5,433 research outputs found
Full polar cap cascade scenario: -ray and X-ray luminosities from spin-powered pulsars
We modify polar cap cascade picture to include the ICS of the higher
generation pairs. In such a ``full-cascade'' scenario, not only the
perpendicular portion of the energy of the pairs goes to high energy radiation
via SR, but the parallel portion of the energy of the pairs can also contribute
to high energy emission via ICS with the soft thermal photons from either the
full neutron star surface or the hot polar cap. An important output of such a
scenario is that the soft tail of the ICS spectrum can naturally result in a
non-thermal X-ray component which can contribute to the luminosities observed
by ROSAT and ASCA. Here we present an analytic description of such a full polar
cap cascade scenario within the framework of Harding & Muslimov acceleration
model. We present the theoretical predictions of the -ray luminosities,
the thermal and non-thermal X-ray luminosities for the known spin-powered X-ray
pulsars. Our results show that the observed different dependences of the high
energy luminosities on the pulsar spin-down luminosities, i.e., and , are well
reproduced. Our model predicts that the {\em pulsed} soft X-rays in the ROSAT
band from most of the millisecond pulsars might be of thermal origin if there
is no strong multipole field components near the surfaces of these pulsars.Comment: 23 pages, emulateapj style, final version to appear in the
Astrophysical Journa
Seismic structure of the southern Gulf of California from Los Cabos block to the East Pacific Rise
Multichannel reflection and coincident wide-angle seismic data collected during the 2002 Premier Experiment, Sea of Cortez, Addressing the Development of Oblique Rifting (PESCADOR) experiment provide the most detailed seismic structure to date of the southern Gulf of California. Multichannel seismic (MCS) data were recorded with a 6-km-long streamer, 480-channel, aboard the R/V Maurice Ewing, and wide-angle data was recorded by 19 instruments spaced every similar to 12 km along the transect. The MCS and wide-angle data reveal the seismic structure across the continent-ocean transition of the rifted margin. Typical continental and oceanic crust are separated by a similar to 75-km-wide zone of extended continental crust dominated by block-faulted basement. Little lateral variation in crustal thicknesses and seismic velocities is observed in the oceanic crust, suggesting a constant rate of magmatic productivity since seafloor spreading began. Oceanic crustal thickness and mean crustal velocities suggest normal mantle temperature (1300 degrees C) and passive mantle upwelling at the early stages of seafloor spreading. The crustal thickness, width of extended continental crust, and predicted temperature conditions all indicate a narrow rift mode of extension. On the basis of upper and lower crust stretching factors, an excess of lower crust was found in the extended continental crust. Total extension along transect 5W is estimated to be similar to 35 km. Following crustal extension, new oceanic crust similar to 6.4-km-thick was formed at a rate of similar to 48 mm a(-1) to accommodate plate separation
Pair Production Absorption Troughs in Gamma-Ray Burst Spectra: A Potential Distance Discriminator
Relativistic bulk motion with large Lorentz factors has recently been
inferred for gamma-ray bursts regardless of whether they are of galactic or
cosmological origin. This conclusion results from calculations of internal pair
production transparency in bursts that usually assume an infinite power-law
source spectrum for simplicity, an approximation that is quite adequate for
some bursts detected by EGRET. However, for a given bulk Lorentz factor
\teq{\Gamma}, photons above the EGRET range can potentially interact with
sub-MeV photons in such calculations. Hence it is essential to accurately
address the spectral curvature in bursts seen by BATSE. In this paper we
present the major properties induced in photon-photon opacity considerations by
such spectral curvature. The observed spectral breaks around 1 MeV turn out to
be irrelevant to opacity in cosmological bursts, but are crucial to estimates
of source transparency in the 1 GeV -- 1 TeV range for sources located in the
galactic halo. We find that broad absorption troughs can arise at these
energies for suitable bulk motion parameters \teq{\Gamma}. Such troughs are
probably an unambiguous signature of a galactic halo population, and if
observed by experiments such as Whipple, MILAGRO and GLAST, would provide
powerful evidence that such bursts are not at cosmological distances.Comment: 10 pages, AASTeX format, including 2 eps figures, ApJLett in pres
On the Origin of X-ray Emission From Millisecond Pulsars in 47 Tuc
The observed spectra and X-ray luminosities of millisecond pulsars in 47 Tuc
can be interpreted in the context of theoretical models based on strong, small
scale multipole fields on the neutron star surface. For multipole fields that
are relatively strong as compared to the large scale dipole field, the emitted
X-rays are thermal and likely result from polar cap heating associated with the
return current from the polar gap. On the other hand, for weak multipole
fields, the emission is nonthermal and results from synchrotron radiation of
pairs created by curvature radiation. The X-ray luminosity, , is
related to the spin down power, , expressed in the form with and for strong and weak
multipole fields respectively. If the polar cap size is of the order of the
length scale of the multipole field, and , the polar cap
temperature is .
