Detection of the gamma-ray binary LS I +61 303 in a low flux state at Very High Energy gamma-rays with the MAGIC Telescopes in 2009

TL;DR

This paper reports the detection of very high energy gamma-ray emission from the binary system LS I +61 303 during a low flux state, indicating a new emission state not solely due to photon-photon absorption.

Contribution

First detection of LS I +61 303 in a low flux state at VHE gamma-rays, expanding understanding of its emission variability.

Findings

Detected gamma-ray signal with 6.3σ significance above 400 GeV.

Flux is about 1.3% of Crab Nebula flux, lower than previous measurements.

Identified a new low emission state not explained by photon-photon absorption.

Abstract

We present very high energy (VHE, E > 100 GeV) {\gamma}-ray observations of the {\gamma}-ray binary system LS I+61 303 obtained with the MAGIC stereo system between 2009 October and 2010 January. We detect a 6.3{\sigma} {\gamma}-ray signal above 400 GeV in the combined data set. The integral flux above an energy of 300 GeV is F(E>300 GeV)=(1.4 +- 0.3stat +- 0.4syst) * 10^{-12} cm^{-2} s^{-1}, which corresponds to about 1.3% of the Crab Nebula flux in the same energy range. The orbit-averaged flux of LS I +61 303 in the orbital phase interval 0.6--0.7, where a maximum of the TeV flux is expected, is lower by almost an order of magnitude compared to our previous measurements between 2005 September and 2008 January. This provides evidence for a new low emission state in LS I +61 303. We find that the change to the low flux state cannot be solely explained by an increase of photon-photon absorption around the compact star.