Analysis of hard X-ray/high energy data from LS I +61{\deg}303 based on implications from its 4.6 yr periodicity

TL;DR

This paper investigates the impact of a 4.6-year periodicity on the high-energy emission behavior of LS I +61°303, revealing how long-term modulation influences spectral state transitions and data analysis of its X-ray and gamma-ray emissions.

Contribution

It demonstrates the importance of accounting for the 4.6-year periodicity when analyzing high-energy data from LS I +61°303, highlighting the modulation of spectral state transitions.

Findings

Long-term periodicity modulates spectral state transitions.

Folding data over large periodic intervals can misrepresent emission behavior.

Analysis of high-energy data must incorporate the 4.6-year cycle.

Abstract

The most peculiar radio characteristics of the TeV emitting high-mass X-ray binary LS I +61{\deg}303 are two periodicities: A large periodic outburst which exhibits the same period as the orbit (phase \Phi) and a second periodicity of 1667 days (phase \Theta) which modulates the orbital phase and amplitude of the large outburst. Recent analysis of the radio spectral index present strong evidence for the presence of the critical transition from optically thick emission (related to a steady jet) to an optically thin outburst (related to a transient jet) as in other microquasars. In parallel, a switch from a low/hard X-ray state to a transitional state (e.g. steep power law state) would be expected. We show how the critical transition from optically thick emission to an optically thin outburst is modulated by \Theta. Folding over a too large \Theta-interval mixes up important information about the outbursts and can yield a false picture of the emission behaviour of the source along the orbit. We therefore analyse the implications of this long period for treatment of hard X-ray/high energy data obtained from LS I +61{\deg}303, e.g. with Fermi-LAT or INTEGRAL, taking into account this long-term periodicity.