958 research outputs found
Formation of phase lags at the cyclotron energies in the pulse profiles of magnetized, accreting neutron stars
Context: Accretion-powered X-ray pulsars show highly energy-dependent and
complex pulse-profile morphologies. Significant deviations from the average
pulse profile can appear, in particular close to the cyclotron line energies.
These deviations can be described as energy-dependent phase lags, that is, as
energy-dependent shifts of main features in the pulse profile. Aims: Using a
numerical study we explore the effect of cyclotron resonant scattering on
observable, energy-resolved pulse profiles. Methods: We generated the
observable emission as a function of spin phase, using Monte Carlo simulations
for cyclotron resonant scattering and a numerical ray-tracing routine
accounting for general relativistic light-bending effects on the intrinsic
emission from the accretion columns. Results: We find strong changes in the
pulse profile coincident with the cyclotron line energies. Features in the
pulse profile vary strongly with respect to the average pulse profile with the
observing geometry and shift and smear out in energy additionally when assuming
a non-static plasma. Conclusions: We demonstrate how phase lags at the
cyclotron energies arise as a consequence of the effects of angular
redistribution of X-rays by cyclotron resonance scattering in a strong magnetic
field combined with relativistic effects. We also show that phase lags are
strongly dependent on the accretion geometry. These intrinsic effects will in
principle allow us to constrain a system's accretion geometry.Comment: 4 pages, 4 figures; updated reference lis
Cyclotron resonant scattering feature simulations. I. Thermally averaged cyclotron scattering cross sections, mean free photon-path tables, and electron momentum sampling
Electron cyclotron resonant scattering features (CRSFs) are observed as
absorption-like lines in the spectra of X-ray pulsars. A significant fraction
of the computing time for Monte Carlo simulations of these quantum mechanical
features is spent on the calculation of the mean free path for each individual
photon before scattering, since it involves a complex numerical integration
over the scattering cross section and the (thermal) velocity distribution of
the scattering electrons.
We aim to numerically calculate interpolation tables which can be used in
CRSF simulations to sample the mean free path of the scattering photon and the
momentum of the scattering electron. The tables also contain all the
information required for sampling the scattering electron's final spin.
The tables were calculated using an adaptive Simpson integration scheme. The
energy and angle grids were refined until a prescribed accuracy is reached. The
tables are used by our simulation code to produce artificial CRSF spectra. The
electron momenta sampled during these simulations were analyzed and justified
using theoretically determined boundaries.
We present a complete set of tables suited for mean free path calculations of
Monte Carlo simulations of the cyclotron scattering process for conditions
expected in typical X-ray pulsar accretion columns (0.01<B/B_{crit}<=0.12,
where B_{crit}=4.413x10^{13} G and 3keV<=kT<15keV). The sampling of the tables
is chosen such that the results have an estimated relative error of at most
1/15 for all points in the grid. The tables are available online at
http://www.sternwarte.uni-erlangen.de/research/cyclo.Comment: A&A, in pres
Character Displacement in the Midst of Substantial Background Evolution in Anolis Lizard Island Populations
Negative interactions between species can generate divergent selection that causes character displacement. However, other processes cause similar divergence. We use spatial and temporal replication across island populations of Anolis lizards to assess the importance of negative interactions in driving trait shifts. Previous work showed that the establishment of Anolis sagrei on islands drove resident Anolis carolinensis to perch higher and evolve larger toepads. To further test the interaction\u27s causality and predictability, we resurveyed a subset of islands nine years later. Anolis sagrei had established on one island between surveys. We found that A. carolinensis on this island now perch higher and have larger toepads. However, toepad morphology change on this island was not distinct from shifts on six other islands whose Anolis community composition had not changed. Thus, the presence of A. sagrei only partly explains A. carolinensis trait variation across space and time. We also found that A. carolinensis on islands with previously established A. sagrei now perch higher than a decade ago, and that current A. carolinensis perch height is correlated with A. sagrei density. Our results suggest that character displacement likely interacts with other evolutionary processes in this system, and that temporal data are key to detecting such interactions
ESA Venus Entry Probe Study
The Venus Entry Probe is one of ESA s Technology Reference Studies (TRS). The purpose of the Technology Reference Studies is to provide a focus for the development of strategically important technologies that are of likely relevance for future scientific missions. The aim of the Venus Entry Probe TRS is to study approaches for low cost in-situ exploration of Venus and other planetary bodies with a significant atmosphere. In this paper, the mission objectives and an outline of the mission concept of the Venus Entry Probe TRS are presented
A diagrammatic view of the equals sign: arithmetical equivalence as a means, not an end
