Makishima-Nakazawa lab seminar Oct.3, 2013

An Introduction to Low-Mass X-ray Binaries

Dipping LMXBs -- Suzaku observation of XB 1916-053

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. 2002The 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., 2006Revnivtsev et al., 2006

In a galaxy field

~ 3%

cataclysmic variable

activebinary

LMXBs outside the Milky Way

Inner Lagrangian point

Rd : 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 Lacc = GMNSM/RNS

disk luminosity Ldisk = ½ Lacc (another half is released close to NS surface)

Lacc ~ 1036 – 1038 erg/s M ~ 10-10 – 10-8 M/yr

Ledd = 4πGMNS mpc/σT (σT: Thomson cross section)

When accretion matter is hydrogen Ledd ~ 1.3E38(MNS/M) erg/s The maximum temperature on a NS surface can be calculated. How? Ledd = 4πσRNS

2 Tmax

4 (Stefan-Boltzmann law, σ: stefan-Boltzmann cross section) Tmax ~ 2-3 keV

Accretion models of LMXBsHow to explain

the observations?

Credit: Gilfanov

Asai+2013Red: clear soft stateBlue: clear hard state

LMXBs show clear high/soft and low/hard states

Bimodality of LMXBsLMXBs 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 thin geometrically thick

Reason of bimodality: thermal instability (positive feedback)

Soft-to-Hard Hard-to-Soft Tgas

Pgas

Disk expand

ngas

Emissivity ~ n2

Cooling

Tgas

Pgas

Disk shrink

ngas

Emissivity ~ n2

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 = 4πRbb

2σTbb4 (S-B law)

(Rbb : 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 Tcolor = κTeff (κ ~ 1.7, Shimura+1995)

2) When Tin occurs somewhere larger than Rin. Rin = ξRin-(xspec fitting)

(ξ ~ 0.412, Kubota+1998)

Makishima+1986

Hard state: Comptonized NS BB emission by hot electron corona+ disk emission (Mitsuda+1989)

Comptonizationinverse Compton scatteringinelastic 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 1) Electron temperature kTe

When hν << kTe, dν/ν ~ kTe /mec2

2) Corona optical depth τ, which matters the scattering times of each photon.

Our study of dipping LMXBsDipping LMXBsLMXBs 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?

2. If yes, can we describe its spectrum with the canonical LMXB soft state model (diskbb+bbody; Mitsuda +1984), with modifications?

3. 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. 8th, 2006

Hardness ratio

Lightcurve

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

Comparison of the spectral shape

The spectrum is in between the soft- and hard-state spectra of Aql X-1.

XB 1916-053 spectrum is softer than other dippers, especially above 10 keV.

XB 1916-053 XB 1916-053

diskbb+bbody

1. Mitsuda +1984 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 +1984

kTin ~ 0.92 keVkTbb ~ 2.35 keV

χ2 /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 Rin is relatively small.

kTbb = 1.28 ± 0.06 keVRbb = 2.2 ± 0.3 kmkTe = 11.3 (+64.5 -4.0) keVτ = 3.1 (+1.5 -2.5)

kTin = 0.68 ± 0.02 keVRin = 10.9 ± 0.6 km (assuming i = 70°)

χ2 /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:kTe >10.0 keVτ < 3.0

kTin = 0.54 ± 0.02 keVRin = 16.7 ± 0.2 km (assuming i = 70°)

kTbb = 1.62 ± 0.03 keVRbb = 1.3 ± 0.1 km

nthcomp(diskbb)+nthcomp(bbody)

χ2 /d.o.f =1.11

Summary

1. We study the spectrum of XB 1916-053, 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 kTe > 10 keV is needed at least on the BB component.

InterpretationX-ray spectra of dipping LMXBs are strongly Comptonized, possibly because their photons pass through hot electron clouds above the accretion disk.

70°