A Precessing Jet Scenario for the Multi-Wavelength Long-Term Modulation of LS I +61$^\circ$303

TL;DR

This paper proposes that the long-term multi-wavelength modulation of LS I +61°303 is caused by a precessing jet, with phase shifts across energies explained by jet structure, and discusses deviations at TeV energies possibly due to magnetic reconnection.

Contribution

It introduces a jet precession model to explain the multi-wavelength long-term modulation and phase shifts in LS I +61°303, supported by multi-wavelength observational analysis.

Findings

Long-term modulation is stable at radio wavelengths.

Phase shifts across energy bands are systematic and energy-dependent.

TeV emission deviations suggest alternative mechanisms like magnetic reconnection.

Abstract

The high-mass X-ray binary LS I +61$^\circ$303 is detected across the electromagnetic spectrum from radio until the very high energy gamma-ray regime. The emission is not only highly variable on many time scales, but is also periodic at all observed wavelengths. Periodic modulation was observed on different time-scales, ranging from hours, over months to several years. The subject of this article is a super-orbital, long-term modulation of ~4.6 years. We review the observation of this periodic modulation at multiple wavelengths and investigate systematic relationships between them. In particular, radio observations reveal that the long-term modulation is a very stable feature of the source. Observations at other wavelengths result in a phase-shift of the modulation pattern that is a systematic function of energy. The stability of this period favors a scenario in which the long-term modulation is the result of a precessing jet giving rise to periodic changes in the Doppler factor, beating with the orbital modulation of the accretion rate. We explain the phase-shifts across energy bands in a scenario with shorter wavelengths originating closer to the base of the presessing jet. A significant deviation of the TeV emission from this trend possibly requires a different explanation related to magnetic reconnection events.