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CfA Press Release
 
 Release No.: 98-02

For Release: Thursday, January 8, 1998

TeV GAMMA RAY ASTRONOMY: NEW LIGHT FROM THE DARK NIGHT SKYTeV GAMMA RAY ASTRONOMY: NEW LIGHT FROM THE DARK NIGHT SKY

AAS Bruno Rossi Prize Lecture
January 8, 1998
Trevor C. Weekes
Smithsonian Institution
Fred Lawrence Whipple Observatory

Lecture Summary

WASHINGTON, DC -- Some of the most exciting results in astrophysics inWASHINGTON, DC -- Some of the most exciting results in astrophysics inthe last few decades have come from that broad area of researchthe last few decades have come from that broad area of researchclassified as "high-energy astrophysics." Since this covers the studyclassified as "high-energy astrophysics." Since this covers the studyof X rays and gamma rays (both of which are strongly absorbed by theof X rays and gamma rays (both of which are strongly absorbed by theEarth's atmosphere), it is generally assumed that high-energyEarth's atmosphere), it is generally assumed that high-energyastrophysics can ONLY be pursued from space. It is surprising,astrophysics can ONLY be pursued from space. It is surprising,therefore, that one of the newest disciplines of high-energytherefore, that one of the newest disciplines of high-energyastrophysics, involving the study of very-high-energy gamma raysastrophysics, involving the study of very-high-energy gamma raysshould be possible using a ground-based technique. The development ofshould be possible using a ground-based technique. The development ofthis discipline is recognized in the award by the Americanthis discipline is recognized in the award by the AmericanAstronomical Society (AAS) of this year's Bruno Rossi Prize. Astronomical Society (AAS) of this year's Bruno Rossi Prize.

Developed by a group of scientists at a number of institutions, butDeveloped by a group of scientists at a number of institutions, butcentered at the Smithsonian Institution's Fred Lawrence Whipplecentered at the Smithsonian Institution's Fred Lawrence WhippleObservatory at Amado, Arizona, the so-called, "atmospheric CherenkovObservatory at Amado, Arizona, the so-called, "atmospheric Cherenkovimaging technique" uses large optical cameras to detect the effects ofimaging technique" uses large optical cameras to detect the effects ofthe interaction of very-high-energy gamma rays with the Earth'sthe interaction of very-high-energy gamma rays with the Earth'satmosphere and hence determine their origin. The sensitivity of thisatmosphere and hence determine their origin. The sensitivity of thistechnique complements high-energy gamma-ray space telescopes, such astechnique complements high-energy gamma-ray space telescopes, such asthe EGRET aboard the Compton Gamma Ray Observatory). the EGRET aboard the Compton Gamma Ray Observatory).

The new technique gives the first glimpse of a violent universe asThe new technique gives the first glimpse of a violent universe asseen in photons a million times the energy of a photon of light. Asseen in photons a million times the energy of a photon of light. Asexpected, these very-high-energy photons come not from ordinary starsexpected, these very-high-energy photons come not from ordinary starsor galaxies, but from cosmic sources undergoing explosive emissionor galaxies, but from cosmic sources undergoing explosive emissionprocesses. processes.

The dominant sources of this energy in the galaxy are supernovaThe dominant sources of this energy in the galaxy are supernovaremnants, the expanding debris that results from the catastrophicremnants, the expanding debris that results from the catastrophicdisintegration of dying stars. The strongest source and the firstdisintegration of dying stars. The strongest source and the firstdetected was the Crab Nebula, which dominates the sky at almostdetected was the Crab Nebula, which dominates the sky at almostevery waveband of photon energy. Although it exploded in 1054 AD, theevery waveband of photon energy. Although it exploded in 1054 AD, theCrab is still a dynamic object, with the ongoing injection ofCrab is still a dynamic object, with the ongoing injection ofrelativistic particles from a rapidly rotating pulsar at its center.relativistic particles from a rapidly rotating pulsar at its center.Three other supernova remnants have been detected with similarThree other supernova remnants have been detected with similarenergies, all in the southern hemisphere.energies, all in the southern hemisphere.

The most surprising results have come from the study of extragalacticThe most surprising results have come from the study of extragalacticobjects known as "Active Galactic Nuclei." These sources are theobjects known as "Active Galactic Nuclei." These sources are thecores of large galaxies and are believed to contain massive blackcores of large galaxies and are believed to contain massive blackholes. holes. more more

For reasons unknown, these black holes are associated with large jetsFor reasons unknown, these black holes are associated with large jetsof relativistic particles; and, in "blazars" (a sub-class of activeof relativistic particles; and, in "blazars" (a sub-class of activegalactic nuclei), these jets happen to be directed towards the Solargalactic nuclei), these jets happen to be directed towards the SolarSystem. Not surprisingly, these are the strongest sources ofSystem. Not surprisingly, these are the strongest sources ofhigh-energy gamma rays, and the study of their gamma-ray signalshigh-energy gamma rays, and the study of their gamma-ray signals(which are highly variable) is providing a new perspective on the(which are highly variable) is providing a new perspective on thedynamics of the jets. By looking directly down a jet, an observer isdynamics of the jets. By looking directly down a jet, an observer iseffectively looking down the bore of the cosmic cannon and witnessingeffectively looking down the bore of the cosmic cannon and witnessingthe same kinds of interactions as seen in the beam of a man-madethe same kinds of interactions as seen in the beam of a man-madeparticle accelerator. particle accelerator.

The Whipple Observatory group has also observed the shortest flareThe Whipple Observatory group has also observed the shortest flareever seen in gamma rays and some of the highest energies everever seen in gamma rays and some of the highest energies everrecorded from a blazar (Markarian 421). Earlier this year, therecorded from a blazar (Markarian 421). Earlier this year, theWhipple Observatory group discovered an extraordinary outburst fromWhipple Observatory group discovered an extraordinary outburst fromanother blazar (Markarian 501), which lasted for more than six months.another blazar (Markarian 501), which lasted for more than six months. Fluctuations in the flares seen on short time-scales (less than 30 Fluctuations in the flares seen on short time-scales (less than 30minutes), coupled with the high energies, places severe constraints onminutes), coupled with the high energies, places severe constraints onthe possible emission mechanisms.the possible emission mechanisms.

Based on the success of the Whipple technique, more than seven groupsBased on the success of the Whipple technique, more than seven groupsworld-wide have built similar telescopes; and, very-high-energyworld-wide have built similar telescopes; and, very-high-energygamma-ray astronomy has become one of the most active and productivegamma-ray astronomy has become one of the most active and productivedisciplines in high-energy astrophysics. The Whipple Observatory groupdisciplines in high-energy astrophysics. The Whipple Observatory groupitself plans to build an array of telescopes in Arizona (the VERITASitself plans to build an array of telescopes in Arizona (the VERITASProject) which will greatly increase observing sensitivity and,Project) which will greatly increase observing sensitivity and,ultimately, offer new insights on the most powerful, most energetic,ultimately, offer new insights on the most powerful, most energetic,most violent processes in the Universe.most violent processes in the Universe.

For more information, contact:

Trevor Weekes, tweekes@cfa.harvard.edu, 520-670-5726
James Cornell, jcornell@cfa.harvard.edu, 617-495-7462

 
 
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