Starke Magnetfelder akkretierender Neutronensterne - Modellierung von Zyklotronlinien

Accreting neutron stars in X-ray binaries are unique astrophysical laboratories for studying the physics of matter under extreme conditions. Not only does their compact nature lead to an amount of gravity only topped by black hole systems; they can also possess extreme magnetic fields, exceeding the highest magnetic field which has ever been produced on Earth by a million times. These magnetic fields dominate the observed radiation characteristics, the most prominent being pulsed emission. The origin and structure of the magnetic fields, however, is still highly enigmatic. The only direct method currently known for probing the magnetic field of a neutron star is the study of cyclotron resonance scattering features. These features, first discovered in the spectrum of the binary system Hercules X-1, have been observed as absorption lines in the spectra of more than a dozen accreting X-ray pulsars. They form due to resonant scattering processes of high energy photons with quantized electrons in the accreted matter at the neutron star poles. Their line energies are approximately proportional to the surface magnetic field strength of the neutron star. Moreover, the analysis of their shapes is a powerful tool for assessing the fascinating but poorly understood physics of accretion. Today, with the access to data from satellites like BeppoSAX, RXTE, INTEGRAL and Suzaku, the diagnostic potential of cyclotron lines has grown anew: with these instruments the observed cyclotron line features have been energetically resolved in detail. On the other hand, explicit physical models to understand their complex observed shapes are lacking. Phenomenological models are used to obtain their characteristic parameters and to determine the magnetic field strength. The underlying physics, however, are extremeley difficult to assess with such an approach. In the scope of this work, cyclotron resonances scattering features are calculated for typical neutron star spectra using Monte Carlo simulations. The line profiles are inferred under the assumptionof physical parameters such as the magnetic field, the accretion geometry, the plasma temperature and optical depth , and the emergent angle of radiation. Based on these simulations, a new interpolation and convolution model is developed for modeling cyclotron lines in X-ray pulsar continua. This model is further implemented as a local model, named cyclomc, into the spectral fitting analysis package XSPEC to allow for a direct comparison with observational data. Results, obtained from fitting cyclotron lines for observations of the X-ray pulsars V0332+53, Cen X-3 and 4U 1907+09 with cyclomc allow for a first glimpse on the physics beyond a phenomenological analysis.Akkretierende Neutronensterne in Roentgendoppelsternsystemen sind einzigartige astrophysikalische Laboratorien fuer das Studium der Physik unter extremen Bedingungen. Nicht nur bedingt ihre Kompaktheit ein Mass an Gravitation das nur noch von Schwarzlochsystemen uebertroffen wird; sie koennen auch extrem starke Magnetfelder, millionenfach staerker als das staerkste bisher auf Erden erzeugte Magnetfeld, haben. Diese Magnetfelder bestimmen die beobachtbaren Strahlungscharakteristika, darunter das wohl auffaelligste die Emission von Strahlungspulsen. Ursprung und Struktur der Magnetfelder sind allerdings bis heute noch sehr raetselhaft. Die einzig derzeit bekannte Methode, das Magnetfeld eines Neutronensterns zu vermessen, basiert auf dem Studium von Zyklotronlinien. Diese Spektrallinien wurden erstmals fuer das Doppelsternsystem Hercules X-1 entdeckt. Seitdem sind Zyklotronlinien fuer mehr als ein Dutzend Roentgenpulsare beobachtet worden. Sie entstehen durch resonante Streuprozesse von hochenergetischen Photonen mit quantisierten Elektronen in der akkretierten Materie an den Polen des Neutronensterns. Die Linienenergien sind nahezu proportional zum Oberflaechenmagnetfeld des Neutronensterns. Die Untersuchung ihrer Profile bietet einen maechtigen Zugang zu der faszinierenden jedoch nur schlecht verstandenen Physik der Akkretion. Der Zugriff auf qualitativ hochwertige Daten von Satelliten wie BeppoSAX, RXTE, INTEGRAL und Suzaku, hat die diagnostische Bedeutung von Zyklotronlinien heutzutage einerseits gesteigert. Andererseits gibt es bisher kein konkretes physikalisches Modell, um ihre komplexen Profile im Detail zu erklaeren. Stattdessen werden die Linienparameter und die Magnetfeldstaerke mit phaenomenologischen Modellen bestimmt. Mit solchen Ansaetzen die zugrunde liegende Physik der Linienentstehung zu erschliessen ist extrem schwierig. Im Rahmen dieser Arbeit werden Zyklotronlinien mit Monte Carlo Simulationen berechnet. Die Linienprofile werden von Parametern wie dem Magnetfeld, der Akkretionsgeometrie, der Plasmatemperatur und optischen Tiefe, und des Austrittswinkels der Photonen abgeleitet. Darauf aufbauend wird ein neues Interpolations- und Faltungsmodell zur Modellierung von Zyklotronlinien in den Spektren von Roentgenpulsaren entwickelt. Dessen Implementierung als lokales Modell, genannt cyclomc, fuer die XSPEC Spektralanalyse-Software ermoeglicht einen direkten Vergleich mit Beobachtungsdaten. Fitresultate fuer die Beobachtungen dreier Roentgenpulsare, V0332+53, Cen X-3 und 4U 1907+09, mit cyclomc erlauben einen ersten Blick auf die grundlegende Physik ueber einen phaenomenologischen Ansatz hinaus

