219 research outputs found
Pair Multiplicities and Pulsar Death
Through a simple model of particle acceleration and pair creation above the
polar caps of rotation-powered pulsars, we calculate the height of the
pair-formation front (PFF) and the dominant photon emission mechanism for the
pulsars in the Princeton catalog. We find that for most low- and moderate-field
pulsars, the height of the pair formation front and the final Lorentz factor of
the primary beam is set by nonresonant inverse Compton scattering (NRICS), in
the Klein-Nishina limit. NRICS is capable of creating pairs over a wide range
of pulsar parameters without invoking a magnetic field more complicated than a
centered dipole, although we still require a reduced radius of curvature for
most millisecond pulsars. For short-period pulsars, the dominant process is
curvature radiation, while for extremely high-field pulsars, it is resonant
inverse Compton scattering (RICS). The dividing point between NRICS dominance
and curvature dominance is very temperature-dependent; large numbers of pulsars
dominated by NRICS at a stellar temperature of K are dominated by
curvature at K. We apply these results to pulsar death-line calculations
and to the issue of particle injection into the Crab Nebula.Comment: 14 pages, 7 figures, to appear 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
Direct simulations of helical Hall-MHD turbulence and dynamo action
Direct numerical simulations of turbulent Hall dynamos are presented. The
evolution of an initially weak and small scale magnetic field in a system
maintained in a stationary turbulent regime by a stirring force at a
macroscopic scale is studied to explore the conditions for exponential growth
of the magnetic energy. Scaling of the dynamo efficiency with the Reynolds
numbers is studied, and the resulting total energy spectra are found to be
compatible with a Kolmogorov type law. A faster growth of large scale magnetic
fields is observed at intermediate intensities of the Hall effect.Comment: 13 pages, 17 figures, ApJ (in press
GCRT J1745-3009 as a Transient White Dwarf Pulsar
A transient radio source in the direction of the Galactic Center, GCRT
J1745-3009, exhibited 5 peculiar consecutive outbursts at 0.33 GHz with a
period of 77.13 minutes and a duration of ~10 minutes for each outburst. It has
been claimed to be the prototype of a hitherto unknown class of transient radio
sources. We interpret it as a transient white dwarf pulsar with a period of
77.13 minutes. The ~10-minute flaring duration corresponds to the epoch when
the radio beam sweeps our line of sight. The bursting epoch corresponds to the
episodes when stronger sunspot-like magnetic fields emerge into the white dwarf
polar cap region during which the pair production condition is satisfied and
the white dwarf behaves like a radio pulsar. It switches off as the pair
production condition breaks down.Comment: minor changes, ApJL, in pres
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
Color-magnetic flux tubes in quark matter cores of neutron stars
We argue that if color-superconducting quark matter exists in the core of a
neutron star, it may contain a high density of flux tubes, carrying flux that
is mostly color-magnetic, with a small admixture of ordinary magnetic flux. We
focus on the two-flavor color-superconducting ("2SC") phase, and assume that
the flux tubes are energetically stable, although this has not yet been
demonstrated. The density of flux tubes depends on the nature of the transition
to the color-superconducting phase, and could be within an order of magnitude
of the density of magnetic flux tubes that would be found if the core were
superconducting nuclear matter. We calculate the cross-section for
Aharonov-Bohm scattering of gapless fermions off the flux tubes, and the
associated collision time and frictional force on a moving flux tube. We
discuss the other forces on the flux tube, and find that if we take in to
account only the forces that arise within the 2SC core region then the
timescale for expulsion of the color flux tubes from the 2SC core is of order
10^10 years.Comment: 28 pages, LaTeX, 1 figure, 2 appendices; added discussion of
energetic stability of flux tube
Pair-production multiplicities in rotation-powered pulsars
We discuss the creation of electron-positron cascades in the context of
pulsar polar cap acceleration models and derive several useful analytic and
semi-analytic results for the spatial extent and energy response of the
cascade. Instead of Monte Carlo simulations, we use an integro- differential
equation which describes the development of the cascade energy spectrum in one
space dimension quite well, when it is compared to existing Monte Carlo models.
We reduce this full equation to a single integral equation, from which we can
derive useful results, such as the energy loss between successive generations
of photons and the spectral index of the response. We find that a simple
analytic formula represents the pair cascade multiplicity quite well, provided
that the magnetic field is below 10^12 Gauss, and that an only slightly more
complex formula matches the numerically-calculated cascade at all other field
strengths. Using these results, we find that cascades triggered by gamma rays
emitted through inverse Compton scattering of thermal photons from the neutron
star's surface, both resonant and non-resonant, are important for the dynamics
of the polar cap region in many pulsars.Comment: to appear in ApJ; 19 pages, 18 figure
PSR 0943+10: a bare strange star?
Recent work by Rankin & Deshpande strongly suggests that there exist strong
``micro-storms'' rotating around the magnetic axis of the 1.1s pulsar PSR
0943+10. Such a feature hints that most probably the large-voltage vacuum gap
proposed by Ruderman & Sutherland (RS) does exist in the pulsar polar cap.
However, there are severe arguments against the formation of the RS-type gap in
pulsars, since the binding energies of both the Fe ions and the electrons in a
neutron star's surface layer is too small to prevent thermionic ejection of the
particles from the surface. Here we propose that PSR 0943+10 (probably also
most of the other ``drifting'' pulsars) might be bare strange stars rather than
normal neutron stars, in which the ``binding energy'' at the surface is merely
infinity either for the case of ``pulsar'' or ``anti-pulsar''. It is further
proposed that identifying a drifting pulsar as an anti-pulsar is the key
criterion to distinguish strange stars from neutron stars.Comment: 4 pages, no figures, LaTeX, accepted 1999 July 9 by ApJ Letter
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
On the Possibility of the Detection of Extinct Radio Pulsars
We explore the possibilities for detecting pulsars that have ceased to
radiate in the radio band. We consider two models: the model with hindered
particle escape from the pulsar surface (first suggested by Ruderman and
Sutherland 1975) and the model with free particle escape (Arons 1981; Mestel
1999). In the model with hindered particle escape, the number of particles that
leave the pulsar magnetosphere is small and their radiation cannot be detected
with currently available instruments. At the same time, for the free particle
escape model, both the number of particles and the radiation intensity are high
enough for such pulsars to be detectable with the presently available receivers
such as GLAST and AGILE spacecrafts. It is also possible that extinct radio
pulsars can be among the unidentified EGRET sources.Comment: 5 pages, 1 figure corrected version of the paper that was published
in Astronomy Letter
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