10,259 research outputs found
Quantum Heisenberg Chain with Long-Range Ferromagnetic Interactions at Low Temperature
A modified spin-wave theory is applied to the one-dimensional quantum
Heisenberg model with long-range ferromagnetic interactions. Low-temperature
properties of this model are investigated. The susceptibility and the specific
heat are calculated; the relation between their behaviors and strength of the
long-range interactions is obtained. This model includes both the
Haldane-Shastry model and the nearest-neighbor Heisenberg model; the
corresponding results in this paper are in agreement with the solutions of both
the models. It is shown that there exists an ordering transition in the region
where the model has longer-range interactions than the HS model. The critical
temperature is estimated.Comment: 17 pages(LaTeX REVTeX), 1 figure appended (PostScript), Technical
Report of ISSP A-274
Early Growth and Efficient Accretion of Massive Black Holes at High Redshift
Black-hole masses of the highest redshift quasars (4 <~ z <~ 6) are estimated
using a previously presented scaling relationship, derived from reverberation
mapping of nearby quasars, and compared to quasars at lower redshift. It is
shown that the central black holes in luminous z >~ 4 quasars are very massive
(>~ 10^9 solar masses). It is argued that the mass estimates of the high-z
quasars are not subject to larger uncertainties than those for nearby quasars.
Specifically, the large masses are not overestimates and the lack of similarly
large black-hole masses in the nearby Universe does not rule out their
existence at high-z. However, AGN host galaxies do not typically appear fully
formed or evolved at these early epochs. This supports scenarios in which black
holes build up mass very fast in a radiatively inefficient (or obscured) phase
relative to the stars in their galaxies. Additionally, upper envelopes of
black-hole mass of approximately 10^{10} solar masses and bolometric luminosity
of ~ 10^{48} erg/s are observed at all redshifts.Comment: 17 pages including 7 figures (5 in color) and 1 table. To appear in
ApJ, v600, January 1, 200
Magnetic properties of quantum Heisenberg ferromagnets with long-range interactions
Quantum Heisenberg ferromagnets with long-range interactions decayin as
in one and two dimensions are investigated by means of the Green's
function method. It is shown that there exists a finite-temperature phase
transition in the region for the -dimensional case and that no
transitions at any finite temperature exist for ; the critical
temperature is also estimated. We study the magnetic properties of this model.
We calculate the critical exponents' dependence on ; these exponents also
satisfy a scaling relation. Some of the results were also found using the
modified spin-wave theory and are in remarkable agreement with each other.Comment: 13 pages(LaTeX REVTeX), 2 figures not included (postscript files
available on request), submitted to Phys.Rev.
Discovery of the accretion-powered millisecond pulsar SWIFT J1756.9-2508 with a low-mass companion
We report on the discovery by the Swift Gamma-Ray Burst Explorer of the
eighth known transient accretion-powered millisecond pulsar, SWIFT
J1756.9-2508, as part of routine observations with the Swift Burst Alert
Telescope hard X-ray transient monitor. The pulsar was subsequently observed by
both the X-Ray Telescope on Swift and the Rossi X-Ray Timing Explorer
Proportional Counter Array. It has a spin frequency of 182 Hz (5.5 ms) and an
orbital period of 54.7 minutes. The minimum companion mass is between 0.0067
and 0.0086 solar masses, depending on the mass of the neutron star, and the
upper limit on the mass is 0.030 solar masses (95% confidence level). Such a
low mass is inconsistent with brown dwarf models, and comparison with white
dwarf models suggests that the companion is a He-dominated donor whose thermal
cooling has been at least modestly slowed by irradiation from the accretion
flux. No X-ray bursts, dips, eclipses or quasi-periodic oscillations were
detected. The current outburst lasted approximately 13 days and no earlier
outbursts were found in archival data.Comment: 13 pages, 2 figures, accepted by Astrophysical Journal Letter
Study of the 12C+12C fusion reactions near the Gamow energy
The fusion reactions 12C(12C,a)20Ne and 12C(12C,p)23Na have been studied from
E = 2.10 to 4.75 MeV by gamma-ray spectroscopy using a C target with ultra-low
hydrogen contamination. The deduced astrophysical S(E)* factor exhibits new
resonances at E <= 3.0 MeV, in particular a strong resonance at E = 2.14 MeV,
which lies at the high-energy tail of the Gamow peak. The resonance increases
the present non-resonant reaction rate of the alpha channel by a factor of 5
near T = 8x10^8 K. Due to the resonance structure, extrapolation to the Gamow
