Chapter 0 Introduction X-RAY BINARIES

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1 X-RAY BINARIES 1

2 Structure of this course 0. Introduction 1. Compact stars: formation and observational appearance. Mass transfer in binaries 3. Observational properties of XRBs 4. Formation and evolution of XRBs 5. Thin accretion disks 6. Accretion onto compact objects 7. Thick disks and accretion flows

3 Lecture Notes at References Accretion power in astrophysics, J,.Frank, A. R. King, & Raine, D. Cambridge University Press, 00 X-ray binaries, W. H. G. Lewin, J. van Paradijs, & E. P. J. van den Heuvel, eds., Cambridge University Press, 1995 Compact stellar X-ray sources, W. H. G. Lewin, M. van der Klis, eds. Cambridge University Press, 006 (preprints in astro-ph archive) Evolutionary Processes in Binary and Multiple Stars, Eggleton, P. Cambridge University Press, 006 3

4 Chapter 0 Introduction 1. Introduction to X-ray Astronomy What are X-rays? X-rays are a highly energetic form of light ~1-5Å, or h~ kev 4

5 It takes gigantic explosions, or intense magnetic or gravitational fields to energize particles to these high temperatures. Such conditions range from the vast spaces between galaxies to the collapsed worlds of neutron stars and black holes. 5

6 Where X-ray sources? (1) Galactic sources - X-ray binaries - Cataclysmic variables - Supernova remnants - Protostellar and young stellar objects - Planets () Extragalactic sources - Active galactic nuclei - Clusters of galaxies (3) The diffuse X-ray background 6

7 . Production of X-rays (1) A high-speed collision between an electron and a proton. () Inverse Compton scattering 7

8 (3) Synchrotron radiation (4) Atomic emission 8

9 3. History of X-ray astronomy (1) Early development 1054 Chinese observed the Crab supernova. 157 Tycho Brahe observed the supernova in Cassiopeia Roentgen discovers X-rays X-rays from the Sun discovered. 196 X-rays from source (Sco X-1) outside the solar system discovered. 9

10 () Important space missions Uhuru, launched on 1 December 1970 in Kenya, is the first earth-orbiting mission dedicated entirely to celestial X-ray astronomy. The Einstein Observatory (HEAO-), was the first fully imaging X-ray telescope put into space. It was a key mission in X-ray astronomy and its scientific outcome completely changed the view of the X-ray sky. 10

11 ROSAT, a Germany/US/UK collaboration, was launched on June 1, The survey obtained by ROSAT was the first X-ray and XUV all-sky survey. During the pointed phase ROSAT made deep observations of a wide variety of objects. ASCA (the Advanced Satellite for Cosmology and Astrophysics), launched on February 0, 1993, was the first satellite to use CCD detectors for X-ray astronomy. 11

12 RXTE (the Rossi X-ray Timing Explorer), launched on December 30, 1995, is designed to facilitate the study of time variability in the emission of X-ray sources. BeppoSAX, launched on April , is the first X-ray mission with a scientific payload covering more than three decades of energy - from 0.1 to 300 kev. 1

13 CXO (Chandra X-ray Observatory), NASA's Advanced X-ray Astrophysics Facility, in honor of S. Chandrasekhar, launched on July 3, The combination of high resolution, large collecting area, and sensitivity to higher energy X-rays makes it possible to study extremely faint sources. XMM-Newton, the European Space Agency's X-ray Multi-Mirror Mission, launched on December , carries high throughput X-ray telescopes with an unprecedented effective area. 13

14 (3) Future missions Spectrum-RG ASTROSAT Astro-H Hard X-ray Modulation Telescope (HXMT) etc 14

15 4. Accretion power in astrophysics The most powerful sources known in astronomical zoo are powered by accretion: the infall of matter onto a compact object under the influence of gravity. Accretion is the most efficient way of liberating energy. Define the energy generating efficiency L Mc 15

16 If a particle falls towards a compact object with mass M and radius R at an infinite distance, the velocity at distance r equals the free fall velocity v ff ( r) GM r Assume that all the kinetic energy is converted into radiation at the surface of the star, the total liberated energy (luminosity) is L 1 Mv ff ( R) GMM R 16

17 The associated accretion efficiency is acc L Mc GM Rc R ~ s R where R s =GM/c is the Schwarzschild radius., Typical parameters for accreting objects Object Mass (M ) Radius R (cm) Efficiency (R s /R) White dwarf Neutron star ~ Black hole (M/M ) ~ The efficiency of nuclear burning (p-p chain) is pp =

18 The Eddington limit Assume (1) spherical, symmetrical accretion, () accreting material being fully ionized hydrogen. The radial force exerted by radiation on an electron due to Thompson scattering is c T L 4r, where 8 e T ( ) 3 mec is the Thompson cross section. 18

19 The scattering cross section for a proton is a factor (m p /m e ) smaller for an electron, but the electrostatic Coulomb force causes protons and electrons to move together. The total inward gravitational force is GM ( m m ) GMm r r p e p The limiting luminosity is reached when GMm r L 4 r c, p T 19

20 i.e., L Edd 38 4GMm pc / T ( M / M ) ergs -1 Accretion will be halted if L > L Edd. For stable accretion, L Edd represents the maximum accretion luminosity that can be reached. 0

21 The spectrum (Why X-ray?) We first define the radiation temperature for a typical photon of a continuum spectrum. In optically thick case, the blackbody temperature for an accreting luminosity L is T L 1/ 4 b ( ) 4R, where and R are the Stefan-Boltzmann constant and the stellar radius, respectively. 1

22 In optically thin case, if the gravitational potential energy is entirely turned into thermal energy, GM( m p R m e ) 3 kt th, where k is the Boltzmann constant, the thermal temperature is T th GMm 3kR p

23 In reality, T b T rad T th. For neutron star (or black hole) accreting binaries, L~ ergs -1, R~10 6 cm, 1keV h ktrad 50 MeV. Most of the emitting photons are in medium to hard X-rays. This is why they are called X-ray binaries (XRBs). 3