11 páginas, 6 figuras, 4 tablas.-- Aceptado para su publicación en ApJ.-- MAGIC Collaboration: et al.We present the results of five years (2005-2009) of MAGIC observations of the BL Lac object PG 1553+113 at very high energies (VHEs, E > 100 GeV). Power law fits of the individual years are compatible with a steady mean photon index \Gamma = 4.27 ± 0.14. In the last three years of data, the flux level above 150 GeV shows a clear variability (probability of constant flux < 0.001%). The flux variations are modest, lying in the range from 4% to 11% of the Crab Nebula flux. Simultaneous optical data also show only modest variability that seems to be correlated with VHE gamma ray variability. We also performed a temporal analysis of (all available) simultaneous Fermi/LAT data of PG 1553+113 above 1 GeV, which reveals hints of variability in the 2008-2009 sample. Finally, we present a combination of the mean spectrum measured at very high energies with archival data available for other wavelengths. The mean spectral energy distribution can be modeled with a one-zone Synchrotron Self Compton (SSC) model, which gives the main physical parameters governing the VHE emission in the blazar jet.The support of the German BMBF and MPG, the Ital-
ian INFN, the Swiss National Fund SNF, and the Span-
ish MICINN is gratefully acknowledged. This work was
also supported by the Marie Curie program, by the
CPAN CSD2007-00042 and MultiDark CSD2009-00064
projects of the Spanish Consolider-Ingenio 2010 pro-
gramme, by grant DO02-353 of the Bulgaria n NSF, by
grant 127740 of the Academy of Finland, by the YIP of
the Helmholtz Gemeinschaft, by the DFG Cluster of Ex-
cellence “Origin and Structure of the Universe”, by the
DFG Collaborative Research Centers SFB823/C4 and
SFB876/C3, and by the Polish MNiSzW grant 745/N-
HESS-MAGIC/2010/0. Additional support for science analysis during the op-
erations phase is gratefully acknowledged from the Istituto Nazionale di Astrofisica in Italy and the Centre
National d’Études Spatiales in France.Peer reviewe
