D. I. Meyer

Detection of Gamma Rays with E > 300 GeV from Markarian 501

The detection of gamma rays of energy greater than 300 GeV from the BL Lacertae object Mrk 501 demonstrates that extragalactic TeV emission is not unique to Mrk 421. During 66 hr of observations between 1995 March and July we measured an average flux of 8.1 ± 1.4 × 10-12 cm-2 s-1 above 300 GeV, a flux that is only 20% of the average Mrk 421 flux. The new gamma-ray source has not been reported by the Compton Gamma Ray Observatory as an emitter of gamma rays at lower energies. There is evidence for variability on timescales of days.

Survey of candidate gamma-ray sources at TeV energies using a high-resolution Cerenkov imaging system - 1988-1991

The steady TeV gamma-ray emission from the Crab Nebula has been used to optimize the sensitivity of the Whipple Observatory atmospheric Cerenkov imaging telescope. Using this method, which is of order 20 times more sensitive than the standard method using a simple non-imaging detector, it is possible to detect the Crab Nebula at a significance level in excess of 6 standard deviations (6 sigma) in under 1 hr on source (with a corresponding time observing a background comparison region); a source one-tenth the strength of the Crab Nebula can be detected at the 4 sigma level after 40 hr on the source (and 40 hr on a background region). A variety of sources have been monitored using this technique over the period 1988-1991, but none were detected apart from the Crab Nebula. Upper limits are presented which in many instances are a factor of 10 below the flux of the Crab Nebula. These upper limits assume steady emission from the source and cannot rule out sporadic gamma-ray emission with short duty cycles.

Multiwavelength Observations of Markarian 421 During a TeV/X-Ray Flare

A TeV flare from the BL Lac object Mrk 421 was detected in May of 1994 by the Whipple Observatory air Cherenkov experiment during which the flux above 250 GeV increased by nearly an order of magnitude over a 2-day period. Contemporaneous observations by ASCA showed the X-ray flux to be in a very high state. We present these results, combined with the first ever simultaneous or nearly simultaneous observations at GeV gamma-ray, UV, IR, mm, and radio energies for this nearest BL Lac object. While the GeV gamma-ray flux increased slightly, there is little evidence for variability comparable to that seen at TeV and X-ray energies. Other wavelengths show even less variability. This provides important constraints on the emission mechanisms at work. We present the multiwavelength spectrum of this gamma-ray blazar for both quiescent and flaring states and discuss the data in terms of current models of blazar emission.

Outburst of TeV photons from Markarian 421

Markarian 421, an active galactic nucleus (AGN) of the BL Lacertae type, is the closest Energetic Gamma Ray Experiment Telescope (EGRET)-detected AGN. It has been monitored by the Whipple Observatory very high energy gamma-ray telescope since its discovery at TeV energies, for approximately 90 nights, totaling 130 hours of observation. Observations from 1993 December 23 to 1994 May 10 showed an average sourve flux only half that of its 1992 discovery level. However, observations on 1994 May 14 and 15 show an increase in flux above this quiescent level by a factor of approximately 10. The timescale of this increase provides the best geometric constraint on the extent of TeV photon emission regions within AGNs. The observation of the high TeV flux occurred 1 day prior to the observation by ASCA of a very high 2-10 keV X-ray flux. This strong TeV outburst is reminiscent of the behavior seen for the stronger 100 MeV-GeV EGRET sources (e.g. 3C 279) but was not anticipated in view of the fact that the six EGRET observations of Mrk 421 from 1991 June to 1993 July showed no evidence for variability.

Upper Limits on TeV Gamma-Ray Emission from Active Galactic Nuclei

The results of a search for TeV gamma-ray emission from 35 active galactic nuclei (AGNs) using the Whipple Observatory High Resolution Atmospheric Cerenkov Camera are reported. Fifteen of these objects have been detected at GeV energies by the EGRET experiment on the {ital Compton} {ital Gamma} {ital Ray} {ital Observatory}. None of the 35 objects gave a signal at the 3 {sigma} level; Mrk 421 remains the only AGN detected at TeV energies. The absence of a TeV signal may imply a change in the primary source spectrum and/or the effect of absorption by pair production on intergalactic infrared photons. {copyright} {ital 1995 The American Astronomical Society.}

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 1.6

Hadron-proton elastic scattering at 50, 100, and 200 GeV/c momentum

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Production cross section and electroweak asymmetry of D* and D mesons produced in e+e- annihilations at 29 GeV

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An upper limit to the infrared background from observations of TeV gamma rays

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Neutral K*(890) and ρ0 meson production in e+e- annihilation at ;s=29GeV

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