BEPPOSAX OBSERVATIONS OF UNPRECEDENTED SYNCHROTRON ACTIVITY IN THE BL-LACERTAE OBJECT MARKARIAN-501

The BL Lacertae object Markarian 501, one of only three extragalactic sources (with Mrk 421 and IES 2344+514) so far detected at TeV energie s, was observed with the BeppoSAX satellite in 1997 April 7, 11, and 1 6 during a phase of high activity at TeV energies, as monitored with t he Whipple, HEGRA, and CAT Cherenkov telescopes. Over the whale 0.1-20 0 keV range, the spectrum was exceptionally hard (alpha less than or e qual to 1, with F-v proportional to nu(-alpha)), indicating that the X -ray power output peaked at (or above) similar to 100 keV. This repres ents a shift of at least 2 orders of magnitude with respect to previou s observations of Mrk 501, a behavior never seen before in this or any other blazar. The overall X-ray spectrum hardens with increasing inte nsity, and at each epoch it is softer at larger energies. The correlat ed variability from soft X-rays to the TeV band points to models in wh ich the same population of relativistic electrons produces the X-ray c ontinuum via synchrotron radiation and the TeV emission by inverse Com pton scattering of the synchrotron photons or other seed photons. For the first lime in any blazar, the synchrotron power is observed to pea k at hard X-ray energies. The large shift of the synchrotron peak freq uency with respect to previous observations of Mrk 501 implies that in trinsic changes in the relativistic electron spectrum caused the incre ase in emitted power. Due to the very high electron energies, the inve rse Compton process is limited by the Klein-Nishina regime. This impli es a quasi-linear (as opposed to quadratic) relation of the variabilit y amplitude in the TeV and hard X-ray ranges (for the synchrotron self -Compton model) and an increase of the inverse Compton peak frequency smaller than that of the synchrotron peak frequency.