Be X-ray Binary Outburst Zoo II

We have continued our recently started systematic study of Be X-ray binary (BeXRB) outbursts. Specifically, we are developing a catalogue of outbursts including their basic properties based on nearly all available X-ray all-sky-monitors. These properties are derived by fitting asymmetric Gaussians to the outburst lightcurves. This model describes most of the outbursts covered by our preliminary catalogue well; only 13% of all datasets show more complex outburst shapes. Analyzing the basic properties, we reveal a strong correlation between the outburst length and the reached peak flux. As an example, we discuss possible models describing the observed correlation in EXO 2030+375

Pulse phase resolved analysis of the HMXB Cen X-3 over two binary orbits

We present a detailed analysis of observations of the high mass X-ray binary Cen X-3 spanning two consecutive binary orbits performed with the RXTE satellite in early March 1997. The PCA and HEXTE light curves both show a clear reduction in count rate after mid-orbit for both binary revolutions. We therefore analyze two broad band spectra for each orbit, before and after mid-orbit. Consistent with earlier observations these four joint PCA and HEXTE spectra can be well described using a phenomenological pulsar continuum model, including an iron emission line and a cyclotron resonance scattering feature. While no strong spectral variations were detected, the second half of orbit 2 shows a tendency toward being softer and more strongly absorbed. In order to follow the orbital phase-dependent evolution of the spectrum in greater detail, we model spectra for shorter exposures, confirming that most spectral parameters show either a gradual or sudden change for the second half of the second orbit. A comparison with a simple wind model indicates the existence of an accretion wake in this system. We also present and discuss high resolution pulse profiles for several different energy bands, as well as their hardness ratios. PCA and HEXTE spectra were created for 24 phase bins and fitted using the same model as in the phase averaged case. Systematic pulse phase-dependent variations of several continuum and cyclotron line parameters were detected, most notably a significant increase of the cyclotron line energy during the early rise of the main peak, followed by a gradual decrease. We show that applying a simple dipole model for the magnetic field is not sufficient to describe our data.Comment: 12 pages, 9 figures, accepted for publication in Ap

Domain-pattern transfer across an artificial magnetoelectric interface

Magnetoelectric interfaces provide efficient pathways for manipulating the magnetic order with lowpower-consuming electric fields. A prime example of this is the voltage-controlled domain-pattern transfer from a multiferroic BiFeO3 film into an amorphous Co0.9Fe0.1 ferromagnetic layer. Here we use a combination of optical second-harmonic generation and magnetic force microscopy to scrutinize the evolution of this transfer under voltage application. The nondestructive nature of our analysis permits the in operando examination of the magnetoelectric poling. We trigger ferroelectric switching events in BiFeO3 and track the evolution of their magnetoelectric transfer to the ferromagnetic domains of Co0.9Fe0.1 with spatial resolution. This reveals a wake-up effect in the magnetoelectric coupling across the interface. It is followed by simultaneous 90° rotations of the in-plane net polarization and net magnetization that progress with the increase of the voltage applied to the device.ISSN:2331-701

