ar X iv : a st ro - p h / 97 07 17 9 v 1 1 6 Ju l 1 99 7 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) γ-ray observations taken with the Whipple Observatory γ-ray telescope, data from the Oriented Scintillation Spectrometer Experiment (OSSE) of the Compton Gamma-Ray Observatory (CGRO), and quicklook results from the All-Sky Monitor (ASM) of the Rossi X-ray Timing Explorer (RXTE) while the Energetic Gamma-Ray Experiment Telescope (EGRET) 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 γ-ray flux observed during this period is the highest ever detected from this object. The VHE γ-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. Department of Physics and Astronomy, Iowa State University, Ames, IA 50011-3160 Department of Physics, University of Leeds, Leeds, LS2 9JT, Yorkshire, England, UK Experimental Physics Department, University College, Belfield, Dublin 4, Ireland 4 Fred Lawrence Whipple Observatory, Harvard-Smithsonian CfA, P.O. Box 97, Amado, AZ 85645-0097 Physics Department, St.Patrick’s College, Maynooth, County Kildare, Ireland E. O. Hulburt Center for Space Research, Code 7650, Naval Research Laboratory, Washington, D.C. 20375 Department of Physics, Purdue University, West Lafayette, IN 47907 Space Radiation Laboratory, California Institute of Technology, Pasadena, CA 91125 NASA/Goddard Flight Center, Code 662, Greenbelt, MD 20771 Universities Space Research Association Department of Physical Sciences, Regional Technical College, Galway, Ireland