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An Introduction to Low-Mass X-ray Binaries

An Introduction to Low-Mass X-ray Binaries . Dipping LMXBs -- Suzaku observation of XB 1916-053. Zhongli Zhang (U. of Tokyo). Makishima-Nakazawa lab seminar Oct.3, 2013. Outline. What is a LMXB? observations, accretion condition, formation scenarios, E ddington luminosity

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An Introduction to Low-Mass X-ray Binaries

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  1. An Introduction to Low-Mass X-ray Binaries Dipping LMXBs -- Suzaku observation of XB 1916-053 Zhongli Zhang (U. of Tokyo) Makishima-Nakazawa lab seminar Oct.3, 2013

  2. Outline • What is a LMXB? • observations, accretion condition, formation scenarios, Eddington luminosity • Accretion models of LMXBs • bimodality of soft/hard states, “western” and “eastern” controversy, comptonizing corona • Our study of dipping LMXBs • motivation, method and results

  3. What is a LMXB? A low-mass (< 1 M) star (MS star, red giant, white dwarf) orbits around a NS or BH (for BH it is called BH binary in JP), and transfers mass onto the compact object through Rochelobe overflow. • Start from discovery of Scorpius X-1 in 1962 (see seminar Vol.52) • Around ~150 LMXBs are identified in the Milky Way Open circles The scale height of LMXBs is larger than HMXBs Grimm et al. 2002

  4. LMXBs outside the Milky Way • They are the most important X-ray source population in non star-bursting galaxies (contributes > 40% of X-ray emission). • They follow the stellar mass distribution (10 ~ few 100 in each galaxy) In a galaxy field Chandra NGC 4278 cataclysmic variable active binary ~ 3% Credit: Zhang Sazonov et al., 2006Revnivtsev et al., 2006

  5. Formation of LMXBs Accretion condition Rd: radius of donor a: separation of two stars (Paczyński 1971) donor is big enough, binary is close enough Inner Lagrangian point Two formation scenarios Dynamical: two-body Interaction in high mass density system (glob. cluster) Primordial: donor expansion (from main-sequence to red giant) angular momentum loss (gravitational radiation and magnetic braking)

  6. Luminosity of LMXBs Accretion luminosity Lacc = GMNSM/RNS Lacc ~ 1036 – 1038 erg/s  M ~ 10-10 – 10-8M/yr disk luminosity Ldisk = ½ Lacc (another half is released close to NS surface) Eddington luminosity: balance between the force of radiation acting outward and the gravitational force acting inward. Ledd= 4πGMNS mpc/σT (σT: Thomson cross section) When accretion matter is hydrogenLedd~ 1.3E38(MNS/M) erg/s The maximum temperature on a NS surface can be calculated. How? Ledd= 4πσRNS2Tmax4 (Stefan-Boltzmann law, σ: stefan-Boltzmann cross section) Tmax ~ 2-3 keV

  7. Accretion models of LMXBs How to explain the observations? Red: clear soft state Blue: clear hard state Asai+2013 Credit: Gilfanov LMXBs show clear high/soft and low/hard states

  8. Bimodality of LMXBs LMXBs is either in soft or hard state, no stable intermediate state! Soft state: high M, kTbb ~ 1-2 keV, accretion from standard disk: optically thick geometrically thin artist image Hard state: low M, kTe ~ 10-50 keV, inner disk region expands to electron corona: optically thingeometrically thick Reason of bimodality: thermal instability (positive feedback) Soft-to-Hard Hard-to-Soft Tgas Tgas Pgas Pgas Cooling  Cooling  Disk expand Disk shrink Emissivity ~ n2  Emissivity ~ n2  ngas ngas

  9. Canonical models of LMXBs Soft state: BB emission from NS surface + multi-temperature BB from inner region of accretion disk (bbody+diskbb, Mitsuda+1984) BB from NS surface (bbody): L = 4πRbb2σTbb4 (S-B law) (Rbb : equivalent to spherical radiation) Makishima+1986 MCD emission (diskbb): superposition of bbody with continuous distribution of disk temperature T(r) ~ r -3/4 Possible corrections: 1) Spectral hardening Tcolor = κTeff(κ~ 1.7, Shimura+1995) 2) When Tin occurs somewhere larger than Rin.Rin = ξRin-(xspec fitting) (ξ ~ 0.412, Kubota+1998)

