The Universe >100 MeV

1
The Universe gt100 MeV

• Brenda Dingus
• Los Alamos National Laboratory

2
EGRET

• Compton Observatory
• 1991-2000
• BATSE, OSSE, and Comptel at lt MeV
• EGRET 30 MeV 30 GeV
• 1st proposed in late 1970s
• Spark Chamber with NaI calorimeter

3
GLAST
Instrument
16 towers ? modularity height/width 0.4 ?
large field-of-view Si-strips fine pitch 228
µm, high efficiency 0.44 X0 front-end ? reduce
multiple scattering 1.05 X0 back-end ? increase
sensitivity gt 1 GeV CsI wide energy range
0.1-100 GeV hodoscopic ? cosmic-ray rejection
? shower leakage correction XTOT 10.1
X0 ? shower max contained lt100GeV segmented
plastic scintillator ? minimize self-veto gt
0.9997 efficiency redundant readout
TKR
CAL
Expected Launch Date 2007 First of 16 towers
delivered March 2005 to integrate and test with
the spacecraft
ACD
4
GLAST Instrument Performance
More than 50 times the sensitivity of
EGRET Large Effective Area (20 MeV gt 300
GeV) Optimized Point Spread Function (0.35o _at_ 1
GeV) Wide Field of View (2.4 sr) Energy
Resolution (DE/E lt 10, E gt100 MeV)
5
Natures Particle Accelerators

• Electromagnetic Processes
• Synchrotron Emission
• E g a (Ee/mec2)2 B
• Inverse Compton Scattering
• E f (Ee/mec2)2 E i
• Bremmstrahlung
• E g 0.5 E e
• Hadronic Cascades
• p g -gt p po -gt e n g
• p p -gt p po -gt e n g

6
Exotic Gamma-Ray Production

• Particle-Antiparticle Annihilation
• WIMP called neutralino, c, is postulated by SUSY
  
• 50 GeVlt mclt few TeV
• Primordial Black Hole Evaporation
• As mass decreases due to Hawking radiation,
  temperature increases causing the mass to
  evaporate faster
• Eventually temperature is high enough to create a
  quark-gluon plasma and hence a flash of
  gamma-rays

7
High Energy Gamma-Ray Astronomy
Typical Multiwavelength Spectrum from High Energy
g-ray source
E 2 dN/dE or n F n

Energy Emitted
Radio
Optical
X-ray
GeV
TeV
Photon Energy
8
Crab Nebula
Electron Energies

• Spinning Neutron Star Fills Nebula with Energetic
  Electrons
• Synchrotron Radiation and Inverse Compton
  Scattering

9
Active Galactic Nuclei

• Massive Black Hole Accelerates Jet of Particles
  to Relativistic Velocities
• gt Synchrotron Emission and Inverse Compton
  and/or Proton Cascades

10
AGN Theory, e.g. WComae Blazar

• Electrons produce gammas via Inverse Compton
  scattering of synchrotron photons
• Protons produce gammas via m synchrotron

Boettcher, Mukherjee, A. Reimer, 2002
11
Gamma-Ray Bursts

• EGRET discovered GeV emission from 4 bright GRBs
  with no evidence of a spectral break at higher
  energies
• One GRB had GeV emission extending for over an
  hour

12
Typical GRB Broad Band Spectra
13
GRB 941017

• M.M. González, B.L. Dingus, Y. Kaneko, R.D.
  Preece, C.D. Dermer and M.S. Briggs, Nature, 424,
  749 (2003)
• This burst is the first observation of a distinct
  higher energy spectral component in a GRB
• Power released in higher energy component is more
  than twice the lower energy component
• Higher energy component decays slower than lower
  energy component
• Peak of higher energy component is above the
  energy range of the detector

-18 to 14 sec
14 to 47 sec
47 to 80 sec
80 to 113 sec
113 to 200 sec
14
GRB GeV-TeV Theories

• Requires GRBs are more energetic phenomena
• Different timescale of low and high energy
  implies an evolving source environment or
  different high energy particles
• Shape of high energy component applies tight
  constraints to ambient densities and magnetic
  fields
• Or evidence of origin of Ultra High Energy Cosmic
  Rays
• More and Higher Energy observations are needed

15
Gamma-Ray Detected Pulsars
16
Pulsars

• Extend of gamma-ray pulsars to of order 100
• Differentiate between different accelerators

17
gt100 MeV Astrophysical Sources

• Active Galactic Nuclei, Gamma Ray Bursts, and
  Pulsars are ONLY identified classes of individual
  sources.
• ¾ of EGRET point sources NOT identified with
  known objects.

Individual Examples of Sources Solar Flare Large
Magellenic Cloud X-ray Binary (?) Cen A (?)
18
Supernova Remnants (SNR)

• SNR are predicted by some to be source of cosmic
  rays
• 19 EGRET sources are positionally coincident with
  SNR
• Probability of chance coincidents 10-5
• Several are non-variable and spectra consistent
  with that expected by SNR
• However, other sources associated with SNR
• Pulsars that might not be known at other
  wavelengths
• Pulsar Wind Nebula accelerate electrons with
  energy of pulsar and the electrons radiate
  gamma-rays.
• See D. Torres et al. Physics Reports 2003 for
  review.

19
Supernova Remnants with GLAST

• Example of GLAST sensitivity to SNR
• Improved spectra to resolve po bump
• Improved localization to resolve correlation with
  dense proton target of molecular cloud

SNR g-Cygni
20
Galactic Plane

• Galactic Diffuse Spectrum of Region blt10 and
  300lt l lt60
• Nucleon-Nucleon (po decay) component should
  dominate above 1 GeV and should have the same
  E-2.7 differential photon spectrum as cosmic
  rays.
• However, the observed flux gt1 GeV is greater
  resulting in an E-2.4 differential photon
  spectrum.
• Strong, Moskolenko, Reimer 2004 require cosmic
  ray flux in galaxy gt2 times local flux
• Other theories such as increasing Inverse Compton
  ruled out by TeV observation of Galactic plane by
  Milagro

Hunter, et al. ApJ 481,205-240
Nucleon-Nucleon
Electron Bremstrahlung
Inverse Compton
Isotropic Diffuse E-2.1 (Extragalactic)
21
Extragalactic Diffuse

• Whats left over?
• Unresolved point sources
• Diffuse sources, both in and out of our galaxy
• No predicted sources can over produce this limit
  of diffuse emission

(Sreekumar et al. 1998)
22
Conclusions

• EGRET detected 300 sources
• 1/4 individual identifications
• Active Galactic Nuclei
• Pulsars
• Gamma-ray bursts
• Large Magellenic Cloud, Solar Flare
• Possibly Cen A and an x-ray binary
• Unidentified Source possibilities include
• Supernova Remnants
• Pulsar Wind Nebula
• Galactic Black Holes
• Galaxy Clusters
• Luminous IR Galaxies

GLAST predicted to detect 10000 sources