The Universe >100 MeV Brenda Dingus Los Alamos National Laboratory

EGRET Compton Observatory BATSE, OSSE, and Comptel at ~< MeV EGRET 30 MeV – 30 GeV 1 st proposed in late 1970s Spark Chamber with NaI calorimeter  e+e+ e–e– calorimeter (energy measurement) particle tracking detectors conversion foil anticoincidence shield Pair-Conversion Telescope

GLAST 16 towers  modularity height/width = 0.4  large field-of-view Si-strips: fine pitch: 228 µm, high efficiency 0.44 X 0 front-end  reduce multiple scattering 1.05 X 0 back-end  increase sensitivity > 1 GeV CsI: wide energy range GeV hodoscopic  cosmic-ray rejection  shower leakage correction X TOT = 10.1 X 0  shower max contained <100GeV segmented plastic scintillator  minimize self-veto > efficiency & redundant readout Instrument TKR CAL ACD Expected Launch Date 2007 First of 16 towers delivered March 2005 to integrate and test with the spacecraft

GLAST Instrument Performance More than 50 times the sensitivity of EGRET Large Effective Area (20 MeV – > 300 GeV) Optimized Point Spread Function ( GeV) Wide Field of View (2.4 sr) Energy Resolution (  E/E < 10%, E >100 MeV)

Electromagnetic Processes: Synchrotron Emission E   (E e /m e c 2 ) 2 B Inverse Compton Scattering E f ~ (E e /m e c 2 ) 2 E i Bremmstrahlung E  ~ 0.5 E e Hadronic Cascades p +   ± +  o +…  e ± +  +  +… p + p  ± +  o +…  e ± +  +  +… Nature’s Particle Accelerators

“Exotic” Gamma-Ray Production Particle-Antiparticle Annihilation WIMP called neutralino,  is postulated by SUSY 50 GeV< m  < 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 q q or  or Z  lines?  

RadioOpticalX-rayGeVTeV E 2 dN/dE or  F  High Energy Gamma-Ray Astronomy Typical Multiwavelength Spectrum from High Energy  -ray source [ Energy Emitted] [ Photon Energy]

Crab Nebula Electron Energies Spinning Neutron Star Fills Nebula with Energetic Electrons  Synchrotron Radiation and Inverse Compton Scattering

Massive Black Hole Accelerates Jet of Particles to Relativistic Velocities => Synchrotron Emission and Inverse Compton and/or Proton Cascades Active Galactic Nuclei

AGN Theory, e.g. WComae Blazar Electrons produce gammas via Inverse Compton scattering of synchrotron photons Protons produce gammas via  synchrotron Boettcher, Mukherjee, & A. Reimer, 2002

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

Typical GRB Broad Band Spectra

GRB 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

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 Pe’er & Waxman 2003 constrain source parameters for Inverse Compton emission of GRB Milagro Sensitivity z=0.2 z=0.02

Gamma-Ray Detected Pulsars

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

>100 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 (?)

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.

Supernova Remnants with GLAST Example of GLAST sensitivity to SNR Improved spectra to resolve  o bump Improved localization to resolve correlation with dense proton target of molecular cloud SNR  -Cygni

Galactic Plane Nucleon-Nucleon Electron Bremstrahlung Inverse Compton Isotropic Diffuse E -2.1 (Extragalactic) Galactic Diffuse Spectrum of Region |b|<10 and 300< l <60 Nucleon-Nucleon (  o 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 >1 GeV is greater resulting in an E -2.4 differential photon spectrum. Strong, Moskolenko, Reimer 2004 require cosmic ray flux in galaxy >2 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,

Extragalactic Diffuse What’s 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)

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