Millisecond Pulsar Binary Systems Research Articles

Pressure Balance and Intrabinary Shock Stability in Rotation-powered-state Redback and Transitional Millisecond Pulsar Binary Systems

Abstract A number of low-mass millisecond pulsar (MSP) binaries in their rotation-powered state exhibit double-peaked X-ray orbital modulation centered at inferior pulsar conjunction. This state, which has been known to persist for years, has recently been interpreted as emission from a shock that enshrouds the pulsar. However, the pressure balance for such a configuration is a crucial unresolved issue. We consider two scenarios for pressure balance: a companion magnetosphere and stellar mass loss with gas dominance. It is found that the magnetospheric scenario requires several kilogauss poloidal fields for isobaric surfaces to enshroud the MSP, as well as for the magnetosphere to remain stable if there is significant mass loss. For the gas-dominated scenario, it is necessary that the companion wind loses angular momentum prolifically as an advection- or heating-dominated flow. Thermal bremsstrahlung cooling in the flow may be observable as a UV to soft X-ray component independent of orbital phase if the mass rate is high. We formulate the general requirements for shock stability against gravitational influences in the pulsar rotation-powered state for the gas-dominated scenario. We explore stabilizing mechanisms, principally irradiation feedback, which anticipates correlated shock emission and companion variability and predicts F γ /F X ≲ 14 for the ratio of pulsar magnetospheric γ-ray to total shock soft-to-hard X-ray fluxes. This stability criterion implies an unbroken extension of X-ray power-law emission to hundreds of keV for some systems. We explore observational discriminants between the gas-dominated and magnetospheric scenarios, motivating contemporaneous radio through γ-ray monitoring of these systems.

3D MODEL ATMOSPHERES FOR EXTREMELY LOW-MASS WHITE DWARFS

We present an extended grid of mean three-dimensional (3D) spectra for low-mass, pure-hydrogen atmosphere DA white dwarfs (WDs). We use CO5BOLD radiation-hydrodynamics 3D simulations covering Teff = 6000-11,500 K and logg = 5-6.5 (cgs units) to derive analytical functions to convert spectroscopically determined 1D temperatures and surface gravities to 3D atmospheric parameters. Along with the previously published 3D models, the 1D to 3D corrections are now available for essentially all known convective DA WDs (i.e., logg = 5-9). For low-mass WDs, the correction in temperature is relatively small (a few per cent at the most), but the surface gravities measured from the 3D models are lower by as much as 0.35 dex. We revisit the spectroscopic analysis of the extremely low-mass (ELM) WDs, and demonstrate that the 3D models largely resolve the discrepancies seen in the radius and mass measurements for relatively cool ELM WDs in eclipsing double WD and WD + milli-second pulsar binary systems. We also use the 3D corrections to revise the boundaries of the ZZ Ceti instability strip, including the recently found ELM pulsators.

γ-ray emission states in the redback millisecond pulsar binary system PSR J1227−4853

Long expected transition states between the rotation powered and accretion powered non-thermal emission in the millisecond pulsar binary systems have been recently observed in the case of three objects PSR J1023+0038, PSR J1824-2452, and PSR J1227-4859. Surprisingly, the transition is related to the significant change in the -ray flux being a factor of a few higher with the presence of an accretion disk. The origin of this enhanced emission seems to be related to the penetration of the inner pulsar magnetosphere by the accretion disk. We propose that the radiation processes, characteristic for the rotation powered pulsar, can co-exist with the presence of an accretion disk in the inner pulsar magnetosphere. In our scenario additional -ray emission is produced by secondary leptons, originated close to the acceleration gap, which Compton up-scatter thermal radiation from the accretion disk to GeV energies. The accretion disk penetrates deep into the pulsar magnetosphere allowing the matter to fall onto the NS surface producing pulsed X-ray emission. We show that the sum of the rotation powered pulsar -ray emission, produced by the primary electrons in the curvature process, and the -ray emission, produced by secondary leptons, can explain the observed high energy radiation from the redback binary pulsar PSR J1227-4853 in the state with evidences of the accretion disk.

