We have used data from the TeV γ-ray flare associated with the active galaxy Markarian 421 observed on 15 May 1996 to place bounds on the possible energy-dependence of the speed of light in the context of an effective quantum gravitational energy scale. The possibility of an observable time dispersion in high energy radiation has recently received attention in the literature, with some suggestions that the relevant energy scale could be less than the Planck mass and perhaps as low as 10 16 GeV. The limits derived here indicate this energy scale to be in excess of 4 × 10 16 GeV at the 95% confidence level. To the best of our knowledge, this constitutes the first convincing limit on such phenomena in this energy regime.

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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 տ 350 GeV) gray observations taken with the Whipple Observatory g-ray telescope, data from the Oriented Scintillation Spectrometer Experiment of the Compton Gamma Ray Observatory, and quick-look results from the All-Sky Monitor of the Rossi X-Ray Timing Explorer, while EGRET did not detect Mrk 501. Short-term optical correlations are not conclusive, but the U-band flux observed with the 1.2 m telescope of the Whipple Observatory was 10% higher than in March. The average energy output of Mrk 501 appears to peak in the 2-100 keV range, which suggests 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 g-ray flux observed during this period is the highest ever detected from this object. The VHE g-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 տ5 TeV.

The Very Energetic Radiation Imaging Telescope Array System (VERITAS) represents an important step forward in the study of extreme astrophysical processes in the universe. It combines the power of the atmospheric Cherenkov imaging technique using a large optical reflector with the power of stereoscopic observatories using arrays of separated telescopes looking at the same shower. The seven identical telescopes in VERITAS, each of aperture 10 m, will be deployed in a filled hexagonal pattern of side 80 m; each telescope will have a camera consisting of 499 pixels with a field of view of 3.5°. VERITAS will substantially increase the catalog of very high energy (E > 100 GeV) c-ray sources and greatly improve measurements of established sources. Ó