NASAs GLAST Program and the Future of GammaRay Astronomy Where Particle Physics Meets Orbiting Astro

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NASAs GLAST Program and the Future of Gamma-Ray
AstronomyWhere Particle Physics Meets Orbiting
Astronomy

• Tim Brennan
• Woodstock Union High School, Woodstock, VT
• GLAST Educational Ambassador
• Northeast Regional AAPT
• April, 2003

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GLAST Gamma-ray Large Area Space Telescope

• Orbits Earth, a la Hubble Space Telescope and
  other scopes that need supra-atmospheric views of
  Cosmos
• Outfitted with gamma-ray detectors, a la SLAC,
  Fermilab, etc.
• Combines expertise of particle physicists,
  astrophysicists, and rocket scientists

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To Be Launched in . . . 2006

• Currently under development at Stanford Linear
  Accelerator facility
• Like all orbiters, extensive testing is necessary
  . . . repairs are costly options.

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Why Orbit? Its not just a distortion problem!
Short Wavelength Long Wavelength High
Frequency Low Frequency High Energy Low
Energy
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High energy? How high?

• Typical Gamma-ray photons have a frequency of 10
  23 Hz, about one billion times higher than a
  visible light photon
• They are, therefore, a billion times more
  energetic than a visible light photon some are
  even more energetic than that!
• A burst of gamma-ray can achieve a luminosity
  of 10 45 watts the equivalent of 10 19 Suns!
• Quasars, notoriously luminous cosmic beasts,
  radiate out, conservatively, 1/1000th the power
  of a GRB!

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A look inside . . .
Source glast.sonoma.edu
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Detection Scheme Features

• Each tower is made of interleaved silicon strip
  detectors and lead converters
• Lead stops gamma-rays and, via collisions,
  creates an electron-positron pair.
• Particles pass through the silicon strips
  generating electric pulses
• Strips are oriented in x direction on one
  layer, then y direction on next
• Path of particle can be reconstructed by looking
  at successive positions in the silicon strips
• Vertex can be precisely reconstructed because the
  strips are narrow ( 200 um)

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Intro to Gamma-ray Astronomy

• Two general types of events . . .
• Steady sources of gamma-rays, seemingly linked to
  active galactic nuclei (AGNs)
• Brief, tumultuous, gamma-ray bursts (GRBs)
  associated with . . . ???

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AGNs

• These nuclei are active home to super-massive
  black holes
• Orientations of host galaxies can mask underlying
  similarities
• GLAST to detect thousands of these, increasing
  the catalog exponentially

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Some sources are nearby . . .

• This is the Milky Way in gamma-rays
• Note the galactic disk and cluster near the
  galactic center
• A composite photo accumulated by the EGRET
  instrument on the Compton Gamma-Ray Observatory

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The First Burst

• Vela satellite fleet launched to detect
• nuclear weapons test in late 60s
• Multiple satellites flown
• allowed crude position
• determination and could
• test for coincidence
• In 1969, data from 1967 found
• which showed a burst that was
• clearly not a clandestine bomb
• test (plot on right)
• 16 bursts found between 1969 and 1972

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The GRB Gallery
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If youve seen one GRB, youve seen ONE GRB.

• Some show the single rapid burst followed by
• a longer secondary burst
• Some are relatively smooth, others spiky
• Durations range from 30 milliseconds to 1000
  seconds

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Compton Gamma Ray Observatory (1991-2000) BATSE
instruments
8 instruments on corners of spacecraft NaI
scintillators that interact with gamma-rays
creating lower energy photons detectable by
photo- multiplier tubes . . . GLASTs
Cesium-Iodide calorimeter is quite similar
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The Big Questions What and Where
Sparse data makes for guessing games
Clearly, dealing with high energy events
But, a clue eventually became apparent
GRBs are evenly spread across the whole sky!
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So, where are they?

• Clearly, they are not bound to the galactic
  plane, as BATSE distribution shows.
• But, could they be part of the galactic halo?
  Their position, in the third dimension, was
  unknown.
• More data were needed the answer was to be found
  in afterglow light.

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Afterglow?
8 hours X-ray image
3 days X-ray image
BeppoSax, Italian Space Agency X-ray spacecraft,
detected the X-ray afterglow of a gamma-ray
burst on February 28, 1997 (GRB970228). While
gamma-rays may only persist for a short duration,
the object will continue to emit less energetic
light for some time, eventually dropping down
into the visible and radio ranges!
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Establishing distances . . . Always the tricky
part!

