211 research outputs found
Atmospheres and Spectra of Strongly Magnetized Neutron Stars II: Effect of Vacuum Polarization
We study the effect of vacuum polarization on the atmosphere structure and
radiation spectra of neutron stars with surface magnetic fields B=10^14-10^15
G, as appropriate for magnetars. Vacuum polarization modifies the dielectric
property of the medium and gives rise to a resonance feature in the opacity;
this feature is narrow and occurs at a photon energy that depends on the plasma
density. Vacuum polarization can also induce resonant conversion of photon
modes via a mechanism analogous to the MSW mechanism for neutrino oscillation.
We construct atmosphere models in radiative equilibrium with an effective
temperature of a few \times 10^6 K by solving the full radiative transfer
equations for both polarization modes in a fully ionized hydrogen plasma. We
discuss the subtleties in treating the vacuum polarization effects in the
atmosphere models and present approximate solutions to the radiative transfer
problem which bracket the true answer. We show from both analytic
considerations and numerical calculations that vacuum polarization produces a
broad depression in the X-ray flux at high energies (a few keV \la E \la a few
tens of keV) as compared to models without vacuum polarization; this arises
from the density dependence of the vacuum resonance feature and the large
density gradient present in the atmosphere. Thus the vacuum polarization effect
softens the high energy tail of the thermal spectrum, although the atmospheric
emission is still harder than the blackbody spectrum because of the non-grey
opacities. We also show that the depression of continuum flux strongly
suppresses the equivalent width of the ion cyclotron line and therefore makes
the line more difficult to observe.Comment: 21 pages, 21 figures; MNRAS; corrected minor typo
Atmospheres and Spectra of Strongly Magnetized Neutron Stars
We construct atmosphere models for strongly magnetized neutron stars with
surface fields G and effective temperatures K. The atmospheres directly determine the characteristics
of thermal emission from isolated neutron stars, including radio pulsars, soft
gamma-ray repeaters, and anomalous X-ray pulsars. In our models, the atmosphere
is composed of pure hydrogen or helium and is assumed to be fully ionized. The
radiative opacities include free-free absorption and scattering by both
electrons and ions computed for the two photon polarization modes in the
magnetized electron-ion plasma. Since the radiation emerges from deep layers in
the atmosphere with \rho\ga 10^2 g/cm, plasma effects can significantly
modify the photon opacities by changing the properties of the polarization
modes. In the case where the magnetic field and the surface normal are
parallel, we solve the full, angle-dependent, coupled radiative transfer
equations for both polarization modes. We also construct atmosphere models for
general field orientations based on the diffusion approximation of the
transport equations and compare the results with models based on full radiative
transport. In general, the emergent thermal radiation exhibits significant
deviation from blackbody, with harder spectra at high energies. The spectra
also show a broad feature (\Delta E/\Ebi\sim 1) around the ion cyclotron
resonance \Ebi=0.63 (Z/A)(B/10^{14}{G}) keV, where and are the atomic
charge and atomic mass of the ion, respectively; this feature is particularly
pronounced when \Ebi\ga 3k\Teff. Detection of the resonance feature would
provide a direct measurement of the surface magnetic fields on magnetars.Comment: 29 pages, 11 figures; corrected factor of 2 in He models: minor
changes to figs 4 and 9 as a result; other very minor change
Equilibrium spin pulsars unite neutron star populations
Many pulsars are formed with a binary companion from which they can accrete
matter. Torque exerted by accreting matter can cause the pulsar spin to
increase or decrease, and over long times, an equilibrium spin rate is
achieved. Application of accretion theory to these systems provides a probe of
the pulsar magnetic field. We compare the large number of recent torque
measurements of accreting pulsars with a high-mass companion to the standard
model for how accretion affects the pulsar spin period. We find that many long
spin period (P > 100 s) pulsars must possess either extremely weak (B < 10^10
G) or extremely strong (B > 10^14 G) magnetic fields. We argue that the
strong-field solution is more compelling, in which case these pulsars are near
spin equilibrium. Our results provide evidence for a fundamental link between
pulsars with the slowest spin periods and strong magnetic fields around
high-mass companions and pulsars with the fastest spin periods and weak fields
around low-mass companions. The strong magnetic fields also connect our pulsars
to magnetars and strong-field isolated radio/X-ray pulsars. The strong field
and old age of our sources suggests their magnetic field penetrates into the
superconducting core of the neutron star.Comment: 6 pages, 4 figures; to appear in MNRA
New dynamical tide constraints from current and future gravitational wave detections of inspiralling neutron stars
Previous theoretical works using the pre-merger orbital evolution of
coalescing neutron stars to constrain properties of dense nuclear matter assume
a gravitational wave phase uncertainty of a few radians, or about a half cycle.