Modulated gamma-ray emission from compact millisecond pulsar binary systems

A significant amount of the millisecond pulsars has been discovered within binary systems. In several such binary systems the masses of the companion stars have been derived allowing to distinguish two classes of objects, called the Black Widow and the Redback binaries. Pulsars in these binary systems are expected to produce winds which, colliding with stellar winds, create conditions for acceleration of electrons. These electrons should interact with the anisotropic radiation from the companion stars producing gamma-ray emission modulated with the orbital period of the binary system. We consider the interaction of a millisecond pulsar (MSP) wind with a very inhomogeneous stellar wind from the companion star within binary systems of the Black Widow and Redback types. It is expected that the pulsar wind should mix efficiently with the inhomogeneous stellar wind. Electrons accelerated in such mixed, turbulent winds can interact with the magnetic field and also strong radiation from the companion star producing not only synchrotron radiation but also gamma-rays in the the Inverse Compton process. Applying numerical methods, we calculated the GeV-TeV gamma-ray spectra and the light curves expected from some millisecond pulsar binary systems. It is concluded that energetic millisecond pulsar binary systems create a new class of TeV gamma-ray sources which could be detectable by the future Cherenkov arrays (e.g. CTA) and possibly also by the extensive campaigns with the present arrays (HESS, MAGIC, VERITAS). However, gamma-ray emission from the millisecond pulsar binary systems is predicted to have different features than those observed in the case of massive TeV gamma-ray binaries such as LS I 303 61 or LS 5039. The maximum in the TeV gamma-ray orbital light curve should appear when the MSP is behind the companion star.

Accretion, ablation and propeller evolution in close millisecond pulsar binary systems

A model for the formation and evolution of binary millisecond radio pulsars in systems with low mass companions (< 0.1 Msun) is investigated using a binary population synthesis technique. Taking into account the non conservative evolution of the system due to mass loss from an accretion disk as a result of propeller action and from the companion via ablation by the pulsar, the transition from the accretion powered to rotation powered phase is investigated. It is shown that the operation of the propeller and ablation mechanisms can be responsible for the formation and evolution of black widow millisecond pulsar systems from the low mass X-ray binary phase at an orbital period of ~0.1 day. For a range of population synthesis input parameters, the results reveal that a population of black widow millisecond pulsars characterized by orbital periods as long as ~0.4 days and companion masses as low as ~0.005 Msun can be produced. The orbital periods and minimum companion mass of this radio millisecond pulsar population critically depend on the thermal bloating of the semi-degenerate hydrogen mass losing component, with longer orbital periods for a greater degree of bloating. Provided that the radius of the companion is increased by about a factor of 2 relative to a fully degenerate, zero temperature configuration, an approximate agreement between observed long orbital periods and theoretical modeling of hydrogen rich donors can be achieved. We find no discrepancy between the estimated birth rates for LMXBs and black widow systems, which on average are $\sim~1.3\times10^{-5} {\rm yr}^{-1}$ and $1.3\times10^{-7} {\rm yr}^{-1}$ respectively.

X-RAY VARIABILITY AND EVIDENCE FOR PULSATIONS FROM THE UNIQUE RADIO PULSAR/X-RAY BINARY TRANSITION OBJECT FIRST J102347.6+003841

We report on observations of the unusual neutron-star binary system FIRST J102347.6+003841 carried out using the XMM-Newton satellite. This system consists of a radio millisecond pulsar in an 0.198-day orbit with a ~0.2 solar-mass Roche-lobe-filling companion, and appears to have had an accretion disk in 2001. We observe a hard power-law spectrum (\Gamma = 1.26(4)) with a possible thermal component, and orbital variability in X-ray flux and possibly hardness of the X-rays. We also detect probable pulsations at the pulsar period (single-trial significance ~4.5 sigma from an 11(2)% modulation), which would make this the first system in which both orbital and rotational X-ray pulsations are detected. We interpret the emission as a combination of X-rays from the pulsar itself and from a shock where material overflowing the companion meets the pulsar wind. The similarity of this X-ray emission to that seen from other millisecond pulsar binary systems, in particular 47 Tuc W (PSR J0024-7204W) and PSR J1740-5340, suggests that they may also undergo disk episodes similar to that seen in J1023 in 2001.