4,814 research outputs found
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
1E 1547.0-5408: a radio-emitting magnetar with a rotation period of 2 seconds
The variable X-ray source 1E 1547.0-5408 was identified by Gelfand & Gaensler
(2007) as a likely magnetar in G327.24-0.13, an apparent supernova remnant. No
X-ray pulsations have been detected from it. Using the Parkes radio telescope,
we discovered pulsations with period P = 2.069 s. Using the Australia Telescope
Compact Array, we localized these to 1E 1547.0-5408. We measure dP/dt =
(2.318+-0.005)e-11, which for a magnetic dipole rotating in vacuo gives a
surface field strength of 2.2e14 G, a characteristic age of 1.4 kyr, and a
spin-down luminosity of 1.0e35 ergs/s. Together with its X-ray characteristics,
these rotational parameters of 1E 1547.0-5408 prove that it is a magnetar, only
the second known to emit radio waves. The distance is ~9 kpc, derived from the
dispersion measure of 830 pc/cc. The pulse profile at a frequency of 1.4 GHz is
extremely broad and asymmetric due to multipath propagation in the ISM, as a
result of which only approximately 75% of the total flux at 1.4 GHz is pulsed.
At higher frequencies the profile is more symmetric and has FWHM = 0.12P.
Unlike in normal radio pulsars, but in common with the other known
radio-emitting magnetar, XTE J1810-197, the spectrum over 1.4-6.6 GHz is flat
or rising, and we observe large, sudden changes in the pulse shape. In a
contemporaneous Swift X-ray observation, 1E 1547.0-5408 was detected with
record high flux, f_X(1-8 keV) ~ 5e-12 ergs/cm^2/s, 16 times the historic
minimum. The pulsar was undetected in archival radio observations from 1998,
implying a flux < 0.2 times the present level. Together with the transient
behavior of XTE J1810-197, these results suggest that radio emission is
triggered by X-ray outbursts of usually quiescent magnetars.Comment: Accepted for publication in ApJ Letter
Contiguous redshift parameterizations of the growth index
The growth rate of matter perturbations can be used to distinguish between
different gravity theories and to distinguish between dark energy and modified
gravity at cosmological scales as an explanation to the observed cosmic
acceleration. We suggest here parameterizations of the growth index as
functions of the redshift. The first one is given by that
interpolates between a low/intermediate redshift parameterization
and a high
redshift constant value. For example, our interpolated form
can be used when including the CMB to the rest of the data while
the form can be used otherwise. It is found that the
parameterizations proposed achieve a fit that is better than 0.004% for the
growth rate in a CDM model, better than 0.014% for
Quintessence-Cold-Dark-Matter (QCDM) models, and better than 0.04% for the flat
Dvali-Gabadadze-Porrati (DGP) model (with ) for the entire
redshift range up to . We find that the growth index parameters
take distinctive values for dark energy models and
modified gravity models, e.g. for the CDM model
and for the flat DGP model. This provides a means for future
observational data to distinguish between the models.Comment: 7 pages, 6 figures, matches PRD accepted versio
Characteristics of Diffuse X-Ray Line Emission within 20 pc of the Galactic Center
Over the last 3 yrs, the Galactic center (GC) region has been monitored with
the Chandra X-Ray Observatory. With 11 Chandra observations through 2002 June,
the total effective exposure reaches ~590 ks, providing significant photon
statistics on the faint, filamentary, diffuse X-ray emission. The true-color
X-ray image and the equivalent width (EW) images for the detected elemental
species demonstrate that the diffuse X-ray features have a broad range of
spatio-spectral properties. Enhancements of the low-ionization-state, or
``neutral'' Fe line emission (E~6.4 keV) to the northeast of Sgr A* can be
interpreted as fluorescence within the dense ISM resulting from irradiation by
hard, external X-ray sources. They may also be explained by emission induced by
the bombardments by high energy particles on the ISM, such as unresolved
supernova (SN) ejecta intruding into dense ISM. The detection of molecular
cloud counterparts to the 6.4 keV Fe line features indicates that these Fe line
features are associated with dense GC clouds and/or active star-forming
regions, which supports the X-ray reflection and/or SN ejecta origins for the
Fe line emission. We detect highly ionized S and Si lines which are generally
coincident with the neutral Fe line emission and the dense molecular clouds in
the northeast of Sgr A*. These hot plasmas are likely produced by massive
star-forming activities and/or SNRs. In contrast, we find that highly ionized
He-like Fe line emission (E~6.7 keV) is primarily distributed along the plane
instead of being concentrated in the northeast of Sgr A*. The implied high
temperature and the alignment along the plane are consistent with the magnetic
confinement model.Comment: 13 pages (ApJ emulator style) including 4 figures (2 color figs).
Accepted by ApJ. For full-quality figures, contact [email protected]
Quasi-periodic X-ray Flares from the Protostar YLW15
With ASCA, we have detected three X-ray flares from the Class I protostar
YLW15. The flares occurred every ~20 hours and showed an exponential decay with
time constant 30-60 ks. The X-ray spectra are explained by a thin thermal
plasma emission. The plasma temperature shows a fast-rise and slow-decay for
each flare with kT_{peak}~4-6 keV. The emission measure of the plasma shows
this time profile only for the first flare, and remains almost constant during
the second and third flares at the level of the tail of the first flare. The
peak flare luminosities L_{X,peak} were ~5-20 * 10^{31} erg s^{-1}, which are
among the brightest X-ray luminosities observed to date for Class I protostars.
