21,334 research outputs found
A BeppoSAX observation of the supersoft source 1E 0035.4-7230
Results from a 37,000 s BeppoSAX Low-Energy Concentrator Spectrometer (LECS)
observation of the supersoft source SMC 13 (=1E 0035.4-7230) in the Small
Magellanic Cloud are reported. The BeppoSAX spectrum is fitted either with a
blackbody spectrum with an effective temperature kT = 26-58 eV, an LTE white
dwarf atmosphere spectrum with kT = 35-50 eV, or a non-LTE white dwarf
atmosphere spectrum with kT = 25-32 eV. The bolometric luminosity is < 8 10^37
erg s-1 and < 3 10^37 erg s^-1 for the LTE and the non-LTE spectrum. We also
applied a spectral fit to combined spectra obtained with BeppoSAX LECS and with
ROSAT PSPC. The kT derived for the non-LTE spectrum is 27-29 eV, the bolometric
luminosity is 1.1-1.2 10^37 erg s^-1. We can exclude any spectrally hard
component with a luminosity > 2 10^35 erg s^-1 (for a bremmstrahlung with a
temperature of 0.5 keV) at a distance of 60 kpc. The LTE temperature is
therefore in the range 5.5+/-0.2 10^5 K and the non-LTE temperature in the
range 3.25+/-0.16 10^5 K. Assuming the source is on the stability line for
atmospheric nuclear burning, we constrain the white dwarf mass from the LTE and
the non-LTE fit to ~1.1 M-solar and ~0.9 M-solar respectively. However, the
temperature and luminosity derived with the non-LTE model for 1E 0035.4-7230 is
consistent with a lower mass M~0.6-0.7 M-solar white dwarf as predicted by Sion
and Starrfield (1994). At the moment, neither of these two alternatives for the
white dwarf mass can be excluded.Comment: 6 pages, accepted by A&A March 30th 199
Luminous supersoft X-ray emission from the recurrent nova U Scorpii
BeppoSAX detected luminous 0.2-2.0 keV supersoft X-ray emission from the
recurrent nova U Sco ~19-20 days after the peak of the optical outburst in
February 1999. U Sco is the first recurrent nova to be observed during a
luminous supersoft X-ray phase. Non-LTE white dwarf atmosphere spectral models
(together with a ~0.5 keV optically thin thermal component) were fitted to the
BeppoSAX spectrum. We find that the fit is acceptable assuming enriched He and
an enhanced N/C ratio. This implies that the CNO cycle was active during the
outburst, in agreement with a thermonuclear runaway scenario. The best-fit
temperature is ~9 10^5 K and the bolometric luminosity those predicted for
steady nuclear burning on a WD close to the Chandrasekhar mass. The fact that
U~Sco was detected as a supersoft X-ray source is consistent with steady
nuclear burning continuing for at least one month after the outburst. This
means that only a fraction of the previously accreted H and He was ejected
during the outburst and that the WD can grow in mass, ultimately reaching the
Chandrasekhar limit. This makes U~Sco a candidate type Ia supernova progenitor.Comment: 4 pages, accepted by A&A Letters 15 June 199
Steady state entanglement in open and noisy quantum systems at high temperature
We show that quantum mechanical entanglement can prevail even in noisy open
quantum systems at high temperature and far from thermodynamical equilibrium,
despite the deteriorating effect of decoherence. The system consists of a
number N of interacting quantum particles, and it can interact and exchange
particles with some environment. The effect of decoherence is counteracted by a
simple mechanism, where system particles are randomly reset to some standard
initial state, e.g. by replacing them with particles from the environment. We
present a master equation that describes this process, which we can solve
analytically for small N. If we vary the interaction strength and the reset
against decoherence rate, we find a threshold below which the equilibrium state
is classically correlated, and above which there is a parameter region with
genuine entanglement.Comment: 5 pages, 3 figure
Domain-Wall Energies and Magnetization of the Two-Dimensional Random-Bond Ising Model
We study ground-state properties of the two-dimensional random-bond Ising
model with couplings having a concentration of antiferromagnetic
and of ferromagnetic bonds. We apply an exact matching algorithm which
enables us the study of systems with linear dimension up to 700. We study
the behavior of the domain-wall energies and of the magnetization. We find that
the paramagnet-ferromagnet transition occurs at compared to
the concentration at the Nishimory point, which means that the
phase diagram of the model exhibits a reentrance. Furthermore, we find no
indications for an (intermediate) spin-glass ordering at finite temperature.Comment: 7 pages, 12 figures, revTe
Phase transitions in diluted negative-weight percolation models
We investigate the geometric properties of loops on two-dimensional lattice
graphs, where edge weights are drawn from a distribution that allows for
positive and negative weights. We are interested in the appearance of spanning
loops of total negative weight. The resulting percolation problem is
fundamentally different from conventional percolation, as we have seen in a
previous study of this model for the undiluted case.
Here, we investigate how the percolation transition is affected by additional
dilution. We consider two types of dilution: either a certain fraction of edges
exhibit zero weight, or a fraction of edges is even absent. We study these
systems numerically using exact combinatorial optimization techniques based on
suitable transformations of the graphs and applying matching algorithms. We
perform a finite-size scaling analysis to obtain the phase diagram and
determine the critical properties of the phase boundary.
We find that the first type of dilution does not change the universality
class compared to the undiluted case whereas the second type of dilution leads
to a change of the universality class.Comment: 8 pages, 7 figure
Entanglement and its dynamics in open, dissipative systems
Quantum mechanical entanglement can exist in noisy open quantum systems at
high temperature. A simple mechanism, where system particles are randomly reset
to some standard initial state, can counteract the deteriorating effect of
decoherence, resulting in an entangled steady state far from thermodynamical
equilibrium. We present models for both gas-type systems and for strongly
coupled systems. We point out in which way the entanglement resulting from such
a reset mechanism is different from the entanglement that one can find in
thermal states. We develop master equations to describe the system and its
interaction with an environment, study toy models with two particles (qubits),
where the master equation can often be solved analytically, and finally examine
larger systems with possibly fluctuating particle numbers. We find that in
gas-type systems, the reset mechanism can produce an entangled steady state for
an arbitrary temperature of the environment, while this is not true in strongly
coupled systems. But even then, the temperature range where one can find
entangled steady states is typically much higher with the reset mechanism.Comment: 30 pages, 15 figure
Spin gases as microscopic models for non-Markovian decoherence
We analyze a microscopic decoherence model in which the total system is
described as a spin gas. A spin gas consists of N classically moving particles
with additional, interacting quantum degrees of freedom (e.g. spins). For
various multipartite entangled probe states, we analyze the decoherence induced
by interactions between the probe- and environmental spins in such spin gases.
We can treat mesoscopic environments (10^5 particles). We present results for a
lattice gas, which could be realized by neutral atoms hopping in an optical
lattice, and show the effects of non-Markovian and correlated noise, as well as
finite size effects.Comment: 4 pages, 4 figure
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