Accretion in Binaries II Classes of X-ray binaries Low-mass (BH and NS) High-mass (BH and NS) X-ray pulsars (NS) Be/X-ray binaries (NS) Distinguishing NS versus BH binaries

X-Ray Binaries Low mass: companion star mass less than one solar mass High mass: companion star mass greater than one solar mass.

matter collects in wake Wind Fed Accretion r acc The wind velocity is generally of the order of a few thousand km/s while the compact object moves at about 200 km/s. The speed of sound in the gas is about 10km/s, thus a bow shock forms around the compact object and this forms at r(acc). Capture will occur within a cylindrical region with its axis along the relative wind direction. Matter collects behind the compact in its orbital motion, in the wake. An accretion disk might form if the accreted gas possesses residual angular momentum. bow shock matter collects in wake

Wind Fed Accretion Matter in wind will accrete if its speed is less than the escape speed from the compact object at the radius of closest approach Vw = wind velocity Vx = velocity of compact object Rc = capture radius

Luminosity If a neutron star binary has the following parameters, Dorb=1012 cm, vw= 1000 km/s, D(orb) is the distance between the compact object and the companion star.

Accreting Pulsars High mass X-ray binary First discovered in X-rays, thus the name “X-ray pulsars”. High mass X-ray binary

Accretion in magnetic field

Magnetosphere boundary Magnetopheric boundary, rM, is where the magnetic pressure balances the ram pressure of accreted matter Assuming spherical accretion and a dipole field with the dipole moment m = BSRS3, where RS and BS are the radius of the star and the field strength at the surface. Used free-fall velocity for v and mass conservation to connect rho and m-dot.

Accretion Torque For disc accretion, the radius of the magnetosphere is expected to be different from that for spherical accretion. It has been shown that the difference can be summarize by a multiplicative numerical factor.

Accretion Torque

Pulse Period Variations

What happens if the spin rate of the pulsar is faster than Keplerian rotation rate at the magnetospheric boundary? Corotation radius lies outside magnetopheric boundary

Equilibrium Period The accreted matter ceases to transfer angular momentum to the neutron star when Millisecond radio pulsars are likely descendents of weak field, spinning neutron stars in a binary system. For a maximum (Eddington) luminosity and typical neutron star mass and radius, we have

Emission Geometry Accreted plasma Shock front Hot spot Settling matter

Pulse Profiles

Landau Levels Quantization of energy due to magnetic field: where is the momentum of the electron parallel to the field, n is the quantum number, and Bcrit is the critical field, For B << Bcrit, the spacing between adjacent levels is

Cyclotron Lines

X-Ray Pulsar Cen X-3 Pulses are modulated at orbital period of 2.09 days

Distinguishing BH vs NS M{x}: mass of the compact object M{o}: mass of the companion star i: inclination angle of the binary orbital plane P{o}: period of the orbital motion K{o}: maximum velocity of the companion star along the line of sight Mass function

Compact object masses