Neutron star low-mass X-ray binaries Rudy Wijnands Anton Pannekoek Institute for Astronomy University of Amsterdam 3 August 2015Lorentz 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: M companion < M accretor –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: M companion >10 M solar –Mass transfer not through Roche-lobe overflow Wind accretion Accretion from circumstellar decretion disk –See talk by Sidoli

NS-LMXBs Roche-lobe overflow Typically main sequence or white dwarf companion Typically low magnetic field neutron star

Symbiotic NS LMXBs Strong stellar wind from low-mass red giant companion Typically high magnetic field neutron star Similar 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% L Edd 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 (> ) increase in accretion rate –Outbursts due to instabilities in the disk During outbursts they behave like the persistent sources RXTE/ASM light curves

Transient NS low-mass X-ray binaries A huge variety in behavior!

Quiescence Outburst X-ray luminosity Transient 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/A Hard color (HC) = C/B Plot HC versus SC Color-color diagram (CD) Plot HC versus L x 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 J years active in at near Eddington luminosities

CCD evolution of XTE J 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 J (0.47 s), 2A (0.59 s), Her X-1 (1.24 s), 4U (7.66 s) –Rest are very slow rotators: the symbiotic NS-LMXBs Postnov et al. 2010

Enoto et al 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 J 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 L x that resemble quiescent NS-LMXBs

Luminosity classes 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 GRO J SAX J Campana et al. 2008