Accretion High Energy Astrophysics empmssl ucl ac uk

Accretion High Energy Astrophysics emp@mssl. ucl. ac. uk http: //www. mssl. ucl. ac. uk/

Introduction • Mechanisms of high energy radiation X-ray sources Supernova remnants Pulsars thermal synchrotron loss rotational energy magnetic dipole

Accretion onto a compact object • Principal mechanism for producing highenergy radiation • Most efficient of energy production known in the Universe. Gravitational potential energy released for body mass M and radius R when mass m accreted

Example - neutron star Accreting mass m=1 kg onto a neutron star: m neutron star mass = 1 solar mass R = 10 km R => ~1016 m Joules, M ie approx 1016 Joules per kg of accreted matter - as electromagnetic radiation

Efficiency of accretion • Compare this to nuclear fusion H => He releases ~ 0. 007 mc 2 ~ 6 x 1014 m Joules - 20 x smaller (for ns) So energy released proportional to M/R ie the more compact a body is, the more efficient accretion will be.

Accretion onto white dwarfs • For white dwarfs, M~1 solar mass and R~10, 000 km so nuclear burning more efficient by factor of ~50. • Accretion still important process however - nuclear burning on surface => nova outburst - accretion important for much of lifetime

Origin of accreted matter • Given M/R, luminosity produced depends. on accretion rate, m. . • Where does accreted matter come from? ISM? No - too small. Companion? Yes.

Accretion onto AGN • Active Galactic Nuclei, M ~ 10 9 solar mass - very compact, very efficient (cf nuclear) - accretes surrounding gas and stars

Fuelling a neutron star • Mass = 1 solar mass 31 observed luminosity = 10 J/s (in X-rays) • Accretion produces ~ 1016 J/kg. 31 16 22 • m = 10 / 10 kg/s ~ 3 x 10 kg/year ~ 10 -8 solar masses per year

The Eddington Luminosity • There is a limit to which luminosity can be produced by a given object, known as the Eddington luminosity. • Effectively this is when the inward gravitational force on matter is balanced by the outward transfer of momentum by radiation.

Eddington Luminosity M r m Fgrav Frad Accretion rate controlled by momentum transferred from radiation to mass Note that R is now negligible wrt r Outgoing photons from M scatter material (electrons and protons) accreting.

Scattering L = accretion luminosity no. photons crossing at r per second photons m -2 s -1 Scattering cross-section will be Thomson cross-section se ; so no. scatterings per sec:

Momentum transferred from photon to particle: hn e-, p Momentum gained by particle per second = force exerted by photons on particles

Eddington Limit radiation pressure = gravitational pull At this point accretion stops, effectively imposing a ‘limit’ on the luminosity of a given body. So the Eddington luminosity is:

Assumptions made • Accretion flow steady + spherically symmetric: eg. in supernovae, LEdd exceeded by many orders of magnitude. • Material fully ionized and mostly hydrogen: heavies cause problems and may reduce ionized fraction - but OK for X-ray sources

What should we use for m? Electrostatic forces between e- and p binds them so act as a pair. Thus: M Joule/sec

Black Holes • Black hole does not have hard surface - so what do we use for R? • Use efficiency parameter, h. then • at a maximum h = 0. 42, typically h = 0. 1 • solar mass bh as efficient as neutron star

Emitted Spectrum • define temperature T rad such that hn~k. Trad • define ‘effective’ BB temp Tb • thermal temperature, Tth such that: =>

Accretion temperatures • Flow optically-thick: • Flow optically-thin:

Accretion energies • In general, • For a neutron star, • assuming

Neutron star spectrum • Thus expect photon energies in range: • similarly for a stellar mass black hole 26 • For white dwarf, L acc ~10 J/s, M~M Sun , R=5 x 10 6 m, • => optical, UV, X-ray sources

Accretion modes in binaries ie. binary systems which contain a compact star, either white dwarf, neutron star or black hole. (1) Roche Lobe overflow (2) Stellar wind - correspond to different types of X-ray binaries

Roche Lobe Overflow • Compact star M 1 and normal star M 2 CM M 2 + M 1 a • normal star expanded or binary separation decreased => normal star feeds compact

Roche equipotentials • Sections in the orbital plane M 1 CM + + v L 1 M 2 +

Accretion disk structure The accretion disk (AD) can be considered as rings or annuli of blackbody emission. Dissipation rate, D(R) R = blackbody flux

Disk temperature Thus temperature as a function of radius T(R): When

Accretion disk formation Matter circulates around the compact object: ang mom outwards matter inwards

• Material transferred has high angular momentum so must lose it before accreting => disk forms • Gas loses ang mom through collisions, shocks, viscosity and magnetic fields: kinetic energy converted into heat and radiated. • Matter sinks deeper into gravity of compact object

Magnetic fields in ADs Magnetic “flux tube”

Mag field characteristics • Magnetic loops rise out of the plane of the disk at any angle – the global field geometry is “tangled” • The field lines confine and carry plasma across the disk • Reconnection and snapping of the loops releases energy into the disk atmosphere – mostly in X-rays • The magnetic field also transfers angular momentum out of the disk system

Disk Luminosity • Energy of particle with mass m in circular orbit at R (=surface of compact object) 1 2 mv 2 = 1 GM 1 m R = 2 E acc 2 • Gas particles start at large distances with negligible energy, thus Ldisk = . MM G 2 R = 1 2 Lacc

Disk structure The other half of the accretion luminosity is released very close to the star. X-ray Hot, optically-thin inner region; emits bremsstrahlung UV optical bulge Outer regions are cool, optically-thick and emit blackbody radiation

Stellar Wind Model Early-type stars have intense and highly supersonic winds. Mass loss rates - 10 -6 to 10 -5 solar masses per year. For compact star - early star binary, compact star accretes if GMm 1 m(v 2 + v 2 ) > w ns r 2

Thus : r acc = 2 GM v 2 w + v 2 ns racc bow shock matter collects in wake

Stellar wind model cont. • Process much less efficient than Roche lobe overflow, but mass loss rates high enough to explain observed luminosities. • 10 -8 solar masses per year is required to produce X-ray luminosities of 10 31 J/s.

Magnetic neutron stars For neutron star with strong mag 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 pulsator

‘Spin-up pulsars’ • Primary accretes material with angular momentum => primary spins-up (rather than spin-down as observed in pulsars) • Rate of spin-up consistent with neutron star primary (white dwarf would be slower) • Cen X-3 ‘classical’ X-ray pulsator

Types of X-ray Binaries 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