Variability Profiles of Millisecond X-Ray Pulsars: Results of Pseudo-Newtonian 3D MHD Simulations

We model the variability profiles of millisecond period X-ray pulsars. We performed three-dimensional magnetohydrodynamic simulations of disk accretion to millisecond period neutron stars with a misaligned magnetic dipole moment, using the pseudo-Newtonian Paczynski-Wiita potential to model general relativistic effects. We found that the shapes of the resulting funnel streams of accreting matter and the hot spots on the surface of the star are quite similar to those for more slowly rotating stars obtained from earlier simulations using the Newtonian potential. The funnel streams and hot spots rotate approximately with the same angular velocity as the star. The spots are bow-shaped (bar-shaped) for small (large) misalignment angles. We found that the matter falling on the star has a higher Mach number when we use the Paczynski-Wiita potential than in the Newtonian case. Having obtained the surface distribution of the emitted flux, we calculated the variability curves of the star, taking into account general relativistic, Doppler and light-travel-time effects. We found that general relativistic effects decrease the pulse fraction (flatten the light curve), while Doppler and light-travel-time effects increase it and distort the light curve. We also found that the light curves from our hot spots are reproduced reasonably well by spots with a gaussian flux distribution centered at the magnetic poles. We also calculated the observed image of the star in a few cases, and saw that for certain orientations, both the antipodal hot spots are simultaneously visible, as noted by earlier authors.Comment: 9 pages, 10 figures, accepted for publication in ApJ; corrected some typo

Three-dimensional Simulations of Accretion to Stars with Complex Magnetic Fields

Disk accretion to rotating stars with complex magnetic fields is investigated using full three-dimensional magnetohydrodynamic (MHD) simulations. The studied magnetic configurations include superpositions of misaligned dipole and quadrupole fields and off-centre dipoles. The simulations show that when the quadrupole component is comparable to the dipole component, the magnetic field has a complex structure with three major magnetic poles on the surface of the star and three sets of loops of field lines connecting them. A significant amount of matter flows to the quadrupole "belt", forming a ring-like hot spot on the star. If the maximum strength of the magnetic field on the star is fixed, then we observe that the mass accretion rate, the torque on the star, and the area covered by hot spots are several times smaller in the quadrupole-dominant cases than in the pure dipole cases. The influence of the quadrupole component on the shape of the hot spots becomes noticeable when the ratio of the quadrupole and dipole field strengths $B_q/B_d\gtrsim0.5$, and becomes dominant when $B_q/B_d\gtrsim1$. In the case of an off-centre dipole field, most of the matter flows through a one-armed accretion stream, forming a large hot spot on the surface, with a second much smaller secondary spot. The light curves may have simple, sinusoidal shapes, thus mimicking stars with pure dipole fields. Or, they may be complex and unusual. In some cases the light curves may be indicators of a complex field, in particular if the inclination angle is known independently. We also note that in the case of complex fields, magnetospheric gaps are often not empty, and this may be important for the survival of close-in exosolar planets.Comment: 13 pages, 21 figures, accepted for publication in MNRA

Pattern of changes in morpho-densitometric parameters of magnocellular neurons of the hypothalamic paraventricular nucleus under restraint stress of different duration

