Astrophysics最新文献

Shocks are ubiquitous in astrophysical environments, and particle acceleration at such astrophysical shocks is related to high-energy phenomena. In particular, the acceleration mechanism and the time evolution of the particle distribution function have been extensively examined. This paper describes a simple analytic method using the one-dimensional Fokker-Planck equation in the test-particle regime. We aim to investigate the evolution of the particle distribution function in the shock upstream, which could be streaming toward Earth along the open magnetic field geometry. The behavior of the analytical solution is examined over a wide range of parameters representing shock structure, such as the shock Mach number, plasma beta, injection fraction into diffusive shock acceleration, and the scale of the upstream magnetic field. The behavior is associated with upstream turbulence for diffusive shock acceleration, as expected. Additionally, pre-accelerated particles could affect the time evolution of the particle distribution only when the radiative or advection losses are small enough for the pre-accelerated distribution to have a flatter power-law slope than the power-law slope based on shock acceleration theory. We also provide a formula for a spherically expanding shock and its relevant application to calculate high-energy emission due to hadronic interactions. We suggest that the simple analytic method could be applied to examine astrophysical shocks with a wide range of plasma parameters.

We investigate vacuum expectation value of the energy-momentum tensor for a massive Dirac field in flat spacetime with a toroidal subspace of a general dimension. Quasiperiodicity conditions with arbitrary phases are imposed on the field operator along compact dimensions. These phases are interpreted in terms of magnetic fluxes enclosed by compact dimensions. The equation of state in the uncompact subspace is of the cosmological constant type. It is shown that, in addition to the diagonal components, the vacuum energy-momentum tensor has nonzero off-diagonal components. In special cases of twisted (antiperiodic) and untwisted (periodic) fields the off diagonal components vanish. For untwisted fields the vacuum energy density is positive and the energy-momentum tensor obeys the strong energy condition. For general values of the phases in the periodicity conditions the energy density and stresses can be either positive or negative. The numerical results are given for a Kaluza-Klein type model with two extra dimensions.

In this paper, we have used the Plane Symmetric LRS Bianchi type I metric to study bulk viscous fluid coupled to a string of clouds within the framework of the f (R) theory of gravity. To obtain the deterministic solutions, some physically plausible conditions like the weak field limit for a point-like source f (R) = R ^3/2 , the very well known expansion-shear scalar proportionality relation and the special form of an average scale factor are taken into account. Furthermore, we have calculated some physical and kinematical parameters along with the energy conditions to study the astrophysical implications of the constructed model and discussed their graphical behaviour, which shows good similarity with recent observational data.

In this paper, we have revisited the Berman’s idea of the variation of Hubble parameter. While previously explored in the context of Λ-varying cosmologies, where scale factor variations yield linear universe expansion, this parametrization has undergone extensive scrutiny. Our investigation, however, explores into its implications in the context of late-time cosmic acceleration, within the framework of classical general relativity, adopting the Friedmann-Lemaître-Robertson-Walker (FLRW) spacetime as our background metric. Our analysis offers a precise solution to Einstein’s field equations (EFEs) in a model-independent way, affording a thorough assessment of both geometrical and physical model parameters. Additionally, this study supplements its findings with graphical representations of the evolving cosmological parameters across flat, closed, and open universe scenarios, all subject to constraints derived from the model parameters. In synthesizing these results, we shed light on the intricate interplay between cosmic acceleration, dark energy, and the parametrization of the Hubble parameter, thereby providing valuable insights into the fundamental mechanics of our universe.

Results are presented from an analysis of manifestations of the activity of the spectral class K2V dwarf HD189733, which has one of the best studied exoplanets of the hot-Jupiter type. Including the activity of the star makes analysis of the characteristics of the planet considerably more complicated (in particular, cool spots on the star’s surface introduce an additional signal in the relation between the depth of a transit and the size of the planet and prevent an exact determination of the radius of the planet). Photometric data for HD189733 in the archive of the TESS mission are analyzed and conclusions are reached on the values of the rotation period P_rot of the star, for which we estimated values of 12.5 and 11.3 days for sectors 41 and 54, respectively. A comparison of these with previous estimates based on earthbound photometric observations suggests that the differences in P_rot at different times are related to the fact of different observation times spots located at different latitudes on the surface of the differentially rotating star predominated. According to the TESS data the amplitude of the brightness variation of HD189733 is 1.6 and 0.9% of the mean brightness level of the star (for sectors 41 and 54, respectively). The standard method was used to estimate the spottedness parameter of the star (2.0 and 1.2% of the star’s surface area) and values of the area of the spotted surface A of the star in absolute terms (9200 MSH and 5400 MSH for sectors 41 and 54, respectively) were obtained. Based on 771 estimates of the star’s brightness in the V band presented in the Kamogata Wide-field Survey, an analysis was made of the long-term activity of HD189733 which yielded indications of the possible existence of activity cycles of duration 1140 and 4220 days (3.1 and 11.6 years, respectively). Based on an analysis of published data it has been concluded that the level of chromospheric activity of HD189733 exceeds the average values typical of other cool dwarfs with analogous values of the color index (B-V) and is higher than the solar value. It is concluded that HD189733 has a moderate (or somewhat higher than average) activity typical of analogous dwarfs in spectral class K but, nevertheless, requires that the properties and evolution of the planet formed around this star be taken into detailed account during modeling, regardless of whether a simplified method or an exact magnetohydrodynamic model is used to estimate them. It should also be noted that the activity of the star is a variable over time (on a time scale comparable to P_rot) and requires estimates that are made simultaneously with the main studies of the exoplanet’s atmosphere.