A comparison of the X-ray properties of millisecond pulsars in globular
clusters and in the Galactic field suggests that the emergence of relatively
strong small scale multipole fields from the neutron star interior may be
correlated with the age and evolutionary history of the underlying neutron
star.Comment: 25 pages, 2 figures, accepted for publication in Ap
Unstable states in QED of strong magnetic fields
We question the use of stable asymptotic scattering states in QED of strong
magnetic fields. To correctly describe excited Landau states and photons above
the pair creation threshold the asymptotic fields are chosen as generalized
Licht fields. In this way the off-shell behavior of unstable particles is
automatically taken into account, and the resonant divergences that occur in
scattering cross sections in the presence of a strong external magnetic field
are avoided. While in a limiting case the conventional electron propagator with
Breit-Wigner form is obtained, in this formalism it is also possible to
calculate -matrix elements with external unstable particles.Comment: Revtex, 7 pages. To appear in Phys. Rev. D53(2
The Wicked Machinery of Government: Malta and the Problems of Continuity under the New Model Administration
This is a study focused on the early years of British rule in Malta (1800-1813). It explores the application to the island of the ânew modelâ of colonial government, one based on direct rule from London mediated by the continuation of existing laws and institutions. Systemic deficiencies are identified. These tended to undermine the effectiveness of direct British rule. This study also reveals, in the context of legal and constitutional continuity, unresolved tensions between modernity and tradition. The political stability of the island was damaged and the possibility of continued British possession was threatened
Three Millisecond Pulsars in FERMI LAT Unassociated Bright Sources
We searched for radio pulsars in 25 of the non-variable, unassociated sources
in the Fermi LAT Bright Source List with the Green Bank Telescope at 820 MHz.
We report the discovery of three radio and gamma-ray millisecond pulsars (MSPs)
from a high Galactic latitude subset of these sources. All of the pulsars are
in binary systems, which would have made them virtually impossible to detect in
blind gamma-ray pulsation searches. They seem to be relatively normal, nearby
(<=2 kpc) millisecond pulsars. These observations, in combination with the
Fermi detection of gamma-rays from other known radio MSPs, imply that most, if
not all, radio MSPs are efficient gamma-ray producers. The gamma-ray spectra of
the pulsars are power-law in nature with exponential cutoffs at a few GeV, as
has been found with most other pulsars. The MSPs have all been detected as
X-ray point sources. Their soft X-ray luminosities of ~10^{30-31} erg/s are
typical of the rare radio MSPs seen in X-rays.Comment: Accepted for publication in ApJ Letter
Spin and magnetization effects in plasmas
We give a short review of a number of different models for treating
magnetization effects in plasmas. In particular, the transition between kinetic
models and fluid models is discussed. We also give examples of applications of
such theories. Some future aspects are discussed.Comment: 18 pages, 1 figure. To appear in Plasma Physics and Controlled
Fusion, Special Issue for the 37th ICPP, Santiago, Chil
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
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