M. Ruderman

NEUTRON STAR MAGNETIC FIELD EVOLUTION, CRUST MOVEMENT, AND GLITCHES

Spinning superfluid neutrons in the core of a neutron star interact strongly with coexisting superconducting protons. One consequence is that the outward (inward) motion of core superfluid neutron vortices during spin-down (spin-up) of a neutron star may alter the cores magnetic field. Such core field changes are expected to result in movements of the stellar crust and changes in the stars surface magnetic field that reflect those in the core below. Observed magnitudes and evolution of the spin-down indices of canonical pulsars are understood as a consequence of such surface field changes. If the growing crustal strains caused by the changing core magnetic field configuration in canonical spinning-down pulsars are relaxed by large-scale crust-cracking events, special properties are predicted for the resulting changes in spin period. These agree with various glitch observations, including glitch activity, permanent shifts in spin-down rates after glitches in young pulsars, the intervals between glitches, families of glitches with different magnitudes in the same pulsar, the sharp drop in glitch intervals and magnitudes as pulsar spin periods approach 0.7 s, and the general absence of glitching beyond this period.

Pulsar death lines and death valley

In most models of a spinning-down neutron stars magnetosphere, the star turns off as a radio pulsar when it can no longer produce electron-positron pairs. The period (P) at turnoff depends upon the magnitude and the structure of the stars surface magnetic field. Only the magnetic dipole component of this field (Bp) is inferred from the observed spin-down rate of a pulsar. Turnoff should occur in a B p −P death valley with well-defined boundaries rather than a death line

The Central engine of gamma-ray bursters

Cosmic gamma-ray bursts (GRBs) are thought to be created when relativistic blast waves that are powered by central engines emit gamma rays between 10 and 10,000 AU from where the explosive energy has been released. To account for the observed duration and variability of GRBs, the central engines must remain active from several to very many seconds and must usually fluctuate strongly in their output on much shorter timescales. We show how neutron stars that are initially rotating differentially at millisecond periods could be such engines, emitting, on the observed timescales, energetic pulses of the right variety and power for as long as the differential motion remains sufficiently vigorous. The energy stored in the differential rotation would be released mainly in sub-bursts, as toroidal magnetic fields are repeatedly wound up to ~1017 G and, only then, pushed to and through the surface by buoyant forces. The same mechanism could also operate in nuclear density tori. The differentially rotating neutron stars or tori could be formed in several ways and at rates sufficiently high to explain the observed frequency of occurrence of GRBs.

Accretion turnoff and rapid evaporation of very light secondaries in low-mass X-ray binaries

The illumination of companion stars in very low mass X-ray binaries by various kinds of radiation from the neighborhood of the neutron star after accretion has terminated or during accretion is considered. If a neutron stars spun-up period approaches 0.001 s, pulsar kHz radiation can quench accretion by pushing surrounding plasma away from the neutron star, and may leave the companion to be evaporated by the high-energy radiation component expected from an isolated millisecond radiopulsar. Expected accretion-powered MeV gamma-rays and e(+ or -) winds may also be effective in evaporating dwarf companions. Neutron star spin-down energy release may sustain the power in these radiation mechanisms even while accretion falls. Accretion-powered soft X-rays may speed the mass loss of highly evolved dwarf companions, particularly those with a large fraction of carbon and oxygen. 30 references.

Neutrino pair emission from finite-temperature neutron superfluid and the cooling of young neutron stars

The neutrons inside neutron stars are almost certainly superfluid below a critical temperature T/subc/approx.10/sup 10/ K. Below T/subc/, pairs of excited neutron quasiparticles may recombine, resulting, if weak neutral currents exist, in the emission of neutrino-antineutrino pairs. We calculate the emissivity associated with this process and compare it with other neutrino emissivities. For neutron star interior temperatures in the range 10/sup 9/-10/sup 10/ K the recombination emissivity can dominate all others. (AIP)

