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High-Energy Astrophysics Lecture 8: Accretion and Jets in Binary Stars

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High-Energy Astrophysics Lecture 8: Accretion and Jets in Binary Stars High-Energy Astrophysics Distribution of X-ray binary stars in the Lecture 8: Accretion and jets in binary Galaxy (RXTE) stars Robert Laing Compact accreting binary systems Primary stars Primary Compact star WD NS BH Group I Group II Luminous (early, Optically faint (blue) Early-type, massive Massive XRB; X-ray Cyg X-1 massive opt countpart) opt counterpart pulsars LMC X-3 (high-mass systems) (low-mass systems) Late-type, low-mass CV (dwarf Low-mass XRB A06200-00 hard X-ray spectra soft X-ray spectra novae) etc. (T>100 million K) (T~30-80 million K) often pulsating non-pulsating For white dwarfs and neutron stars with low-mass primaries, need to X-ray eclipses no X-ray eclipses consider magnetic field strength. Galactic plane Gal. Centre + bulge Population I older, population II Evidence for black holes Accretion mechanisms in binary systems z Analysis of binary star orbits from primary star z Roche-lobe overflow occurs in a binary system spectral lines (dependent on inclination, but the containing a compact object (white dwarf, neutron mass function gives a lower limit). star or black hole) and a primary star which is on the giant branch. z Mass of unseen companion > 3 solar masses (sometimes by large factors). z Primary expands so that its surface reaches the inner Lagrange point (saddle point in the z Hence cannot be neutron stars supported by neutron degeneracy pressure. gravitational potential between the stars). z 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 Mass transfer by Roche lobe overflow Roche equipotentials M1 CM M2 +++ v L 1 > M 2 M1 Accretion mechanisms - stellar winds Bondi-Hoyle accretion radius z If the primary star is within its Roche lobe, but is z Material flowing with speed v past a compact object losing mass rapidly via a stellar wind, then some of mass M. fraction of the wind can be captured by the compact z Accretion only possible if kinetic + potential energy companion. < 0, i.e. -7 -5 z Typical mass-loss rates are between 10 and 10 v2/2 < GM/r or r < 2GM/ v2 solar masses per year for stars between 15 and 60 a a solar masses. z For a stellar wind, the fraction accreted is π 2 π 2 2 2 2 4 2 4 2 4 z These systems are high-mass X-ray binaries, ~ ra /4 r ~ G MS /r vW ~ MS vS / MP vW and have X-ray luminosities of 1029 - 1031 W where MS, MP are the masses of the primary and secondary stars, vS and vW are the speeds of the secondary and wind. z Note crude assumptions: vW >> vS and MP >> MS Consequences of accretion radius Thus : 2GM r acc = z Observed luminosity depends linearly on the mass- 22 v wns + v loss rate. racc z Therefore very sensitive to wind speed. z 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 2 Accretion near the central object Black hole and neutron star disks z Boundary layer. z Accretion column Magnetised neutron stars and white dwarfs; accretion at magnetic poles. z Advection Black holes Effects of magnetic fields Effects of magnetic fields 2 z Compact stars (neutron stars or white dwarfs; not z Numbers for a 1.4 solar mass neutron star black holes) often have a strong surface magnetic accreting at the Eddington rate: field. This can have a major effect on accretion. L = 1.8 x 1031 W z Wind accretion onto a compact secondary. Assume Accretion efficiency = 0.1 field is dipolar, hence energy density B = 108 T 2 µ 6 umag ~ (B /2 0)(R/r) (R is secondary radius). R = 10 km ρ 2 z This is ~ kinetic energy density in the wind, vw /2 at the Alfven radius. The accretion rate is Hence rA = 1000 km and the immediate vicinity of the neutron star is magnetically dominated. 2 ξ = 4πr ρv, so z Hence material must flow close to the poles of the r = (2π2/Gµ 2)1/7 (B4R12/Mξ)1/7 A 0 dipole field in an accretion column. z In extreme cases, no accretion disk forms. Observational tests of disk accretion Magnetic neutron stars z Eclipse mapping Use eclipse by companion star to For neutron star with strong magnetic field, disk study spatial and velocity structure of disk (primarily disrupted in inner parts. accretion onto white dwarfs). z Doppler tomography Observe velocity structure of Material is spectral line; use change of direction caused by channeled orbital motion to reconstruct emission distribution along field and velocity field. lines and falls z Integrated disk spectra onto star at magnetic z Lyman edges For face-on disks, expect a poles discontinuity in the spectrum at the wavelength of This is where most radiation is produced. Lyman alpha because of the abrupt change in opacity. Compact object spinning => X-ray pulsar; spun up by disk. z Quasi-periodic oscillations 3 Doppler tomography - model images Results of Doppler tomography Quasi-periodic oscillations observed in a The last stages of accretion low-mass X-ray binary Intensity Quasi-periodic oscillations as expected for black hole accretion Power spectrum Spin vs orbital period for X-ray pulsars High-mass X-ray binaries (high-mass X-ray binaries) z Young population, short-lived OB primaries, mostly in spiral arms. z Mostly X-ray pulsars (next lecture). z Roche-lobe overflow, supergiant and Be systems with different mass-loss mechanisms. z Spin periods 66 ms - 1000 s; orbital periods > 1 day. z Spin-up and spin-down are both possible. z Magnetic field confirmed from cyclotron absorption. 4 Low-mass X-ray binaries Eclipses z Brightest X-ray sources in the Galaxy z Neutron star secondary z Few contain pulsars (either low magnetic field or magnetic and spin axes are aligned) z All Roche-lobe overflow z Eclipses and dips => orbital period z Bursts with typical duration 10 - 30 s (thermonuclear runaway) => not a black hole. Microquasars: jet formation in binary stars Superluminal motions in GRS1915+015 Superluminal motions in GRS1915+015 Accretion and jets in a microquasar 5 Hard state, radio oscillations and steady State transitions and jets jets in GRS1915+015 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). z Very hard and intermediate Disk + PL in varying proportions; QPO’s. Radio? z Hard Disk + weak PL; radio suppressed. z Low/hard and Off PL dominates; highly variable. Radio low-level, steady, flat-spectrum jets,. Radio flares associated with state transitions? Radio component motions in Sco X1 Radio images of SS433 Average component speed = 0.45c; θ = 44o 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 SS433 W50 - supernova remnant around SS433 6 SS433 z Unique object because jet velocity is determined both from proper motions (radio) and Doppler shift of spectral lines (optical). 4 z Therefore, bulk motion of 10 K gas as well as relativistic electrons. z Precession of jet axis with 163-day period 7.
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