28 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
Polarization Sweeps in Rotation Powered Pulsars
We re-examine the characteristic polarization angle sweep of rotation-powered
pulsars and calculate the expected deviations from this sweep caused by
aberrational effects and by polar-cap current flow. We find that in addition to
the previously known phase shift of the entire sweep by , aberration shifts the polarization angle itself by . Similarly, current flow above the polar cap
shifts the polarization sweep by , potentially providing a method of directly measuring the magnitude of
the current. The competition between these two effects produces a potentially
observable signature in the polarization angle sweep. Although these effects
may appear similar to orthogonal mode shifts, they are an independent
phenomenon with distinct observational characteristics.Comment: 23 pages, 8 figures; accepted by Ap
Adjustment of the electric current in pulsar magnetospheres and origin of subpulse modulation
The subpulse modulation of pulsar radio emission goes to prove that the
plasma flow in the open field line tube breaks into isolated narrow streams. I
propose a model which attributes formation of streams to the process of the
electric current adjustment in the magnetosphere. A mismatch between the
magnetospheric current distribution and the current injected by the polar cap
accelerator gives rise to reverse plasma flows in the magnetosphere. The
reverse flow shields the electric field in the polar gap and thus shuts up the
plasma production process. I assume that a circulating system of streams is
formed such that the upward streams are produced in narrow gaps separated by
downward streams. The electric drift is small in this model because the
potential drop in narrow gaps is small. The gaps have to drift because by the
time a downward stream reaches the star surface and shields the electric field,
the corresponding gap has to shift. The transverse size of the streams is
determined by the condition that the potential drop in the gaps is sufficient
for the pair production. This yields the radius of the stream roughly 10% of
the polar cap radius, which makes it possible to fit in the observed
morphological features such as the "carousel" with 10-20 subbeams and the
system of the core - two nested cone beams.Comment: 8 pages, 1 figur
A Tale of Two Current Sheets
I outline a new model of particle acceleration in the current sheet
separating the closed from the open field lines in the force-free model of
pulsar magnetospheres, based on reconnection at the light cylinder and
"auroral" acceleration occurring in the return current channel that connects
the light cylinder to the neutron star surface. I discuss recent studies of
Pulsar Wind Nebulae, which find that pair outflow rates in excess of those
predicted by existing theories of pair creation occur, and use those results to
point out that dissipation of the magnetic field in a pulsar's wind upstream of
the termination shock is restored to life as a viable model for the solution of
the "" problem as a consequence of the lower wind 4-velocity implied by
the larger mass loading.Comment: 17 pages, 6 figures, Invited Review, Proceedings of the "ICREA
Workshop on The High-Energy Emission from Pulsars and their Systems", Sant
Cugat, Spain, April 12-16, 201
Current Flow and Pair Creation at Low Altitude in Rotation Powered Pulsars' Force-Free Magnetospheres: Space-Charge Limited Flow
(shortened) We report the results of an investigation of particle
acceleration and electron-positron plasma generation at low altitude in the
polar magnetic flux tubes of Rotation Powered Pulsars, when the stellar surface
is free to emit whatever charges and currents are demanded by the force-free
magnetosphere. We observe novel behavior. a) When the current density is less
than the Goldreich-Julian (GJ) value (0<j/j_{GJ}<1), space charge limited
acceleration of the current carrying beam is mild, with the full GJ charge
density being comprised of the charge density of the beam, co-existing with a
cloud of electrically trapped particles with the same sign of charge as the
beam. The voltage drops are on the order of mc^2/e, and pair creation is
absent. b) When the current density exceeds the GJ value (j/j_{GJ}>1), the
system develops high voltage drops, causing emission of gamma rays and intense
bursts of pair creation. The bursts exhibit limit cycle behavior, with
characteristic time scales somewhat longer than the relativistic fly-by time
over distances comparable to the polar cap diameter (microseconds). c) In
return current regions, where j/j_{GJ}<0, the system develops similar bursts of
pair creation. In cases b) and c), the intermittently generated pairs allow the
system to simultaneously carry the magnetospherically prescribed currents and
adjust the charge density and average electric field to force-free conditions.
