Observations of the Crab pulsar between 25 and 100 GeV with the MAGIC I telescope

Abstract

We report on the observation of gamma-rays above 25 GeV from the Crab pulsar (PSR B0532+ 21) using the MAGIC I telescope. Two data sets from observations during the winter period 2007/2008 and 2008/2009 are used. In order to discuss the spectral shape from 100 MeV to 100 GeV, one year of public Fermi Large Area Telescope (Fermi-LAT) data are also analyzed to complement the MAGIC data. The extrapolation of the exponential cutoff spectrum determined with the Fermi-LAT data is inconsistent with MAGIC measurements, which requires a modification of the standard pulsar emission models. In the energy region between 25 and 100 GeV, the emission in the P1 phase (from -0.06 to 0.04, location of the main pulse) and the P2 phase (from 0.32 to 0.43, location of the interpulse) can be described by power laws with spectral indices of -3.1 +/- 1.0(stat) +/- 0.3(syst) and -3.5 +/- 0.5(stat) +/- 0.3(syst), respectively. Assuming an asymmetric Lorentzian for the pulse shape, the peak positions of the main pulse and the interpulse are estimated to be at phases -0.009 +/- 0.007 and 0.393 +/- 0.009, while the full widths at half- maximum are 0.025 +/- 0.008 and 0.053 +/- 0.015, respectively.