Very high energy outburst of Markarian 501 in May 2009

¹ Department of Physics and Astronomy, Barnard College, Columbia University, NY 10027, USA
² Physics Department, McGill University, Montreal, QC H3A 2T8, Canada
³ Department of Physics, Washington University, St. Louis, MO 63130, USA
⁴ Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA
⁵ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁶ DESY, Platanenallee 6, 15738 Zeuthen, Germany
⁷ Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for Astrophysics, Amado, AZ 85645, USA
⁸ Department of Physics and Astronomy and the Bartol Research Institute, University of Delaware, Newark, DE 19716, USA
⁹ School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
¹⁰ Centre for Space Research, North-West University, Private Bag X6001, 2520 Potchefstroom, South Africa
¹¹ Santa Cruz Institute for Particle Physics and Department of Physics, University of California, Santa Cruz, CA 95064, USA
¹² Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA
¹³ School of Physics, National University of Ireland Galway, University Road, Galway, Ireland
¹⁴ Institute of Physics and Astronomy, University of Potsdam, 14476 Potsdam-Golm, Germany
¹⁵ Astronomy Department, Adler Planetarium and Astronomy Museum, Chicago, IL 60605, USA
¹⁶ Department of Physics and Astronomy, Purdue University, West Lafayette, IN 47907, USA
¹⁷ School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA
¹⁸ Department of Astronomy and Astrophysics, 525 Davey Lab, Pennsylvania State University, University Park, PA 16802, USA
¹⁹ Department of Physics, California State University – East Bay, Hayward, CA 94542, USA
²⁰ Department of Physics and Astronomy, University of Iowa, Van Allen Hall, Iowa City, IA 52242, USA
²¹ Physics Department, Columbia University, New York, NY 10027, USA
²² Department of Physics and Astronomy, University of Utah, Salt Lake City, UT 84112, USA
²³ Department of Physics and Astronomy, DePauw University, Greencastle, IN 46135-0037, USA
²⁴ School of Physics and Center for Relativistic Astrophysics, Georgia Institute of Technology, 837 State Street NW, Atlanta, GA 30332-0430, USA
²⁵ Department of Physics, Anderson University, 1100 East 5th Street, Anderson, IN 46012, USA
²⁶ Department of Life and Physical Sciences, Galway-Mayo Institute of Technology, Dublin Road, Galway, Ireland
²⁷ Department of Physics, Grand Valley State University, Allendale, MI 49401, USA
²⁸ Enrico Fermi Institute, University of Chicago, Chicago, IL 60637, USA
²⁹ NASA/Goddard Space-Flight Center, Code 661, Greenbelt, MD 20771, USA
³⁰ Instituto de Astronomia y Fisica del Espacio, Casilla de Correo 67, Sucursal 28, (C1428ZAA) Ciudad Autónoma de Buenos Aires, Argentina
³¹ Physics Department, California Polytechnic State University, San Luis Obispo, CA 94307, USA
³² Department of Physical Sciences, Cork Institute of Technology, Bishopstown, Cork, Ireland
³³ Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA
³⁴ University of Maryland, College Park/NASA GSFC, College Park, MD 20742, USA
³⁵ Rice University, Department of Physics and Astronomy, PO Box 1892, Houston, TX 77251-1892, USA
³⁶ Department of Physics and Department of Astronomy, University of Maryland, College Park, MD 20742, USA
³⁷ Laboratoire Leprince-Ringuet, École Polytechnique, CNRS/IN2P3, 91128 Palaiseau, France
³⁸ W. W. Hansen Experimental Physics Laboratory, Kavli Institute for Particle Astrophysics and Cosmology, Department of Physics and SLAC National Accelerator Laboratory, Stanford University, Stanford, CA 94305, USA
³⁹ Max-Planck-Institut für Physik, 80805 München, Germany

Received: 19 April 2016
Accepted: 6 June 2016

Abstract

The very high energy (VHE; E> 100 GeV) blazar Markarian 501 was observed between April 17 and May 5 (MJD 54 938–54 956), 2009, as part of an extensive multi-wavelength campaign from radio to VHE. Strong VHE γ-ray activity was detected on May 1st with Whipple and VERITAS, when the flux (E> 400 GeV) increased to 10 times the pre-flare baseline flux (3.9 × 10^-11 ph cm^-2 s^-1), reaching five times the flux of the Crab Nebula. This coincided with a decrease in the optical polarization and a rotation of the polarization angle by 15°. This VHE flare showed a fast flux variation with an increase of a factor ~4 in 25 min, and a falling time of ~50 min. We present the observations of the quiescent state previous to the flare and of the high state after the flare, focusing on the flux and spectral variability from Whipple, VERITAS, Fermi-LAT, RXTE, and Swift combined with optical and radio data.