Research

In order to pursue these investigations, research focusses on interacting compact binary stars, particularly cataclysmic variables (CVs) and X-ray binaries (XRBs). Specific topics include:

• Multiwavelength spectroscopic and photometric observations
• Modelling accretion discs and the accretion process
• Evolutionary studies
• Population synthesis studies

Accretion is a key process in the formation of many astronomical objects and lies at the heart of extreme environment astrophysics. It powers AGN, the most luminous persistent sources in the Universe, gives rise to the wide range of variability timescales observed in compact interacting binary stars, and is key to understanding star and planet formation. For accretion to proceed, in general the material must rid itself of angular momentum, hence the flow usually forms a viscous accretion disc. A long-standing issue in accretion physics is the nature of the viscosity mechanism. If the accreting material is ionised, it is widely accepted that a turbulent magnetohydrodynamic dynamo operates, and provides a viscosity mechanism far more effective than molecular viscosity. For cool, quiescent discs the gas flow is probably quieter, but as these discs are much more difficult to observe, relatively little is known about them. Another complication is presented by magnetic fields anchored on the accreting object which can disrupt the smooth disc-like flow and significantly alter the accretion geometry. We study quiescent and outbursting accretion discs in various types of CV in an attempt to understand these processes.

Several hundred wind-fed high mass X-ray binaries (HMXB) and Roche-lobe overflow-fed low mass X-ray binaries (LMXB) are known within the Galaxy and many individual systems have been well studied for decades. Their broad categorisation is clear: the donors in HMXBs are either O/B supergiants or Be stars, whilst the accretors are mostly X-ray pulsars (neutron stars, NS), or in a handful of systems black holes, BH. Those in eccentric orbits usually exhibit periodic or aperiodic X-ray outburst behaviour. In LMXBs, the donors are generally main sequence F-K stars, and the accretors are a mix of NS and BH. The LMXBs show a variety of characteristics: a few are persistent X-ray pulsars, others only reveal pulsations during X-ray bursts; their spectral and temporal variability allows characterisation as 'Z' or 'Atoll' sources, with a variety of QPO behaviour; and many systems show thermal-viscous instability disc outbursts, analogous to those in dwarf novae. However, the overall picture of the XB population in the Galaxy is confused by absorption and distance uncertainties. It is not easy to distinguish bulge, disk and spiral arm populations, and the varied star formation histories and metallicities in these regions give rise to very different phenomena. The key to progress in understanding XBs beyond the level of individual systems and into the realms of entire populations is to consider those systems in external galaxies. In this way the characteristics of complete and homogenous populations may be obtained, whose selection effects are known and quantifiable. XBs in external galaxies offer to XB research what studies of extrasolar planets offer to solar system science: the ability to escape the constraints of the 'single sample', be it a single solar system's population of planets or a single galaxy's populations of X-ray binaries. We are therefore pursuing studies of XRBs in the Magellanic Clouds, in the Andromeda Galaxy, and beyond.

Recent highlights of our binary star research include:

Haswell

With postgrad Smith, using phase resolved spectroscopy of the CVs IY UMa and VW Hyi to characterise their dwarf nova outbursts and locate the emission regions.

The figure shows trailed spectrograms and Doppler tomograms of emission lines from VW Hyi

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Haswell

With postgrad Foulkes, carrying out smooth particle hydrodynamic simulations of eccentric precessing accretion discs to reveal the origin of superhump modulations in CVs and double degenerate binaries. For more information see http://www.physics.open.ac.uk/FHMR.

The figure shows two dimensional accretion disc surface dissipation animation

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Haswell

Multiwavelength target-of-opportunity observations of soft X-ray transients used to reveal the evolution of the outburst and characterise disc processes.

Multiwavelength spectral energy distribution of a soft X-ray transient in outburst

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Norton

With Haswell, elucidating an alternative face-on, stream-fed intermediate polar model for two supposed ultra-short period binaries.

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Norton

With postgrad Parker, carrying out a survey of X-ray orbital modulation in asynchronous magnetic CVs in order to characterize the mode of accretion.

The figure shows the ASCA X-ray lightcurve of BG CMi (in low and high energy bands) folded at the orbital period, and corresponding power spectra

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Norton

Modelling the accretion flows in magnetic CVs and using this to characterize the spin period / magnetic moment parameter space in these systems. For further information, see http://www.physics.open.ac.uk/ipflows.html

Simulations exploring the variety of accretion flows possible in magnetic cataclysmic variables

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Norton

With postgrad Val Baker, determining the mass of the neutron star in the eclipsing X-ray pulsar SMC X-1, following earlier determinations for Cen X-3 and Vela X-1.

The figure shows the radial velocity curve of the giant star in the eclipsing X-ray pulsar SMC X-1

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Kolb

The development of a powerful binary star evolution and population synthesis computer code: BiSEPS, which incorporates simplified descriptions of all evolutionary phases and is supported by full 1D stellar models.

The figure shows a schematic illustrating an example of binary star evolution through a common envelope phase

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Kolb

Investigation of the CV period distribution and a possible clarification of its structure through an understanding of the angular momentum transport provided by circumbinary discs.

The figure shows the minimum period part of the observational and theoretical CV period distribution.

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Kolb

An investigation of post common envelope binaries and the orbital period vs donor mass distribution for different white dwarf mass intervals.

The figure shows the post common-envelope configuration of currently forming CVs; orbital period vs. donor mass for different white dwarf mass intervals

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Kolb

With Barnard and postgrad Shaw Greening, characterisation of the X-ray binary population of the Andromeda galaxy

The figure shows multiwavelength images of the Andromeda Galaxy showing the location of numerous X-ray binaries

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C

Clark

Discovery of bright supergiant counterparts to ultra-luminous X-ray sources in M51.

The figure shows the identification of the counterpart to a ULX in M51

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Barnard

With Kolb, development of a means of distinguishing neutron star systems from black holes systems and applying this to the M31 X-ray binary population.

Figure: Disc accreting binaries show two basic types of power spectrum, depending on the accretion rate, enabling neutron star and black hole accretors to be distinguished as the maximum accretion rate depends on the mass of the compact object

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Barnard

With Haswell and Kolb, discovery of a precessing accretion disc in an X-ray binary in M31.

The figure shows deep X-ray dips indicating the presence of a precessing accretion disc in the M31 XRB Bo158