2,838 research outputs found
Evidence for precession of the isolated neutron star RX J0720.4-3125
The XMM-Newton spectra of the isolated neutron star RX J0720.4-3125 obtained
over 4.5 years can be described by sinusoidal variations in the inferred
blackbody temperature, the size of the emitting area and the depth of the
absorption line with a period of 7.1 +/- 0.5 years, which we suggest to be the
precession period of the neutron star. Precession of a neutron star with two
hot spots of different temperature and size, probably not located exactly in
antipodal positions, may account for the variations in the X-ray spectra,
changes in the pulsed fraction, shape of the light curve and the phase-lag
between soft and hard energy bands observed from RX J0720.4-3125. An
independent sinusoidal fit to published and new pulse timing residuals from a
coherent analysis covering ~12 years yields a consistent period of 7.7 +/- 0.6
years supporting the precession model.Comment: Accepted for publication in A&A Letters, 5 pages, 5 figure
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
Critical behavior in colloid-polymer mixtures: theory and simulation
We extensively investigated the critical behavior of mixtures of colloids and
polymers via the two-component Asakura-Oosawa model and its reduction to a
one-component colloidal fluid using accurate theoretical and simulation
techniques. In particular the theoretical approach, hierarchical reference
theory [Adv. Phys. 44, 211 (1995)], incorporates realistically the effects of
long-range fluctuations on phase separation giving exponents which differ
strongly from their mean-field values, and are in good agreement with those of
the three-dimensional Ising model. Computer simulations combined with
finite-size scaling analysis confirm the Ising universality and the accuracy of
the theory, although some discrepancy in the location of the critical point
between one-component and full-mixture description remains. To assess the limit
of the pair-interaction description, we compare one-component and two-component
results.Comment: 15 pages, 10 figures. Submitted to Phys. Rev.
Wall-Fluid and Liquid-Gas Interfaces of Model Colloid-Polymer Mixtures by Simulation and Theory
We perform a study of the interfacial properties of a model suspension of
hard sphere colloids with diameter and non-adsorbing ideal polymer
coils with diameter . For the mixture in contact with a planar hard
wall, we obtain from simulations the wall-fluid interfacial free energy,
, for size ratios and 1, using
thermodynamic integration, and study the (excess) adsorption of colloids,
, and of polymers, , at the hard wall. The interfacial
tension of the free liquid-gas interface, , is obtained following
three different routes in simulations: i) from studying the system size
dependence of the interfacial width according to the predictions of capillary
wave theory, ii) from the probability distribution of the colloid density at
coexistence in the grand canonical ensemble, and iii) for statepoints where the
colloidal liquid wets the wall completely, from Young's equation relating
to the difference of wall-liquid and wall-gas interfacial
tensions, . In addition, we calculate , and using density functional theory and a scaled particle
theory based on free volume theory. Good agreement is found between the
simulation results and those from density functional theory, while the results
from scaled particle theory quantitatively deviate but reproduce some essential
features. Simulation results for obtained from the three
different routes are all in good agreement. Density functional theory predicts
with good accuracy for high polymer reservoir packing fractions,
but yields deviations from the simulation results close to the critical point.Comment: 23 pages, 10 figures, REVTEX. Fig 5a changed. Final versio
The signature of 44Ti in Cassiopeia A revealed by IBIS/ISGRI on INTEGRAL
We report the detection of both the 67.9 and 78.4 keV 44Sc gamma-ray lines in
Cassiopeia A with the INTEGRAL IBIS/ISGRI instrument. Besides the robustness
provided by spectro-imaging observations, the main improvements compared to
previous measurements are a clear separation of the two 44Sc lines together
with an improved significance of the detection of the hard X-ray continuum up
to 100 keV. These allow us to refine the determination of the 44Ti yield and to
constrain the nature of the nonthermal continuum emission. By combining
COMPTEL, BeppoSAX/PDS and ISGRI measurements, we find a line flux of (2.5 +/-
0.3)*10(-5) cm(-2) s(-1) leading to a synthesized 44Ti mass of 1.6
(+0.6-0.3)*10(-4) solar mass. This high value suggests that Cas A is peculiar
in comparison to other young supernova remnants, from which so far no line
