Twin-peak quasiperiodic oscillations as an internal resonance

Two inter-related peaks occur in high-frequency power spectra of X-ray lightcurves of several black-hole candidates. We further explore the idea that a non-linear resonance mechanism, operating in strong-gravity regime, is responsible for these quasi-periodic oscillations (QPOs). By extending the multiple-scales analysis of Rebusco, we construct two-dimensional phase-space sections, which enable us to identify different topologies governing the system and to follow evolutionary tracks of the twin peaks. This suggests that the original (Abramowicz and Kluzniak) parametric-resonance scheme can be viewed as an ingenuous account of the QPOs model with an internal resonance. We show an example of internal resonance in a system with up to two critical points, and we describe a general technique that permits to treat other cases in a systematical manner. A separatrix divides the phase-space sections into regions of different topology: inside the libration region the evolutionary tracks bring the observed twin-peak frequencies to an exact rational ratio, whereas in the circulation region the observed frequencies remain off resonance. Our scheme predicts the power should cyclically be exchanged between the two oscillations. Likewise the high-frequency QPOs in neutron-star binaries, also in black-hole sources one expects, as a general property of the non-linear model, that slight detuning pushes the twin-peak frequencies out of sharp resonance.Comment: Accepted for publication in A&A; 11 pages, 6 figure

Diffusion of Mn interstitials in (Ga,Mn)As epitaxial layers

Magnetic properties of thin (Ga,Mn)As layers improve during annealing by out-diffusion of interstitial Mn ions to a free surface. Out-diffused Mn atoms participate in the growth of a Mn-rich surface layer and a saturation of this layer causes an inhibition of the out-diffusion. We combine high-resolution x-ray diffraction with x-ray absorption spectroscopy and a numerical solution of the diffusion problem for the study of the out-diffusion of Mn interstitials during a sequence of annealing steps. Our data demonstrate that the out-diffusion of the interstitials is substantially affected by the internal electric field caused by an inhomogeneous distribution of charges in the (Ga,Mn)As layer.Comment: 11 pages, 5 figure

Instrumentation for study of nanomaterials in NPI REZ (New laboratory for material study in Nuclear Physics Institute in REZ)

Nano-sized materials become irreplaceable component of a number of devices for every aspect of human life. The development of new materials and deepening of the current knowledge require a set of specialized techniques-deposition methods for preparation/modification of the materials and analytical tools for proper understanding of their properties. A thoroughly equipped research centers become the requirement for the advance and development not only in nano-sized field. The Center of Accelerators and Nuclear Analytical Methods (CANAM) in the Nuclear Physics Institute (NPI) comprises a unique set of techniques for the synthesis or modification of nanostructured materials and systems, and their characterization using ion beam, neutron beam and microscopy imaging techniques. The methods are used for investigation of a broad range of nano-sized materials and structures based on metal oxides, nitrides, carbides, carbon-based materials (polymers, fullerenes, graphenes, etc.) and nano-laminate composites (MAX phases). These materials can be prepared at NPI using ion beam sputtering, physical vapor deposition and molecular beam epitaxy. Based on the deposition method and parameters, the samples can be tuned to possess specific properties, e.g., composition, thickness (nm-μm), surface roughness, optical and electrical properties, etc. Various nuclear analytical methods are applied for the sample characterization. RBS, RBS-channeling, PIXE, PIGE, micro-beam analyses and Transmission Spectroscopy are accomplished at the Tandetron 4130MC accelerator, and additionally the Neutron Depth Profiling (NDP) and Prompt Gamma Neutron Activation (PGNA) analyses are performed at an external neutron beam from the LVR-15 research reactor. The multimode AFM facility provides further surface related information, magnetic/electrical properties with nano-metric precision, nano-indentation, etc

Production and characterization of micro-size pores for ion track etching applications

For many years the applications of ion track etch materials have increased considerably, like charged particles detection, molecular identification with nanopores, ion track filters, magnetic studies with nanowires and so on. Over the materials generally used as track detector, the Poly-Allyl-Diglycol Carbonate (PADC), offers many advantages, like its nearly 100 % detection efficiency for charged particle, a high resistance to harsh environment, the lowest detection threshold, a high abrasion resistance and a low production costs. All of these properties have made it particularly attractive material, even if due to its brittleness, obtaining a thin film (less than 500 μm) is still a challenge. In this work, PADC foils have been exposed to a-particles emitted by a thin radioactive source of 241Am and to C ions from the Tandetron 4130 MC accelerator. The latent tracks generated in the polymer have been developed using a standard etching procedure in 6.25 NaOH solution. The dependence of the ion tracks' geometry on the ion beam energy and fluence has been evaluated combining the information obtained through a semiautomatic computer script that selects the etched ion tracks according to their diameter and mean grey value and nanometric resolution images by atomic force microscopy

