21,175 research outputs found
The Birmingham-CfA cluster scaling project - I: gas fraction and the M-T relation
We have assembled a large sample of virialized systems, comprising 66 galaxy
clusters, groups and elliptical galaxies with high quality X-ray data. To each
system we have fitted analytical profiles describing the gas density and
temperature variation with radius, corrected for the effects of central gas
cooling. We present an analysis of the scaling properties of these systems and
focus in this paper on the gas distribution and M-T relation. In addition to
clusters and groups, our sample includes two early-type galaxies, carefully
selected to avoid contamination from group or cluster X-ray emission. We
compare the properties of these objects with those of more massive systems and
find evidence for a systematic difference between galaxy-sized haloes and
groups of a similar temperature. We derive a mean logarithmic slope of the M-T
relation within R_200 of 1.84+/-0.06, although there is some evidence of a
gradual steepening in the M-T relation, with decreasing mass. We recover a
similar slope using two additional methods of calculating the mean temperature.
Repeating the analysis with the assumption of isothermality, we find the slope
changes only slightly, to 1.89+/-0.04, but the normalization is increased by
30%. Correspondingly, the mean gas fraction within R_200 changes from
(0.13+/-0.01)h70^-1.5 to (0.11+/-0.01)h70^-1.5, for the isothermal case, with
the smaller fractional change reflecting different behaviour between hot and
cool systems. There is a strong correlation between the gas fraction within
0.3*R_200 and temperature. This reflects the strong (5.8 sigma) trend between
the gas density slope parameter, beta, and temperature, which has been found in
previous work. (abridged)Comment: 27 pages, accepted for publication in MNRAS; uses longtable.sty &
lscape.st
Quantum computation over continuous variables
This paper provides necessary and sufficient conditions for constructing a
universal quantum computer over continuous variables. As an example, it is
shown how a universal quantum computer for the amplitudes of the
electromagnetic field might be constructed using simple linear devices such as
beam-splitters and phase shifters, together with squeezers and nonlinear
devices such as Kerr-effect fibers and atoms in optical cavities. Such a device
could in principle perform `quantum floating point' computations. Problems of
noise, finite precision, and error correction are discussed.Comment: 9 pages, Te
Efficient estimation of nearly sparse many-body quantum Hamiltonians
We develop an efficient and robust approach to Hamiltonian identification for
multipartite quantum systems based on the method of compressed sensing. This
work demonstrates that with only O(s log(d)) experimental configurations,
consisting of random local preparations and measurements, one can estimate the
Hamiltonian of a d-dimensional system, provided that the Hamiltonian is nearly
s-sparse in a known basis. We numerically simulate the performance of this
algorithm for three- and four-body interactions in spin-coupled quantum dots
and atoms in optical lattices. Furthermore, we apply the algorithm to
characterize Hamiltonian fine structure and unknown system-bath interactions.Comment: 8 pages, 2 figures. Title is changed. Detailed error analysis is
added. Figures are updated with additional clarifying discussion
The Origin of Soft X-rays in DQ Herculis
DQ Herculis (Nova Herculis 1934) is a deeply eclipsing cataclysmic variable
containing a magnetic white dwarf primary. The accretion disk is thought to
block our line of sight to the white dwarf at all orbital phases due to its
extreme inclination angle. Nevertheless, soft X-rays were detected from DQ Her
with ROSAT PSPC. To probe the origin of these soft X-rays, we have performed
Chandra ACIS observations. We confirm that DQ Her is an X-ray source. The bulk
of the X-rays are from a point-like source and exhibit a shallow partial
eclipse. We interpret this as due to scattering of the unseen central X-ray
source, probably in an accretion disk wind. At the same time, we observe what
appear to be weak extended X-ray features around DQ Her, which we interpret as
an X-ray emitting knot in the nova shell.Comment: 18 pages including 4 figures, accepted for publication in
Astrphyisical Journa
Cyclic networks of quantum gates
In this article initial steps in an analysis of cyclic networks of quantum
logic gates is given. Cyclic networks are those in which the qubit lines are
loops. Here we have studied one and two qubit systems plus two qubit cyclic
systems connected to another qubit on an acyclic line. The analysis includes
the group classification of networks and studies of the dynamics of the qubits
in the cyclic network and of the perturbation effects of an acyclic qubit
acting on a cyclic network. This is followed by a discussion of quantum
algorithms and quantum information processing with cyclic networks of quantum
gates, and a novel implementation of a cyclic network quantum memory. Quantum
sensors via cyclic networks are also discussed.Comment: 14 pages including 11 figures, References adde
Tight Binding Hamiltonians and Quantum Turing Machines
This paper extends work done to date on quantum computation by associating
potentials with different types of computation steps. Quantum Turing machine
Hamiltonians, generalized to include potentials, correspond to sums over tight
binding Hamiltonians each with a different potential distribution. Which
distribution applies is determined by the initial state. An example, which
enumerates the integers in succession as binary strings, is analyzed. It is
seen that for some initial states the potential distributions have
quasicrystalline properties and are similar to a substitution sequence.Comment: 4 pages Latex, 2 postscript figures, submitted to Phys Rev Letter
Born Again Protoplanetary Disk Around Mira B
The Mira AB system is a nearby (~107 pc) example of a wind accreting binary
star system. In this class of system, the wind from a mass-losing red giant
star (Mira A) is accreted onto a companion (Mira B), as indicated by an
accretion shock signature in spectra at ultraviolet and X-ray wavelengths.
Using novel imaging techniques, we report the detection of emission at
mid-infrared wavelengths between 9.7 and 18.3 m from the vicinity of Mira
B but with a peak at a radial position about 10 AU closer to the primary Mira
A. We interpret the mid-infrared emission as the edge of an optically-thick
accretion disk heated by Mira A. The discovery of this new class of accretion
disk fed by M-giant mass loss implies a potential population of young planetary
systems in white-dwarf binaries which has been little explored, despite being
relatively common in the solar neighborhood.Comment: Accepted for Ap
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