The aim of the work was to define the nature of changes in morpho-densitometric parameters of magnocellular neurons of the hypothalamic paraventricular nucleus in rats at 6, 15 and 21 weeks of restricted living space. Materials and methods. A total of 55 male Wistar rats aged 6-10 months were used and divided into 4 groups (1 – intact control, 10 rats; 2, 3 and 4 – experimental groups of 15 rats each, which were in conditions of restricted environmental space for 6, 15 and 21 weeks, respectively). Morpho-densitometric analysis of 5-μm-thick rat brain sections was performed after preliminary standard histological processing and 48 hours of halocyanine-chrome staining by Einarson using an AxioImager-M2 microscope (Carl Zeiss, Germany). Images of magnocellular neurons of the hypothalamic paraventricular nuclei (PVN), which were topographically identified according to the stereotaxic atlas of the rat brain, were captured using a highly sensitive AxioCam-ERc 5s video camera (Carl Zeiss, Germany) and recorded as a computer file using AxioVision 40 V 4.8.2.0 software (license number 3005339). Quantitative data on the area of neurons, their nuclei and nucleoli, the content of heterogeneous DNA and RNA in the cell cytoplasm, nuclei and nucleoli were obtained in a semi-automatic mode using the open source software ImageJ (National Institutes of Health, USA). Results. Chronic stress condition modelled in rats for 6, 15, and 21 weeks (limited living space, inactivity, and complicated communications) resulted in a decrease in the area of cytoplasm and nuclei of magnocellular neurons of the hypothalamic PVN. The decrease was directly proportional to restrain stress duration: cytoplasm and nuclei by 18 % and 9 % at week 6 and by 40 % and 25 % at week 21, respectively, and only cytoplasm by 23 % at 15 weeks. On the contrary, the area of neuronal nuclei was increased by 31 % at 6 weeks of stress and 33 % at 15 weeks, and by 15 % at 21 weeks of stress compared to the control. The content of nucleic acids in the cytoplasm of magnocellular neurons of the hypothalamic PVN in the group of rats with 6-week restriction was 39 % less than in the control group, 34 % less than that in the group with 15-week restriction, and 42 % less than that in the group of 21-week restrain stress. In the nuclei of cells after 6 and 15 weeks of restriction, the content was decreased approximately equally by 37 % and 35 %, respectively, and long-term restriction (21 weeks) contributed to a 41 % decrease. In the neuron nucleoli, the content of nucleic acids was changed similarly: in the group of 6 weeks, a decrease of 40 % was characteristic, after 15 weeks – by 39 %, and after 21 weeks – by 43 %. Conclusions. Changes in the morphometric parameters of magnocellular neurons of the hypothalamic PVN in rats with long-term limitation of living space are characterized by time dependence and multidirectionality, which is related to the functional features of the neuron structural components (cytoplasm, nucleus and nucleoli). There is a progressive decrease in the cytoplasmic area of PVN neurons and their nuclei, while the nucleolar area increases with increasing length of living space limitation. The content of nucleic acids in the studied structures of the hypothalamic PVN magnocellular neurons after prolonged stress at each time point (6, 15, 21 weeks) becomes less than the control values by more than a third. Comparison of nucleic acid content in the neuron cytoplasm, nuclei, and nucleoli shows a narrow range of variations in the parameters of the experimental group rats between different periods

Pathogenetic characteristics of the hormonal profile in rats subjected to restraint stress of different duration

The aim of the work is to ascertain the character of changes in the hormonal profile (concentrations of corticosterone, insulin, and adrenocorticotropic hormone) in conjunction with metabolic alterations and elevated blood pressure, which are induced in rats by restraint stress at 6th, 15th and 21st weeks of environmental space restriction. Materials and methods. A total of 55 normotensive male Wistar rats, aged 6–10 months, were assigned into 4 experimental groups (1 – intact control (10 rats); 2, 3 and 4 (15 rats each exposed to restricted space allowance)). All the animals were subjected to blood pressure (BP), blood glucose level and body weight measurements twice: at the stage of forming groups and at the 6th, 15th and 21st weeks. Plasma hormone concentrations (insulin, corticosterone, and ACTH) were examined by the immunoenzymatic method using commercial kits (Monobind, USA). Results. The body weight of the animals was significantly reduced by 20.72 % after 6 weeks of space allowance restriction, it was restored to baselines by the 15th week and exceeded control values by 26.1 % at the 21st week. BP levels showed an increasing trend, a dynamic increase in systolic pressure by 7 %, 17 % and 26 % was detected, respectively, as well as diastolic from the 15th week to the 21st week by 21.4 % and 37.0 %, respectively. Glucose concentration was within the euglycemic range. Changes in the hormonal profile showed an increase in the concentration of ACTH by more than 50 % and a decrease in insulin – by 34 % at the 6th week with a subsequent twofold decrease in the insulin concentration (at week 15) and a further more than twofold increase in ACTH at the 21st week. As for changes in the concentration of corticosterone, a peak increase of 3.77 times was noticed at the 15th week, followed by a decrease and restoration to the normative values by the 21st week. Conclusions. Even minor and unremarkable continuously acting stressors, which cannot be coped, become important triggers for hormonal profile and carbohydrate metabolism alterations as well as for a persistent increase in blood pressure, which manifest first by hypoinsulinemia, an increase in the level of ACTH, and a constant concentration of corticosterone. Long-term stress exposure contributes to a transient “peak” increase in the corticosterone level, a significant increase in insulin and a sustained increase in ACTH. Multidirectional changes in the levels of the studied hormones occur amidst a gradual increase in blood pressure and a stable increase in the level of glycemia