energy E_G = 1.5 MeV is quite uncertain. An experimental approach based on an
underground accelerator placed in a salt mine in combination with a high
efficiency detection setup could provide data over the full E_G energy range.Comment: 4 Pages, 4 figures, accepted for publication in Phys. Rev. Let
Swift observations of the SFXT SAX J1818.6-1703 in outburst
We present the Swift observations of the supergiant fast X-ray transient
(SFXT) SAX J1818.6-1703 collected during the most recent outburst, which
occurred on May 6 2009. In particular, we present broad-band spectroscopic and
timing analysis as well as a Swift/XRT light curve that spans more than two
weeks of observations. The broad-band spectral models and length of the
outburst resemble those of the prototype of the SFXT class, XTE J1739-302,
further confirming SAX J1818.6-1703 as a member of this class.Comment: Proceedings of 'X-Ray Astronomy 2009, Present Status, multiwavelength
approach and future perspectives', September 7 - 11, 2009, Bologna, Ital
Interrelation between the pseudogap and the incoherent quasi-particle features of high-Tc superconductors
Using a scenario of a hybridized mixture of localized bipolarons and
conduction electrons, we demonstrate for the latter the simultaneous appearance
of a pseudogap and of strong incoherent contributions to their quasi-particle
spectrum which arise from phonon shake-off effects. This can be traced back to
temporarily fluctuating local lattice deformations, giving rise to a
double-peak structure in the pair distribution function, which should be a key
feature in testing the origin of these incoherent contributions, recently seen
in angle-resolved photoemission spectroscopy (ARPES).Comment: 4 pages, 3 figures, to be published in Phys. Rev. Let
The mass surface density in the local disk and the chemical evolution of the Galaxy
We have studied the effect of adopting different values of the total baryonic
mass surface density in the local disk at the present time in a model for the
chemical evolution of the Galaxy. We have compared our model results with the
G-dwarf metallicity distribution, the amounts of gas, stars, stellar remnants,
infall rate and SN rate in the solar vicinity, and with the radial abundance
gradients and gas distribution in the disk. This comparison strongly suggests
that the value of the total baryonic mass surface density in the local disk
which best fits the observational properties should lie in the range 50-75 Msun
pc-2, and that values outside this range should be ruled out.Comment: 6 pages, LaTeX, 3 figures, accepted for publication in the
Astrophysical Journal, uses emulateapj.st
Corrections to the apparent value of the cosmological constant due to local inhomogeneities
Supernovae observations strongly support the presence of a cosmological
constant, but its value, which we will call apparent, is normally determined
assuming that the Universe can be accurately described by a homogeneous model.
Even in the presence of a cosmological constant we cannot exclude nevertheless
the presence of a small local inhomogeneity which could affect the apparent
value of the cosmological constant. Neglecting the presence of the
inhomogeneity can in fact introduce a systematic misinterpretation of
cosmological data, leading to the distinction between an apparent and true
value of the cosmological constant. We establish the theoretical framework to
calculate the corrections to the apparent value of the cosmological constant by
modeling the local inhomogeneity with a solution. Our assumption
to be at the center of a spherically symmetric inhomogeneous matter
distribution correspond to effectively calculate the monopole contribution of
the large scale inhomogeneities surrounding us, which we expect to be the
dominant one, because of other observations supporting a high level of isotropy
of the Universe around us.
By performing a local Taylor expansion we analyze the number of independent
degrees of freedom which determine the local shape of the inhomogeneity, and
consider the issue of central smoothness, showing how the same correction can
correspond to different inhomogeneity profiles. Contrary to previous attempts
to fit data using large void models our approach is quite general. The
correction to the apparent value of the cosmological constant is in fact
present for local inhomogeneities of any size, and should always be taken
appropriately into account both theoretically and observationally.Comment: 16 pages,new sections added analyzing central smoothness and accuracy
of the Taylor expansion approach, Accepted for publication by JCAP. An essay
based on this paper received honorable mention in the 2011 Essay Context of
the Gravity Research Foundatio
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