PG 1553+113: five years of observations with MAGIC

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 $\pm$ 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.