# 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.

## Key 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.

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Source: https://tomesphere.com/paper/1908.03111