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X-ray binaries

X-ray binaries. Based on: Compact Stellar X-Ray Sources', eds. W.H.G. Lewin and M. van der Klis, Cambridge University Press Tauris & van den Heuvel: arXiv:0303456 Mc Clintock & Remillard: arXiv:0306213 Van der Klis arXiv:0410551, Hasinger & van der Klis 1989 A&A Psaltis arXiv: arXiv:0410536

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X-ray binaries

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  1. X-ray binaries Based on: Compact Stellar X-Ray Sources', eds. W.H.G. Lewin and M. van der Klis, Cambridge University Press Tauris & van den Heuvel: arXiv:0303456 Mc Clintock & Remillard: arXiv:0306213 Van der Klis arXiv:0410551, Hasinger & van der Klis 1989 A&A Psaltis arXiv: arXiv:0410536 Fender+ 2004 arXiv:0409360

  2. Basic facts and discovery P=4.84 s Po=2.087 days • Sco-X1 discovered in one of the first X-ray observation of the sky (1962) • ~100 bright (Fx>10-10 cgs) X-ray sources in the Galaxy, most discovered already by Uhuru (1971) • 1034<LX<1038 erg/s • NS in a binary hypothesis confirmed soon by discovery of X-ray pulsating emission and regular eclipses (Cen-X3, 1972)

  3. X-ray pulsars

  4. X-ray pulsars

  5. X-ray pulsars

  6. X-ray pulsars

  7. Masses in binaries

  8. Neutron star masses

  9. X-ray binaries

  10. HMXB, LMXB

  11. X-ray binaries

  12. Accretion and B field

  13. Accretion and B field

  14. Accretion and B field The material coming out from the companion star (blue arrows) is captured by the NS. The particle are deviated from the original trajectory and converge behind the NS. There they collide, loosing their energies and then fall toward the NS. AS they come closer the grav. Field accelerates them to very high energies. In the second panel the NS is surrounded by a strong B fiels, the incoming matter is very hot and cannot penetrate the magnetosphere. The matter move along B lines and continue to accelerate. B lines converge to poles and the particles are there focused, forming an accretion column. The density is high and the collisions frequent. The particles loose energy in form of X-rays. Other particles loose their energy impacting the NS.

  15. Accretion and B field • When a strongly magnetic neutron star accretes plasma from acompanion star orthe interstellar medium, its magnetic field becomes dynamically important close tothe stellar surface and determines the properties of the accretion flow. The radiusat which the effects of the magnetic field dominate all others is called the Alfvenradius. • For thin-disk accretion onto a neutron star, the Alfven radius is defined as the radiusat which magnetic stresses remove efficiently the angular momentum of the accretingmaterial • Fora surface magneticfield strength of 1012G and a mass accretion rate ~Eddingtoncritical rate, the Alfven radius is ∼100 neutron-star radii. • If the stellarspin frequency is smaller than the orbital frequency of matter at the interactionradius, then the accreting material is forced into corotation with the star and ischanneled along field lines onto the magnetic poles. An accretion-powered pulsaris produced • if the stellar spin frequencyis larger than the orbitalfrequency of matter at the interaction radius, then the material cannot overcome thecentrifugal barrier in order to accrete onto the star. Matter eventually escapes theneutron star in the form of a wind. “Propeller” regime

  16. High mass X-ray binaries

  17. HMXB

  18. HMXB • A compact object can accrete matter from a companion star that does not fillits Roche lobe, if the latter star is losing mass in the form ofa stellar wind. For thisprocess to result in a compact star that is a bright X-ray source, the companion starhas to be massive (≥ 10 M⊙) in order to drive a strong wind. In this configuration,the optical luminosity of the companion star dominates the total emission from thesystem and the rate of mass transfer is determined by the strength and speed of thewind and the orbital separation. Such systems are called High-Mass X-ray Binaries. • ~150 HMXB known, ~30 with good orbital parametes • because neutron stars in HMXBs accrete for a relativelyshort period of time, their magnetic fields do not evolve awayfrom their high birthvalues, and hence these neutron stars appear mostly as accretion-powered pulsars.~40 pulsating HMXB with P=10-300 sec (0.07s-20min) • Porb<10days • The lifetimes of HMXBs are determined by the evolution of the high-masscompanions and are short (∼105 − 107yr) • HMXBs are distributed along the galactic plane, as young stellar populationsdo

  19. HMXB X-ray spectra The accretion is disrupted at hundreds NS radii and most matter is funneled into NS poles, on relatively small areas. The average spectrum of persistent HMXB can be approximated by a broken power law: With =1.2+/-0.2 c~20 keV F~12 keV Cold/warm absorption from the star wind Iron features Cyclotron features

  20. Cyclotron lines • For neutron-star B fielf of≃1012G, the cyclotron energy on the stellar surface is≃11.6 keV and the continuums pectra areexpected to show evidence for harmonically related cyclotron resonances catteringfeatures (or cyclotron lines) in the X-rays. • Observation of such featureswas anticipated from the early days of X-ray astronomy and expected to lead todirect measurements B (e.g.,Trumper et al. 1978).

