Long-term monitoring of the high-energy gamma-ray emission from LS I +61{\deg} 303 and LS 5039

TL;DR

This paper reports on 30 months of Fermi LAT observations of LS I +61° 303 and LS 5039, revealing new gamma-ray behaviors, flux variations, and spectral characteristics that challenge existing models and require new theoretical explanations.

Contribution

It provides the first detailed long-term gamma-ray monitoring of these binaries, identifying flux changes, spectral features, and the need for new models to explain the observed behaviors.

Findings

Detected increased flux in LS I +61° 303 in March 2009

Observed a decline in orbital flux modulation over time

Identified spectral cutoff features in the gamma-ray emission

Abstract

The Fermi Large Area Telescope (LAT) reported the first definitive GeV detections of the binaries LS I +61\degree 303 and LS 5039 in the first year after its launch in June, 2008. These detections were unambiguous as a consequence of the reduced positional uncertainty and the detection of modulated gamma-ray emission on the corresponding orbital periods. An analysis of new data from the LAT, comprising 30 months of observations, identifies a change in the gamma-ray behavior of LS I +61\degree 303. An increase in flux is detected in March 2009 and a steady decline in the orbital flux modulation is observed. Significant emission up to 30GeV is detected by the LAT; prior datasets led to upper limits only. Contemporaneous TeV observations no longer detected the source, or found it -in one orbit- close to periastron, far from the phases at which the source previously appeared at TeV energies. The detailed numerical simulations and models that exist within the literature do not predict or explain many of these features now observed at GeV and TeV energies. New ideas and models are needed to fully explain and understand this behavior. A detailed phase-resolved analysis of the spectral characterization of LS I +61\degree 303 in the GeV regime ascribes a power law with an exponential cutoff spectrum along each analyzed portion of the system's orbit. The on-source exposure of LS 5039 is also substantially increased with respect to our prior publication. In this case, whereas the general gamma-ray properties remain consistent, the increased statistics of the current dataset allows for a deeper investigation of its orbital and spectral evolution.