3,595 research outputs found
The Influence of Stellar Wind Variability on Measurements of Interstellar O VI Along Sightlines to Early-Type Stars
A primary goal of the FUSE mission is to understand the origin of the O VI
ion in the interstellar medium of the Galaxy and the Magellanic Clouds. Along
sightlines to OB-type stars, these interstellar components are usually blended
with O VI stellar wind profiles, which frequently vary in shape. In order to
assess the effects of this time-dependent blending on measurements of the
interstellar O VI lines, we have undertaken a mini-survey of repeated
observations toward OB-type stars in the Galaxy and the Large Magellanic Cloud.
These sparse time series, which consist of 2-3 observations separated by
intervals ranging from a few days to several months, show that wind variability
occurs commonly in O VI (about 60% of a sample of 50 stars), as indeed it does
in other resonance lines. However, in the interstellar O VI 1032
region, the O VI 1038 wind varies only in 30% of the cases. By
examining cases exhibiting large amplitude variations, we conclude that
stellar-wind variability {\em generally} introduces negligible uncertainty for
single interstellar O VI components along Galactic lines of sight, but can
result in substantial errors in measurements of broader components or blends of
components like those typically observed toward stars in the Large Magellanic
Cloud. Due to possible contamination by discrete absorption components in the
stellar O VI line, stars with terminal velocities greater than or equal to the
doublet separation (1654 km/s) should be treated with care.Comment: Accepted for publication in the Astrophysical Journal Lette
On characteristic initial data for a star orbiting a black hole
We take further steps in the development of the characteristic approach to
enable handling the physical problem of a compact self-gravitating object, such
as a neutron star, in close orbit around a black hole. We examine different
options for setting the initial data for this problem and, in order to shed
light on their physical relevance, we carry out short time evolution of this
data. To this end we express the matter part of the characteristic gravity code
so that the hydrodynamics are in conservation form. The resulting gravity plus
matter relativity code provides a starting point for more refined future
efforts at longer term evolution. In the present work we find that,
independently of the details of the initial gravitational data, the system
quickly flushes out spurious gravitational radiation and relaxes to a
quasi-equilibrium state with an approximate helical symmetry corresponding to
the circular orbit of the star.Comment: 20 pages, 10 figure
Understanding possible electromagnetic counterparts to loud gravitational wave events: Binary black hole effects on electromagnetic fields
In addition to producing loud gravitational waves (GW), the dynamics of a
binary black hole system could induce emission of electromagnetic (EM)
radiation by affecting the behavior of plasmas and electromagnetic fields in
their vicinity. We here study how the electromagnetic fields are affected by a
pair of orbiting black holes through the merger. In particular, we show how the
binary's dynamics induce a variability in possible electromagnetically induced
emissions as well as an enhancement of electromagnetic fields during the
late-merge and merger epochs. These time dependent features will likely leave
their imprint in processes generating detectable emissions and can be exploited
in the detection of electromagnetic counterparts of gravitational waves.Comment: 12 page
Deuterium toward the WD0621-376 sight line: Results from the Far Ultraviolet Spectroscopic Explorer (FUSE) Mission
Far Ultraviolet Spectroscopic Explorer observations are presented for
WD0621-376, a DA white dwarf star in the local interstellar medium (LISM) at a
distance of about 78 pc. The data have a signal-to-noise ratio of about 20-40
per 20 km/s resolution element and cover the wavelength range 905-1187 \AA.
LISM absorption is detected in the lines of D I, C II, C II*, C III, N I, N II,
N III, O I, Ar I, and Fe II. This sight line is partially ionized, with an
ionized nitrogen fraction of > 0.23. We determine the ratio (2). Assuming a standard interstellar
oxygen abundance, we derive . Using the
value of N(H I) derived from EUVE data gives a similar D/H ratio. The D I/N I
ratio is (2).Comment: accepted for publication in the ApJ
Low Redshift Intergalactic Absorption Lines in the Spectrum of HE0226-4110
We present an analysis of the FUSE and STIS E140M spectra of HE0226-4110
(z=0.495). We detect 56 Lyman absorbers and 5 O VI absorbers. The number of
intervening O VI systems per unit redshift with W>50 m\AA is dN(O VI)/dz~ 11.
