The accretion regime of LS 5039: 3-D SPH simulations

TL;DR

This study uses 3D SPH simulations to explore how a black hole in LS 5039 accretes stellar wind, revealing accretion rates consistent with observed gamma-ray emissions and highlighting the importance of gamma-ray absorption near periastron.

Contribution

First detailed 3D hydrodynamical simulations of wind accretion in LS 5039 assuming a black hole, linking accretion rates to gamma-ray production.

Findings

Accretion rate follows classical Bondi-Hoyle-Littleton model.

Accretion rates around 10^{16} g/s near periastron.

Gamma-ray absorption near periastron affects observed flux.

Abstract

LS 5039 is a TeV gamma-ray binary with extended radio emission. It consists of a compact object in the mildly eccentric (e=0.35), 3.9-day orbit around a massive O star. The nature of the compact object is not yet established. In this paper, assuming that the compact object is a black hole, we study the accretion of O-star wind by the black hole, by performing three-dimensional Smoothed Particle Hydrodynamics (SPH) simulations. In order to roughly emulate the effect of the stellar radiation effectively canceling the stellar gravity, we assume that the O star's gravity does not exert on the wind. The wind particles are ejected with half the observed terminal velocity in a narrow range of azimuthal and vertical angles toward the black hole, in order to emulate the wind significantly slower than the terminal speed, and optimize the resolution and computational efficiency of simulations. We find that the mass-accretion rate closely follows the classical Bondi-Hoyle-Littleton accretion rate, which is of the order of 10^{16}g/s around periastron. The accretion rate at this level would provide jets enough power to produce the gamma-rays detected by HESS. Since the accretion peak occurs near the periastron passage, we need a strong gamma-ray absorption around periastron in order for the microquasar scenario to be consistent with the observed orbital modulation of the TeV gamma-ray flux.