Simulations of spectral lines from an eccentric precessing accretion disc

Abstract

Two-dimensional smoothed particle hydrodynamics (SPH) simulations of a precessing accretion disc in a q= 0.1 binary system (such as XTE J1118+480) reveal complex and continuously varying shape, kinematics and dissipation. The stream–disc impact region and disc spiral density waves are prominent sources of energy dissipation. The dissipated energy is modulated on the period Psh= (P−1orb−P−1prec)−1 with which the orientation of the disc relative to the mass donor repeats. This superhump modulation in dissipation energy has a variation in amplitude of ∼10 per cent relative to the total dissipation energy and evolves, repeating exactly only after a full disc precession cycle. A sharp component in the light curve is associated with centrifugally expelled material falling back and impacting the disc. Synthetic trailed spectrograms reveal two distinct ‘S-wave’ features, produced respectively by the stream gas and the disc gas at the stream–disc impact shock. These S-waves are non-sinusoidal, and evolve with disc precession phase. We identify the spiral density wave emission in the trailed spectrogram. Instantaneous Doppler maps show how the stream impact moves in velocity space during an orbit. In our maximum entropy Doppler tomogram, the stream impact region emission is distorted, and the spiral density wave emission is suppressed. A significant radial velocity modulation of the whole line profile occurs on the disc precession period. We compare our SPH simulation with a simple three-dimensional model; the former is appropriate for comparison with emission lines, while the latter is preferable for skewed absorption lines from precessing discs.