224 research outputs found
Faraday rotation: effect of magnetic field reversals
The standard formula for the rotation measure, RM, which determines the
position angle, , due to Faraday rotation, includes
contributions only from the portions of the ray path where the natural modes of
the plasma are circularly polarized. In small regions of the ray path where the
projection of the magnetic field on the ray path reverses sign (called QT
regions) the modes are nearly linearly polarized. The neglect of QT regions in
estimating RM is not well justified at frequencies below a transition frequency
where mode coupling changes from strong to weak. By integrating the
polarization transfer equation across a QT region in the latter limit, I
estimate the additional contribution needed to correct this
omission. In contrast with a result proposed by \cite{BB10}, is
small and probably unobservable. I identify a new source of circular
polarization, due to mode coupling in an asymmetric QT region. I also identify
a new circular-polarization-dependent correction to the dispersion measure at
low frequencies.Comment: 25 pages 1 figure, accepted for publication in The Astrophysical
Journa
Electrodynamics of Magnetars: Implications for the Persistent X-ray Emission and Spindown of the Soft Gamma Repeaters and Anomalous X-ray Pulsars
(ABBREVIATED) We consider the structure of neutron star magnetospheres
threaded by large-scale electrical currents, and the effect of resonant Compton
scattering by the charge carriers (both electrons and ions) on the emergent
X-ray spectra and pulse profiles. In the magnetar model for the SGRs and AXPs,
these currents are maintained by magnetic stresses acting deep inside the star.
We construct self-similar, force-free equilibria of the current-carrying
magnetosphere with a power-law dependence of magnetic field on radius, B ~
r^(-2-p), and show that a large-scale twist softens the radial dependence to p
< 1. The spindown torque acting on the star is thereby increased in comparison
with a vacuum dipole. We comment on the strength of the surface magnetic field
in the SGR and AXP sources, and the implications of this model for the narrow
measured distribution of spin periods. A magnetosphere with a strong twist,
B_\phi/B_\theta = O(1) at the equator, has an optical depth ~ 1 to resonant
cyclotron scattering, independent of frequency (radius), surface magnetic field
strength, or charge/mass ratio of the scattering charge. When electrons and
ions supply the current, the stellar surface is also heated by the impacting
charges at a rate comparable to the observed X-ray output of the SGR and AXP
sources, if B_{dipole} ~ 10^{14} G. Redistribution of the emerging X-ray flux
at the ion and electron cyclotron resonances will significantly modify the
emerging pulse profile and, through the Doppler effect, generate a non-thermal
tail to the X-ray spectrum. The sudden change in the pulse profile of SGR
1900+14 after the 27 August 1998 giant flare is related to an enhanced optical
depth to electron cyclotron scattering, resulting from a sudden twist imparted
to the external magnetic field.Comment: 31 January 2002, minor revisions, new section 5.4.
Understanding the Geometry of Astrophysical Magnetic Fields
Faraday rotation measurements have provided an invaluable technique with
which to measure the properties of astrophysical magnetized plasmas.