emission from 44Ti decay has been unambiguously detected.Comment: 5 pages, 4 figures, Accepted for publication in ApJ
Single-shot readout of electron spin states in a quantum dot using spin-dependent tunnel rates
We present a method for reading out the spin state of electrons in a quantum
dot that is robust against charge noise and can be used even when the electron
temperature exceeds the energy splitting between the states. The spin states
are first correlated to different charge states using a spin dependence of the
tunnel rates. A subsequent fast measurement of the charge on the dot then
reveals the original spin state. We experimentally demonstrate the method by
performing read-out of the two-electron spin states, achieving a single-shot
visibility of more than 80%. We find very long triplet-to-singlet relaxation
times (up to several milliseconds), with a strong dependence on in-plane
magnetic field.Comment: 4 pages, 4 figure
Detection of single electron spin resonance in a double quantum dot
Spin-dependent transport measurements through a double quantum dot are a
valuable tool for detecting both the coherent evolution of the spin state of a
single electron as well as the hybridization of two-electron spin states. In
this paper, we discuss a model that describes the transport cycle in this
regime, including the effects of an oscillating magnetic field (causing
electron spin resonance) and the effective nuclear fields on the spin states in
the two dots. We numerically calculate the current flow due to the induced spin
flips via electron spin resonance and we study the detector efficiency for a
range of parameters. The experimental data are compared with the model and we
find a reasonable agreement.Comment: 7 pages, 5 figures. To be published in Journal of Applied Physics,
proceedings ICPS 200
Heritability of Smoking Initiation and Nicotine Dependence
In contrast to other aspects of smoking behavior, little attention has been paid to the genetics of nicotine dependence. In this paper, three models (single liability dimension, independent liability dimension and combined model) have been applied to data on smoking initiation and nicotine dependence (n = 1572 Dutch twin pairs, mean age 30.5). A combined model best described the data. This model postulates a smoking initiation dimension and a nicotine dependence dimension, which are not independent. For both males and females, individual differences in smoking initiation were explained by genetic (44%), shared environmental (51%) and unique environmental (5%) influences. The nicotine dependence dimension was influenced only by genetic (75%) and unique environmental (25%) factors. The substantial impact of genetic factors on nicotine dependence emphasizes the need for further research to localize and identify specific genes and pathways involved in nicotine dependence. © 2005 Springer Science+Business Media, Inc
Control and Detection of Singlet-Triplet Mixing in a Random Nuclear Field
We observe mixing between two-electron singlet and triplet states in a double
quantum dot, caused by interactions with nuclear spins in the host
semiconductor. This mixing is suppressed by applying a small magnetic field, or
by increasing the interdot tunnel coupling and thereby the singlet-triplet
splitting. Electron transport involving transitions between triplets and
singlets in turn polarizes the nuclei, resulting in striking bistabilities. We
extract from the fluctuating nuclear field a limitation on the time-averaged
spin coherence time T2* of 25 ns. Control of the electron-nuclear interaction
will therefore be crucial for the coherent manipulation of individual electron
spins.Comment: 4 pages main text, 4 figure
Measuring the cosmic ray acceleration efficiency of a supernova remnant
Cosmic rays are the most energetic particles arriving at earth. Although most
of them are thought to be accelerated by supernova remnants, the details of the
acceleration process and its efficiency are not well determined. Here we show
that the pressure induced by cosmic rays exceeds the thermal pressure behind
the northeast shock of the supernova remnant RCW 86, where the X-ray emission
is dominated by synchrotron radiation from ultra-relativistic electrons. We
determined the cosmic-ray content from the thermal Doppler broadening measured
with optical spectroscopy, combined with a proper-motion study in X- rays. The
measured post-shock proton temperature in combination with the shock velocity
does not agree with standard shock heating, implying that >50% of the
post-shock pressure is produced by cosmic rays.Comment: Published in Science express, 10 pages, 5 figures and 2 table
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