It is recommended in the mathematics education literature that pupils be presented with equality statements that can be assessed for numerical balance by attending to notational structure rather than computation. I describe an alternative, diagrammatic approach in which pupils do not assess statements but instead use them to make substitutions of notation. I report on two trials of a computer-based task conducted with pairs of pupils and highlight two findings. First, the pupils found it useful to articulate the distinct substitutive effects of commutative (âswapâ, âswitchâ) and partitional (âsplitâ, âseparateâ) statements when working on the task. Secondly, the pupils did not notice that some of the statements presented were in fact false, which suggests their substituting activities were independent of numerical equivalence conceptions. This demonstrates that making substitutions offers task designers a mathematical utility for equality statements that is distinct from, but complementary to, assessing numerical balance
Evaluation of antigens for the serodiagnosis of kala-azar and oriental sores by means of the indirect immunofluorescence antibody test (IFAT)
Antigens and corresponding sera were collected from travellers with leishmaniasis returning to Germany from different endemic areas of the old world. The antigenicity of these Leishmania strains, which were maintained in Syrian hamsters, was compared by indirect immunofluorescence (IFAT). Antigenicity was demonstrated by antibody titres in 18 sera from 11 patients. The amastigotic stages of nine strains of Leishmania donovani and four strains of Leishmania tropica were compared with each other and with the culture forms of insect flagellates (Strigomonas oncopelti and Leptomonas ctenocephali). Eighteen sera from 11 patients were available for antibody determination with these antigens. The maximal antibody titres in a single serum varied considerably depending on which antigen was used for the test. High antibody levels could only be maintained when Leishmania donovani was employed as the antigen, but considerable differences also occurred between the different strains of this species. The other antigens were weaker. No differences in antigenicity between amastigotes and promastigotes of the same strain were observed. It is important to select suitable antigens. Low titres may be of doubtful specificity and are a poor baseline for the fall in titre which is an essential index of effective treatment.Wir sammelten Parasiten und Seren von Reisenden, die aus verschiedenen endemischen Gebieten der Alten Welt mit einer Leishmaniasis nach Deutschland zurĂŒckkehrten. Die AntigenaktivitĂ€ten der isolierten und fortlaufend in Goldhamstern gehaltenenLeishmania-StĂ€mme wurden im indirekten Immunofluoreszenztest (IFAT) verglichen. Die AntigenitĂ€t wurde an Hand von Antikörpertitern in 18 Serumproben von 11 Patienten bewiesen. Neun StĂ€mme desLeishmania donovani-Komplexes und vierLeishmania tropica-Isolate wurden in ihrem amastigoten Stadium miteinander verglichen. Hinzu kamen zwei Insekten-Flagellaten als Kulturformen:Strigomonas oncopelti undLeptomonas ctenocephali. 18 Serumproben von 11 Patienten standen fĂŒr die Antikörperbestimmung mit diesen Antigenen zur VerfĂŒgung. Die maximalen Titerhöhen variierten in ein- und derselben antiserumprobe zum Teil erheblich, je nachdem, welches Antigen fĂŒr den Test benutzt wurde. Hohe Antikörpertiter konnten nur erhalten werden, wennLeishmania donovani als Antigen vorlag, es ergaben sich aber auch zwischen den einzelnen StĂ€mmen dieser Leishmaniaart erhebliche Unterschiede in der AntigenaktivitĂ€t. Antigene anderer Art erwiesen sich als wenig wirksam. Zwischen amastigoten und promastigoten Entwicklungsformen einesLeishmania donovani-Stammes konnten keine Unterschiede in der AntigenaktivitĂ€t erkannt werden. FĂŒr den Nachweis möglichst hoher Antikörpertiter im IFAT ist die Auswahl geeigneter Antigene von ausschlaggebender Bedeutung. Niedrige Titer erschweren deren Beurteilung als spezifisch und sind eine schlechte Ausgangsposition fĂŒr die Beobachtung des obligatorischen Titerabfalles nach erfolgreicher Therapie
X-ray emission from magnetized neutron star atmospheres at low mass accretion rates. I. Phase-averaged spectrum
Recent observations of X-ray pulsars at low luminosities allow, for the first
time, to compare theoretical models for the emission from highly magnetized
neutron star atmospheres at low mass accretion rates ( g s) with the broadband X-ray data. The purpose of this paper
is to investigate the spectral formation in the neutron star atmosphere at low
and to conduct a parameter study of physical properties of the
emitting region. We obtain the structure of the static atmosphere, assuming
that Coulomb collisions are the dominant deceleration process. The upper part
of the atmosphere is strongly heated by the braking plasma, reaching
temperatures of 30-40 keV, while its denser isothermal interior is much cooler
(~2 keV). We numerically solve the polarized radiative transfer in the
atmosphere with magnetic Compton scattering, free-free processes, and
non-thermal cyclotron emission due to possible collisional excitations of
electrons. The strongly polarized emitted spectrum has a double-hump shape that
is observed in low-luminosity X-ray pulsars. A low-energy "thermal" component
is dominated by extraordinary photons that can leave the atmosphere from deeper
layers due to their long mean free path at soft energies. We find that a
high-energy component is formed due to resonant Comptonization in the heated
non-isothermal part of the atmosphere even in the absence of collisional
excitations. The latter, however, affect the ratio of the two components. A
strong cyclotron line originates from the optically thin, uppermost zone. A fit
of the model to NuSTAR and Swift/XRT observations of GX 304-1 provides an
accurate description of the data with reasonable parameters. The model can thus
reproduce the characteristic double-hump spectrum observed in low-luminosity
X-ray pulsars and provides insights into spectral formation.Comment: 18 pages, 10 figures, A&A accepte
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