52,533 research outputs found
The hydrogen atom in electric and magnetic fields : Pauli's 1926 article
The results obtained by Pauli, in his 1926 article on the hydrogen atom, made
essential use of the dynamical so(4) symmetry of the bound states. Pauli used
this symmetry to compute the perturbed energy levels of an hydrogen atom in a
uniform electric field (Stark effect) and in uniform electric and magnetic
fields. Although the experimental check of the single Stark effect on the
hydrogen atom has been studied experimentally, Pauli's results in mixed fields
have been studied only for Rydberg states of rubidium atoms in crossedfields
and lithium atoms in parallel fields.Comment: 11 pages, latex file, 2 figure
Detection of X-ray Emission from the Very Old Pulsar J0108-1431
PSR J0108-1431 is a nearby, 170 Myr old, very faint radio pulsar near the
"pulsar death line" in the P-Pdot diagram. We observed the pulsar field with
the Chandra X-ray Observatory and detected a point source (53 counts in a 30 ks
exposure, energy flux (9+/-2)\times 10^{-15} ergs cm^{-2} s^{-1} in the 0.3-8
keV band) close to the radio pulsar position. Based on the large X-ray/optical
flux ratio at the X-ray source position, we conclude that the source is the
X-ray counterpart of PSR J0108-1431.The pulsar spectrum can be described by a
power-law model with photon index Gamma \approx 2.2 and luminosity L_{0.3-8
keV} \sim 2\times 10^{28} d_{130}^2 ergs s^{-1}, or by a blackbody model with
the temperature kT\approx 0.28 keV and bolometric luminosity L_{bol} \sim
1.3\times 10^{28} d_{130}^2 ergs s^{-1}, for a plausible hydrogen column
density NH = 7.3\times 10^{19} cm^{-2} (d_{130}=d/130 pc). The pulsar converts
\sim 0.4% of its spin-down power into the X-ray luminosity, i.e., its X-ray
efficiency is higher than for most younger pulsars. From the comparison of the
X-ray position with the previously measured radio positions, we estimated the
pulsar proper motion of 0.2 arcsec yr^{-1} (V_\perp \sim 130 d_{130} km
s^{-1}), in the south-southeast direction.Comment: 8 pages, 9 figures, accepted to ApJ; minor revisions in Sections 2.2
and 3.
Analysis of economics of a TV broadcasting satellite for additional nationwide TV programs
The influence of a TV broadcasting satellite, transmitting four additional TV networks was analyzed. It is assumed that the cost of the satellite systems will be financed by the cable TV system operators. The additional TV programs increase income by attracting additional subscribers. Two economic models were established: (1) each local network is regarded as an independent economic unit with individual fees (cost price model) and (2) all networks are part of one public cable TV company with uniform fees (uniform price model). Assumptions are made for penetration as a function of subscription rates. Main results of the study are: the installation of a TV broadcasting satellite improves the economics of CTV-networks in both models; the overall coverage achievable by the uniform price model is significantly higher than that achievable by the cost price model
Pulsar Polar Cap Heating and Surface Thermal X-Ray Emission I. Curvature Radiation Pair Fronts
We investigate the effect of pulsar polar cap (PC) heating produced by
positrons returning from the upper pair formation front. Our calculations are
based on a self-consistent treatment of the pair dynamics and the effect of
electric field screening by the returning positrons. We calculate the resultant
X-ray luminosities, and discuss the dependence of the PC heating efficiencies
on pulsar parameters, such as characteristic spin-down age, spin period, and
surface magnetic field strength. In this study we concentrate on the regime
where the pairs are produced in a magnetic field by curvature photons emitted
by accelerating electrons. Our theoretical results are not in conflict with the
available observational X-ray data and suggest that the effect of PC heating
should significantly contribute to the thermal X-ray fluxes from middle-aged
and old pulsars. The implications for current and future X-ray observations of
pulsars are briefly outlined.Comment: 28 pages, 7 figures, accepted for publication in Ap
Evidence flow graph methods for validation and verification of expert systems
The results of an investigation into the use of evidence flow graph techniques for performing validation and verification of expert systems are given. A translator to convert horn-clause rule bases into evidence flow graphs, a simulation program, and methods of analysis were developed. These tools were then applied to a simple rule base which contained errors. It was found that the method was capable of identifying a variety of problems, for example that the order of presentation of input data or small changes in critical parameters could affect the output from a set of rules
Phase resolved spectroscopy of the Vela pulsar with XMM-Newton
The ~10^4 y old Vela Pulsar represents the bridge between the young Crab-like
and the middle-aged rotation powered pulsars. Its multiwavelength behaviour is
due to the superposition of different spectral components. We take advantage of
the unprecedented harvest of photons collected by XMM-Newton to assess the Vela
Pulsar spectral shape and to study the pulsar spectrum as a function of its
rotational phase. As for the middle-aged pulsars Geminga, PSR B0656+14 and PSR
B1055-52 (the "Three Musketeers"), the phase-integrated spectrum of Vela is
well described by a three-component model, consisting of two blackbodies
(T_bb1=(1.06+/-0.03)x10^6 K, R_bb1=5.1+/-0.3 km, T_bb2=(2.16+/-0.06)x10^6 K,
R_bb2=0.73+/-0.08 km) plus a power-law (gamma=2.2+/-0.3). The relative
contributions of the three components are seen to vary as a function of the
pulsar rotational phase. The two blackbodies have a shallow 7-9% modulation.
