Hα emission variability in the γ-ray binary LS I +61 303

Abstract

LS I +61 303 is an exceptionally rare example of a high-mass X-ray binary that also exhibits MeV-TeV emission, making it one of only a handful of "γ-ray binaries." Here we present Hα spectra that show strong variability during the 26.5 day orbital period and over decadal timescales. We detect evidence of a spiral density wave in the Be circumstellar disk over part of the orbit. The Hα 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. © 2010. The American Astronomical Society. All righrs reserved.