The total energy released in each flare was 3-6*10^{36} ergs. The first flare
is well reproduced by the quasi-static cooling model, which is based on solar
flares, and it suggests that the plasma cools mainly radiatively, confined by a
semi-circular magnetic loop of length ~14 Ro with diameter-to-length ratio
\~0.07. The two subsequent flares were consistent with the reheating of the
same magnetic structure as of the first flare. The large-scale magnetic
structure and the periodicity of the flares imply that the reheating events of
the same magnetic loop originate in an interaction between the star and the
disk due to the differential rotation.Comment: Accepted by ApJ, 9 pages incl. 4 ps figure
Charge Exchange Spectra of Hydrogenic and He-like Iron
We present H-like Fe XXVI and He-like Fe XXV charge-exchange spectra
resulting from collisions of highly charged iron with N2 gas at an energy of 10
eV/amu in an electron beam ion trap. Although individual high-n emission lines
are not resolved in our measurements, we observe that the most likely level for
Fe25+ --> Fe24+ electron capture is n~9, in line with expectations, while the
most likely value for Fe26+ --> Fe25+ charge exchange is significantly higher.
In the Fe XXV spectrum, the K-alpha emission feature dominates, whether
produced via charge exchange or collisional excitation. The K-alpha centroid is
lower in energy for the former case than the latter (6666 versus 6685 eV,
respectively), as expected because of the strong enhancement of emission from
the forbidden and intercombination lines, relative to the resonance line, in
charge-exchange spectra. In contrast, the Fe XXVI high-n Lyman lines have a
summed intensity greater than that of Ly-alpha, and are substantially stronger
than predicted from theoretical calculations of charge exchange with atomic H.
We conclude that the angular momentum distribution resulting from electron
capture using a multi-electron target gas is significantly different from that
obtained with H, resulting in the observed high-n enhancement. A discussion is
presented of the relevance of our results to studies of diffuse Fe emission in
the Galactic Center and Galactic Ridge, particularly with ASTRO-E2/Suzaku.Comment: 16 pages, 4 figures (3 color), accepted by Ap
Non-Gaussian signatures of Tachyacoustic Cosmology
I investigate non-Gaussian signatures in the context of tachyacoustic
cosmology, that is, a noninflationary model with superluminal speed of sound. I
calculate the full non-Gaussian amplitude , its size ,
and corresponding shapes for a red-tilted spectrum of primordial scalar
perturbations. Specifically, for cuscuton-like models I show that , and the shape of its non-Gaussian amplitude peaks for
both equilateral and local configurations, the latter being dominant. These
results, albeit similar, are quantitatively distinct from the corresponding
ones obtained by Magueijo {\it{et. al}} in the context of superluminal bimetric
models.Comment: Some comments and references added. Matches the version published in
JCA
New X-ray views of the Galactic center observed with Suzaku
We report the diffuse X-ray emissions from the Sgr A and B regions observed
with Suzaku. From the Sgr A region, we found many K-shell transition lines of
iron and nickel. The brightest are K alpha lines from FeI, FeXXV and FeXXVI at
6.4 keV, 6.7 keV and 6.9 keV. In addition, K alpha lines of NiI and NiXXVII, K
beta of FeI, FeXXV and FeXXVI, and K gamma of FeXXV and FeXXVI are detected for
the first time. The center energy of K alpha of FeXXV favors collisional
excitation as the origin for this line emission. The ionization temperature
determined from the flux ratio of K alpha of FeXXV and FeXXVI is similar to the
electron temperature determined from the flux ratio of K alpha and K beta of
FeXXV, which are in the range of 5-7 keV. Consequently, the Galactic Center
diffuse X-rays (GCDX) are consistent with emission from a plasma nearly in
ionization equilibrium. The radio complex Sgr B region also exhibits K alpha
lines of FeI, FeXXV and FeXXVI. The 6.7 keV line (FeXXV) map exhibits a local
excess at (l,b) = (0.612, 0.01), and could be a new young SNR. The 6.4 keV
image is clumpy with local excesses near Sgr B2 and at (l,b) = (0.74, -0.09).
Like Sgr B2, this latter excess may be another X-ray reflection Nebulae (XRN).Comment: 9 pages, 8 figures, to be published in the proceedings of Galactic
Center Workshop 200
Curvature perturbations from ekpyrotic collapse with multiple fields
A scale-invariant spectrum of isocurvature perturbations is generated during
collapse in the ekpyrotic scaling solution in models where multiple fields have
steep negative exponential potentials. The scale invariance of the spectrum is
realized by a tachyonic instability in the isocurvature field. This instability
drives the scaling solution to the late time attractor that is the old
ekpyrotic collapse dominated by a single field. We show that the transition
from the scaling solution to the single field dominated ekpyrotic collapse
automatically converts the initial isocurvature perturbations about the scaling
solution to comoving curvature perturbations about the late-time attractor. The
final amplitude of the comoving curvature perturbation is determined by the
Hubble scale at the transition.Comment: 15 pages, 3 figures, a reference added, to be published in CQG, a
remark on the comoving density perturbation correcte
Formation of fractal structure in many-body systems with attractive power-law potentials
We study the formation of fractal structure in one-dimensional many-body
systems with attractive power-law potentials. Numerical analysis shows that the
range of the index of the power for which fractal structure emerges is limited.
Dependence of the growth rate on wavenumber and power-index is obtained by
linear analysis of the collisionless Boltzmann equation, which supports the
numerical results.Comment: accepted by PR
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