The O VI systems unambiguously trace hot gas only in one case. For the 4 other
O VI systems, photoionization and collisional ionization models are viable
options to explain the observed column densities of the O VI and the other
ions. If the O VI systems are mostly photoionized, only a fraction of the
observed O VI will contribute to the baryonic density of the warm-hot ionized
medium (WHIM) along this line of sight. Combining our results with previous
ones, we show that there is a general increase of N(O VI) with increasing b(O
VI). Cooling flow models can reproduce the N-b distribution but fail to
reproduce the observed ionic ratios. A comparison of the number of O I, O II, O
III, O IV, and O VI systems per unit redshift show that the low-z IGM is more
highly ionized than weakly ionized. We confirm that photoionized O VI systems
show a decreasing ionization parameter with increasing H I column density. O VI
absorbers with collisional ionization/photoionization degeneracy follow this
relation, possibly suggesting that they are principally photoionized. We find
that the photoionized O VI systems in the low redshift IGM have a median
abundance of 0.3 solar. We do not find additional Ne VIII systems other than
the one found by Savage et al., although our sensitivity should have allowed
the detection of Ne VIII in O VI systems at T~(0.6-1.3)x10^6 K (if CIE
applies). Since the bulk of the WHIM is believed to be at temperatures T>10^6
K, the hot part of the WHIM remains to be discovered with FUV--EUV metal-line
transitions.Comment: Accepted for publication in the ApJS. Full resolution figures
available at
http://www.journals.uchicago.edu/ApJ/journal/preprints/ApJS63975.preprint.pd
Robustness of the Blandford-Znajek mechanism
The Blandford-Znajek mechanism has long been regarded as a key ingredient in
models attempting to explain powerful jets in AGNs, quasars, blazzars etc. In
such mechanism, energy is extracted from a rotating black hole and dissipated
at a load at far distances. In the current work we examine the behaviour of the
BZ mechanism with respect to different boundary conditions, revealing the
mechanism robustness upon variation of these conditions. Consequently, this
work closes a gap in our understanding of this important scenario.Comment: 7 pages, accepted in CQ
Hamiltonian Relaxation
Due to the complexity of the required numerical codes, many of the new
formulations for the evolution of the gravitational fields in numerical
relativity are not tested on binary evolutions. We introduce in this paper a
new testing ground for numerical methods based on the simulation of binary
neutron stars. This numerical setup is used to develop a new technique, the
Hamiltonian relaxation (HR), that is benchmarked against the currently most
stable simulations based on the BSSN method. We show that, while the length of
the HR run is somewhat shorter than the equivalent BSSN simulation, the HR
technique improves the overall quality of the simulation, not only regarding
the satisfaction of the Hamiltonian constraint, but also the behavior of the
total angular momentum of the binary. The latest quantity agrees well with
post-Newtonian estimations for point-mass binaries in circular orbits.Comment: More detailed description of the numerical implementation added and
some typos corrected. Version accepted for publication in Class. and Quantum
Gravit
Gravitational waveforms with controlled accuracy
A partially first-order form of the characteristic formulation is introduced
to control the accuracy in the computation of gravitational waveforms produced
by highly distorted single black hole spacetimes. Our approach is to reduce the
system of equations to first-order differential form on the angular
derivatives, while retaining the proven radial and time integration schemes of
the standard characteristic formulation. This results in significantly improved
accuracy over the standard mixed-order approach in the extremely nonlinear
post-merger regime of binary black hole collisions.Comment: Revised version, published in Phys. Rev. D, RevTeX, 16 pages, 4
figure
Unpolarized light in quantum optics
We present a new derivation of the unpolarized quantum states of light, whose
general form was first derived by Prakash and Chandra [Phys. Rev. A 4, 796
(1971)]. Our derivation makes use of some basic group theory, is
straightforward, and offers some new insights.Comment: 3 pages, REVTeX, presented at ICQO'200
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