Astronomy beyond photons

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Major new facilities that detect signals from the cosmos other than electromagnetic radiation are bringing new fields into the forefront of astronomy. Neutrino observatories study the energy-producing thermonuclear reactions at the center of the sun with detectors utilizing chlorine, gallium, and pure water, the latter making use of Cerenkov radiation from recoil electrons. The pioneering Homestake mine experiment and the huge Super-Kamiokande experiment are important examples. Neutrino astronomers detected a flash of neutrinos from the collapse of a star in the supernova SN 1987A and hope to see extragalactic flashes from gamma-ray bursts.

Cosmic ray observatories study highly energetic charged particles (mostly protons) entering the atmosphere from the Galaxy and probably extragalactic sources. The element abundances at energies < 1 GeV provide a lifetime (~ 107 yr) for their storage in the Galaxy. The highest energy particles initiate extensive air showers (EAS) of particles in the earth's atmosphere, facilitating their study with detector arrays covering 103 km2, such as the HiRes Fly's Eye and the Auger project. The most energetic such particles, ~10 to 300 EeV (1019 to 3 x 1020 eV) are probably extragalactic in origin and may arrive from the approximate directions of their origin. Small EAS initiated by TeV gamma rays high in the atmosphere produce Cerenkov radiation observed with ground based mirror-PMT systems, i.e., TeV photon astronomy.

Gravitational waves (G waves) are predicted by Einstein's general theory of relativity and searches for them have so far not reached the needed sensitivities. However, binary radio pulsars do exhibit decaying orbits that clearly indicate the loss of energy through the emission of gravitational waves. Resonant bar detectors were employed in the first searches and now huge (4 km) laser interferometers such as LIGO are reaching sensitivity levels that could detect neutron star mergers in the Virgo cluster, ~55 MLY distant. The compact-star mergers or stellar collapses that probably give rise to gamma-ray bursts could also generate detectable pulses of gravitational radiation. Low frequency studies could detect binary compact star systems, mergers of supermassive black holes, and background radiation from the early universe. The planned NASA/ESA Laser Interferometer Space Antenna

(LISA) mission is intended to carry out such studies.

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