version is called cyclotron radiation
Lucy Fortson Extreme Astronomy Short Course
September 26, 2004
high energy emission mechanisms (3)
 Compton Scattering - “rebound radiation”
o
o
A high-energy photon hits a low-energy electron. The photon loses energy, and
the electron gains some.
Inverse Compton Scattering: A low-energy photon hits a relativistic electron.
The photon gains energy, becoming an X- or gamma-ray.
Lucy Fortson Extreme Astronomy Short Course
September 26, 2004
The spectral keys
 Supernova remnant Cas A observed in gamma rays
o
What emission mechanism is at work?
 Dotted line
o neutral pion decay
 Dashed line
o Bremsstrahlung and
Compton, B=1.6
mG
 Solid line
o Brem + Compton,
B=1 mG
Lucy Fortson Extreme Astronomy Short Course
September 26, 2004
Time domain
 Multiwavelength light curves of seven pulsars seen in HE gammas
o
o
measure intensity versus time
pulsars repeat, so build up peaks
Lucy Fortson Extreme Astronomy Short Course
September 26, 2004
The multiwavelength story of AG(N)
 Active galaxies have very high
luminosities
o
o
large amount of star formation
accretion driven jets
 Multiwavelength analysis helps
figure out which is which:
o
Arp 220 versus Centaurus A
Lucy Fortson Extreme Astronomy Short Course
September 26, 2004
Active Galactic Nuclei
(3c219 courtesy NRAO/HST)
• Giant elliptical
galaxies
• Black hole at
center
• Relativistic
jets, accretion
power
Lucy Fortson Extreme Astronomy Short Course
September 26, 2004
Active galaxy’s Shocking blobs
 Jets of M87
o
o
knots of material ejected out of
central core propagate down
the jet
seen side on
 What happens when jet is
aimed at Earth?
 Blazar!
o
beamed gamma ray and x-ray
emission
Lucy Fortson Extreme Astronomy Short Course
September 26, 2004
blazar Light Curves
 Building up data from
different wavelengths
over time
o
o
variability seen on minutes,
days, years
correlations in flares
between wavelengths
Lucy Fortson Extreme Astronomy Short Course
September 26, 2004
Mrk 501 spectral energy distribution
 Correlation in variability between
synchrotron and g-ray emission
naturally explained by IC:
o
Same population of electrons
produce both components.
 g-Ray measurements provide
separate constraint on electron
energy, breaks degeneracies.
Lucy Fortson Extreme Astronomy Short Course
September 26, 2004
Blazar Emission Mechanisms
Current paradigm:
Synchrotron Self Compton
External Compton
Proton Induced Cascades
Proton Synchrotron
Energetics, mechanism for
jet formation and
collimation, nature of the
plasma, and particle
acceleration mechanisms
are still poorly understood
(Buckley,
Science,
Lucy Fortson Extreme Astronomy
Short
Course 1998)
September 26, 2004
AGNs: The Central Engine?
 More than
phenomenological
understanding of
radiative processes
 VHE g-rays provide
probes of strong gravity
close to the central
engine
Lucy Fortson Extreme Astronomy Short Course
September 26, 2004
Summary
 Multiwavelength astronomy is relatively new
o
radio since 1950’s, x-ray, gamma-ray since 1970’s
o
coordination amongst different astronomy cultures
 Just learning the best ways to utilize MWL as tool
• targets of opportunity
o
o
merging data archives from different groups
time-domain (building lightcurves) is resource intensive
 MWL is extreme because it pulls together all the information
we have on different objects:
MWL WILL DOMINATE THE FUTURE OF
ASTRONOMY
Lucy Fortson Extreme Astronomy Short Course
September 26, 2004
Image Resources for M ulti wave length Astronomy Presentation

Tour of the Universe Images
o Cool Cosmos websit e wit h FABU LOUS detail ed info:
http:/ /coolcosmos.ipac.calt ech.edu
o Hubb le sit e wit h a ll of i ts usual help:
h ttp :/ / h u b b le s ite .o rg
h ttp :/ / h u b b le s ite .o rg / s ci.d .te ch / b e h in d _th e _p ictu re s / m e a nin g _o f_
co lor/ filte rs .s h tm l
o Astronomy Picture of the D ay websit e:
http:/ /antwrp.gsfc.nasa .gov/apod/ap020920 .html
o Great Exp lorations in Math and S cience websit e:
http:/ /www. lhsgems.org/IUtour.html

Electro magnetic Spectrum, Emission Mechanisms
o GLAST expe riment EPO webpag e:
http:/ /gla st.sonoma.edu/resour ces/presentations .html