However, recent studies of the signal from GW170817 and next generation
detector sensitivities indicate actual phase uncertainties at least twenty
times better. Using these refined estimates, we show that future observations
of nearby sources like GW170817 may be able to reveal neutron star properties
beyond just radius and tidal deformability, such as the matter composition
and/or presence of a superfluid inside neutron stars, via tidal excitation of
g-mode oscillations. Data from GW170817 already limits the amount of orbital
energy that is transferred to the neutron star to <2x10^47 erg and the g-mode
tidal coupling to Qmode<10^-3 at 50 Hz (5x10^48 erg and 4x10^-3 at 200 Hz), and
future observations and detectors will greatly improve upon these constraints.
In addition, analysis using general parameterization models that have been
applied to the so-called p-g instability show that the instability already
appears to be restricted to regimes where the mechanism is likely to be
inconsequential; in particular, we show that the number of unstable modes is
<<100 at <~100 Hz, and next generation detectors will essentially rule out this
mechanism (assuming that the instability remains undetected). Finally, we
illustrate that measurements of tidal excitation of r-mode oscillations in
nearby rapidly rotating neutron stars are within reach of current detectors and
note that even non-detections will limit the inferred inspiralling neutron star
spin rate to <20 Hz, which will be useful when determining other parameters
such as neutron star mass and tidal deformability.Comment: 7 pages, 4 figures; accepted for publication in Physical Review
R-Mode Oscillations and Spindown of Young Rotating Magnetic Neutron Stars
Recent work has shown that a young, rapidly rotating neutron star loses
angular momentum to gravitational waves generated by unstable r-mode
oscillations. We study the spin evolution of a young, magnetic neutron star
including both the effects of gravitational radiation and magnetic braking
(modeled as magnetic dipole radiation). Our phenomenological description of
nonlinear r-modes is similar to, but distinct from, that of Owen et al. (1998)
in that our treatment is consistent with the principle of adiabatic invariance
in the limit when direct driving and damping of the mode are absent. We show
that, while magnetic braking tends to increase the r-mode amplitude by spinning
down the neutron star, it nevertheless reduces the efficiency of gravitational
wave emission from the star. For B >= 10^14 (\nus/300 Hz)^2 G, where \nus is
the spin frequency, the spindown rate and the gravitational waveforms are
significantly modified by the effect of magnetic braking. We also estimate the
growth rate of the r-mode due to electromagnetic (fast magnetosonic) wave
emission and due to Alfven wave emission in the neutron star magnetosphere. The
Alfven wave driving of the r-mode becomes more important than the gravitational
radiation driving when B >= 10^13 (\nus/150 Hz)^3 G; the electromagnetic wave
driving of the r-mode is much weaker. Finally, we study the properties of local
Rossby-Alfven waves inside the neutron star and show that the fractional change
of the r-mode frequency due to the magnetic field is of order 0.5 (B/10^16 G)^2
(\nus/100 Hz)^-2.Comment: 18 pages, 4 figures; ApJ, accepted (v544: Nov 20, 2000); added two
footnotes and more discussion of mode driving by Alfven wave
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