1,464 research outputs found
Locating very high energy gamma ray sources with arc minute accuracy
The angular accuracy of gamma-ray detectors is intrinsically limited by the physical processes involved in photon detection. Although a number of point-like sources were detected by the COS-B satellite, only two were unambiguously identified by time signature with counterparts at longer wavelengths. By taking advantage of the extended longitudinal structure of Very High Energy gamma-ray showers, measurements in the TeV energy range can pinpoint source coordinates to arc minute accuracy. This was demonstrated using Cerenkov air shower imaging techniques. With two telescopes in coincidence, the individual event circular probable error will be 0.13 deg. The half-cone angle of the field of view is effectively 1 deg
Hercules X-1: Pulsed gamma-rays detected above 150 GeV
The 1.24 second binary pulsar Her X-1, first observed in X-rays in 1971 by UHURU has now been seen as a sporadic gamma ray source from 1 TeV up to at least 500 TeV. In addition, reprocessed optical and infrared pulses are seen from the companion star HZ Herculis. Thus measurements of the Her X-1/HZ Herculis system span 15 decades in energy, rivaling both the Crab pulsar and Cygnus X-3 in this respect for a discrete galactic source
Search for TeV Gamma-Rays from Shell-Type Supernova Remnants
If cosmic rays with energies <100 TeV originate in the galaxy and are
accelerated in shock waves in shell-type supernova remnants (SNRs), gamma-rays
will be produced as the result of proton and electron interactions with the
local interstellar medium, and by inverse Compton emission from electrons
scattering soft photon fields. We report on observations of two supernova
remnants with the Whipple Observatory's 10 m gamma-ray telescope. No
significant detections have been made and upper limits on the >500 GeV flux are
reported. Non-thermal X-ray emission detected from one of these remnants
(Cassiopeia A) has been interpreted as synchrotron emission from electrons in
the ambient magnetic fields. Gamma-ray emission detected from the
Monoceros/Rosette Nebula region has been interpreted as evidence of cosmic-ray
acceleration. We interpret our results in the context of these observations.Comment: 4 pages, 2 figures, to appear in the proceedings of 26th
International Cosmic Ray Conference (Salt Lake City, 1999
Observation of the Askaryan Effect: Coherent Microwave Cherenkov Emission from Charge Asymmetry in High Energy Particle Cascades
We present the first direct experimental evidence for the charge excess in
high energy particle showers predicted nearly 40 years ago by Askaryan. We
directed bremsstrahlung photons from picosecond pulses of 28.5 GeV electrons at
the SLAC Final Focus Test Beam facility into a 3.5 ton silica sand target,
producing electromagnetic showers several meters long. A series of antennas
spanning 0.3 to 6 GHz were used to detect strong, sub-nanosecond radio
frequency pulses produced whenever a shower was present. The measured electric
field strengths are consistent with a completely coherent radiation process.
The pulses show 100% linear polarization, consistent with the expectations of
Cherenkov radiation. The field strength versus depth closely follows the
expected particle number density profile of the cascade, consistent with
emission from excess charge distributed along the shower. These measurements
therefore provide strong support for experiments designed to detect high energy
cosmic rays and neutrinos via coherent radio emission from their cascades.Comment: 10 pages, 4 figures. Submitted to Phys. Rev. Let
Very High Energy Gamma-Ray Emission from the Blazar Markarian 421
Very high energy gamma-ray emission from the BL Lac object Markarian 421 has
been detected over three observing seasons on 59 nights between April 1992 and
June 1994 with the Whipple 10-meter imaging Cherenkov telescope. During its
initial detection in 1992, its flux above 500 GeV was
1.610photons cm s. Observations in 1993 confirmed
this level of emission. For observations made between December 1993 and April
1994, its intensity was a factor of 2.20.5 lower. Observations on 14 and
15 May, 1994 showed an increase over this quiescent level by a factor of
10 (Kerrick et al. 1995). This strong outburst suggests that 4 episodes
of increased flux measurements on similar time scales in 1992 and 1994 may be
attributed to somewhat weaker outbursts. The variability of the TeV gamma-ray
emission from Markarian 421 stands in contrast to EGRET observations (Lin et
al. 1994) which show no evidence for variability.Comment: gzip compressed tar file including LaTeX text and 4 postscript
figures (14 pages total incl. 4 tables), accepted for publication in the
Astrophysical Journal. Contact address is [email protected]
Characteristics of the Multi-Telescope Coincidence Trigger of the HEGRA IACT System
The HEGRA--collaboration is operating a system of imaging atmospheric
Cherenkov telescopes to search for sources of TeV-gamma-rays. Air showers are
observed in stereoscopic mode with several telescopes simultaneously. To
trigger the telescope system a versatile two-level trigger scheme has been
implemented, which allows a significant reduction of the energy threshold with
respect to single telescopes. The technical implementation of this trigger
scheme and the performance of the trigger system are described. Results include
the dependence of single- and multi-telescope trigger rates on the trigger
thresholds, on the orientation of the telescopes, and on the type of the
primary particle.Comment: 17 Pages, 10 figures, Late
Multiwavelength Observations of a Flare from Markarian 501
We present multiwavelength observations of the BL Lacertae object Markarian
501 (Mrk 501) in 1997 between April 8 and April 19. Evidence of correlated
variability is seen in very high energy (VHE, E > 350 GeV) gamma-ray
observations taken with the Whipple Observatory gamma-ray telescope, data from
the Oriented Scintillation Spectrometer Experiment of the Compton Gamma-Ray
Observatory, and quicklook results from the All-Sky Monitor of the Rossi X-ray
Timing Explorer while the Energetic Gamma-Ray Experiment Telescope did not
detect Mrk 501. Short term optical correlations are not conclusive but the
U-band flux observed with the 1.2m telescope of the Whipple Observatory was 10%
higher than in March. The average energy output of Mrk 501 appears to peak in
the 2 keV to 100 keV range suggesting an extension of the synchrotron emission
to at least 100 keV, the highest observed in a blazar and ~100 times higher
than that seen in the other TeV-emitting BL Lac object, Mrk 421. The VHE
gamma-ray flux observed during this period is the highest ever detected from
this object. The VHE gamma-ray energy output is somewhat lower than the 2-100
keV range but the variability amplitude is larger. The correlations seen here
do not require relativistic beaming of the emission unless the VHE spectrum
extends to >5 TeV.Comment: 10 pages, 2 figures, accepted for publication in ApJ Letter
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