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

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.

💡 Research Summary

This paper presents a detailed spectroscopic study of H-alpha emission variability in the gamma-ray binary system LS I +61 303, a rare high-mass X-ray binary that also emits at MeV-TeV energies. The analysis is based primarily on 83 high-resolution (R~12,000) spectra obtained over 35 consecutive nights in 2008, covering a full 26.5-day orbital cycle, supplemented by earlier and later datasets for long-term context.

The key findings revolve around two distinct variable components in the H-alpha line profile. First, the researchers detect evidence of a one-armed spiral density wave in the Be star’s circumstellar disk, visible as an S-shaped pattern in the residual spectra after subtracting the mean profile. This feature is present during roughly half the orbit (phase 0.9 to 0.6, using the conventional Taylor & Gregory radio phase convention) and is attributed to tidal perturbation of the disk by the compact companion.

The second and more dramatic discovery is a transient, high-velocity “red shoulder” emission component that appears abruptly just before apastron, around phase 0.6. This feature, which can increase the total H-alpha equivalent width by up to 50%, is characterized by a large redshift (up to ~440 km/s) and a broad line width (FWHM > 329 km/s), indicating its origin in a turbulent, high-velocity region distinct from the quasi-Keplerian Be disk.

The core of the paper investigates the physical origin of this enigmatic red shoulder. The authors systematically test and discuss several hypotheses:

1. Accretion Disk around the Compact Object: This is ruled out because the observed radial velocity of the feature is far too high to correspond to the orbital motion of the compact companion.
2. Mass Transfer Stream: A tidal stream launched from the Be disk at periastron is considered. However, the observed large line width and the strong orbit-to-orbit variability in the strength and timing of the feature are inconsistent with expectations for a collimated stream.
3. Pulsar Wind Nebula: The interaction between the relativistic wind from a putative pulsar companion and the Be star’s wind could form a shock (a compact pulsar wind nebula) that emits H-alpha. This remains a plausible scenario.
4. Infalling Gas Plume: The authors propose a new possibility: gas tidally stripped at periastron may follow a trajectory and later fall back onto the opposite side of the Be disk. The impact of this infalling plume could produce the observed redshifted, broadened emission. Simple estimates of the infall velocity are roughly consistent with the observations.

The study also documents significant long-term (decadal) variability in the overall H-alpha emission strength, correlated with changes in the Be disk’s size and density. The paper concludes that the H-alpha emission in LS I +61 303 is a complex composite, originating not only from the Be disk itself but also from dynamical structures induced by the strong binary interaction, such as spiral density waves and likely a shocked region or infalling material related to the compact object’s passage. These observations provide crucial insights into the mass transfer and energetic processes at work in this unique gamma-ray binary.