  21. Intermediate mass X-ray binaries

  22. Low Mass X-ray Binary providesObservational Evidence of NS Structure Neutron star primary Evolved red dwarf secondary Roche point Accretion disk

  23. LMXB: properties • 150 known LMXB (2001): • 130 in the Galaxy, • 13 in globular clusters, • 2 in LMC • 63 are X-ray bursters • 75 transient (not always observable) • 11 with a black hole (& 8 possible candidates) • Typical luminosity 1036-1038 erg/s • Soft X-ray spectra • Accretion process: Roche-lobe overflow • Orbital periods: from 11 minutes to 17 days

  24. Formation of LMXB • Direct: Birth as binary system • More massive star ⇒compact object • Less massive star fills Roche radius ⇒mass-transfer ⇒LMXB • Capture: • Birth of more massive star alone ⇒ compact object • Close encounter ⇒capture of second star • High star density ⇒happens almost only in globular clusters

  25. Transients LMXB • Fraction of transients among the BH systems is >than the fraction of transients among NS systemsand their outbursts aretypically longer and rarer. • BH transients inquiescence are significantly fainter than NS transients. • These differences are caused by the different massratios of the members of the binary systems between the two populations as well asby the presence of an event horizon in BH systems. The prevailing model of transient sources is based on the disk instability modelof illuminated accretion disks (van Paradijs 1996; King+ 1996):accretion flows that extend to large radii ( > 109 − 1010cm) from thecompact object have T< 104K, at which theanomalous opacity related to the ionization of H renders them susceptible to a thermal instability. At the off-cycle of the instability, materialpiles up at the outer edges of the accretion disk with very little mass accretedby the central object: quiescent phase. When the diskbecomes unstable, the accretion flow evolves towards the central object at the viscoustimescale, and the system becomes a bright X-ray source in outburst.

  26. Bursts from LMXB

  27. EXO0748-676 origin of X-ray bursts circumstellar material

  28. ISM z = 0.35 z = 0.35 ISM z = 0.35 Gravitationally Redshifted Neutron Star Absorption Lines • XMM-Newton found red-shifted X-ray absorption features • Cottam et al. (2002, Nature, 420, 51): • - observed 28 X-ray bursts from EXO 0748-676 • Fe XXVI & Fe XXV • (n = 2 – 3) and O VIII • (n = 1 – 2) transitions • with z = 0.35 • Red plot shows: • - source continuum • - absorption features • from circumstellar gas • Note: z = (l-lo)/loand l/lo = (1 – 2GM/c2r)-1/2

  29. X-ray absorption lines Low T bursts Fe XXV & O VIII (T < 1.2 keV) High T busts Fe XXVI (T > 1.2 keV) quiescence low-ionization circumstellar absorber redshifted, highly ionized gas z = 0.35 due to NS gravity suggests: M = 1.4 – 1.8 M R = 9 – 12 km

  30. Bursts from LMXB • Two Types of bursts: • Type I: thermonuclear explosion of He on the neutron starThe material that is accreted on the surface of a weakly-magnetic neutronstar may be compressed to densities and temperatures for which the thermonuclearburning of helium is unstable. The ignition of helium results in a rapid (∼1 s)increase in the X-ray luminosity of the neutron star, followed by a slower (∼tensof seconds) decay that reflects the cooling of the surface layers that ignited. During bursts coherent oscillations of the observed X-ray fluxes are oftendetected. In burstsfrom two ultra-compact millisecond pulsars, in which the spin frequencies of the starsare known, the asymptotic values of the burst oscillation frequencies are nearly equalto the spin frequencies of the NS • Type II: instabilities of accretion flow onto the neutron star

  31. Spectral and timing properties X-ray timing properties are correlated with X-ray spectral states. Source states are qualitatively different, recurring patterns of spectral and timing characteristics. They arise from qualitatively different inner flow configurations.

  32. Spectral and timing properties: QPOs

  33. Spectral and timing properties • Z sources on time scales of hours to a day or so trace out roughly Z shaped tracks (Fig. 2.4c) in CD/HIDs consisting of three branches connected end-to-end and called horizontal branch, normal branch and flaring branch (HB, NB, FB). kHz QPOs and a15-60Hz QPO called HBO occur on the HB and upper NB, an ∼6Hz QPO called NBO on the lower NB, and mostly power-law noise <1Hz on the FB • At high Lx atoll sources trace out a well-defined, curved banana branch in the CD/HIDs

  34. LMXB spectra • For weak (<109 G) B fields the accretion disk may touch or come close to the NS surface and the accreting matter is distributed over large areas. • No pulsations • Partially Comptonized spectrum

  35. mmsec pulsars • millisecond radio pulsars were most often found in binarieswith evolved, low-masswhite dwarf companions (Bhattacharya & van den Heuvel 1991), which were thoughtto be the descendents of LMXBs. • The discovery, withRXTE, of highly coherent pulsations in the X-ray fluxes ofLMXBs during thermonuclear X-ray bursts (Strohmayer et al.1996) provided the then strongest evidence for the presence of neutron stars with millisecond spin periods in LMXBs. • However,the first bona fidemillisecond, accretion powered pulsar was discovered onlyin 1998, in a transientultracompact binary SAX J1808.4−3658

  36. Black hole binaries

  37. BH binaries

  38. BH binaries • Found in HMXB, LMXB. • 3 persistent (Cyg X-1, LMC X-3, LMC X-1) • many LMXB X-ray Novae (A0620-00, from 50 Crabs to 1uCrab!).

