x-ray binaries

1. x-ray binaries • • • References: • 1. Bhattacharya & van den Heuvel, Phys Reports, vol 203, 1,1991 • 2.X-ray Binaries, edited by Lewin, van Paradijs, and van den Heuvel, 1995, Cambridge university press. • • • •

2. Evidence for Black-Holes • If a compact object has mass greater than the maximum allowed for a NS (3 Mo -- for causality based EoS) then the object is most likely a BH; x-ray binaries offer one of the best evidence for the existence of black-holes. (Mass is determined using the Kepler’s law.) • Orbits of individual stars at the center of our Galaxy provide compelling evidence for the existence of supermassive BHs. • Keplerian rotation profile is the central disk of NGC 4258 (a mega-maser galaxy) as the only other case where we are confident that there is a massive BH at the center. Mass function: If you know the mass of star “1” (from optical observations) then You can determine the minimum Mass of star “2” by taking sin i=1 i: orbital inclination angle. P: orbital period. v1: observed velocity of star “1”.

3. High-mass x-ray binary (HMXB) • NS accretes from wind of its massive star companion. • The wind is disrupted at Req, where ram pressure Equals the magnetic pressure, and is channeled onto the magnetic pole which results in pulsed emission. (The majority of HMXBs are x-ray pulsars.) • Hard spectra upto ~ 10-20 kev; emission from polar cap. • Cyclotron lines have been seen in a dozen or more systems -- Ecyclo ~ 11.6 B12 kev; the magnetic field found from this is ~ few times 1012 Gauss. • Spin period -- fraction of a sec to 103s; Porb ~ 1-200 days. Cyclotron frequency:

4. HMXB continued (order of magnitude estimates) 1. Energy production efficiency onto a NS and BH. 10% for NS; 6%--42% for BHs. 2. Effective temperature for LEddington & NS radius. 3. Wind fed mass accretion rate in a binary system. • Many x-ray pulsars show spin-up. (some have spin-down phase perhaps because of wind fluctuation leading to disk spin reversal). Derive the above result in the class, and also the Bondi calculation.

5. 4. Bondi accretion rate (spherical inflow). • The accretion rate when the object is moving through the ISM with speed V is: • X-ray transients:

6. Corona Low mass star filling Roche lobe Accretion disk Low-mass x-ray binary (LMXB)

11. Mass determination in a binary system a: semi-major axis P: orbital period i: orbital inclination angle v1,obs: line of sight speed Kepler’s Law: If we know the velocity of the 2nd star, we know a 2nd mass Function, similar to the one above, and we can determine The mass ratio (m_1/m_2); we still need the orbital inclination In order to determine the masses individually.

12. 1. This is a low mass x-ray binary system (the companion star is low mass which Supplying gas to the compact star via Roche-lobe overflow). 2. Milli-sec pulsars have been spun-up by the accreted gas. 3. Magnetic field must be low for the NS to be spun-up to milli-sec period.

13. from Charles & Seward, “Exploring the x-ray Universe”, Cambridge press.

14. from Charles & Seward, “Exploring the x-ray Universe”, Cambridge press.

15. Correlation between spin-up rate and x-ray lumninosity (from Charles & Seward, “Exploring the x-ray Universe”, Cambridge press)