Characterizing the VHE emission of LS I +61 303 using VERITAS observations

TL;DR

This paper presents long-term VERITAS observations of the gamma-ray binary LS I +61 303, analyzing its variable TeV emission over multiple orbital and superorbital periods to understand its emission mechanisms.

Contribution

It provides a comprehensive analysis of over 220 hours of VERITAS data, revealing the variability of TeV emission across different orbital and superorbital phases, and discusses implications for emission models.

Findings

TeV emission varies with orbital phase, peaking near apastron.

Long-term data shows superorbital modulation of TeV flux.

Spectral variations suggest complex emission mechanisms.

Abstract

The TeV gamma-ray binary LS I +61 303, approximately 2 kpc from Earth, consists of a low mass compact object in an eccentric orbit around a massive Be star. LS I +61 303 exhibits modulated VHE gamma-ray emission around its 26.5 days orbit, with strongest TeV emission during its apastron passage (orbital phases {\phi}=0.55-0.65). Multiple flaring episodes with nightly flux variability at TeV energies have been observed since its detection in 2006. GeV, X-ray, and radio emission have been detected along the entire orbit, enabling detailed study of the orbital modulation pattern and its super-orbital period. Previously reported TeV baseline emission and spectral variations may indicate a neutron star flip-flop scenario, in which the binary system switches between accretor and propeller phases at different phases of the orbit. Since September 2007, VERITAS has observed LS I +61 303 over three additional seasons, accruing 220+ hours of data during different parts of its orbit. In this work, we present a summary of recent and long-term VERITAS observations of LS I +61 303. This analysis includes a discussion of the observed variation of TeV emission during different phases of the orbit, and during different superorbital phases.