966 research outputs found
Polarization of Thermal X-rays from Isolated Neutron Stars
Since the opacity of a magnetized plasma depends on polarization of
radiation, the radiation emergent from atmospheres of neutron stars with strong
magnetic fields is expected to be strongly polarized. The degree of linear
polarization, typically ~10-30%, depends on photon energy, effective
temperature and magnetic field. The spectrum of polarization is more sensitive
to the magnetic field than the spectrum of intensity. Both the degree of
polarization and the position angle vary with the neutron star rotation period
so that the shape of polarization pulse profiles depends on the orientation of
the rotational and magnetic axes. Moreover, as the polarization is
substantially modified by the general relativistic effects, observations of
polarization of X-ray radiation from isolated neutron stars provide a new
method for evaluating the mass-to-radius ratio of these objects, which is
particularly important for elucidating the properties of the superdense matter
in the neutron star interiors.Comment: 7 figures, to be published in Ap
Rational approximation and arithmetic progressions
A reasonably complete theory of the approximation of an irrational by
rational fractions whose numerators and denominators lie in prescribed
arithmetic progressions is developed in this paper. Results are both, on the
one hand, from a metrical and a non-metrical point of view and, on the other
hand, from an asymptotic and also a uniform point of view. The principal
novelty is a Khintchine type theorem for uniform approximation in this context.
Some applications of this theory are also discussed
Chandra Phase-Resolved Spectroscopy of the Crab Pulsar
We present the first phase-resolved study of the X-ray spectral properties of
the Crab Pulsar that covers all pulse phases. The superb angular resolution of
the Chandra X-ray Observatory enables distinguishing the pulsar from the
surrounding nebulosity, even at pulse minimum. Analysis of the pulse-averaged
spectrum measures interstellar X-ray extinction due primarily to photoelectric
absorption and secondarily to scattering by dust grains in the direction of the
Crab Nebula. We confirm previous findings that the line-of-sight to the Crab is
underabundant in oxygen, although more-so than recently measured. Using the
abundances and cross sections from Wilms, Allen & McCray (2000) we find [O/H] =
(3.33 +/-0.25) x 10**-4. Analysis of the spectrum as a function of pulse phase
measures the low-energy X-ray spectral index even at pulse minimum -- albeit
with large statistical uncertainty -- and we find marginal evidence for
variations of the spectral index. The data are also used to set a new (3-sigma)
upper limit to the temperature of the neutron star of log T(infinity) < 6.30.Comment: 20 Pages including 7 figures. Accepted for publication in the
Astrophysical Journa
The Prospects for X-ray Polarimetry and its Potential use for Understanding Neutron Stars
We review the state of the art for measuring the X-ray polarization of neutron stars. We discuss how valuable precision measurements of the degree and position angle of polarization as a function of energy and, where relevant, of pulse phase, would provide deeper insight into the details of the emission mechanisms. We then review the current state of instrumentation and its potential for obtaining relevant data. Finally, we conclude our discussion with some opinions as to future directions
Adsorbing vs. nonadsorbing tracers for assessing pesticide transport in arable soils
The suitability of two different tracers to mimic the behavior of pesticides in agricultural soils and to evidence the potential for preferential flow was evaluated in outdoor lysimeter experiments. The herbicide atrazine [6âchloroâNâethylâNâČâ(1âmethylethyl)â1,3,5âtriazineâ2,4âdiamine] was used as a model compound. Two tracers were used: a nonadsorbing tracer (bromide) and a weakly adsorbing dye tracer (uranine). Two soils that are expected to show a different extent of macropore preferential flow were used: a wellâdrained sandyâloamy Cambisol (gravel soil) and a poorly drained loamy Cambisol (moraine soil). Conditions for preferential flow were promoted by applying heavy simulated rainfall shortly after pesticide application. In some of the experiments, preferential flow was also artificially simulated by injecting the solutes through a narrow tube below the root zone. With depth injection, preferential leaching of atrazine occurred shortly after application in both soil types, whereas with surface application, it occurred only in the moraine soil. Thereafter, atrazine transport was mainly through the porous soil matrix, although contributions of preferential flow were also observed. For all the application approaches and soil types, after 900 d, <3% of the applied amount of atrazine was recovered in the drainage water. Only uranine realistically illustrated the early atrazine breakthrough by transport through preferential flow. Uranine broke through during the first intense irrigation at the same time as atrazine. Bromide, however, appeared earlier than atrazine in some cases. The use of dye tracers as pesticide surrogates might assist in making sustainable decisions with respect to pesticide application timing relative to rainfall or soil potential for preferential flow