  39. BH binaries light curves

  40. BH binaries transients • 6 X-ray novae detected by Rossi-XTE ASM • U 1543-47:clean example of a classic light curve with ane-folding decay time of ≈ 14 days. • XTE J1859+226:another classic light curve that doesshow a secondary maximum (at about 75 days after discovery).Note the intensevariability near the primary maximum. • XTE J1118+480:One of five X-ray novaethat remained in a hard state throughout the outburst and failed to reach the HSstate. Note the prominent precursor peak. • GRO J1655-40:double peakedprofile During the first maximum strong flaring and intense non-thermal emission (VH state). • XTE J1550-564:The complex profile includes two dominant peaks

  41. BH binaries high/soft state • High accretion rates. • Geometrically thin, optically thick disk, Tmax~107K, 1 keV X-rays • Multicolor disk model, estimate rin from normalization, T, inclination and distance • Weak variability, f-1, no or weak QPO

  42. BH binaries low/hard state • Lower accretion rates, a few% of Eddington • Hard, non-thermal power law component (∼1.7) • steep cut-off near 100 keV • Comptonization of soft photons by a hot optically thin plasma. Disk is faint or undetected. • presence of a compact and quasi-steady radio jet (first in GRS1915, then Cyg X-1 and others). Flat radio spectral index • Strong variability

  43. BHB quiescent state • BHB spends most of its life in this state, L-1030.5 - 1033.5 ergs/s, 10-8 outburst L!! • L/Ledd ~10-8 • Hard spectrum, =1.5-2.1 • Quiescent state may be just an extremely low state • In the quiescent state thedisk is truncated at some larger radius and the interior volume is filled with a hot(Te ∼100 keV) advection dominated accretion flow or ADAF. Most of theenergy released via viscous dissipation remains in the accreting gas rather than beingradiated away (as in a thin disk). The bulk of the energy is advected with the flow and it is lost in the BH. Radiative efficiency <0.1-1%.

  44. BH binaries very high state • Both disk and power law component present, both with a luminosity >0.1 LEdd • Steep power law component, =2.5 up to 1MeV: Compton scattering in a non-thermal corona • QPOs in both disk and power law component in the range 0.1-30Hz, both LFQPO and HFQPO. Persistent. Organized emission region. • LFQPO<<Keplerian f. BH 10 M⊙, anorbital frequency near 3 Hz coincides with a disk radius near100 Rg , while theexpected radius for maximum X-ray emission 1-10 Rg. Disk oscillations, spiral waves. • HFQPO: often commensurate frequencies. Resonance phenomenon of GR oscillations. • Explosive formation of radio jets: the instabilitythat causes impulsive jets is somehow associated with the VHS state

  45. HFQPOs

  46. BH binaries spectral states • thehigh/soft(HS) state, a high intensity state dominated by thermalemission from an accretion disk; • thelow/hard(LH) state, a low intensity statedominated by power law emission and rapid variability; • thequiescentstatean extraordinarily faint state also dominated by power lawemission; • thevery high(VH) state; • theintermediatestate

  47. Jets and radio emission in BHB • Relativistic, superluminal jets. • Non-thermal, polarized radio spectra, indicating shock-accelerated e- emitting synchrotron • Very clear correlation between the presence of jets and the X-ray spectral state ofthe accretion flows. Jets appear when theX-ray spectra of the sources indicate emission from hot electrons (∼100 keV) • The mechanism responsible for the heating of electrons in the accretion flowmay be related to the formation of an outflow, as is the case both for magneticallyactive accretion disks

  48. Jets, disks and spectral states

  49. Jets, disks and spectral states • i low state steady jet Ljet ∝ LX0.5 • ii motion nearly vertical. After a peak motion nearly horizontal to the left, Source move in the VHS/IS. Jet persist. • iii source approaches the jet line between Jet producing and jet free states. Velocity increases. Propagation of an internal shock. • iv source is in the soft state and no jet is produced. Refill of disk. • The thin disk extend close to the BH. Following phase iv sources drop in intensity to reach the canonical LS. • Inner disk is ejected resulting in a disappearence of the inner disk, transition to LS, jet launch.

  50. Relativistic iron lines • The first broad Fe Kα line observed for either a BHB or an AGN wasreported in the spectrum of Cyg X-1 based onEXOSATdata. This result that inspired Fabian et al. (1989) to investigate the production of sucha line in the near vicinity of a Schwarzschild BH, a result that was later generalizedby Laor (1991) to include the Kerr metric. • Beppo-SAX discovered relativistic lines in several BHB: SAXJ1711+3808, XTEJ1909+094,GRS1915+105, V4641Sgr • XMM and Chandra: CCD and gratings In many cases ISCO consistent with non-spinning BH Detection of “smeared edges”

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