Luminosity dependent accretion state change in GRO J1008–57

In a former publication, we have analyzed the transient neutron star X-ray binary GRO J1008–57 using all available RXTE-, Swift-, and Suzaku-data. As we have found, the source’s spectral components, i.e., a power-law with high exponential cutoff and a black-body, are strongly correlated with the hard X-ray flux (15–50 keV). We update the analytical description of these dependence, including a change in the photon index behaviour from a flat to a logarithmic function. The flux, where the change occurs, is consistent with the onset of the black-body emission. Thus, a change of the accretion state always occurs in GRO J1008–57 at a particular flux level.We thank the Bundesministerium für Wirtschaft und Technologie for support via Deutsches Zentrum für Luft- und Raumfahrt grant 50OR1113 and the Deutscher Akademischer Austauschdienst

Two-Dimensional Structure of Disulfides and Thiols on Gold(111)

In order to find factors which determine the two-dimensional structure of self-assembled monolayers (SAMs), several classes of thiols and disulfides on gold (111) have been investigated by atomic force microscopy (AFM). SAMs were formed from a series of symmetrical and asymmetrical diethylalkanoate disulfides, -hydroxy- and -carboxyalkanethiols, diacetylene disulfides, and different anthracene terminated thiols and disulfides. In all the cases, two-dimensional crystalline structures could be resolved; even for an asymmetrical diethylalkanoate disulfide that had a chain length difference of five methylene units. The lattices were analyzed quantitatively. Two distinctly different types of crystalline structures were observed, namely, a hexagonal and a centered rectangular lattice. For the diethylalkanoate disulfides with short alkyl chains (n <- 10) both structural phases were observed, domains with a hexagonal lattice existing simultaneously with centered rectangular domains. The length of the alkyl chain determined the probability of finding disulfides in the hexagonal structure. This dependence on the shape of the molecules as well as the clear contrast of SAMs of asymmetric disulfides suggest that the AFM tip penetrates into the SAMs and probes, at least partially, the interior of the layers. With the atomic force microscope no difference was observed between SAMs formed from thiols and those from disulfides

Paracrine or virus-mediated induction of decorin expression by endothelial cells contributes to tube formation and prevention of apoptosis in collagen lattices

Resting endothelial cells express the small proteoglycan biglycan, whereas sprouting endothelial cells also synthesize decorin, a related proteoglycan. Here we show that decorin is expressed in endothelial cells in human granulomatous tissue. For in vitro investigations, the human endothelium-derived cell line, EA.hy 926, was cultured for 6 or more days in the presence of 1% fetal calf serum on top of or within floating collagen lattices which were also populated by a small number of rat fibroblasts. Endothelial cells aligned in cord-like structures and developed cavities that were surrounded by human decorin. About 14% and 20% of endothelial cells became apoptotic after 6 and 12 days of co-culture, respectively. In the absence of fibroblasts, however, the extent of apoptosis was about 60% after 12 days, and cord-like structures were not formed nor could decorin production be induced. This was also the case when lattices populated by EA.hy 926 cells were maintained under one of the following conditions: 1) 10% fetal calf serum; 2) fibroblast-conditioned media; 3) exogenous decorin; or 4) treatment with individual growth factors known to be involved in angiogenesis. The mechanism(s) by which fibroblasts induce an angiogenic phenotype in EA.hy 926 cells is (are) not known, but a causal relationship between decorin expression and endothelial cell phenotype was suggested by transducing human decorin cDNA into EA.hy 926 cells using a replication-deficient adenovirus. When the transduced cells were cultured in collagen lattices, there was no requirement of fibroblasts for the formation of capillary-like structures and apoptosis was reduced. Thus, decorin expression seems to be of special importance for the survival of EA.hy 926 cells as well as for cord and tube formation in this angiogenesis model