• As a gamma-ray burst decays and gives way to
  other types of light, other telescopes can make
  observations.
• X-ray telescopes like BeppoSAX, Chandra, and XMM
  Newton, were able to see sources in x-rays.
• Ground-based radio and optical telescopes could
  identify the galaxies within which these bursts
  had occurred.
• Conclusion Bursts had occurred in galaxies that
  are, typically, billions of light-years away!

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The new problem
So, these are very powerful objects (1000x
luminosity of quasars) which are at very great
distances.
What can do that?
Hypernova
Binary neutron star merger
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The Supernova Connection
GRB011121
Afterglow faded like supernova
Data showed presence of gas like a stellar wind
Indicates some sort of supernova and not a NS/NS
merger
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Evidence mounts for Supernova sources

• Iron-absorption lines in x-ray spectrum of
  GRB990705
• Chandra X-ray Observatory detects iron emission
  lines in GRB991216
• XMM Newton detects silicon, argon, sulfur, etc.
  in GRB011211, indications of supernova event

Source The Brightest Explosions in the
Universe, N. Gehrels, L. Piro, P.
Leonard, Scientific American, December, 2002
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So, a supernova creating a neutron star or black
hole is a natural candidate for a GRB progenitor.

• If GRB progenitor is beamed . . .
• Energy estimate is pushed downward
• But, by necessity then, the Universe
• must be populated with much larger
• numbers of GRBS than have been spotted

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Lets review What do we know about GRBs?
The data seem to indicate two kinds of GRBs

• Those with burst durations less than 2 seconds

• Those with burst durations more than 2 seconds

Short bursts tend to produce harder gamma rays,
as predicted by the NS/NS merger model
Long bursts tend to produce softer gamma rays,
as predicted by the hypernova merger model
There is a great need to study the short duration
events. Will we find, as in the case of the
long-duration events, some corroborating
evidence for our model?
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Many difficult questions remain

• There are many times more supernovae than there
  are GRBs Not every supernova creates a GRB. So,
  why some and not others?
• Why the variation in GRB spectra?
• How, exactly, can a neutron star pair merger or
  supernova collapse, create the GRBs?
• Some GRBs last for an hour or more! Very little
  is understood about this phenomena.
• Some GRBs are extremely short and, again, more
  information is needed.

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GLAST Program Goals

• New techniques will allow for greater precision
  of locations of gamma-ray objects
• Greater coverage of the gamma-ray spectrum (both
  lower and higher energy thresholds than previous
  missions)
• Increase the gamma-ray catalog
• Increase understanding of gamma-ray sources

Source glast.sonoma.edu
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Acknowledgments

• Special thanks to Phil Plait, at Sonoma State
  University, for his guidance, information, and
  use of his slides!
• Thanks to Lynn Cominsky, also at Sonoma State,
  for her training and guidance on the GLAST
  project.
• For more information about the GLAST program see
• www.glast.sonoma.edu.
• And a special thank you to Donna Young of Tufts
  University and the Chandra X-ray Telescope
  Educational Outreach Program for her insight and
  support.

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Magazine Articles on Gamma-ray Bursts

• Check out The Brightest Explosions in the
  Universe, by Neil Gehrels, Luigi Piro, and Peter
  Leonard, December, 2002, Scientific American
• And Stalking Cosmic Explosions, by Govert
  Schilling, February, 2003, Astronomy

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Recent Books about Gamma-ray Bursts

• The Biggest Bangs The Mystery of Gamma-Ray
  Bursts, the Most Violent Explosions Ever,
  Jonathan I. Katz, Oxford University Press, 2002
• Flash! The Hunt for the Biggest Explosions in
  the Universe, Govert Schilling, Cambridge
  University Press, 2002

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Magazine Articles on High-Energy Astronomy

• Quasars Explained, by William Keel, February,
  2003, Astronomy
• Astronomys Phantom Foul Balls Ultrahigh-energy
  Cosmic Rays, by Ivan Semeniuk, March, 2003, Sky
  Telescope
• Super X-ray Vision, by Michael Klesius,
  December, 2002, National Geographic