Wind accretion: Theory and Observations

A review of wind accretion in HMXB is presented. We focus on different regimes of quasi-spherical accretion onto a NS: supersonic (Bondi) accretion, which takes place when the captured matter cools down rapidly and falls supersonically towards the NS magnetosphere, and subsonic (settling) accretion which occurs when the plasma remains hot until it meets the magnetospheric boundary. The two regimes are separated by a limit in X-ray luminosity at about 4 10^{36} erg/s. In subsonic accretion, which works a hot quasi-spherical shell must form around the magnetosphere, and the actual accretion rate onto the NS is determined by the ability of the plasma to enter the magnetosphere due to the Rayleigh-Taylor instability. Two regimes of subsonic accretion are possible, depending on the plasma cooling mechanism (Compton or radiative) near the magnetopshere. The transition from the high-luminosity regime with Compton cooling to the low-luminosity (L_x < 3times 10^35 erg/s) regime with radiative cooling can be responsible for the onset of the off states repeatedly observed in several X-ray pulsars, such as Vela X-1, GX 301-2 and 4U 1907+09. The triggering of the transition may be due to a switch in the X-ray beam pattern in response to a change in the optical depth in the accretion column with changing luminosity. We also show that in the settling accretion theory, bright X-ray flares (10^{38}-10^{40} ergs) observed in SFXT may be produced by sporadic capture of magnetized stellar-wind plasma. At sufficiently low accretion rates, magnetic reconnection can enhance the magnetospheric plasma entry rate, resulting in copious production of X-ray photons, strong Compton cooling and ultimately in unstable accretion of the entire shell. A bright flare develops on the free-fall time scale in the shell, and the typical energy released in an SFXT bright flare corresponds to the mass of the shell.

INTEGRAL observations of SS433: systems parameters and nutation of supercritical accretion disk

Based on multiyear INTEGRAL observations of SS433 in 2003-2011, a composite IBIS/ISGRI 18-60 keV orbital light curve is constructed around zero precessional phases psi_{pr}= 0 at the maximim accretion disk opening angle. It shows a peculiar shape with significant excess near the orbital phase phi_orb= 0.25, which is not seen in the softer 2-10 keV energy band. The 40-60 keV orbital light curve demonstrates two almost equal humps at phases sim 0.25 and sim 0.75, most likely due to nutation effects of the accretion disk. The nutational variability of SS433 in 15-50 keV with a period of 6.290 days is independently found from timing analysis of Swift/BAT data. The change of the off-eclipse 18-60 keV X-ray flux with the precessional phase shows a double-wave form with strong primary maximum at psi_{pr}= 0 and weak but significant secondary maximum at psi_{pr}= 0.6. A weak variability of the 18-60 keV flux in the middle of the orbital eclipse correlated with the disk precessional phase is also observed. The joint analysis of the broadband 18-60 keV orbital and precessional light curves confirms the presence of a hot extended corona in the central parts of the supercritical accretion disk and constrains the binary mass ratio in SS433 in the range 0.5>q>0.3, suggesting the black hole nature of the compact object.

Variable neutron star free precession in Hercules X-1 from evolution of RXTE X-ray pulse profiles with phase of the 35-day cycle

Accretion of matter onto the surface of a freely precessing NS with a complex non-dipole magnetic field can explain the change of X-ray pulse profiles of Her X-1 observed by RXTE with the phase of the 35-day cycle. We demonstrate this using all available measurements of X-ray pulse profiles in the 9-13 keV energy range obtained with the RXTE/PCA. The measured profiles guided the elaboration of a geometrical model and the definition of locations of emitting poles, arcs, and spots on the NS surface which satisfactorily reproduce the observed pulse profiles and their dependence on free precession phase. We have found that the observed trend of the times of the 35-day turn-ons on the O-C diagram, which can be approximated by a collection of consecutive linear segments around the mean value, can be described by our model by assuming a variable free precession period, with a fractional period change of about a few percent. We propose that the 2.5% changes in the free precession period that occur on time scales of several to tens of 35-day cycles can be related to wandering of the principal inertia axis of the NS body due to variations in the patterns of accretion onto the NS surface. The closeness of periods of the disk precession and the NS free precession can be explained by the presence of a synchronization mechanism in the system, which modulates the dynamical interaction of the gas streams and the accretion disk with the NS free precession period.

Do we see accreting magnetars in X-ray pulsars?

Strong magnetic field of accreting neutron stars ($10^{14}$ G) is hard to probe by X-ray spectroscopy but can be indirectly inferred from spin-up/spin-down measurement in X-ray pulsars. The existing observations of slowly rotating X-ray pulsars are discussed. It is shown that magnetic fields of neutron stars derived from these observations (or lower limits in some cases) fall within the standard $10^{12}$-$10^{13}$ G range. Claims about the evidence for accreting magnetars are critically discussed in the light of recent progress in understanding of accretion onto slowly rotating neutron stars in the subsonic regime.

