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.

Time-dependent spectral-feature variations of stars displaying the B[e] phenomenon

“…A.17). The line-profile variations are affected by orbital and precession period (Foulkes et al 2004;Regály et al 2011). Similar geometry show some short periodic cataclysmic variables, SU UMa stars, or some of the transient low-mass X-ray binaries.…”

Doppler and modulation tomography of XTE J1118+480 in quiescence

“…5) 2 yr after the Zu02 observations were consistent with the precession period but ruled out the 26‐d period. Simulations by Foulkes et al (2004) give convincing evidence that such long time‐scale modulations of broad emission lines can be directly linked to the precession of an eccentric, asymmetric disc. In essence, the precession alters the angle at which we observe the elliptical orbit of matter in the disc, thereby causing the velocities we observe at the blue and red ‘edges’ of the disc to oscillate with the precession period.…”

Optical spectroscopy of flares from the black hole X-ray transient A0620-00 in quiescence

“…Smith (1999) have attempted to compute such a map and find that the Doppler map of an eccentric disc is shifted with respect to the centre of mass of the system with asymmetric enhancements in intensity around the edge of the disc. More recently, Foulkes et al (2004) have performed detailed 2‐D smoothed particle hydrodymanical (SPH) simulations of an eccentric precessing disc in a binary system. The synthetic trailed spectra show two distinct non‐sinusoidal S‐wave features, which evolve with the disc precession phase that are produced by the gas stream and the disc gas at the stream‐disc impact shock.…”