Jupiter's X-ray and EUV auroras monitored by Chandra, XXM-Newton, and Hisaki satellite
Jupiter's X-ray auroral emission in the polar cap region results from particles which have undergone strong field-aligned acceleration into the ionosphere. The origin of precipitating ions and electrons and the time variability in the X-ray emission are essential to uncover the driving mechanism for the high-energy acceleration. The magnetospheric location of the source field line where the X-ray is generated is likely affected by the solar wind variability. However, these essential characteristics are still unknown because the long-term monitoring of the X-rays and contemporaneous solar wind variability has not been carried out. In April 2014, the first long-term multiwavelength monitoring of Jupiter's X-ray and EUV auroral emissions was made by the Chandra X-ray Observatory, XMM-Newton, and Hisaki satellite. We find that the X-ray count rates are positively correlated with the solar wind velocity and insignificantly with the dynamic pressure. Based on the magnetic field mapping model, a half of the X-ray auroral region was found to be open to the interplanetary space. The other half of the X-ray auroral source region is magnetically connected with the prenoon to postdusk sector in the outermost region of the magnetosphere, where the Kelvin-Helmholtz (KH) instability, magnetopause reconnection, and quasiperiodic particle injection potentially take place. We speculate that the high-energy auroral acceleration is associated with the KH instability and/or magnetopause reconnection. This association is expected to also occur in many other space plasma environments such as Saturn and other magnetized rotators
On the accretion flow geometry in A0535+26
The geometry of accretion flow in the Be/X-ray transient A0535+26 is
explored. It is shown that neither moderate nor giant X-ray flaring events
observed in the system can be interpreted within the spherically symmetrical
accretion model and hence the formation of an accretion disk around the neutron
star magnetosphere during the both types of flares is required. The accretion
disk can be formed at the periastron if (i) the expansion velocity of the Be
star envelope in the equatorial plane is V_wr < 150 km/s and (ii) the parameter
accounting for the accretion flow inhomogeneities, xi, satisfies the following
condition: xi > 0.16 (Mdot_17)^-1/7, where (Mdot_17)^-1/7 is the rate of mass
capture by the neutron star expressed in units of 1017 g/s. We suggest that the
`missing' outburst phenomenon can be associated with the spherically
symmetrical accretion onto the interchange-stable magnetosphere of the neutron
star. The average spin up rate of the neutron star during moderate flares < 3.5
x 10^-12 Hz/s is predicted.Comment: 6 pages, published in A&A 372, 227 (2001
K^0 pi^0 Sigma^+ and K^*0 Sigma^+ photoproduction off the proton
The exclusive reactions and , leading to the p 4 final state, have
been measured with a tagged photon beam for incident energies from threshold up
to 2.5 GeV. The experiment has been performed at the tagged photon facility of
the ELSA accelerator (Bonn). The Crystal Barrel and TAPS detectors were
combined to a photon detector system of almost 4 geometrical acceptance.
Differential and total cross sections are reported. At energies close to the
threshold, a flat angular distribution has been observed for the reaction
suggesting dominant s-channel production.
and higher lying hyperon states have been observed. An
enhancement in the forward direction in the angular distributions of the
reaction indicates a -channel exchange
contribution to the reaction mechanism. The experimental data are in reasonable
agreement with recent theoretical predictions.Comment: 11 pages, 13 figures, submitted to EPJ
Measurement of polarisation observables in photoproduction off the proton
The reaction is studied in the
photon energy range from threshold. Linearly polarised photon beams from
coherent bremsstrahlung enabled the first measurement of photon beam
asymmetries in this reaction up to MeV. In addition, the
recoil hyperon polarisation was determined through the asymmetry in the weak
decay up to MeV. The data are
compared to partial wave analyses, and the possible impact on the
interpretation of a recently observed cusp-like structure near the
thresholds is discussed.Comment: 6 pages, 5 figures. References [8,9,10,11] which were not on the
original submission are now include
Measurement of the Beam Asymmetry in the Forward Direction for pi0 Photoproduction
Photoproduction of neutral pions has been studied with the CBELSA/TAPS
detector for photon energies between 0.92 and 1.68~GeV at the electron
accelerator ELSA. The beam asymmetry~ has been extracted for of the ~meson and for . The new beam asymmetry data improve the world database for
photon energies above 1.5~GeV and, by covering the very forward region, extend
previously published data for the same reaction by our collaboration. The
angular dependence of shows overall good agreement with the SAID
parameterization.Comment: 11 pages, 10 figures, published in Phys. Rev. C, included LEPS data
and MAID 2007 predictions for comparison with our dat
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