Unfortunately, typical observations provide information only about the
density-weighted average of the magnetic field component parallel to the line
of sight. As a result, the magnetic field geometry along the line of sight, and
in many cases even the location of the rotating material, is poorly
constrained. Frequently, interpretations of Faraday rotation observations are
dependent upon underlying models of the magnetic field being probed (e.g.,
uniform, turbulent, equipartition). However, we show that at sufficiently low
frequencies, specifically below roughly 13 (RM/rad m^-2)^(1/4) (B/G)^(1/2) MHz,
the character of Faraday rotation changes, entering what we term the
``super-adiabatic regime'' in which the rotation measure is proportional to the
integrated absolute value of the line-of-sight component of the field. As a
consequence, comparing rotation measures at high frequencies with those in this
new regime provides direct information about the geometry of the magnetic field
along the line of sight. Furthermore, the frequency defining the transition to
this new regime, nu_SA, depends directly upon the local electron density and
magnetic field strength where the magnetic field is perpendicular to the line
of sight, allowing the unambiguous distinction between Faraday rotation within
and in front of the emission region. Typical values of nu_SA range from 10 kHz
to 10 GHz, depending upon the details of the Faraday rotating environment. In
particular, for resolved AGN, including the black holes at the center of the
Milky Way (Sgr A*) and M81, nu_SA ranges from roughly 10 MHz to 10 GHz, and
thus can be probed via existing and up-coming ground-based radio observatories.Comment: 13 pages, 5 figures, submitted to Ap
Resonant Cyclotron Radiation Transfer Model Fits to Spectra from Gamma-Ray Burst GRB870303
We demonstrate that models of resonant cyclotron radiation transfer in a
strong field (i.e. cyclotron scattering) can account for spectral lines seen at
two epochs, denoted S1 and S2, in the Ginga data for GRB870303. Using a
generalized version of the Monte Carlo code of Wang et al. (1988,1989b), we
model line formation by injecting continuum photons into a static
plane-parallel slab of electrons threaded by a strong neutron star magnetic
field (~ 10^12 G) which may be oriented at an arbitrary angle relative to the
slab normal. We examine two source geometries, which we denote "1-0" and "1-1,"
with the numbers representing the relative electron column densities above and
below the continuum photon source plane. We compare azimuthally symmetric
models, i.e. models in which the magnetic field is parallel to the slab normal,
with models having more general magnetic field orientations. If the bursting
source has a simple dipole field, these two model classes represent line
formation at the magnetic pole, or elsewhere on the stellar surface. We find
that the data of S1 and S2, considered individually, are consistent with both
geometries, and with all magnetic field orientations, with the exception that
the S1 data clearly favor line formation away from a polar cap in the 1-1
geometry, with the best-fit model placing the line-forming region at the
magnetic equator. Within both geometries, fits to the combined (S1+S2) data
marginally favor models which feature equatorial line formation, and in which
the observer's orientation with respect to the slab changes between the two
epochs. We interpret this change as being due to neutron star rotation, and we
place limits on the rotation period.Comment: LaTeX2e (aastex.cls included); 45 pages text, 17 figures (on 21
pages); accepted by ApJ (to be published 1 Nov 1999, v. 525
A Chandra Search for Coronal X Rays from the Cool White Dwarf GD 356
We report observations with the Chandra X-ray Observatory of the single,
cool, magnetic white dwarf GD 356. For consistent comparison with other X-ray
observations of single white dwarfs, we also re-analyzed archival ROSAT data
for GD 356 (GJ 1205), G 99-47 (GR 290 = V1201 Ori), GD 90, G 195-19 (EG250 = GJ
339.1), and WD 2316+123 and archival Chandra data for LHS 1038 (GJ 1004) and GD
358 (V777 Her). Our Chandra observation detected no X rays from GD 356, setting
the most restrictive upper limit to the X-ray luminosity from any cool white
dwarf -- L_{X} < 6.0 x 10^{25} ergs/s, at 99.7% confidence, for a 1-keV
thermal-bremsstrahlung spectrum. The corresponding limit to the electron
density is n_{0} < 4.4 x 10^{11} cm^{-3}. Our re-analysis of the archival data
confirmed the non-detections reported by the original investigators. We discuss
the implications of our and prior observations on models for coronal emission
from white dwarfs. For magnetic white dwarfs, we emphasize the more stringent
constraints imposed by cyclotron radiation. In addition, we describe (in an
appendix) a statistical methodology for detecting a source and for constraining
the strength of a source, which applies even when the number of source or
background events is small.Comment: 27 pages, 4 figures, submitted to the Astrophysical Journa
Atomic wave packet dynamics in finite time-dependent optical lattices
Atomic wave packets in optical lattices which are both spatially finite and
time-dependent exhibit many striking similarities with light pulses in photonic
crystals. We analytically characterize the transmission properties of such a
potential geometry for an ideal gas in terms of a position-dependent band
structure. In particular, we find that at specific energies, wave packets at
the center of the finite lattice may be enclosed by pairs of band gaps. These
act as mirrors between which the atomic wave packet is reflected, thereby
effectively yielding a matter wave cavity. We show that long trapping times may
be obtained in such a resonator and investigate the collapse and revival
dynamics of the atomic wave packet by numerical evaluation of the Schr\"odinger
equation
Annihilation Emission from the Galactic Black Hole
Both diffuse high energy gamma-rays and an extended electron-positron
annihilation line emission have been observed in the Galactic Center (GC)
region. Although X-ray observations indicate that the galactic black hole Sgr
A is inactive now, we suggest that Sgr A can become active when a
captured star is tidally disrupted and matter is accreted into the black hole.
