3,007 research outputs found
Inflaton perturbations in brane-world cosmology with induced gravity
We study cosmological perturbations in the brane models with an induced
Einstein-Hilbert term on a brane. We consider an inflaton confined to a de
Sitter brane in a five-dimensional Minkowski spacetime. Inflaton fluctuations
excite Kaluza-Klein modes of bulk metric perturbations with mass and where is an
integer. There are two branches ( branches) of solutions for the
background spacetime. In the branch, which includes the self-accelerating
universe, a resonance appears for a mode with due to a spin-0
perturbation with . The self-accelerating universe has a distinct
feature because there is also a helicity-0 mode of spin-2 perturbations with
. In the branch, which can be thought as the Randall-Sundrum
type brane-world with the high energy quantum corrections, there is no
resonance. At high energies, we analytically confirm that four-dimensional
Einstein gravity is recovered, which is related to the disappearance of van
Dam-Veltman-Zakharov discontinuity in de Sitter spacetime.
On sufficiently small scales, we confirm that the lineariaed gravity on the
brane is well described by the Brans-Dicke theory with in
branch and in branch, respectively, which confirms the
existence of the ghost in branch. We also study large scale perturbations.
In branch, the resonance induces a non-trivial anisotropic stress on the
brane via the projection of Weyl tensor in the bulk, but no instability is
shown to exist on the brane.Comment: 20 pages, 4 figure
Scalar cosmological perturbations in the Gauss-Bonnet braneworld
We study scalar cosmological perturbations in a braneworld model with a bulk
Gauss-Bonnet term. For an anti-de Sitter bulk, the five-dimensional
perturbation equations share the same form as in the Randall-Sundrum model,
which allows us to obtain metric perturbations in terms of a master variable.
We derive the boundary conditions for the master variable from the generalized
junction conditions on the brane. We then investigate several limiting cases in
which the junction equations are reduced to a feasible level. In the low energy
limit, we confirm that the standard result of four-dimensional Einstein gravity
is reproduced on large scales, whereas on small scales we find that the
perturbation dynamics is described by the four-dimensional Brans-Dicke theory.
In the high energy limit, all the non-local contributions drop off from the
junction equations, leaving a closed system of equations on the brane. We show
that, for inflation models driven by a scalar field on the brane, the
Sasaki-Mukhanov equation holds on the high energy brane in its original
four-dimensional form.Comment: 18 pages, v2: minor changes, reference added, v3: comments and
references added, accepted for publication in JCA
Rotation and X-ray emission from protostars
The ASCA satellite has recently detected variable hard X-ray emission from
two Class I protostars in the rho Oph cloud, YLW15 (IRS43) and WL6, with a
characteristic time scale ~20h. In YLW15, the X-ray emission is in the form of
quasi-periodic energetic flares, which we explain in terms of strong magnetic
shearing and reconnection between the central star and the accretion disk. In
WL6, X-ray flaring is rotationally modulated, and appears to be more like the
solar-type magnetic activity ubiquitous on T Tauri stars. We find that YLW15 is
a fast rotator (near break-up), while WL6 rotates with a significantly longer
period. We derive a mass M_\star ~ 2 M_\odot and \simlt 0.4 M_\odot for the
central stars of YLW15 and WL6 respectively. On the long term, the interactions
between the star and the disk results in magnetic braking and angular momentum
loss of the star. On time scales t_{br} ~ a few 10^5 yrs, i.e., of the same
order as the estimated duration of the Class~I protostar stage. Close to the
birthline there must be a mass-rotation relation, t_{br} \simpropto M_\star,
such that stars with M_\star \simgt 1-2 M_\odot are fast rotators, while their
lower-mass counterparts have had the time to spin down. The rapid rotation and
strong star-disk magnetic interactions of YLW15 also naturally explain the
observation of X-ray ``superflares''. In the case of YLW15, and perhaps also of
other protostars, a hot coronal wind (T~10^6 K) may be responsible for the VLA
thermal radio emission. This paper thus proposes the first clues to the
rotation status and evolution of protostars.Comment: 13 pages with 6 figures. To be published in ApJ (April 10, 2000 Part
1 issue
Suzaku Observations of Ejecta-Dominated Galactic Supernova Remnant G346.6-0.2
We present here the results of the X-ray analysis of Galactic supernova
remnant G346.6-0.2 observed with {\it Suzaku}. K-shell emission lines of Mg,
Si, S, Ca and Fe are detected clearly for the first time. Strong emission lines
of Si and S imply that X-ray emission nature of G346.6-0.2 is ejecta-dominated.
