7,426 research outputs found
High-Altitude Emission from Pulsar Slot Gaps: The Crab Pulsar
We present results of a 3D model of optical to gamma-ray emission from the
slot gap accelerator of a rotation-powered pulsar. Primary electrons
accelerating to high-altitudes in the unscreened electric field of the slot gap
reach radiation-reaction limited Lorentz factors of 2 x 10^7, while
electron-positron pairs from lower-altitude cascades flow along field lines
interior to the slot gap. The curvature, synchrotron and inverse Compton
radiation of both primary electrons and pairs produce a broad spectrum of
emission from infra-red to GeV energies. Both primaries and pairs undergo
cyclotron resonant absorption of radio photons, allowing them to maintain
significant pitch angles. Synchrotron radiation from pairs with a power-law
energy spectrum with Lorentz factors 10^2 - 10^5, dominate the spectrum up to
10 MeV. Synchrotron and curvature radiation of primaries dominates from 10 MeV
up to a few GeV. We examine the energy-dependent pulse profiles and
phase-resolved spectra for parameters of the Crab pulsar as a function of
magnetic inclination and viewing angle, comparing to broad-band data. In most
cases, the pulse profiles are dominated by caustics on trailing field lines. We
also explore the relation of the high-energy and the radio profiles, as well as
the possibility of caustic formation in the radio cone emission. We find that
the Crab pulsar profiles and spectrum can be reasonably well reproduced by a
model with viewing angle 45 degrees and inclination angle 100 or 80 degrees.
This model predicts that the slot gap emission below 200 MeV will exhibit
correlations in time and phase with the radio emission.Comment: 35 pages, 7 figures, accepted for publication in Astrophysical
Journa
Radio Quiet Pulsars with Ultra-Strong Magnetic Fields
The notable absence of radio pulsars having measured magnetic dipole surface
field strengths above Gauss naturally raises the
question of whether this forms an upper limit to pulsar magnetization. Recently
there has been increasing evidence that neutron stars possessing higher dipole
spin-down fields do in fact exist, including a growing list of anomalous X-ray
pulsars (AXPs) with long periods and spinning down with high period
derivatives, implying surface fields of -- Gauss.
Furthermore, the recently reported X-ray period and period derivative for the
Soft Gamma-ray Repeater (SGR) source SGR1806-20 suggest a surface field around
Gauss. None of these high-field pulsars have yet been detected as
radio pulsars. We propose that high-field pulsars should be radio-quiet because
electron-positron pair production in their magnetospheres, thought to be
essential for radio emission, is efficiently suppressed in ultra-strong fields
( Gauss) by the action of photon splitting, a
quantum electrodynamical process in which a photon splits into two. Our
computed radio quiescence boundary in the radio pulsar diagram,
where photon splitting overtakes pair creation, is located just above the
boundary of the known radio pulsar population, neatly dividing them from the
AXPs. We thus identify a physical mechanism that defines a new class of
high-field radio-quiet neutron stars that should be detectable by their pulsed
emission at X-ray and perhaps -ray energies.Comment: 4 pages, including one figure and one table, in AASTeX emulatapj
format, Astrophysical Journal Letters, in pres
Magnetic Photon Splitting: Computations of Proper-time Rates and Spectra
The splitting of photons in the presence of an intense magnetic field has
recently found astrophysical applications in polar cap models of gamma-ray
pulsars and in magnetar scenarios for soft gamma repeaters. Numerical
computation of the polarization-dependent rates of this third order QED process
for arbitrary field strengths and energies below pair creation threshold is
difficult: thus early analyses focused on analytic developments and simpler
asymptotic forms. The recent astrophysical interest spurred the use of the
S-matrix approach by Mentzel, Berg and Wunner to determine splitting rates. In
this paper, we present numerical computations of a full proper-time expression
for the rate of splitting that was obtained by Stoneham, and is exact up to the
pair creation threshold. While the numerical results derived here are in accord
with the earlier asymptotic forms due to Adler, our computed rates still differ
by as much as factors of 3 from the S-matrix re-evaluation of Wilke and Wunner,
reflecting the extreme difficulty of generating accurate S-matrix numerics for
fields below about \teq{4.4\times 10^{13}}Gauss. We find that our proper-time
rates appear very accurate, and exceed Adler's asymptotic specializations
significantly only for photon energies just below pair threshold and for
supercritical fields, but always by less than a factor of around 2.6. We also
provide a useful analytic series expansion for the scattering amplitude valid
at low energies.Comment: 13 pages, AASTeX format, including 3 eps figures, ApJ in pres
High magnetic field pulsars and magnetars: a unified picture
We propose a unified picture of high magnetic field radio pulsars and
magnetars by arguing that they are all rotating high-field neutron stars, but
have different orientations of their magnetic axes with respective to their
rotation axes. In strong magnetic fields where photon splitting suppresses pair
creation near the surface, the high-field pulsars can have active inner
accelerators while the anomalous X-ray pulsars cannot. This can account for the
very different observed emission characteristics of the anomalous X-ray pulsar
1E 2259+586 and the high field radio pulsar PSR J1814-1744. A predicted
consequence of this picture is that radio pulsars having surface magnetic field
greater than about G should not exist.Comment: 5 pages, emulateapj style, accepted for publication in the ApJ
Letter
McLean Sea-land Service and related services
Thesis (M.B.A.)--Boston Universit
A summary of research relating to first grade reading, 1932-1963
Thesis (Ed.M.)--Boston UniversityProblem: An attempt to compare the rate of learning of colorful and abstract words of children in grade one.
Materials:
1. Thirty words were selected to be used in three groups of ten words each. Each list consisted of five colorful and five abstract words. Colorful words are those which have much meaning for children and abstract words are those with little meaning. The words were printed on 3 x 11 inch flash cards in lower case letters.
2. Check sheets were made with spaces for checking the retention of words at three different times and pictures were made to enrich the colorful words.
3. Directions were prepared and given to each teacher who participated in the study.
4. Kuhlmann-Anderson Intelligence Test: Battery A.
5. Detroit Word Recognition Test: Form A [TRUNCATED
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