Compact Stars. Lecture 4
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1 Compact Stars Lecture 4
2 X-ray binaries We have talked about the basic structure of accretion disks in X-ray binaries and evolutionary scenarios of low mass and high mass XRBs I will now present the observational review of X-ray binaries. We will then discuss more some specific aspects of the alpha-disks theory
3 Radiation of the disk The temperature profile of the disk scales with radius as R-3/4 Maximum temperature: X-rays Wavelength =1-25 Å, energy h = kev
4 Why X-rays? Optically thick case: the blackbody temperature is 1/4 L T bb = 2 4 R Optically thin case: gravitational potential enegy is turned into thermal energy GM m p me 3 = k T th R For accreting binaries, L~10 10 erg/s, R~10 cm. Tbb<Trad<Tth.. 1 kev < Trad < 50 MeV. Therefore the photon frequency hν is in X-rays. 6
5 Mechanisms of X-ray emission Bremsstrahlung Compton upscattering Synchrotron Atomic emission Details: e.g, Rybicki & Lightman, Radiative processes in astrophysics
6 History of X-ray astronomy 1054: Crab Supernova, observed by Chinese 1572: Supernova in Cassiopeia, observed by Tycho Brahe 1895: X-rays discovered by Roentgen 1949: detection of X-rays from the Sun 1962: detection of first X-ray source outside the Solar system, Cyg X : Nobel prize for Riccardo Giacconi, for his pioneering contributions to X-ray instrumentation
7 Important X-ray observatories Uhuru. Worked from 1970 to 1973, first mission dedicated to X-ray astronomy. The X-ray sources are collected in the 4U Catalog Einstein (HEAO-2). Worked from 1978 to First X-ray imaging telescope in space ROSAT Collected over 150,000 sources in the All-sky survey catalog. Detected isolated neutron stars and showed morphology of supernova remnants.
8 X-ray observatories ASCA First satellite that used CCD detectors for X-ray astronomy Beppo SAX Sensitive from 0.1 to 300 kev. Provided first accurate positions of gamma ray bursts. Chandra. Launched in Studies 100 times fainter sources than other instruments XMM-Newton. From Large effective area; the catalog contains over half million sources.
9 Future observatories Astro-sat: planned by India Astro-H: planned by JAXA Athena+ planned by NASA/ESA (?)
10 Observational characteristics All sky map from Uhuru More than 300 X-ray binaries in the Galaxy, with luminosities erg/s Galactic plane, center, globular clusters Some in other galaxies (LMC, SMC)
11 Galactic and extragalactic sources Chandra image of M83 with point-like NS and BH X-ray sources X-ray binary Cen X-3, discovered in Orbital modulation of 2.08 days, neutron star pulsations with 4.84 sec.
12 HMXBs from Integral Obscured sources: large intrinsic absorption Fast X-ray transients: flares of duration ~few hrs; luminosity increases form to 1037 erg/s
13 Pulsar spectra The energy spectrum is: N(E) =N0 E-a E<= Ec N(E) = N0 E-a exp [-(E-Ec)/Ef] E>Ec The power law index a=0-1 Cutoff energies in the range of kev
14 Spectral components Soft X-ray absorption, parameterized by hydrogen column density and distance Fluorescent iron line at kev, doe to emission from ionized iron in the circumstellar matter
15 Cyclotron lines Form due to resonant scattering of the line-of-sight X-ray photons on the electrons embedded in magnetic field Detected at kev E0 = 11.6 B12 /(1+z) kev Depending on the neutron star's surface magnetic field strength
16 Neutron star LMXB Contain Galactic bulge X-ray sources, X-ray bursters, soft X-ray transients HMXBs Numerous sources are in globular clusters Observed properties may depend on the viewing angle: X-ray dips, eclipses LMXBs
17 Spectral variability NS LMXBs are divided into Atoll and Z -sources, based on color-color diagram Atolls have lower luminosities, power-law spectra; Z's have thermal spectra Soft: log F( kev)/f( kev) Hard: log F( kev)/f( kev) Evolution of a source reflects the changes of mass accretion rate Muno et al. 2001
18 X-ray bursts Rise time ~1 s, decay times >~ 10 s; intervals hrs-days BB temperature decreases during burst decay Bursts are due to thermonuclear explosions of H/He on the NS surface