The ejecta-dominated emission is well fitted with a combined model consisting
of thermal plasma in non-equilibrium ionization and a non-thermal component,
which can be regarded as synchrotron emission with a photon index of
. Absorbing column density of is obtained from the best-fitting implying a high-density medium,
high electron temperature of keV, and ionization timescale
of indicating that this
remnant may be far from full ionization equilibrium. The relative abundances
from the ejecta show that the remnant originates from a Type Ia supernova
explosion.Comment: 7 pages, 4 figur
Simulation of I-V Hysteresis Branches in An Intrinsic Stack of Josephson Junctions in High Superconductors
I-V characteristics of the high T superconductor
BiSrCaCO shows a strong hysteresis, producing many
branches. The origin of hysteresis jumps is studied by use of the model of
multi-layered Josephson junctions proposed by one of the authors (T. K.). The
charging effect at superconducting layers produces a coupling between the next
nearest neighbor phase-differences, which determines the structure of
hysteresis branches. It will be shown that a solution of phase motions is
understood as a combination of rotating and oscillating phase-differences, and
that, at points of hysteresis jumps, there occurs a change in the number of
rotating phase-differences. Effects of dissipation are analyzed. The
dissipation in insulating layers works to damp the phase motion itself, while
the dissipation in superconducting layers works to damp relative motions of
phase-differences. Their effects to hysteresis jumps are discussed.Comment: 18 pages, Latex, 8 figures. To be appear in Phys.Rev.B Vol.60(1999
A Hot Helium Plasma in the Galactic Center Region
Recent X-ray observations by the space mission Chandra confirmed the
astonishing evidence for a diffuse, hot, thermal plasma at a temperature of 9.
K (8 keV) found by previous surveys to extend over a few hundred parsecs
in the Galactic Centre region. This plasma coexists with the usual components
of the interstellar medium such as cold molecular clouds and a soft (~0.8 keV)
component produced by supernova remnants, and its origin remains uncertain.
First, simple calculations using a mean sound speed for a hydrogen-dominated
plasma have suggested that it should not be gravitationally bound, and thus
requires a huge energy source to heat it in less than the escape time. Second,
an astrophysical mechanism must be found to generate such a high temperature.
No known source has been identified to fulfill both requirements. Here we
address the energetics problem and show that the hot component could actually
be a gravitationally confined helium plasma. We illustrate the new prospects
this opens by discussing the origin of this gas, and by suggesting possible
heating mechanisms.Comment: 9 pages, accepted for publication in APJ
Primordial fluctuations in bulk inflaton model
An inflationary brane model driven by a bulk inflaton with exponential
potential is proposed. We find a family of exact solutions that describe
power-law inflation on the brane. These solutions enable us to derive exact
solutions for metric perturbations analytically. By calculating scalar and
tensor perturbations, we obtain a spectrum of primordial fluctuations at the
end of the inflation. The amplitudes of scalar and tensor perturbations are
enhanced in the same way if the energy scale of the inflation is sufficiently
higher than the tension of the brane. Then the relative amplitude of scalar and
tensor perturbations is not suppressed even for high-energy inflation. This is
a distinguishable feature from the inflation model driven by inflaton on the
brane where tensor perturbations are suppressed for high-energy inflation. We
also point out that massive Kaluza-Klein modes are not negligible at
high-frequencies on 3-space of our brane.Comment: 16 pages, 3 figures, reference adde
A Catalog of Diffuse X-ray-Emitting Features within 20 pc of Sgr A*: Twenty Pulsar Wind Nebulae?
