Discovery and modelling of disc precession in the M31 X-ray binary Bo 158

Barnard, Robin; Foulkes, S. B.; Haswell, C. A.; Kolb, Ulrich; Osborne, J. P. and Murray, J. R. (2006). Discovery and modelling of disc precession in the M31 X-ray binary Bo 158. In: Proceedings of the 'The X-ray Universe 2005', 26-30 Sep 2005, El Escorial, Madrid, Spain.



The low mass X-ray binary (LMXB) associated with the M31 globular cluster Bo 158 is known to exhibit intensity dips on a ~2.78 hr period. This is due to obscuration of the X-ray source on the orbital period by material on the outer edge of the accretion disc. However, the depth of dipping varied from <10% to ~83% in three archival XMM-Newton observations of Bo 158. Previous work suggested that the dip depth was anticorrelated with the X-ray luminosity. However, we present results from three new XMM-Newton observations that suggest that the evolution of dipping is instead due to precession of the accretion disc. Such precession is expected in neutron star LMXBs with mass ratios <0.3 (i.e. with orbital periods <4 hr), such as the Galactic dipping LMXB 4U 1916-053. We simulated the accretion disc of Bo 158 using cutting-edge 3D smoothed particle hydrodynamics (SPH), and using the observed parameters. Our results show disc variability on two time-scales. The disc precesses in a prograde direction on a period of 81±3 hr. Also, a radiatively-driven disc warp is present in the inner disc, which undergoes retrograde precession on a ~31 hr period. From the system geometry, we conclude that the dipping evolution is driven by the disc precession. Hence we predict that the dipping behaviour repeats on a ~81 hr cycle.

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