Models for X-Ray Emission from Isolated Pulsars

A model is proposed for the observed combination of power-law and thermal emission of keV X-rays from rotationally powered pulsars. For γ-ray pulsars with accelerators very many stellar radii above the neutron star surface, 100 MeV curvature γ-rays from e- or e+ flowing starward out of such accelerators are converted to e± pairs on closed field lines all around the star. These pairs strongly affect X-ray emission from near the star in two ways. (1) The pairs are a source of synchrotron emission immediately following their creation in regions where B ~ 1010 G. This emission, in the photon energy range 0.1 keV EX 5 MeV, has a power-law spectrum with energy index 0.5 and X-ray luminosity that depends on the backflow current and is typically ~1033 ergs s-1. (2) The pairs ultimately form a cyclotron resonance blanket surrounding the star except for two holes along the open field line bundles that pass through it. In such a blanket, the gravitational pull on e± pairs toward the star is balanced by the hugely amplified push of outflowing surface-emitted X-rays wherever cyclotron resonance occurs. Because of it, the neutron star is surrounded by a leaky hohlraum of hot blackbody radiation with two small holes, which prevents direct X-ray observation of a heated polar cap of a γ-ray pulsar. Weakly spin-modulated radiation from the blanket together with more strongly spin-modulated radiation from the holes through it would then dominate observed low-energy (0.1-10 keV) emission. For non-γ-ray pulsars, in which no such accelerators with their accompanying extreme relativistic backflow toward the star are expected, optically thick e± resonance blankets should not form (except in special cases very close to the open field line bundle). From such pulsars, blackbody radiation from both the warm stellar surface and the heated polar caps should be directly observable. In these pulsars, details of the surface magnetic field evolution, especially of polar cap areas, become relevant to observations. The models are compared to X-ray data from Geminga, PSR 1055-52, PSR 0656+14, PSR 1929+10, and PSR 0950+08.

Late evolution of very low mass X-ray binaries sustained by radiation from their primaries

The accretion-powered radiation from the X-ray pulsar system Her X-1 (McCray et al. 1982) is studied. The changes in the soft X-ray and gamma-ray flux and in the accompanying electron-positron wind are discussed. These are believed to be associated with the inward movement of the inner edge of the accretion disk corresponding to the boundary with the neutron stars corotating magnetosphere (Alfven radius). LMXB evolution which is self-sustained by secondary winds intercepting the radiation emitted near an LMXB neutron star is investigated as well. 59 refs.

Origin and radio pulse properties of millisecond pulsars

Millisecond pulsars may be formed by the accretion induced collapse of massive white dwarfs or from neutron stars spun-up by accretion from low-mass companions. Because the solid crust of a neutron star is expected to be moved by strong stresses which build up during spin-up or spin-down, the expected surface magnetic field structures are quite different for millisecond pulsars formed in these two different scenarios. During prolonged spin-up the moving crust compresses all stellar surface magnetic field into a small region around the spin axis

Photon opacity in surfaces of magnetic neutron stars

Approximate expressions are derived for free-free, bound-free, and Thomson cross-sections of photons by gaseous matter in the presence of superstrong magnetic fields. For photons in modes whose electric field polarization is perpendicular to this magnetic field, the cross-section is reduced by approximately the squared ratio of the photon frequency to the electron cyclotron frequency if this ratio is small.

Millisecond Pulsar Alignment: PSR J0437–4715

The abundances of both orthogonal and nearly aligned neutron stars among the disk population millisecond pulsars appear to be excessively large, relative to those of canonical pulsars. Both of these excesses are expected in strongly spun-up neutron stars. Their surface magnetic field evolution mirrors the changes in the core magnetic field configuration caused by the strong interaction between a cores spinning-up superfluid neutrons and its magnetized superconducting protons. We discuss special observable properties of such a nearly aligned millisecond pulsar and propose an explanation of the apparent paradox for interpreting observations of the very close millisecond pulsar PSR J0437-4715: a strongly modulated X-ray light curve and an extraordinarily broad radio-pulse structure that covers more than three-fourths of the spin period. The former is usually associated with emission polar caps that are being eclipsed by the stars own rotation, while the latter strongly supports alignment and a viewing angle that allows uninterrupted observation of the polar cap.