We have carried out a spectral classification by the Activity Types for a subsample of Blazars from the BZCAT v.5 Catalogue, namely the BL Lac (BLL, Lacertids) candidates, designated in the Catalogue as BZB subtype objects. The classification is based on the Sloan Digital Sky Survey (SDSS) homogeneous medium-resolution optical spectroscopy and along with the standard BPT-type diagnostic diagrams, we have applied our newly introduced fine classification scheme with subtypes of Quasars and considering many more features. Out of 1151 BZB objects, 552 having SDSS spectra were classified. After new classification, 259 (46.1%) of 552 have not changed their optical class, and 293 (53.1%) of these objects have changed their optical class. Having this new information on the optical classification we suggest to change the classification of some objects in BZCAT: for 130 BZB objects to BZG, for 18 BZB objects to BZQ and for 145 BZB objects to BZU.

The properties of isospin-asymmetric cold nuclear matter are studied in terms of the relativistic mean-field theory in which, besides the fields of σ, ω, and ρ mesons, and the isovector, Lorentz-scalar field of the δ-meson is also taken into account. The properties of purely nucleonic np matter are studied as a function of the baryon density n_B and the asymmetry parameter α, as well as the properties of electrically neutral β-equilibrium npe μ matter as a function of the baryon density n_B. For different values of n_B and a, such characteristics of np matter as the energy per baryon, the specific energy owing to isospin asymmetry, the effective proton and neutron masses, and the specific binding energy, are determined. It is shown that the energy owing to the asymmetry for a fixed value of α is a monotonically increasing function of the baryon density n_B. For npem matter, the effective proton and neutron masses ({M}_{p}^{left(effright)},{M}_{n}^{left(effright)},) the specific binding energy E_bind, the symmetry energy E_sym, the quantitative fraction of protons Y_p = n_p/n_B are studied, as well as the average meson fields (widetilde{upsigma },widetilde{upomega },widetilde{updelta },) and (widetilde{uprho }) as functions of the baryon density n_B.

This paper presents the reconstruction of the quintessence dark energy potential in a model- independent way. Reconstruction relies on a Gaussian process and on available expansion rate data. Specifically, 40-point values of H(z) are used, consisting of a 30-point sample deduced from a differential age method and an additional 10-point sample obtained from the radial BAO method. Results are obtained for two kernel functions and for three different values of H₀. This sheds light on the H₀ tension problem indicating that it is not just a numerical problem. The model-independent reconstruction of the potential can serve as a reference to constraint available models and construct new ones. Various possibilities, including V(ϕ) ~ e^-λϕ , are compared with the reconstructions here obtained, which is notably the first truly model-independent reconstruction of the quintessence dark energy potential. This allows the selection of new models that can be interesting for cosmology. The method can be extended to reconstruct the potential of related dark energy models, to be considered in future work.

The Vlasov equations are used to determine the transverse and longitudinal dielectric permeabilities of a hot, rarefied ultrarelativistic electron-positron plasma. The Fermi distribution is used as the unperturbed distribution function of the electrons and positrons. It is shown that, owing to the abundant production of electron-positron pairs, the plasma frequency of the ultrarelativistic plasma increases linearly with rising temperature. Here the ratio of the frequency at which the spectral density of the equilibrium emission is maximal to the plasma frequency remains fixed and equals 28. It is shown that in this sort of plasma, in particular in the matter of the early universe in the ultrarelativistic electronpositron epoch, the spectrum of the radiation differs very little from black-body radiation.

This paper is an analysis of data from multicolor photometric observations of the asynchronous polar IGR J19552+0044 obtained over 59 nights from 2019-2022 at the Crimean Astrophysical Observatory, as well as at the Terskol Peak, Sanglokh, and Lesniki Observatories, and data from the TESS space telescope obtained on 27 days in 2022. It is shown that the maximum amplitude of the brightness oscillations at the rotation period of the white dwarf (~2^m.5-3^m) is observed in the Ic band and is essentially close to zero in the B band. The rotation period of the white dwarf is refined to 0.05645350(14) days. A dependence of the amplitude of the brightness curve of the rotation period on the phase of the synodic period is detected.