15 research outputs found
Toward the Quasi-Steady State Electrodynamics of a Neutron Star
We consider the electrodynamics of a rotating magnetized neutron star with allowance for the quasi-steady state regime of charged particle ejection from the polar cap into the inner magnetosphere. We derive basic equations within the framework of a general relativistic formalism and present the analytic solutions. We demonstrate that the quasi-steady state supply of electrons into the region of open field lines reduces the total power put into the acceleration of primary particles, although the total power corresponding to a steady state solution in general relativity is increased compared to the flat-space solution. Also, we illustrate that temporarily modulated ejection of electrons from the stellar surface results in substantial modulation of an accelerating electric field and charge density along the open field lines. We briefly outline the implications of this effect for the pulsars
Pulsar Polar Cap Heating and Surface Thermal X-Ray Emission I. Curvature Radiation Pair Fronts
We investigate the effect of pulsar polar cap (PC) heating produced by
positrons returning from the upper pair formation front. Our calculations are
based on a self-consistent treatment of the pair dynamics and the effect of
electric field screening by the returning positrons. We calculate the resultant
X-ray luminosities, and discuss the dependence of the PC heating efficiencies
on pulsar parameters, such as characteristic spin-down age, spin period, and
surface magnetic field strength. In this study we concentrate on the regime
where the pairs are produced in a magnetic field by curvature photons emitted
by accelerating electrons. Our theoretical results are not in conflict with the
available observational X-ray data and suggest that the effect of PC heating
should significantly contribute to the thermal X-ray fluxes from middle-aged
and old pulsars. The implications for current and future X-ray observations of
pulsars are briefly outlined.Comment: 28 pages, 7 figures, accepted for publication in Ap
Pulsar Polar Cap Heating and Surface Thermal X-Ray Emission II. Inverse Compton Radiation Pair Fronts
We investigate the production of electron-positron pairs by inverse Compton
scattered (ICS) photons above a pulsar polar cap (PC) and surface heating by
returning positrons. This paper is a continuation of our self-consistent
treatment of acceleration, pair dynamics and electric field screening above
pulsar PCs. We calculate the altitude of the inverse Compton pair formation
fronts, the flux of returning positrons and present the heating efficiencies
and X-ray luminosities. We revise pulsar death lines implying cessation of pair
formation, and present them in surface magnetic field-period space. We find
that virtually all known radio pulsars are capable of producing pairs by
resonant and non-resonant ICS photons radiated by particles accelerated above
the PC in a pure star-centered dipole field, so that our ICS pair death line
coincides with empirical radio pulsar death. Our calculations show that ICS
pairs are able to screen the accelerating electric field only for high neutron
star surface temperatures and magnetic fields. We argue that such screening at
ICS pair fronts occurs locally, slowing but not turning off acceleration of
particles until screening can occur at a curvature radiation (CR) pair front at
higher altitude. In the case where no screening occurs above the PC surface, we
anticipate that the pulsar gamma-ray luminosity will be a substantial fraction
of its spin-down luminosity. The X-ray luminosity resulting from PC heating by
ICS pair fronts is significantly lower than the PC heating luminosity from CR
pair fronts, which dominates for most pulsars. PC heating from ICS pair fronts
is highest in millisecond pulsars, which cannot produce CR pairs, and may
account for observed thermal X-ray components in the spectra of these old
pulsars.Comment: 29 pages, 10 figures, accepted for publication in Ap
Pulsar X-Ray and Gamma-Ray Pulse Profiles: Constraint on Obliquity and Observer Angles
We model the thermal X-ray profiles of Geminga, Vela and PSR 0656+14, which
have also been detected as gamma-ray pulsars, to constrain the phase space of
obliquity and observer angles required to reproduce the observed X-ray pulsed
fractions and pulse widths. These geometrical constraints derived from the
X-ray light curves are explored for various assumptions about surface
temperature distribution and flux anisotropy caused by the magnetized
atmosphere. We include curved spacetime effects on photon trajectories and
magnetic field. The observed gamma-ray pulse profiles are double peaked with
phase separations of 0.4 - 0.5 between the peaks. Assuming that the gamma-ray
profiles are due to emission in a hollow cone centered on the magnetic pole, we
derive the constraints on the phase space of obliquity and observer angles, for
different gamma-ray beam sizes, required to produce the observed gamma-ray peak
phase separations. We compare the constraints from the X-ray emission to those
derived from the observed gamma-ray pulse profiles, and find that the
overlapping phase space requires both obliquity and observer angles to be
smaller than 20-30 degrees, implying gamma-ray beam opening angles of at most
30-35 degrees.Comment: 29 pages, 9 embedded figures, AASTEX v.4, To appear in ApJ, June 20,
1998 (Vol. 499
Particle Acceleration Zones Above Pulsar Polar Caps: Electron and Positron Pair Formation Fronts
We investigate self-consistent particle acceleration near a pulsar polar cap
(PC) by the electrostatic field due to the effect of inertial frame dragging.