19 Quasi-periodic oscillations QPOs are intensity fluctuations with a preferred frequency HOBs (Horizontal branch QPOs): 5-60 Hz, correlating with X-ray intensity Burst oscillations: represent spin frequency of NS
20 Kilo-hertz QPOs Show twin peaks in Hz range Beat frequency model: v2 is the Keplerian frequency of accreting matter at some radius (i.e. sonic or magnetospheric); v1 is the beat frequency between v2 and NS spin frequency Relativistic disk precession model: v2 is the orbital frequency of matter at some radius and v1 is the periastron precession of the orbit
21 Black hole binaries Classified to both LMXBs or HMXBs Can be transient or persistent sources About 30 are known, in our Galaxy and Magellanic Clouds
22 X-ray novae Flux increase by 2 orders of magnitude in several days Decline time of ~ months Many of them are recurrent Possibly outbursts are due to accretion disk instability
23 State transitions Spectra composed of disk black body component and power-law tail Hard X-rays possibly originate in a separate medium, i.e. Corona above the disk
24 State transitions
25 Hysteresis effect Hard->soft transition is made at much higher luminosity than soft-> hard
26 Comparison of X-ray characteristics Feature HMXBs LMXBs X-ray spectrum Hard, kt >= 15 kev, or power-law index of 0-1 Soft, kt = 5-10 kev Time variability Regular pulsations, no X-ray Often X-ra bursts, quasibursts, often X-ray eclipses periodic oscillations Optical spectrum Stellar-like Reprocessing Optical counterpart Massive, early-type star (O, B), Lopt/Lx = Faint stars, Lopt/Lx = Orbital period 1d - 1yr 10 min 10 d Distribution Concentrated towards Concentrated towards 7 Galactic plane, age < 10 yrs Galactic center, age >109 yrs
27 BH diagnostics Lack of type 1 X-ray bursts In general, much softer spectra High energy power-law tail State transitions: high-soft and low-hard For a given orbital period, BH sources are ~100 times dimmer than NS sources
28 Eddington limit Infalling matter is ionized hydrogen The upward force is exerted by the radiation flux, due to the Thomson scatterig on electrons F x= L x T 2 4 r c which is the number of collinsions per electron per unit time ( T = 0.66 x cm2 ), multiplied by photon momentum p These electrons communicate with protons by electrostatic coupling
29 Eddington limit Accretion occurs if the gravity exceeds the photon force. F grav = G M X mp r 2 The critical luminosity is therefore 4 G M x m p 38 M X 1 L Edd = = erg s T M Sun and is called Eddington limit (Eddington 1926) The same limit applies to massive stars supported in hydrostatic equilibrium by radiation pressure
30 Magnitude of viscosity Viscous stresses are generted via thermal and turbulent motions In cylindrical coordinates, the viscous stress tensor r component is Tr = - r ' The kinematic viscosity, according to Shakura & Sunyaev (1973) prescription, is cs H with a constant ~0.1.
31 Magnitude of viscosity There is no reason to believe that viscosity is constant in time and throughout the disk. Probably, hydrodynamic mechanisms cannot produce sustained turbulence in differentailly rotating disks Balbus & Hawley (1991) studied magnetorotational instabilities (MRI) and found they are effective in driving turbulences
32 Magnitude of viscosity For Keplerian disk, we have '= (3/2) and from hydrostatic equlibrium, cs2 = P/ = H2 2 Therefore, the shear stress is Tr = 3/2 P and viscosity magnitude can be estimated as Tr P The stress computed in the shearing-box MHD simulations consists of Maxwell (magnetic) and Reynolds (turbulent) parts. Recent simulatiuons show that consistent values of alpha are obtained with total pressure (Hirose et al. 2009; Jiang et al. 2013)
33 Next week More about steady state and time dependent accretion disk models. Dwarf novae. Mini Journal Club please volunteer Suggested articles: this week's arxiv Hadronic Gamma-Ray and Neutrino Emission from Cygnus X-3 (ApJ paper) A New Accretion Disk Around the Missing Link Binary Pulsar PSR J (submitted to ApJ Letters) Composite profile of the Fe Kα spectral line emitted from a binary system of supermassive black holes Advances in Space Research
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