H-alpha Emission Variability in the gamma-ray Binary LS I +61 303

TL;DR

This study analyzes H-alpha emission variability in the gamma-ray binary LS I +61 303, revealing orbital and long-term changes, a spiral density wave, and a significant emission burst, providing insights into the system's circumstellar environment.

Contribution

It presents detailed spectroscopic observations of H-alpha variability, including evidence of a spiral density wave and a transient emission feature, offering new understanding of the Be star's disk and compact object interactions.

Findings

Detection of strong H-alpha variability over orbital and decadal timescales.

Identification of a spiral density wave in the Be circumstellar disk.

Observation of a dramatic redshifted emission burst near apastron.

Abstract

LS I +61 303 is an exceptionally rare example of a high mass X-ray binary (HMXB) that also exhibits MeV-TeV emission, making it one of only a handful of "gamma-ray binaries". Here we present H-alpha spectra that show strong variability during the 26.5 day orbital period and over decadal time scales. We detect evidence of a spiral density wave in the Be circumstellar disk over part of the orbit. The H-alpha line profile also exhibits a dramatic emission burst shortly before apastron, observed as a redshifted shoulder in the line profile, as the compact source moves almost directly away from the observer. We investigate several possible origins for this red shoulder, including an accretion disk, mass transfer stream, and a compact pulsar wind nebula that forms via a shock between the Be star's wind and the relativistic pulsar wind.