Astrophysics and Space Science最新文献

We study the effects of pair creation on the radiation emerging from black holes under the assumption that the magnetic fields are vortex driven. In particular, for a sufficiently broad range of supermassive black holes, we investigated the energies at which photons undergo decay under the influence of a strong magnetic field, producing electron-positron pairs. Depending on particular physical parameters, it has been shown that in certain scenarios high or very high energy emission generated by black holes will be strongly suppressed, thus, will be unable to escape a zone where radiation is generated. In particular, photons with energies exceeding (sim 1text{ GeV}) will never leave the magnetosphere if they are generated at the scale 10(R_{g}) and the threshold is of the order of (1text{ TeV}), if the emission is produced at (sim 100; R_{g}). Analysing the process versus the black hole mass, assuming the region (100; R_{g}), it has been shown that for the considered lowest mass, the photons with energies (250text{ GeV}) will never leave the black hole and for the considered highest mass the corresponding value is (sim 250text{ TeV}).

Recently, a novel class of modified gravity theories has been proposed, wherein Einstein’s General Relativity (GR) is extended by incorporating a quadratic energy–momentum term of the form (T_{mu nu }T^{mu nu } ), coupled via a constant parameter (alpha ). The corresponding field equations deviate from the Einstein equations only in the presence of matter. Analytical studies indicate that, without interaction, the energy-momentum squared term remains subdominant, mainly enabling non-singular Big Bang scenarios. In this work, we investigate this framework in a homogeneous and isotropic cosmological background. We show that, in its minimal form, the theory does not naturally explain late-time cosmic acceleration. Although a cosmological constant can remedy this, it introduces an effective dark energy component with positive pressure during the matter era, distorting large-scale structure formation. To overcome this, we derive an analytical dark energy form by redefining its equation of state and imposing boundary conditions consistent with early- and late-time cosmology. The resulting phenomenological model alleviates the coincidence and fine-tuning problems and ensures classical stability. Observational constraints confirm good agreement with current data, though a statefinder analysis shows that, while the model mimics (Lambda )CDM today, it deviates in the far future as the acceleration rate increases.

The nature of dark matter (DM) and its effects on Milky Way galactic evolution are outstanding problems in astrophysics, space science, and cosmology. Quantitative models for the distribution of DM in the solar system are needed to clarify the role of DM in the Milky Way and to enable and guide direct and indirect DM detection searches. The dynamics, energetics, and composition of the solar corona are important elements in the Sun-heliosphere connection. By adopting a kinetic collisionless approach, we present the first quantitative model for the spatial distribution of DM in the solar corona. Our results show that the predicted DM mass density can be a fraction of plasma mass densities predicted by models of the solar corona. We find that the DM density in the lower corona is mass-dependent with the largest DM density occurring for a DM mass on the order of several proton masses.

We present an investigation of the classical nova V615 Vul that erupted on July 29, 2024. The results obtained are based on our multicolor observations at AI SAS, the M. R. Štefánik Observatory in Hlohovec and MAO NASU, as well as photometry from the AAVSO and ASAS-SN database. We also present spectral observations obtained with the 1.3-meter telescope at Skalnaté Pleso Observatory, when the very broad asymmetric H_α emission line dominated during the nova’s decline. From the constructed light curves, we determined the brightness decay rates at 2 and 3 magnitudes, obtaining (t_{2,V} = 5^{d}).2, (t_{3,V} = 8^{d}).9 and (t_{3,B} = 11^{d}).6, which corresponds to very fast novae. The light curve shows a variation with a period of 6.5 days from day 12 to about day 90 after the outburst maximum. We also detected and analyzed short-term variations in (V), (R_{c}) and (I_{c}) starting around day 35. We suppose that the found oscillation of 0^d.2238 ± 0.005 is the orbital period of V615 Vul. We calculated color indices and estimated the color temperature of the system. The tracks in the (V-R_{c}) and (R_{c}-I_{c}) diagrams exhibit loop-like variations caused by the broad H_α emission line, while the tracks in (U-B)/(B-V) diagram reflect changes in color temperature during the outburst - behavior typical of many cataclysmic variables and novae. We also estimated the main parameters of the system, such as the absolute magnitude, distance, extinction, and mass of the white dwarf.

