5.2 X-ray Binaries and Cataclysmic Variables

X-ray binaries and cataclysmic variables are cosmic duos that light up the sky. These systems feature a compact object stealing matter from a companion star, creating fireworks in X-rays, optical, and ultraviolet wavelengths.

From pulsars to novae, these celestial pairs showcase the wild side of accretion. They reveal how matter behaves in extreme conditions, offering a front-row seat to the physics of compact objects and their gravitational influence on nearby stars.

Types of X-ray Binaries and Cataclysmic Variables

X-ray Binary Systems

• X-ray binary consists of a compact object (neutron star or black hole) accreting matter from a companion star
• High-mass X-ray binary (HMXB) involves a compact object orbiting a massive star (typically O or B type)
• HMXB accretion occurs primarily through stellar wind capture
• Low-mass X-ray binary (LMXB) features a compact object orbiting a low-mass star (usually K or M type)
• LMXB accretion happens mainly through Roche lobe overflow
• Cataclysmic variable comprises a white dwarf accreting matter from a low-mass companion star
• Cataclysmic variables exhibit periodic outbursts due to instabilities in the accretion disk

Characteristics and Differences

• X-ray binaries emit most of their energy in the X-ray spectrum
• HMXBs have shorter lifespans (millions of years) due to the rapid evolution of massive stars
• LMXBs have longer lifespans (billions of years) because of the slower evolution of low-mass stars
• Cataclysmic variables primarily emit in optical and ultraviolet wavelengths
• X-ray binaries often show more stable accretion rates compared to cataclysmic variables
• Cataclysmic variables display more frequent and dramatic outbursts than X-ray binaries

Accretion Mechanisms

Accretion Column Formation

• Accretion column forms when accreting material follows magnetic field lines of a neutron star
• Material funnels towards the magnetic poles of the neutron star
• Accretion column shape depends on the strength of the magnetic field and accretion rate
• Strong magnetic fields create narrow, tall columns
• Weaker magnetic fields result in wider, shorter columns
• X-ray emission originates from the base of the accretion column due to shock heating

Mass Transfer Processes

• Roche lobe overflow occurs when a star expands beyond its Roche lobe
• Roche lobe defines the region where material remains gravitationally bound to a star in a binary system
• Material crossing the Roche lobe falls towards the compact object
• Stellar wind accretion involves compact object capturing part of the stellar wind from its companion
• Wind accretion efficiency depends on wind velocity and orbital separation
• Roche lobe overflow leads to more efficient mass transfer compared to wind accretion
• Wind accretion dominates in HMXBs, while Roche lobe overflow prevails in LMXBs

Observable Phenomena in X-ray Binaries

Pulsating X-ray Sources

• X-ray pulsars result from rotating neutron stars with strong magnetic fields
• Pulsations occur due to the misalignment of the neutron star's rotation and magnetic axes
• Pulse periods range from milliseconds to thousands of seconds
• Accretion-powered pulsars show variations in pulse period due to changes in accretion rate
• Beam geometry affects the observed pulse profile (pencil beam, fan beam)
• X-ray pulsars provide insights into neutron star properties (magnetic field strength, spin period)

Quasi-Periodic Oscillations

• Quasi-periodic oscillations (QPOs) manifest as peaks in the power spectrum of X-ray light curves
• QPOs occur at frequencies ranging from Hz to kHz
• Low-frequency QPOs (0.1-100 Hz) associate with accretion disk dynamics
• High-frequency QPOs (100-1000 Hz) relate to orbital motion near the inner edge of the accretion disk
• QPOs provide information about the accretion flow and compact object properties
• Twin kHz QPOs observed in some neutron star systems offer clues about strong-field gravity effects

Subclasses of Cataclysmic Variables

Eruptive Phenomena

• Dwarf nova outbursts result from thermal-viscous instabilities in the accretion disk
• Dwarf nova outbursts increase brightness by 2-5 magnitudes and last for days to weeks
• Nova eruptions occur when accreted hydrogen on the white dwarf surface undergoes thermonuclear runaway
• Classical novae increase in brightness by 7-15 magnitudes and do not recur for thousands of years
• Recurrent novae show repeated outbursts on timescales of decades due to higher accretion rates

Magnetic Cataclysmic Variables

• Magnetic cataclysmic variables feature white dwarfs with strong magnetic fields
• Polars (AM Her stars) have magnetic fields strong enough to prevent the formation of an accretion disk
• Intermediate polars (DQ Her stars) have weaker magnetic fields, allowing partial disk formation
• Magnetic fields in polars synchronize the white dwarf's rotation with the orbital period
• Intermediate polars exhibit asynchronous rotation between the white dwarf and orbital period
• Magnetic cataclysmic variables show distinct X-ray and polarized optical emission due to accretion along magnetic field lines