PG 1553+113: five years of observations with MAGIC
MAGIC Collaboration (and others): J. Aleksi\'c (1), E. A. Alvarez (2),, L. A. Antonelli (3), P. Antoranz (4), M. Asensio (2), M. Backes (5), J. A., Barrio (2), D. Bastieri (6), J. Becerra Gonz\'alez (7,8), W. Bednarek (9), A., Berdyugin (10), K. Berger (7,8), E. Bernardini (11)

TL;DR
This paper reports five years of MAGIC observations of PG 1553+113, revealing steady spectral shape but modest flux variability at very high energies, correlated optical and gamma-ray variability, and modeling with a one-zone SSC model.
Contribution
It provides the first long-term VHE gamma-ray observational dataset of PG 1553+113 and models its emission with a one-zone SSC framework.
Findings
VHE flux shows modest variability of 4-11% of Crab flux.
Spectral index remains steady at 4.27 ± 0.14 over five years.
Correlated optical and VHE gamma-ray variability observed.
Abstract
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…
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