Long-term restraint stress as a predictor of insulin resistance development in experimental rats

The aim of the work was to define the pattern of changes and pathogenetic mechanisms of glycemic curve shape during a glucose tolerance test in rats at 6, 15, and 21 weeks of environmental space restriction. Materials and methods. A total of 55 male Wistar rats aged 6–10 months were used and divided into 4 groups (1 – intact control, 10 rats; 2, 3 and 4 of 15 rats each, which were in conditions of restricted environmental space for 6, 15 and 21 weeks, respectively). Fasting plasma levels of glucose (test strips Test Strip II on a Glucocard glucometer (ARKRAY Inc., Japan)), insulin and corticosterone (by an immunoenzymatic method using commercial kits for rats from Monobind, USA) were measured, glucose tolerance tests (GTT) were performed, the HOMA index and the insulin/corticosterone ratio were calculated. Results. The 6-week environmental space restriction, immobility and complex communication relations resulted in a hypoinsulinemic-hypoglycemic type of the glycemic curve with high tissue glucose sensitivity in animals which were in the conditions described (the HOMA index was significantly decreased to 0.21 versus 0.55 in the control), the characteristic delayed hyperglycemic peak at the 30th min of the test slowly returned to the euglycemic level by only the 90th min against the 30th min in the controls. The 15-week restriction caused a hypoinsulinemic curve with preserved tissue glucose sensitivity (the HOMA index 0.33 versus 0.55 in the controls). Alterations in postprandial glucose absorption were manifested by the highest glucose concentration at the 15th min of the test, a very slow decrease in the glucose level in the post-absorption period not being restored to baseline values by the 90th min. The 21-week restriction shaped a diabetic type of the glycemic curve, impaired tissue glucose sensitivity (a substantial and significant increase in the calculated HOMA index from 0.55 in the controls to 0.95) and resulted in the development of insulin resistance. Conclusions. The environmental space restriction, immobility and complex communication relations gradually induce a range of disorders in animals and change the glycemic response shape to glucose load, impair prandial insulin secretion, augment hyperglycemic effects of counterinsular hormones (corticosterone), and realize dysfunctional manifestations of prandial/postprandial glycemia. 6 weeks of restrictions and challenges shape the hypoinsulinemic-hypoglycemic type of the glycemic curve with high tissue glucose sensitivity, while 15 weeks result in the hypoinsulinemic curve with preserved tissue glucose sensitivity. 21 weeks show the diabetic type of the glycemic curve with impaired tissue glucose sensitivity and the development of insulin resistance

Thin accretion disc with a corona in a central magnetic field

We study the steady-state structure of an accretion disc with a corona surrounding a central, rotating, magnetized star. We assume that the magneto-rotational instability is the dominant mechanism of angular momentum transport inside the disc and is responsible for producing magnetic tubes above the disc. In our model, a fraction of the dissipated energy inside the disc is transported to the corona via these magnetic tubes. This energy exchange from the disc to the corona which depends on the disc physical properties is modified because of the magnetic interaction between the stellar magnetic field and the accretion disc. According to our fully analytical solutions for such a system, the existence of a corona not only increases the surface density but reduces the temperature of the accretion disc. Also, the presence of a corona enhances the ratio of gas pressure to the total pressure. Our solutions show that when the strength of the magnetic field of the central neutron star is large or the star is rotating fast enough, profiles of the physical variables of the disc significantly modify due to the existence of a corona.Comment: Accepted for publication in Astrophysics & Space Scienc

