Multiwavelength astronomy is extreme astronomy!

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Multiwavelength astronomy is extreme astronomy!

• OUTLINE
• Importance of Multiwavelength Astronomy
• Some Basics
• A Picture of our Universe
• How Images are Made
• How Photons are Made
• In particular, Gamma Rays
• Image isnt Everything
• Spectral and Time domain information
• The Blazar Example

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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
• Need multiwavelength astronomy to piece together
  whole picture
• Because we cant always see all wavelengths, we
  use other wavelengths to detect objects.

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What does the em-spectrum tell us?

• Transports energy
• Electric and magnetic fields oscillate thats
  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

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Putting it into perspective
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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.
• Can indirectly see gamma-rays from ground
  through airshowers.

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A Picture of our universe

• Theres 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.
• Lets take a brief tour

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Moon
Galaxy
Heliosphere
GRB
IGM
Mag Field
Stellar BH
GCBH
Earth
Planets
ISM
IPM
Nebula
AGN
Supernova
Solar wind
Sun
Photons
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Moon
Visible
UV
Infra-red
Radio
X-ray
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Sun
Visible
EUV
X-ray
Radio
IR
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X-ray
Jupiter
Visible
Infrared
Radio
Saturn
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• Interplanetary Medium
• Dust
• Gas
• Magnetic Fields
• Cosmic Rays

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Open cluster Pleiades - M45 at 380 ly
radio
x-ray
visible
ultra violet
near IR
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Planetary Nebula Dumbbell - M27 at 1250 ly
x-ray
Radio
Far IR
visible
near IR
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Emission Nebula (M17 - Omega Nebula)
5000 light years away in Sagittarius
x-ray
radio
mid-IR
far IR
near IR
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UV
Crab Nebula M1 - 6300 ly in Taurus
Gamma-ray
X-ray
Radio
Visible
Radio, Visible, X-ray
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visible
Multi-wavelength Milkyway
low x-ray
short radio
ultraviolet
long radio
infrared
gamma-ray
gamma sources
x-ray
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Andromeda Galaxy M31 - 2.9 mil ly
ultraviolet
mid-IR
visible
radio
x-ray
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Centaurus A 10 mil ly
X-Ray
UV
Vis Hubble
Vis Ground
Mid IR
Near IR
Radio
Gamma Ray
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More than just a pretty picture

• An image is made up of pixels containing
  number of photons received by a detector
• 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.

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The peculiar Centaurus A

• This peculiar galaxy resulted from merging an
  elliptical and spiral galaxy

• Colors tell us
• blue - new stars
• red - old stars
• black - dust lanes

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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
• radio confirms high energy electrons in mag field
• x-ray indicates temperatures of blast millions of
  degrees

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A little isnt 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
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Image isnt everything

• Images only tell part of the story
• 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
• intensity versus energy (spectrum)
• intensity versus time (light curve)
• But to understand all this - we need to know how
  photons are made!

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How photons are made or modified

• Thermal Radiation
• Nuclear, atomic or molecular excitations
• (absorption, emission lines)
• Acceleration or de-acceleration of charged
  particles
• Elementary particle decay
• Scattering (gain or lose energy)

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Thermal radiation

• Anything above absolute zero emits EM radiation
• Stars, gas, planets, YOU!
• Blackbody Radiation
• The hotter an object the higher the intensity
• The hotter an object the higher frequency the
  peak emission.

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Emission and absorption effects

• A spectrum may be modified by medium it passes
  through
• Thermal spectrum of Sun from photosphere is
  modified
• by its chemical elements to produce absorption
  lines
• by the corona (hot plasma) to produce emission
  lines

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Extreme effects from extreme astronomy

• The high energy world invades the imagination
• The Hulk created by gamma rays
• Fantastic Four gain powers by exposure to cosmic
  rays

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How Gamma-rays are made

• Gamma-rays are emitted through four basic
  processes
• 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
• Acceleration of charged particles
• Bremsstrahlung - field around nucleus
• Synchrotron - static magnetic field
• Compton scattering - EM field of photon

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High Energy emission mechanisms (1)

• Bremsstrahlung - breaking radiation
• Radiation is emitted when charged particles
  accelerate in the field of an ion

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high energy emission mechanisms (2)

• Synchrotron - magnetic spin radiation
• Caused by a relativistic electron as it spirals
  around a magnetic field line
• Non-relativistic version is called cyclotron
  radiation

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high energy emission mechanisms (3)

• Compton Scattering - rebound radiation
• 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.

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The spectral keys

• Supernova remnant Cas A observed in gamma rays
• What emission mechanism is at work?

• Dotted line
• neutral pion decay
• Dashed line
• Bremsstrahlung and Compton, B1.6 mG
• Solid line
• Brem Compton, B1 mG

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Time domain

• Multiwavelength light curves of seven pulsars
  seen in HE gammas
• measure intensity versus time
• pulsars repeat, so build up peaks

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The multiwavelength story of AG(N)

• Active galaxies have very high luminosities
• large amount of star formation
• accretion driven jets
• Multiwavelength analysis helps figure out which
  is which
• Arp 220 versus Centaurus A

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Active Galactic Nuclei
(3c219 courtesy NRAO/HST)

• Giant elliptical galaxies
• Black hole at center
• Relativistic jets, accretion power

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Active galaxys Shocking blobs

• Jets of M87
• knots of material ejected out of central core
  propagate down the jet
• seen side on
• What happens when jet is aimed at Earth?
• Blazar!
• beamed gamma ray and x-ray emission

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blazar Light Curves

• Building up data from different wavelengths over
  time
• variability seen on minutes, days, years
• correlations in flares between wavelengths

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Mrk 501 spectral energy distribution

• Correlation in variability between synchrotron
  and g-ray emission naturally explained by IC
• Same population of electrons produce both
  components.
• g-Ray measurements provide separate constraint on
  electron energy, breaks degeneracies.

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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, 1998)
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AGNs The Central Engine?

• More than phenomenological understanding of
  radiative processes
• VHE g-rays provide probes of strong gravity
  close to the central engine

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Summary

• Multiwavelength astronomy is relatively new
• radio since 1950s, x-ray, gamma-ray since 1970s
• Just learning the best ways to utilize MWL as
  tool
• coordination amongst different astronomy cultures
• targets of opportunity
• 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

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