Test particles gain energy from the electric field parallel to the open
magnetic field lines and lose energy by both curvature radiation (CR) and
resonant and non-resonant inverse Compton scattering (ICS) with soft thermal
X-rays from the neutron star (NS) surface. Gamma-rays radiated by electrons
accelerated from the stellar surface produce pairs in the strong magnetic
field, which screen the electric field beyond a pair formation front (PFF).
Some of the created positrons can be accelerated back toward the surface and
produce gamma-rays and pairs that create another PFF above the surface. We find
that ICS photons control PFF formation near the surface, but due to the
different angles at which the electron and positron scatter the soft photons,
positron initiated cascades develop above the surface and screen the
accelerating electric field. Stable acceleration from the NS surface is
therefore not possible in the presence of dominant ICS energy losses. However,
we find that stable acceleration zones may occur at some distance above the
surface, where CR dominates the electron and positron energy losses, and there
is up-down symmetry between the electron and positron PFFs. We examine the
dependence of CR-controlled acceleration zone voltage, width and height above
the surface on parameters of the pulsar and its soft X-ray emission. For most
pulsars, we find that acceleration will start at a height of 0.5 - 1 stellar
radii above the NS surface.Comment: 46 pages, 12 embedded figures, accepted for publication in Ap
Mechanisms for High-frequency QPOs in Neutron Star and Black Hole Binaries
We explain the millisecond variability detected by Rossi X-ray Timing
Explorer (RXTE) in the X-ray emission from a number of low mass X-ray binary
systems (Sco X-1, 4U1728-34, 4U1608-522, 4U1636-536, 4U0614+091, 4U1735-44,
4U1820-30, GX5-1 and etc) in terms of dynamics of the centrifugal barrier, a
hot boundary region surrounding a neutron star. We demonstrate that this region
may experience the relaxation oscillations, and that the displacements of a gas
element both in radial and vertical directions occur at the same main
frequency, of order of the local Keplerian frequency. We show the importance of
the effect of a splitting of the main frequency produced by the Coriolis force
in a rotating disk for the interpretation of a spacing between the QPO peaks.
We estimate a magnitude of the splitting effect and present a simple formula
for the whole spectrum of the split frequencies. It is interesting that the
first three lowest-order overtones fall in the range of 200-1200 Hz and match
the kHz-QPO frequencies observed by RXTE. Similar phenomena should also occur
in Black Hole (BH) systems, but, since the QPO frequency is inversely
proportional to the mass of a compact object, the frequency of the
centrifugal-barrier oscillations in the BH systems should be a factor of 5-10
lower than that for the NS systems. The X-ray spectrum formed in this region is
a result of upscattering of a soft radiation (from a disk and a NS surface) off
relatively hot electrons in the boundary layer. We also briefly discuss some
alternative QPO models, including a possibility of acoustic oscillations in the
boundary layer, the proper stellar rotation, and g-mode disk oscillations.Comment: The paper is coming out in the Astrophysical Journal in the 1st of
May issue of 199