As a consequence the galactic black hole could be a powerful source of
relativistic protons. We are able to explain the current observed diffuse
gamma-rays and the very detailed 511 keV annihilation line of secondary
positrons by collisions of such protons, with appropriate injection times
and energy. Relativistic protons could have been injected into the ambient
material if the black hole captured a 50M star at several tens million
years ago. An alternative possibility is that the black hole continues to
capture stars with 1M every hundred thousand years. Secondary
positrons produced by collisions at energies \ga 30 MeV are cooled down
to thermal energies by Coulomb collisions, and annihilate in the warm neutral
and ionized phases of the interstellar medium with temperatures about several
eV, because the annihilation cross-section reaches its maximum at these
temperatures. It takes about ten million years for the positrons to cool down
to thermal temperatures so they can diffuse into a very large extended region
around the Galactic center. A much more recent star capture may be also able to
account for recent TeV observations within 10 pc of the galactic center as well
as for the unidentified GeV gamma-ray sources found by EGRET at GC. The
spectral difference between the GeV flux and the TeV flux could be explained
naturally in this model as well.Comment: Accepted by ApJ on March 24, 200
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
Ontology Based Data Access in Statoil
Ontology Based Data Access (OBDA) is a prominent approach to query databases which uses an ontology to expose data in a conceptually clear manner by abstracting away from the technical schema-level details of the underlying data. The ontology is âconnectedâ to the data via mappings that allow to automatically translate queries posed over the ontology into data-level queries that can be executed by the underlying database management system. Despite a lot of attention from the research community, there are still few instances of real world industrial use of OBDA systems. In this work we present data access challenges in the data-intensive petroleum company Statoil and our experience in addressing these challenges with OBDA technology. In particular, we have developed a deployment module to create ontologies and mappings from relational databases in a semi-automatic fashion; a query processing module to perform and optimise the process of translating ontological queries into data queries and their execution over either a single DB of federated DBs; and a query formulation module to support query construction for engineers with a limited IT background. Our modules have been integrated in one OBDA system, deployed at Statoil, integrated with Statoilâs infrastructure, and evaluated with Statoilâs engineers and data
Observations of Non-radial Pulsations in Radio Pulsars
We introduce a model for pulsars in which non-radial oscillations of high
spherical degree (l) aligned to the magnetic axis of a spinning neutron star
reproduce the morphological features of pulsar beams. In our model, rotation of
the pulsar carries a pattern of pulsation nodes underneath our sightline,
reproducing the longitude stationary structure seen in average pulse profiles,
while the associated time-like oscillations reproduce "drifting
subpulses"--features that change their longitude between successive pulsar
spins. We will show that the presence of nodal lines can account for observed
180 degree phase jumps in drifting subpulses and their otherwise poor phase
stability, even if the time-like oscillations are strictly periodic. Our model
can also account for the "mode changes" and "nulls" observed in some pulsars as
quasiperiodic changes between pulsation modes of different l or radial overtone
n, analogous to pulsation mode changes observed in oscillating white dwarf
stars. We will discuss other definitive and testable requirements of our model
and show that they are qualitatively supported by existing data. While
reserving judgment until the completion of quantitative tests, we are inspired
enough by the existing observational support for our model to speculate about
the excitation mechanism of the non-radial pulsations, the physics we can learn
from them, and their relationship to the period evolution of pulsars.Comment: 28 pages, 9 figures (as separate png files), Astrophysical Journal,
in pres
- âŠ