Biochar and biochar with N-fertilizer affect soil N2O emission in Haplic Luvisol

The benefits of biochar application are well described in tropical soils, however there is a dearth of information on its effects in agricultural temperate soils. An interesting and little explored interaction may occur in an intensive agriculture setting; biochar addition may modify the effect of commonplace N-fertilization.We conducted a field experiment to study the effects of biochar application at the rate of 0, 10 and 20 t ha−1 (B0, B10 and B20) in combination with 0, 40 and 80 kg N ha−1 of N-fertilizer (N0, N40, N80).We followed nitrous oxide (N2O) emissions, analysed a series of soil physicochemical properties and measured barley yield in a Haplic Luvisol in Central Europe. Seasonal cumulative N2O emissions from B10N0 and B20N0 treatments decreased by 27 and 25% respectively, when compared to B0N0. Cumulative N2O emissions from N40 and N80 combined with B10 and B20 were also lower by 21, 19 and 25, 32%, respectively compared to controls B0N40 and B0N80. Average pH was significantly increased by biochar addition. Increased soil pH and reduces NO−3 content seen in biochar treatments could be the two possible mechanisms responsible for reduced N2O emissions. There was a statistically significant increase of soil water content in B20N0 treatment compared to B0N0 control, possibly as a result of larger surface area and the presence of microspores having altered pore size distribution and water-holding capacity of the soil. Application of biochar at the rate of 10 t ha−1 had a positive effect on spring barley grain yield

The model constraints from the observed trends for the quasi-periodic oscillation in RE J1034+396

We analyze the time variability of the X-ray emission of RE J1034+396 -- an active galactic nucleus with the first firm detection of a quasi-periodic oscillations (QPO). Based on the results of a wavelet analysis, we find a drift in the QPO central frequency. The change in the QPO frequency correlates with the change in the X-ray flux with a short time delay. The data specifically suggest a linear dependence between the QPO period and the flux, and this gives important constraints on the QPO models. In particular, it excludes explanation in terms of the orbiting hot spot model close to a black hole. Linear structures such as shocks, spiral waves, or very distant flares are favored.Comment: Astronomy & Astrophysics, in pres

Near-infrared polarimetry setting constraints on the orbiting spot model for Sgr A* flares

Context: Recent near-infrared polarization measurements of SgrA* show that its emission is significantly polarized during flares and consists of a non- or weakly polarized main flare with highly polarized sub-flares. The flare activity suggests a quasi-periodicity of ~20 minutes in agreement with previous observations. Aims: By simultaneous fitting of the lightcurve fluctuations and the time-variable polarization angle, we address the question of whether these changes are consistent with a simple hot spot/ring model, in which the interplay of relativistic effects plays the major role, or whether some more complex dependency of the intrinsic emissivity is required. Methods: We discuss the significance of the 20min peak in the periodogram of a flare from 2003. We consider all general relativistic effects that imprint on the polarization degree and angle and fit the recent polarimetric data, assuming that the synchrotron mechanism is responsible for the intrinsic polarization and considering two different magnetic field configurations. Results: Within the quality of the available data, we think that the model of a single spot in addition to an underlying ring is favoured. In this model the broad near-infrared flares of Sgr A* are due to a sound wave that travels around the MBH once while the sub-flares, superimposed on the broad flare, are due to transiently heated and accelerated electrons which can be modeled as a plasma blob. Within this model it turns out that a strong statement about the spin parameter is difficult to achieve, while the inclination can be constrained to values > 35 deg on a 3sigma level.Comment: accepted by A&A for publicatio

Investigation of magneto-structural phase transition in FeRh by reflectivity and transmittance measurements in visible and near-infrared spectral region

Magneto-structural phase transition in FeRh epitaxial layers was studied optically. It is shown that the transition between the low-temperature antiferromagnetic phase and the high-temperature ferromagnetic phase is accompanied by a rather large change of the optical response in the visible and near-infrared spectral ranges. This change is consistent with ab initio calculations of reflectivity and transmittance. Phase transition temperatures in a series of FeRh films with thicknesses ranging from 6 to 100 nm is measured thereby demonstrating the utility of the method to quickly characterise samples. Spatially resolved imaging of their magnetic properties with a micrometer resolution shows that the phase transition occurs at different temperatures in different parts of the sample