We present a catalog of 34 diffuse features identified in X-ray images of the
Galactic center taken with the Chandra X-ray Observatory. Several of the
features have been discussed in the literature previously, including 7 that are
associated with a complex of molecular clouds that exhibits fluorescent line
emission, 4 that are superimposed on the supernova remnant Sgr A East, 2 that
are coincident with radio features that are thought to be the shell of another
supernova remnant, and one that is thought to be a pulsar wind nebula only a
few arcseconds in projection from Sgr A*. However, this leaves 20 features that
have not been reported previously. Based on the weakness of iron emission in
their spectra, we propose that most of them are non-thermal. One long, narrow
feature points toward Sgr A*, and so we propose that this feature is a jet of
synchrotron-emitting particles ejected from the supermassive black hole. For
the others, we show that their sizes (0.1-2 pc in length for D=8 kpc), X-ray
luminosities (between 10^32 and 10^34 erg/s, 2-8 keV), and spectra (power laws
with Gamma=1-3) are consistent with those of pulsar wind nebulae. Based on the
star formation rate at the Galactic center, we expect that ~20 pulsars have
formed in the last 300 kyr, and could be producing pulsar wind nebulae. Only
one of the 19 candidate pulsar wind nebulae is securely detected in an archival
radio image of the Galactic center; the remainder have upper limits
corresponding to L_R<la10^31 erg/s. These radio limits do not strongly
constrain their natures, which underscores the need for further multi-
wavelength studies of this unprecedented sample of Galactic X-ray emitting
structures.Comment: 14 pages, 8 figures, 5 in color. Submitted to Ap
Discovery of luminous pulsed hard X-ray emission from anomalous X-ray pulsars 1RXS J1708-4009, 4U 0142+61 and 1E 2259+586 by INTEGRAL and RXTE
We report on the discovery of hard spectral tails for energies above 10 keV
in the total and pulsed spectra of anomalous X-ray pulsars 1RXS J1708-4009, 4U
0142+61 and 1E 2259+586 using RXTE PCA (2-60 keV) and HEXTE (15-250 keV) data
and INTEGRAL IBIS ISGRI (20-300 keV) data. Improved spectral information on 1E
1841-045 is presented. The pulsed and total spectra measured above 10 keV have
power-law shapes and there is so far no significant evidence for spectral
breaks or bends up to ~150 keV. The pulsed spectra are exceptionally hard with
indices measured for 4 AXPs approximately in the range -1.0 -- 1.0. We also
reanalyzed archival CGRO COMPTEL (0.75-30 MeV) data to search for signatures
from our set of AXPs. No detections can be claimed, but the obtained
upper-limits in the MeV band indicate that for 1RXS J1708-4009, 4U 0142+61 and
1E 1841-045 strong breaks must occur somewhere between 150 and 750 keV.Comment: Accepted for publication in ApJ; 19 pages; 4 Tables; 15 Figures (6
color
Rapidly-Varying Speed of Sound, Scale Invariance and Non-Gaussian Signatures
We show that curvature perturbations acquire a scale invariant spectrum for
any constant equation of state, provided the fluid has a suitably
time-dependent sound speed. In order for modes to exit the physical horizon,
and in order to solve the usual problems of standard big bang cosmology, we
argue that the only allowed possibilities are inflationary (albeit not
necessarily slow-roll) expansion or ekpyrotic contraction. Non-Gaussianities
offer many distinguish features. As usual with a small sound speed,
non-Gaussianity can be relatively large, around current sensitivity levels. For
DBI-like lagrangians, the amplitude is negative in the inflationary branch, and
can be either negative or positive in the ekpyrotic branch. Unlike the power
spectrum, the three-point amplitude displays a large tilt that, in the
expanding case, peaks on smallest scales. While the shape is predominantly of
the equilateral type in the inflationary branch, as in DBI inflation, it is of
the local form in the ekpyrotic branch. The tensor spectrum is also generically
far from scale invariant. In the contracting case, for instance, tensors are
strongly blue tilted, resulting in an unmeasurably small gravity wave amplitude
on cosmic microwave background scales.Comment: 41 pages, 12 figures. v4: Few typos in equations (7.39) correcte
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