Theory of wind accretion

A review of wind accretion in high-mass X-ray binaries is presented. We focus attention to different regimes of quasi-spherical accretion onto the neutron star: the supersonic (Bondi) accretion, which takes place when the captured matter cools down rapidly and falls supersonically toward NS magnetospghere, and subsonic (settling) accretion which occurs when plasma remains hot until it meets the magnetospheric boundary. Two regimes of accretion are separated by an X-ray luminosity of about $4times10^{36}$ erg/s. In the subsonic case, which sets in at low luminosities, a hot quasi-spherical shell must be formed around the magnetosphere, and the actual accretion rate onto NS is determined by ability of the plasma to enter the magnetosphere due to Rayleigh-Taylor instability. We calculate the rate of plasma entry the magnetopshere and the angular momentum transfer in the shell due to turbulent viscosity appearing in the convective differentially rotating shell. We also discuss and calculate the structure of the magnetospheric boundary layer where the angular momentum between the rotating magnetosphere and the base of the differentially rotating quasi-spherical shell takes place. We show how observations of equilibrium X-ray pulsars Vela X-1 and GX 301-2 can be used to estimate dimensionless parameters of the subsonic settling accretion theory, and obtain the width of the magnetospheric boundary layer for these pulsars.

Quasi-spherical accretion in low-luminosity X-ray pulsars: Theory vs. observations

Quasi-spherical subsonic accretion can be realized in slowly rotating wind-fed X-ray pulsars (XPSRs) at X-ray luminosities <4 10^{36} erg/s. In this regime the accreting matter settles down subsonically onto the rotating magnetosphere, forming an extended quasi-static shell. The shell mediates the angular momentum removal from the rotating NS magnetosphere by shear turbulent viscosity in the boundary layer or via large-scale convective motions. In the last case the differential rotation law in the shell is close to iso-angular-momentum rotation. The accretion rate through the shell is determined by the ability of the plasma to enter the magnetosphere due to Rayleigh-Taylor instabilities while taking cooling into account. Measurements of spin-up/spin-down rates of quasi-spherically wind accreting XPSRs in equilibrium with known orbital periods (like e.g. GX 301-2 and Vela X-1) enable determination of the main dimensionless parameters of the model and the NS magnetic field. For equilibrium pulsars with independent measurements of the magnetic field, the stellar wind velocity from the companion can be estimated without the use of complicated spectroscopic measurements. For non-equilibrium pulsars, a maximum possible spin-down torque exerted on the accreting NS exists. From observations of the spin-down rate and X-ray luminosity in such pulsars (GX 1+4, SXP 1062, 4U 2206+54, etc.) a lower limit on the NS magnetic field is derived, which in all cases turns out to be close to the standard one and in agreement with cyclotron line measurements. The model explains the existence of super slowly rotating XPSRs without the need to hypothesize on additional accretion properties and magnetar-like magnetic fields in accreting neutron stars.

On the nature of `off states in slowly rotating low-luminosity X-ray pulsars

We elaborate on a recently proposed model for subsonic quasi-spherical accretion onto slowly rotating pulsars, in which accretion is mediated through a hot quasi-static shell above the neutron star magnetosphere. We show that under the same external conditions, two regimes of subsonic accretion are possible, depending on if plasma cooling in the transition zone is dominated by Compton or radiative processes. We suggest that a transition from the higher luminosity Compton cooling regime to the lower luminosity radiative cooling regime can be responsible for the onset of the `off-states repeatedly observed in several low luminosity slowly accreting pulsars, such as Vela X-1, GX 301-2 and 4U 1907+09. We further suggest that the triggering of the transition may be due to a switch in the X-ray beam pattern in response to a change in the optical depth in the accretion column with changing luminosity.

Quasi-spherical accretion in X-ray pulsars

Quasi-spherical accretion in wind-fed X-ray pulsars is discussed. At X-ray luminosities <4 10^{36} erg/s, a hot convective shell is formed around the neutron star magnetosphere, and subsonic settling accretion regime sets in. In this regime, accretion rate onto neutron star is determined by the ability of plasma to enter magnetosphere via Rayleigh-Taylor instability. A gas-dynamic theory of settling accretion is constructed taking into account anisotropic turbulence. The angular momentum can be transferred through the quasi-static shell via large-scale convective motions initiating turbulence cascade. The angular velocity distribution in the shell is found depending on the turbulent viscosity prescription. Comparison with observations of long-period X-ray wind-fed pulsars shows that an almost iso-angular-momentum distribution is most likely realized in their shells. The theory explains long-term spin-down in wind- fed accreting pulsars (e.g. GX 1+4) and properties of short-term torque-luminosity correlations. The theory can be applied to slowly rotating low-luminosity X-ray pulsars and non-stationary accretion phenomena observed in some SFXTs.

Wind accretion in symbiotic X-ray binaries

The properties of wind accretion in symbiotic X-ray binaries (SyXBs) consisting of red-giant and magnetized neutron star (NS) are discussed. The spin-up/spin-down torques applied to NS are derived based on a hydrodynamic theory of quasi-spherical accretion onto magnetized NSs. In this model, a settling subsonic accretion proceeds through a hot shell formed around the NS magnetosphere. The accretion rate onto the NS is determined by the ability of the plasma to enter the magnetosphere.Due to large Reynolds numbers in the shell, the interaction of the rotating magnetosphere with plasma initiates a subsonic turbulence. The convective motions are capable of carrying the angular momentum through the shell. We carry out a population synthesis of SyXBs in the Galaxy with account for the spin evolution of magnetized NS. The Galactic number of SyXBs with bright (M_v<1) low-mass red-giant companion is found to be from sim 40 to 120, and their birthrate is sim 5times 10^{-5}-10^{-4} per year. According to our model, among known SyXBs, Sct X-1 and IRXS J180431.1-273932 are wind-fed accretors. GX 1+4 lies in the transition from the wind-fed SyXBs to SyXBs in which the giants overflow their Roche lobe. The model successfully reproduces very long NS spins (such as in IGR J16358-4724 and 4U 1954+31) without the need to invoke very strong magnetic fields.