Variable very high energy gamma-ray emission from Markarian 501

© The American Astronomical Society. We would like to thank the IAC for the excellent working conditions on the La Palma Observatory Roque de los Muchachos. We are grateful to the RXTE ASMteamfor their quick-look results. The support of the German BMBF andMPG, the Italian INFN, and the Spanish CICYT is gratefully acknowledged. This work was also supported by ETH research grant TH-34/04-3 and by Polish grant MNiI 1P03D01028. In addition, the authors want to thank Deirdre Horan, Martin Tluczykont, and Fre´de´ric Piron for providing Mrk 501 data from WHIPPLE, HEGRA CT1, and HEGRA CT System and CAT, respectively, and for useful discussions. ; The blazar Mrk 501 was observed at energies above 0.10 TeV with the MAGIC Telescope from 2005 May through July. The high sensitivity of the instrument enabled the determination of the flux and spectrum of the source on a night-by-night basis. Throughout our observational campaign, the flux from Mrk 501 was found to vary by an order of magnitude. Intranight flux variability with flux-doubling times down to 2 minutes was observed during the two most active nights, namely, June 30 and July 9. These are the fastest flux variations ever observed in Mrk 501. The similar to 20 minute long flare of July 9 showed an indication of a 4 +/- 1 minute time delay between the peaks of F(< 0.25 TeV) and F(> 1.2 TeV), which may indicate a progressive acceleration of electrons in the emitting plasma blob. The flux variability was quantified for several energy ranges and found to increase with the energy of the gamma-ray photons. The spectra hardened significantly with increasing flux, and during the two most active nights, a spectral peak was clearly detected at 0.43 +/- 0.06 and 0.25 +/- 0.07 TeV, respectively, for June 30 and July 9. There is no evidence of such a spectral feature for the other nights at energies down to 0.10 TeV, thus suggesting that the spectral peak is correlated with the source luminosity. These observed characteristics could be accommodated in a synchrotron ...