16,247 research outputs found
Wormhole Core, Extra Dimensions, and Physical Universe
We created a model of several dimensional physical universe. The extra
dimensions associated with the four dimensional physical universe is assumed to
have modified Gidding-Strominger wormhole core. This core is separated by a
flexible wall, but it allows the adiabatic pressure generated in the wormhole
to penetrate in the extra dimensions. We assume that the extra dimensions are a
contracting Robinson-Walker space. We show that the associated physical
universe accelerates under certain restriction of the parameters introduced.
The extra dimensional space is very large at the begining, however at present
time this space will be very reduced. As a result the physical universe will
appear to us four dimensional the way we observe it now.Comment: 8 pages, Festschrift Contributio
Searching for the reionization sources
Using a reionization model simultaneously accounting for a number of
experimental data sets, we investigate the nature and properties of
reionization sources. Such model predicts that hydrogen reionization starts at
z \approx 15, is initially driven by metal-free (PopIII) stars, and is 90%
complete by z \approx 8. We find that a fraction f_\gamma >80% of the ionizing
power at z > 7 comes from haloes of mass M<10^9 M_sun predominantly harbouring
PopIII stars; a turnover to a PopII-dominated phase occurs shortly after, with
this population, residing in M>10^9 M_sun haloes, yielding f_\gamma \approx 60%
at z=6. Using Lyman-break broadband dropout techniques, J-band detection of
sources contributing to 50% (90%) of the ionizing power at z \sim 7.5 requires
to reach a magnitude J_{110,AB} = 31.2 (31.7), where about 15 (30) (PopIII)
sources/arcmin^2 are predicted. We conclude that z>7 sources tentatively
identified in broadband surveys are relatively massive (M \approx 10^9 M_sun)
and rare objects which are only marginally (\approx 1%) adding to the
reionization photon budget.Comment: Extended discussions. Accepted to MNRAS Letter
Evolution of X-ray spectra of Cygnus X-3 with radio flares
Cygnus X-3, among the X-ray binaries, is one of the brightest in the radio
band, repeatedly exhibiting huge radio flares. The X-ray spectra shows two
definite states, low (correspondingly hard) and high (correspondingly soft).
During the hard state the X-ray spectra shows a pivoting behaviour correlated
to the radio emission. In the high state the X-ray spectra shows a gamut of
behaviour which controls the radio flaring activity of the source. The complete
evolution of the X-ray spectra along with the radio flaring activity is
reported here, for the first time for this source.Comment: Bibliography has been correctly adde
Regular and Singular Pulse and Front Solutions and Possible Isochronous Behavior in the Short-Pulse Equation: Phase-Plane, Multi-Infinite Series and Variational Approaches
In this paper we employ three recent analytical approaches to investigate the
possible classes of traveling wave solutions of some members of a family of
so-called short-pulse equations (SPE). A recent, novel application of
phase-plane analysis is first employed to show the existence of breaking kink
wave solutions in certain parameter regimes. Secondly, smooth traveling waves
are derived using a recent technique to derive convergent multi-infinite series
solutions for the homoclinic (heteroclinic) orbits of the traveling-wave
equations for the SPE equation, as well as for its generalized version with
arbitrary coefficients. These correspond to pulse (kink or shock) solutions
respectively of the original PDEs.
Unlike the majority of unaccelerated convergent series, high accuracy is
attained with relatively few terms. And finally, variational methods are
employed to generate families of both regular and embedded solitary wave
solutions for the SPE PDE. The technique for obtaining the embedded solitons
incorporates several recent generalizations of the usual variational technique
and it is thus topical in itself. One unusual feature of the solitary waves
derived here is that we are able to obtain them in analytical form (within the
assumed ansatz for the trial functions). Thus, a direct error analysis is
performed, showing the accuracy of the resulting solitary waves. Given the
importance of solitary wave solutions in wave dynamics and information
propagation in nonlinear PDEs, as well as the fact that not much is known about
solutions of the family of generalized SPE equations considered here, the
results obtained are both new and timely.Comment: accepted for publication in Communications in Nonlinear Science and
Numerical Simulatio
Detection of anti-correlated hard X-ray time lag in Cygnus X-3
The wide-band X-ray spectra of the high mass X-ray binary Cygnus X-3 exhibits
a pivoting behavior in the `low' (as well as `hard') state, correlated to the
radio emission. The time scale of the soft and hard X-rays' anti-correlation,
which gave rise to the pivoting feature, was found to be less than a day from
the monitoring observations by RXTE--ASM and CGRO--BATSE. In this Letter we
report the detection of a lag of 1000s in the anti-correlation of
the hard X-ray emission (20--50 keV) to that of the soft X-ray emission (2--7
keV), which may be attributed to the viscous time scale of flow of matter in
the accretion disk. This suggests the geometrical picture of a truncated
accretion disc with a Compton cloud inside the disc, the relative sizes of
which determine the spectral shape. Any change in the disc structure will take
place in a viscous time scale, with corresponding anti-correlated change in the
Compton cloud. We also report the pivoting in the spectra in one span of a
pointed observation when an episode of the rearranging of the accretion system
is serendipitously observed. This is the first such observation of hard X-ray
delay seen in the persistent Galactic microquasars, within the precincts of the
hard state.Comment: Accepted in The Astrophysical Journal (Letters): in pres
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