Periodic very high energy gamma-ray emission from LS I +61 303 observed with the MAGIC telescope

TL;DR

This paper reports extensive MAGIC telescope observations of LS I +61 303, revealing periodic very high energy gamma-ray emission synchronized with its orbital period, with detailed spectral and variability analysis.

Contribution

It provides the first comprehensive gamma-ray light curve covering all orbital phases and confirms the periodicity of VHE emission in LS I +61 303.

Findings

Detected gamma-ray emission across all orbital phases.

Measured a consistent photon index of about 2.6.

Found no intra-night variability down to 30 minutes.

Abstract

The MAGIC collaboration has recently reported the discovery of gamma-ray emission from the binary system LS I +61 303 in the TeV energy region. Here we present new observational results on this source in the energy range between 300 GeV and 3 TeV. In total 112 hours of data were taken between September and December 2006 covering 4 orbital cycles of this object. This large amount of data allowed us to produce an integral flux light curve covering for the first time all orbital phases of LS I +61 303. In addition, we also obtained a differential energy spectrum for two orbital phase bins covering the phase range 0.5<phi<0.6 and 0.6<phi<0.7. The photon index in the two phase bins is consistent within the errors with an average index Gamma=2.6+-0.2_{stat}+-0.2_{sys}. LS I +61 303 was found to be variable at TeV energies on timescales of days. These new MAGIC measurements allowed us to search for intra-night variability of the VHE emission; however, no evidence for flux variability on timescales down to 30 minutes was found. To test for possible periodic structures in the light curve, we apply the formalism developed by Lomb and Scargle to the LS I +61 303 data taken in 2005 and 2006. We found the LS I +61 303 data set to be periodic with a period of (26.8+-0.2) days (with a post-trial chance probability of 10^{-7}), close to the orbital period.