We also elucidate the conditions for pair creating beam flow to be steady,
finding that such steady flows can occupy only a small fraction of the current
density parameter space of the force-free magnetospheric model. The generic
polar flow dynamics and pair creation is strongly time dependent. The model has
an essential difference from almost all previous quantitative studies, in that
we sought the accelerating voltage as a function of the applied current.Comment: 35 pages, 29 figures. Accepted for publication in MNRAS. Added new
appendix, several minor changes in the tex
A Further Study of Relative longitude shift of Pulsar Beams
It is of great important to study pulsar beam shapes if we are concerned with
emission theories and pulsar birth rate. Both observations and/or the inverse
Compton scattering model show that different emission components are emitted
from different heights. The relative longitude phase shifts due to different
heights of emission components and to the toroidal velocity of electron are
considered in this paper. Several possible observational features arising from
the phase shift effects are presented. The emission beams may not have circular
cross sections although the emission regions may be symmetric with respect to
the magnetic axes.Comment: 7 pages, 7 figures, LaTeX, to be published in Chinese Journal of
Astronomy and Astrophysics (2001), http://vega.bac.pku.edu.cn/~rxx
The theory of pulsar winds and nebulae
We review current theoretical ideas on pulsar winds and their surrounding
nebulae. Relativistic MHD models of the wind of the aligned rotator, and of the
striped wind, together with models of magnetic dissipation are discussed. It is
shown that the observational signature of this dissipation is likely to be
point-like, rather than extended, and that pulsed emission may be produced. The
possible pulse shapes and polarisation properties are described. Particle
acceleration at the termination shock of the wind is discussed, and it is
argued that two distinct mechanisms must be operating, with the first-order
Fermi mechanism producing the high-energy electrons (above 1 TeV) and either
magnetic annihilation or resonant absorption of ion cyclotron waves responsible
for the 100 MeV to 1 TeV electrons. Finally, MHD models of the morphology of
the nebula are discussed and compared with observation.Comment: 33 pages, to appear in Springer Lecture Notes on "Neutron stars and
pulsars, 40 years after the discovery", ed W.Becke
Pulsed Gamma Rays from the Original Millisecond and Black Widow Pulsars: a case for Caustic Radio Emission?
We report the detection of pulsed gamma-ray emission from the fast
millisecond pulsars (MSPs) B1937+21 (also known as J1939+2134) and B1957+20
(J1959+2048) using 18 months of survey data recorded by the \emph{Fermi} Large
Area Telescope (LAT) and timing solutions based on radio observations conducted
at the Westerbork and Nan\c{c}ay radio telescopes. In addition, we analyzed
archival \emph{RXTE} and \emph{XMM-Newton} X-ray data for the two MSPs,
confirming the X-ray emission properties of PSR B1937+21 and finding evidence
() for pulsed emission from PSR B1957+20 for the first time. In
both cases the gamma-ray emission profile is characterized by two peaks
separated by half a rotation and are in close alignment with components
observed in radio and X-rays. These two pulsars join PSRs J0034-0534 and
J2214+3000 to form an emerging class of gamma-ray MSPs with phase-aligned peaks
in different energy bands. The modeling of the radio and gamma-ray emission
profiles suggests co-located emission regions in the outer magnetosphere.Comment: Accepted for publication in the Astrophysical Journa
The High Time Resolution Universe Pulsar Survey IV: Discovery and polarimetry of millisecond pulsars
We present the discovery of six millisecond pulsars (MSPs) in the High Time
Resolution Universe (HTRU) survey for pulsars and fast transients carried out
with the Parkes radio telescope. All six are in binary systems with
approximately circular orbits and are likely to have white dwarf companions.
PSR J1017-7156 has a high flux density and a narrow pulse width, making it
ideal for precision timing experiments. PSRs J1446-4701 and J1125-5825 are
coincident with gamma-ray sources, and folding the high-energy photons with the
radio timing ephemeris shows evidence of pulsed gamma-ray emission. PSR
J1502-6752 has a spin period of 26.7 ms, and its low period derivative implies
that it is a recycled pulsar. The orbital parameters indicate it has a very low
mass function, and therefore a companion mass much lower than usually expected
for such a mildly recycled pulsar. In addition we present polarisation profiles
for all 12 MSPs discovered in the HTRU survey to date. Similar to previous
observations of MSPs, we find that many have large widths and a wide range of
linear and circular polarisation fractions. Their polarisation profiles can be
highly complex, and although the observed position angles often do not obey the
rotating vector model, we present several examples of those that do. We
speculate that the emission heights of MSPs are a substantial fraction of the
light cylinder radius in order to explain broad emission profiles, which then
naturally leads to a large number of cases where emission from both poles is
observed.Comment: Update to correct affiliation for CAASTRO. 16 pages, 18 figures.
Accepted for publication in MNRA
The Implementation of a Fast-folding Pipeline for Long-period Pulsar Searching in the PALFA Survey
The Pulsar Arecibo L-Band Feed Array (PALFA) survey, the most sensitive blind search for radio pulsars yet conducted, is ongoing at the Arecibo Observatory in Puerto Rico. The vast majority of the 180 pulsars discovered by PALFA have spin periods shorter than 2 s. Pulsar surveys may miss long-period radio pulsars owing to the summing of a finite number of harmonic components in conventional Fourier analyses (typically ~16), or as a result of the strong effect of red noise at low modulation frequencies. We address this reduction in sensitivity by using a time-domain search technique: the fast-folding algorithm (FFA). We designed a program that implements an FFA-based search in the PALFA processing pipeline and tested the efficiency of the algorithm by performing tests under both ideal, white-noise conditions, as well as with real PALFA observational data. In the two scenarios, we show that the time-domain algorithm has the ability to outperform the FFT-based periodicity search implemented in the survey. We perform simulations to compare the previously reported PALFA sensitivity with that obtained using our new FFA implementation. These simulations show that for a pulsar having a pulse duty cycle of roughly 3%, the performance of our FFA pipeline exceeds that of our FFT pipeline for pulses with dispersion measure lesssim 40 pc cmâ3 and for periods as short as ~500 ms, and that the survey sensitivity is improved by at least a factor of two for periods gsim 6 s. Early results from the implementation of the algorithm in PALFA, including discoveries, are also presented in this paper