Magneto-centrifugally driven winds: comparison of MHD simulations with theory

Stationary magnetohydrodynamic (MHD) outflows from a rotating, conducting Keplerian accretion disk threaded by B-field are investigated numerically by time-dependent, axisymmetric (2.5D) simulations using a Godunov-type code. A large class of stationary magneto-centrifugally driven winds are found where matter is accelerated from a thermal speed at the disk to much larger velocity, greater than the fast magnetosonic speed and larger than the escape speed. The flows are approximately spherical outflows with only small collimation within the simulation region. Numerical results are shown to coincide with the theoretical predictions of ideal, axisymmetric MHD to high accuracy. Investigation of the influence of outer boundary conditions, particularly that on the toroidal component of magnetic field shows that the commonly used ``free'' boundary condition leads to artificial magnetic forces which can act to give spurious collimation. New boundary conditions are proposed which do not generate artificial forces. Artificial results may also arise for cases where the Mach cones on the outer boundaries are partially directed into the simulation region.Comment: 19 pages, 18 figures, emulapj.sty is use

Magnetic Field Limitations on Advection Dominated Flows

Recent papers discussing advection dominated accretion flows (ADAF) as a solution for astrophysical accretion problems should be treated with some caution because of their uncertain physical basis. The suggestions underlying ADAF involve ignoring the magnetic field reconnection in heating of the plasma flow, assuming electron heating due only to binary Coulomb collisions with ions. Here, we analyze the physical processes in optically thin accretion flows at low accretion rates including the influence of an equipartition turbulent magnetic field. For these conditions there is continuous destruction of magnetic flux by reconnection. The reconnection is expected to significantly heat the electrons which can efficiently emit magnetobremstrahlung radiation. Because of this electron emission, the radiative efficiency of the ADAF is not small. We suggest that the small luminosities of nearby galactic black holes is due to outflows rather than ADAF accretion.Comment: 7 pages, 3 figures, Submitted to Ap

ЭФФЕКТИВНЫЙ АЛГОРИТМ ДИАГНОСТИКИ – КЛЮЧ К ОЦЕНКЕ РАСПРОСТРАНЕННОСТИ ОПУХОЛИ ПЕЧЕНИ И ЭФФЕКТА ЛЕЧЕНИЯ

Представлен анализ современных подходов к диагностике гепатоцеллюлярной карциномы (ГЦК). Подробно описаны возможности различных методов исследования (УЗИ, РКТ и МРТ) в выявлении ГЦК малых размеров (≤ 2см). Подчеркивается исключительно важная роль динамического внутривенного контрастирования, в частности, применения гепатоспецифических МР-контрастных препаратов. Подробно описаны характерные черты отображения типичных гиперваскулярных вариантов ГЦК, что позволяет уверенно диагностировать эту опухоль в большинстве случаев без морфологической верификации. Комбинация семиотических МР-признаков (гипоинтенсивность в Т 1, гиперинтенсивность при ДВ-МРТ, диффузное гетерогенное контрастирование всего объема опухоли в артериальную фазу и гипоинтенсивность отображения опухоли в гепатоспецифическую фазу) позволяет увеличить чувствительность, специфичность и точность метода МРТ до 100%, 91,7% и 97,1% соответственно

Accretion, Outflows, and Winds of Magnetized Stars

Many types of stars have strong magnetic fields that can dynamically influence the flow of circumstellar matter. In stars with accretion disks, the stellar magnetic field can truncate the inner disk and determine the paths that matter can take to flow onto the star. These paths are different in stars with different magnetospheres and periods of rotation. External field lines of the magnetosphere may inflate and produce favorable conditions for outflows from the disk-magnetosphere boundary. Outflows can be particularly strong in the propeller regime, wherein a star rotates more rapidly than the inner disk. Outflows may also form at the disk-magnetosphere boundary of slowly rotating stars, if the magnetosphere is compressed by the accreting matter. In isolated, strongly magnetized stars, the magnetic field can influence formation and/or propagation of stellar wind outflows. Winds from low-mass, solar-type stars may be either thermally or magnetically driven, while winds from massive, luminous O and B type stars are radiatively driven. In all of these cases, the magnetic field influences matter flow from the stars and determines many observational properties. In this chapter we review recent studies of accretion, outflows, and winds of magnetized stars with a focus on three main topics: (1) accretion onto magnetized stars; (2) outflows from the disk-magnetosphere boundary; and (3) winds from isolated massive magnetized stars. We show results obtained from global magnetohydrodynamic simulations and, in a number of cases compare global simulations with observations.Comment: 60 pages, 44 figure