  10. Hard state: Comptonized NS BB emission by hot electron corona + disk emission (Mitsuda+1989) Comptonization inverse Compton scattering inelastic interaction of lower energy photon with higher energy electron, and get energy from the electron electron corona weak disk mass accretion is nearly free-fall and spherical The electron temperature kTe is always between kTϒ and kTp Two parameters affect the photon energy after Comptonization Electron temperature kTe When hν << kTe, dν/ν ~ kTe /mec2 2) Corona optical depth τ, which matters the scattering times of each photon.

  11. Our study of dipping LMXBs Dipping LMXBs • LMXBs with periodic dips in X-ray intensity. Compared to normal LMXBs, they have higher inclinations. Normally show harder persistent spectrum. Picture from “ADC source”: progressive covering of accretion disk corona by the donor. NS emission is totally hidden by disk. ADC is a too special design. Trigo+2006: obscuration of central bbody emission, by ionized structure on the disk. (ADC is not needed!) simple and beautiful! donor

  12. Motivation Based on Trigo+2006, no accretion disk corona is needed. Then are dipping LMXBs similar to other LMXBs? Can we distinguish their spectral states (soft/hard) like in non-dipping LMXBs? Especially, can we finda dipper in the soft state? If yes, can we describe its spectrum with the canonical LMXB softstate model (diskbb+bbody; Mitsuda +1984), with modifications? If any, what is its spectral difference compared to non-dippers? Is the spectrum more Comptonized due to high inclination?

  13. Target selection Out of ~ 10 dippers known in the Galaxy, five were observed by Suzaku. Of them, we chose the most luminous one: XB 1916-053 XB 1916-053 Suzaku OBS on Nov. 8th, 2006 Orbital period: ~ 50 min (Church et al. 1998) Inclination angle: 60° ~ 80° (Smale et al. 1988) Distance: 9.3 kpc (Yoshida 1993) Lightcurve Hardness ratio

  14. Comparison of the spectral shape XB 1916-053 spectrum is softer than other dippers, especially above 10 keV. XB 1916-053 XB 1916-053 The spectrum is in between the soft- and hard-state spectra of Aql X-1.

  15. Non-dip spectrum fitting diskbb+bbody Mitsuda +1984 kTin ~ 0.92 keV kTbb ~ 2.35 keV χ2 /d.o.f = 3.06 Mitsuda +1984 model fits the data well till 15 keV. Above 15 keVstrong positive residual is detected, which requires modification of Mitsuda model, i.e., with Comptonization.

  16. diskbb+nthcomp(bbody) kTbb= 1.28 ± 0.06 keV Rbb = 2.2 ± 0.3 km kTe = 11.3 (+64.5-4.0) keV τ = 3.1 (+1.5-2.5) kTin = 0.68 ± 0.02 keV Rin = 10.9 ± 0.6 km (assuming i = 70°) χ2 /d.o.f =1.13 The Mitsuda model becomes successful when the BB component is allowed to be significantly Comptonized. Inner disk radius Rin is relatively small.

  17. nthcomp(diskbb)+nthcomp(bbody) Common corona: kTe >10.0 keV τ < 3.0 kTin = 0.54 ± 0.02 keV Rin= 16.7 ± 0.2 km (assuming i = 70°) kTbb= 1.62 ± 0.03 keV Rbb = 1.3 ± 0.1 km χ2 /d.o.f =1.11 The model remains successful when the disk component is also Comptonized, giving more reasonable physical parameters. Allowing the two components to be Comptonized by different coronae, we get similar results.

  18. Summary We study the spectrum of XB 1916-053, as the most luminous dipping LMXB so far observed with Suzaku. We confirmed that this object is clearly in the soft state. To fit the spectrum with the standard Mitsuda+84 model for the soft state of non-dipping LMXBs, a strong Comptonizationwith kTe > 10 keV is needed at least on the BB component. Interpretation X-ray spectra of dipping LMXBs are strongly Comptonized, possibly because their photons pass through hot electron clouds above the accretion disk. 70°

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