Neutron star low-mass X-ray binaries

Rudy WijnandsAnton Pannekoek Institute for Astronomy

University of Amsterdam

3 August 2015 Lorentz center, Leiden

Low- versus high-mass X-ray binaries

• Low-mass X-ray binaries– Traditional: companion has a mass of <1 solar mass – More accurate: Mcompanion < Maccretor

– Mass transfer through Roche-lobe overflow (but not always)– Accretor can be a neutron star or a black hole

• For BH-LMXBs see talk by Tomsick

• High-mass X-ray binaries– Traditional: Mcompanion >10 Msolar

– Mass transfer not through Roche-lobe overflow• Wind accretion• Accretion from circumstellar decretion disk

– See talk by Sidoli

NS-LMXBs

Roche-lobe overflowTypically main sequence or white dwarf companionTypically low magnetic field neutron star

Symbiotic NS LMXBs

Strong stellar wind from low-mass red giant companionTypically high magnetic field neutron starSimilar physics as in the wind accreting HMXBs?

E.g., see short review of Postnov et al. 2010

Different ways to classify neutron-star LMXBs

• Persistent versus transient LMXBs• Correlated spectral and variability behavior

– Z versus Atoll sources– >1% LEdd

• Pulsars versus non-pulsating sources– Symbiotic LMXBs– AMXPs, including the tMSPs

• Luminosity classes• All classifications are closely linked!

Persistent and transient LMXBs• Most low-mass X-ray binaries are not always visible• They are usually very dim no or very little accretion of matter

– See talk by Degenaar

• They become occasionally very bright huge (>104-6) increase in accretion rate– Outbursts due to instabilities in the disk

• During outbursts they behave like the persistent sources

RX

TE

/AS

M li

ght c

urve

s

Transient NS low-mass X-ray binaries

A huge variety in behavior!

Quiescence

Outburst

X-r

ay lu

min

osit

yTransient low-mass X-ray binaries

Time in days

Accretion spectra in NS-LMXBs• Complex subject and no consensus about what

can be inferred from the accretion spectra– Neutron star properties– Accretion physics

• Similar hard and soft

states as BH transients

Bouchacourt et al. 1984

Broad band spectral analysis • Often limitation on the X-ray data

– Only limited energy resolution– Often limited number of photons

• Use broad-band spectral analysis– Good: model independent– Bad: instrument dependent

• Basic Z-Atoll classification in LMXBs– Correlated X-ray spectral and X-ray timing behavior

• Hasinger & van der Klis 1989

– Note: often non-experts only focus on the X-ray spectral behavior but forget the rapid X-ray variability part

X-ray colors

Neutron star

Corona

Disk

Corona

Neutron star

Soft state

Hard state

Soft color (SC) = B/AHard color (HC) = C/B

Plot HC versus SC • Color-color diagram (CD)

Plot HC versus Lx

• Hardness-intensity diagram (HID)

No standard bands!

A B C

After Lin et al. 2010

Near the Eddington limit 1%-50% of the Eddington limit

XTE J1701-462

1.5 years active in 2006-2007 at near Eddington luminosities

CCD evolution of XTE J1701-462

First ever transient Z source!Difference between Z and atoll sources is in long-term Mdot!

(note: rapid X-ray variability properties support this)

Homan et al. 2007

Accreting X-ray pulsars• Most NS-LMXBs do not pulsate• A dozen or so are slow pulsars

– Four odd-balls • GRO J1744-28 (0.47 s), 2A 1822-371 (0.59 s), Her X-1 (1.24 s),

4U 1626-67 (7.66 s)

– Rest are very slow rotators: the symbiotic NS-LMXBs Postnov et al. 2010

Enoto et al. 2014

AMXPs

Accreting millisecond X-ray pulsars• ~10% of the NS-LMXBs are AMXPs

– Spin frequency mostly > 100 Hz– Also including Terzan 5 X-2 that has a spin frequency 11 Hz

• All AMXPs are transient– Although HETE J1900.1-2455 is a quasi-persistent source– Most AMXPs are not bright in outburst

• One ‘classical’ AMXP is millisecond radio pulsar– M28I– Also called transitional millisecond pulsars (see talk Archibald)

• How do the other tMSPs fit in?– Accreting phase has Lx that resemble quiescent NS-LMXBs

Luminosity classes

Class Lx

(2-10 keV)

Comments

Very bright >1038 erg s-1

> 50% LEdd

• Z sources• Cyg like are brighter than Sco like

Bright 1037-38 erg s-1

~10-50% LEdd

• Atoll sources• GX Atoll sources are the brightest• AMXPs fall in this class although most are faint at peak of outburstFaint 1036-37 erg s-1

~1-10% LEdd

Very faint 1034-36 erg s-1

~0.01-1% LEdd

• Also called sub-luminous LMXBs• See talk by Heinke

Quiescence <1034 erg s-1 <0.01% LEdd

• Very low level accretion or no accretion at all (see talk by Degenaar)• Most tMSPs

Arbitrary, inspired by observations!

Note the energy range used

All classes contain transients and persistent sources

~

Some open problems• What causes the tracks in Z and Atoll sources?• Transient/persistent behavior still hardly understood• Sub-luminous X-ray binaries not understood• Why only 10% of NS-LMXBs are AMXPs?

– Do non-pulsating NS-LMXBs have no B field?

• How do tMSP fit in?– New way to probe very low accretion rates– Where are the bright outbursts?

• Selection effect?

• What phenomena are only accretion related and which are also influenced by the nature of the compact object?

Osaki et al. 2001

1995

GRO J0422+32SAX J1808.4-3658

Campana et al. 2008