Distribution of X-ray binary stars in the Galaxy (RXTE) High-Energy Astrophysics Lecture 8: Accretion and jets in binary stars

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1 High-Energy Astrophysics Lecture 8: Accretion and jets in binary stars Distribution of X-ray binary stars in the Galaxy (RXTE) Robert Laing Primary Compact accreting binary systems Compact star WD NS BH Early-type, massive Massive XRB; X-ray Cyg X-1 pulsars LMC X-3 Late-type, low-mass CV (dwarf Low-mass XRB A novae) etc. For white dwarfs and neutron stars with low-mass primaries, need to consider magnetic field strength. Primary stars Group I Group II Luminous (early, Optically faint (blue) massive opt countpart) opt counterpart (high-mass systems) (low-mass systems) hard X-ray spectra soft X-ray spectra (T>100 million K) (T~30-80 million K) often pulsating non-pulsating X-ray eclipses no X-ray eclipses Galactic plane Gal. Centre + bulge Population I older, population II Evidence for black holes Analysis of binary star orbits from primary star spectral lines (dependent on inclination, but the mass function gives a lower limit). Mass of unseen companion > 3 solar masses (sometimes by large factors). Hence cannot be neutron stars supported by neutron degeneracy pressure. Accretion mechanisms in binary systems Roche-lobe overflow occurs in a binary system containing a compact object (white dwarf, neutron star or black hole) and a primary star which is on the giant branch. Primary expands so that its surface reaches the inner Lagrange point (saddle point in the gravitational potential between the stars). Material can then flow from the giant to the compact companion. The Roche lobe is the equipotential surface which meets the inner Lagrange point. 1

2 Mass transfer by Roche lobe overflow Roche equipotentials M 1 CM + + M + v L 1 M > M 1 Accretion mechanisms - stellar winds If the primary star is within its Roche lobe, but is losing mass rapidly via a stellar wind, then some fraction of the wind can be captured by the compact companion. Typical mass-loss rates are between 10-7 and 10-5 solar masses per year for stars between 15 and 60 solar masses. These systems are high-mass X-ray binaries, and have X-ray luminosities of W Bondi-Hoyle accretion radius Material flowing with speed v past a compact object of mass M. Accretion only possible if kinetic + potential energy < 0, i.e. v / < GM/r a or r a < GM/ v For a stellar wind, the fraction accreted is ~ πr a /πr ~ G M S /r v W ~ M S v S / M P v W where M S, M P are the masses of the primary and secondary stars, v S and v W are the speeds of the secondary and wind. Note crude assumptions: v W >> v S and M P >> M S Thus : r acc = GM v + v w ns r acc Consequences of accretion radius Observed luminosity depends linearly on the massloss rate. Therefore very sensitive to wind speed. In practice, stars of M < 15 solar masses have too little mass loss to produce strong X-ray sources. bow shock matter collects in wake

3 Accretion near the central object Black hole and neutron star disks Boundary layer. Accretion column Magnetised neutron stars and white dwarfs; accretion at magnetic poles. Advection Black holes Effects of magnetic fields Compact stars (neutron stars or white dwarfs; not black holes) often have a strong surface magnetic field. This can have a major effect on accretion. Wind accretion onto a compact secondary. Assume field is dipolar, hence energy density u mag ~ (B /µ 0 )(R/r) 6 (R is secondary radius). This is ~ kinetic energy density in the wind, ρv w / at the Alfven radius. The accretion rate is ξ = πr ρv, so r A = (π /Gµ 0 ) 1/7 (B R 1 /Mξ) 1/7 Effects of magnetic fields Numbers for a 1. solar mass neutron star accreting at the Eddington rate: L = 1.8 x W Accretion efficiency = 0.1 B = 10 8 T R = 10 km Hence r A = 1000 km and the immediate vicinity of the neutron star is magnetically dominated. Hence material must flow close to the poles of the dipole field in an accretion column. In extreme cases, no accretion disk forms. Magnetic neutron stars For neutron star with strong magnetic field, disk disrupted in inner parts. Material is channeled along field lines and falls onto star at magnetic poles This is where most radiation is produced. Compact object spinning => X-ray pulsar; spun up by disk. Observational tests of disk accretion Eclipse mapping Use eclipse by companion star to study spatial and velocity structure of disk (primarily accretion onto white dwarfs). Doppler tomography Observe velocity structure of spectral line; use change of direction caused by orbital motion to reconstruct emission distribution and velocity field. Integrated disk spectra Lyman edges For face-on disks, expect a discontinuity in the spectrum at the wavelength of Lyman alpha because of the abrupt change in opacity. Quasi-periodic oscillations 3

4 Doppler tomography - model images Results of Doppler tomography The last stages of accretion Quasi-periodic oscillations observed in a low-mass X-ray binary Intensity Quasi-periodic oscillations as expected for black hole accretion Power spectrum High-mass X-ray binaries Young population, short-lived OB primaries, mostly in spiral arms. Mostly X-ray pulsars (next lecture). Roche-lobe overflow, supergiant and Be systems with different mass-loss mechanisms. Spin periods 66 ms s; orbital periods > 1 day. Spin-up and spin-down are both possible. Magnetic field confirmed from cyclotron absorption. Spin vs orbital period for X-ray pulsars (high-mass X-ray binaries)

5 Low-mass X-ray binaries Eclipses Brightest X-ray sources in the Galaxy Neutron star secondary Few contain pulsars (either low magnetic field or magnetic and spin axes are aligned) All Roche-lobe overflow Eclipses and dips => orbital period Bursts with typical duration s (thermonuclear runaway) => not a black hole. Microquasars: jet formation in binary stars Superluminal motions in GRS Superluminal motions in GRS Accretion and jets in a microquasar 5

6 State transitions and jets Correlations between X-ray state (accretion) and radio-emission/morphology (jets). Two X-ray spectral components: disk (kt 1 kev black body) and power law (α 1.5; Comptonised). Very hard and intermediate Disk + PL in varying proportions; QPO s. Radio? Hard Disk + weak PL; radio suppressed. Low/hard and Off PL dominates; highly variable. Radio low-level, steady, flat-spectrum jets,. Radio flares associated with state transitions? Hard state, radio oscillations and steady jets in GRS Radio component motions in Sco X1 Radio images of SS33 Average component speed = 0.5c; θ = o Core flares; material travels down the jet with speed >0.95c Galactic analogue of FRII radio source 0.9 solar mass primary + neutron star Precession in SS33 W50 - supernova remnant around SS33 6

7 SS33 Unique object because jet velocity is determined both from proper motions (radio) and Doppler shift of spectral lines (optical). Therefore, bulk motion of 10 K gas as well as relativistic electrons. Precession of jet axis with 163-day period 7