The cooler blackbody, possibly related to the bulk of the neutron star surface,
has a complex modulation, with two peaks per period, separated by ~0.35 in
phase, the radio pulse occurring exactly in between. The hotter blackbody,
possibly originating from a hot polar region, has a nearly sinusoidal
modulation, with a single, broad maximum aligned with the second peak of the
cooler blackbody, trailing the radio pulse by ~0.15 in phase. The non thermal
component, magnetospheric in origin, is present only during 20% of the pulsar
phase and appears to be opposite to the radio pulse. XMM-Newton phase-resolved
spectroscopy unveils the link between the thermally emitting surface of the
neutron star and its charge-filled magnetosphere, probing emission geometry as
a function of the pulsar rotation. This is a fundamental piece of information
for future 3-dimensional modeling of the pulsar magnetosphere.Comment: 27 pages, 9 figures. Accepted for publication in Ap
Luttinger liquid versus charge density wave behaviour in the one-dimensional spinless fermion Holstein model
We discuss the nature of the different ground states of the half-filled
Holstein model of spinless fermions in 1D. In the metallic regime we determine
the renormalised effective coupling constant and the velocity of the charge
excitations by a density-matrix renormalisation group (DMRG) finite-size
scaling approach. At low (high) phonon frequencies the Luttinger liquid is
characterised by an attractive (repulsive) effective interaction. In the
charge-density wave Peierls-distorted state the charge structure factor scales
to a finite value indicating long-range order.Comment: 2 pages, 3 figures, submitted to SCES'0
The Gamma Ray Pulsar Population
We apply a likelihood analysis to pulsar detections, pulsar upper limits, and
diffuse background measurements from the OSSE and EGRET instruments on the
Compton Gamma Ray Observatory to constrain the luminosity law for gamma-ray
pulsars and some properties of the gamma-ray pulsar population. We find that
the dependence of luminosity on spin period and dipole magnetic field is much
steeper at OSSE than at EGRET energies (50-200 keV and >100 MeV, respectively),
suggesting that different emission mechanisms are responsible for low- and
high-energy gamma-ray emission. Incorporating a spin-down model and assuming a
pulsar spatial distribution, we estimate the fraction of the Galactic gamma-ray
background due to unidentified pulsars and find that pulsars may be an
important component of the OSSE diffuse flux, but are most likely not important
at EGRET energies. Using measurements of the diffuse background flux from these
instruments, we are able to place constraints on the braking index, initial
spin period, and magnetic field of the Galactic pulsar population. We are also
able to constrain the pulsar birthrate to be between 1/(25 yr) and 1/(500 yr).
Our results are based on a large gamma-ray beam, but they do not scale in a
simple way with beam size. With our assumed beam size, the implied gamma-ray
efficiency for the EGRET detections is no more than 20%. We estimate that about
20 of the 169 unidentified EGRET sources are probably gamma-ray pulsars. We use
our model to predict the pulsar population that will be seen by future
gamma-ray instruments and estimate that GLAST will detect roughly 750 gamma-ray
pulsars as steady sources, only 120 of which are currently known radio pulsars.Comment: 32 pages, including figures. submitted to Ap
A spectroscopic investigation of the O-type star population in four Cygnus OB associations. II. Determination of the fundamental parameters
Aims. Having established the binary status of nineteen O-type stars located
in four Cygnus OB associations, we now determine their fundamental parameters
to constrain their properties and their evolutionary status. We also
investigate their surface nitrogen abundances, which we compare with other
results from the literature obtained for galactic O-type stars. Methods. Using
optical spectra collected for each object in our sample and some UV data from
the archives, we apply the CMFGEN atmosphere code to determine their main
properties. For the binary systems, we have disentangled the components to
obtain their individual spectra and investigate them as if they were single
stars. Results. We find that the distances of several presumably single O-type
stars seem poorly constrained because their luminosities are not in agreement
with the "standard" luminosities of stars with similar spectral types. The ages
of these O-type stars are all less than 7 Myrs. Therefore, the ages of these
stars agree with those, quoted in the literature, of the four associations,
except for CygOB8 for which the stars seem older than the association itself.
However, we point out that the distance of certain stars is debatable relative
to values found in the literature. The N content of these stars put in
perspective with N contents of several other galactic O-type stars seems to
draw the same five groups as found in the "Hunter" diagram for the O and B-type
stars in the LMC even though their locations are obviously different. We
determine mass-loss rates for several objects from the Halpha line and UV
spectra. Finally, we confirm the "mass discrepancy" especially for O stars with
masses smaller than 30 Msun. .Comment: 11 pages, and 26 pages of Appendix. A&A in pres
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