ar X iv : a st ro - p h / 97 10 33 1 v 1 2 9 O ct 1 99 7 BeppoSAX Observations of Unprecedented Synchrotron Activity in the BL Lac Object Mkn 501

The BL Lac object Mkn 501, one of the only three extragalactic sources (with Mkn 421 and 1ES 2344+514) so far detected at TeV energies, was observed with the BeppoSAX satellite on 7, 11, and 16 April 1997 during a phase of high activity at TeV energies, as monitored with the Whipple, HEGRA and CAT Cherenkov telescopes. Over the whole 0.1-200 keV range the spectrum was exceptionally hard (α ≤ 1, with Fν ∝ ν ) indicating that the X-ray power output peaked at (or above) ∼100 keV. This represents a shift of at least two orders of magnitude with respect to previous observations of Mkn 501, a behavior never seen before in this or any other blazar. The overall X-ray spectrum hardens with increasing intensity and, at each epoch, it is softer at Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 ITESRE-CNR, Via Gobetti 101, I-40129 Bologna, Italy ESA ESTEC, Space Science Department, Astrophysics Division, Postbus 299, NL-2200 AG Noordwijk, The Netherlands Osservatorio Astronomico di Brera, Via Bianchi 46, I-22055 Merate (Lecco), Italy Osservatorio Astronomico di Brera, Via Brera 28, I-20121 Milan, Italy Department of Physics, University of Milan at Como, Via Lucini 3, I-22100 Como, Italy SAX SDC, Via Corcolle 19, I-00131, Rome, Italy IFCTR-CNR, Via Bassini 15, I-20133 Milan, Italy NASA/Goddard Space Flight Center, Greenbelt, MD 20771