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

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

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 Roche lobe 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 Grimm et al The scale height of LMXBs is larger than HMXBs

 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) Chandra Credit: Zhang NGC 4278 Sazonov et al., 2006 Revnivtsev et al., 2006 In a galaxy field ~ 3% cataclysmic variable active binary LMXBs outside the Milky Way

Inner Lagrangian point R d : radius of donor a : separation of two stars (Paczyński 1971) donor is big enough, binary is close enough Accretion condition 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) Formation of LMXBs

Eddington luminosity: balance between the force of radiation acting outward and the gravitational force acting inward. Luminosity of LMXBs Accretion luminosity L acc = GM NS M/R NS disk luminosity L disk = ½ L acc (another half is released close to NS surface) L acc ~ – erg/s  M ~ – M  /yr L edd = 4πGM NS m p c/σ T (σ T: Thomson cross section) When accretion matter is hydrogen L edd ~ 1.3E 38 (M NS /M  ) erg/s The maximum temperature on a NS surface can be calculated. How? L edd = 4πσR NS 2 T max 4 (Stefan-Boltzmann law, σ: stefan-Boltzmann cross section) T max ~ 2-3 keV

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

Bimodality of LMXBs LMXBs is either in soft or hard state, no stable intermediate state! Soft state: high M, kT bb ~ 1-2 keV, accretion from standard disk: optically thick geometrically thin artist image Hard state: low M, kT e ~ keV, inner disk region expands to electron corona: optically thin geometrically thick Reason of bimodality: thermal instability (positive feedback) Soft-to-HardHard-to-Soft T gas  P gas  Disk expand n gas  Emissivity ~ n 2  Cooling  T gas  P gas  Disk shrink n gas  Emissivity ~ n 2  Cooling 

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

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 kT e is always between kT ϒ and kT p Two parameters affect the photon energy after Comptonization 1)Electron temperature kT e When hν << kT e, dν/ν ~ kT e /m e c 2 2) Corona optical depth τ, which matters the scattering times of each photon.

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

Based on Trigo+2006, no accretion disk corona is needed. Then are dipping LMXBs similar to other LMXBs? 1.Can we distinguish their spectral states (soft/hard) like in non- dipping LMXBs? Especially, can we find a dipper in the soft state? 1.If yes, can we describe its spectrum with the canonical LMXB soft state model (diskbb+bbody; Mitsuda +1984), with modifications? 1.If any, what is its spectral difference compared to non-dippers? Is the spectrum more Comptonized due to high inclination? Motivation

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

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

diskbb+bbody 1.Mitsuda model fits the data well till 15 keV. 2.Above 15 keV strong positive residual is detected, which requires modification of Mitsuda model, i.e., with Comptonization. Non-dip spectrum fitting Mitsuda kT in ~ 0.92 keV kT bb ~ 2.35 keV χ 2 /d.o.f = 3.06

1.The Mitsuda model becomes successful when the BB component is allowed to be significantly Comptonized. 2.Inner disk radius R in is relatively small. kT bb = 1.28 ± 0.06 keV R bb = 2.2 ± 0.3 km kT e = 11.3 ( ) keV τ = 3.1 ( ) kT in = 0.68 ± 0.02 keV R in = 10.9 ± 0.6 km (assuming i = 70°) χ 2 /d.o.f =1.13 diskbb+nthcomp(bbody)

1.The model remains successful when the disk component is also Comptonized, giving more reasonable physical parameters. 2.Allowing the two components to be Comptonized by different coronae, we get similar results. Common corona: kT e >10.0 keV τ < 3.0 kT in = 0.54 ± 0.02 keV R in = 16.7 ± 0.2 km (assuming i = 70°) kT bb = 1.62 ± 0.03 keV R bb = 1.3 ± 0.1 km nthcomp(diskbb)+nthcomp(bbody) χ 2 /d.o.f =1.11

Summary 1.We study the spectrum of XB , as the most luminous dipping LMXB so far observed with Suzaku. 2.We confirmed that this object is clearly in the soft state. 3.To fit the spectrum with the standard Mitsuda+84 model for the soft state of non-dipping LMXBs, a strong Comptonization with kT e > 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°