Angular momentum transport is a fundamental process shaping the structure, evolution, and lifespans of stars and disks across a wide range of astrophysical systems. Be stars offer a valuable environment for studying viscous transport of angular momentum in massive stars, thanks to their rapid rotation, observable decretion disks, and likely absence of strong magnetic fields. This study analyzes angular momentum loss in 40 Be binary simulations spanning a range of orbital separations and companion masses, using a smoothed-particle hydrodynamics (SPH) code. A novel framework is introduced to define the outer disk edge based on the behaviour of the azimuthal velocity, streamlining the analysis of angular momentum transport within the system. Applying this framework reveals that systems with smaller truncation radii tend to reaccrete a larger fraction of their angular momentum during dissipation, thereby inhibiting the stars ability to regulate its surface rotation. Modification of this rate may alter the star’s mass-injection duty cycle or long-term evolutionary track. Finally, a subset of the simulations were post-processed using the Monte Carlo radiative transfer code HDUST, generating synthetic observables including H(alpha ) line profiles, V-band polarization, and UV polarization. Suggestions for observational verification of the dynamical results are demonstrated using the simulated data.

In this work, we investigate the thermodynamic properties of a non-rotating hairy Bardeen black hole, highlighting deviations from the predictions of standard general relativity due to the presence of additional parameters. Specifically, we analyze the influence of the electric charge (Q ), the coupling constant (beta ), and the model parameter (eta ) on the black hole’s mass, temperature, sparsity parameter and entropy. While the overall qualitative behavior of these quantities remains consistent, we find that both (Q ) and (eta ) tend to decrease the mass and temperature, whereas (beta ) exerts an opposite effect by increasing them. Furthermore, using the entropic force approach, we derive a novel expression for the black hole entropy, which encapsulates the modifications to the underlying gravitational interaction. We then examine how the parameters (Q ), (beta ), and (eta ) affect the circular motion of photons. Our results show that the radius of stable circular orbits increases with (Q ) and (eta ), while the radius of unstable circular orbits decreases with (Q ) and increases with (beta ). Additionally, the critical impact parameter is found to grow with increasing (Q ), but diminish with increasing (beta ). We also study the variations in the Keplerian frequency of photons orbiting the black hole under the influence of these parameters. For small radial distances (r ), the charge (Q ) reduces the frequency, while for intermediate and large (r ), it causes an increase. A similar trend is observed for the model parameter (eta ) for small values of (r), whereas the coupling constant (beta ) produces the opposite effect across these regimes. At the end of the paper, we derive the modified Friedmann equation from the entropy of the studied black hole.

The origin of the hard, bright X-ray emission that defines the (gamma ) Cas analog class of Be stars remains an outstanding question in Be star literature. This work explores the possibility that the X-ray flux is produced by accretion onto a white dwarf companion. We use three-dimensional smoothed particle hydrodynamics simulations to model the prototype (gamma ) Cas system assuming a white dwarf companion and investigate the accretion of the circumstellar material by the secondary star. We contrast these results to a model for 59 Cyg, a non-(gamma ) Cas Be star system with a stripped companion. We find that the secondary stars in both systems form disk-like accretion structures with Keplerian characteristics, similar to those seen in the Be decretion disks. We also find that white dwarf accretion can produce X-ray fluxes that are consistent with the observed values for (gamma ) Cas, while the predicted X-ray luminosities are significantly lower for the non-degenerate companion in 59 Cyg. In addition, using the three-dimensional radiative transfer code, hdust, we find that these models produce H(alpha ) emission consistent with the observations for both (gamma ) Cas and 59 Cyg, and that the predicted polarization degrees across optical and UV wavelengths are at detectable levels. Finally, we discuss the impact that future UV spectropolarimetry missions could have on our understanding of these systems.

In this paper, we explore the cosmological evolution of a viscous dark energy model within the framework of (f(Q, C)) gravity, employing a two-fluid approach. The model incorporates non-metricity and boundary contributions to the total action, represented by the scalar quantities (Q) and (C). The viscosity in the dark energy fluid is introduced to investigate the impact of bulk viscosity on cosmic expansion and late-time acceleration. Field equations are derived in a modified FLRW background, and the dynamics of key cosmological quantities such as energy density, pressure, and the effective equation of state (EoS) parameter are analyzed. Observational constraints on (H(z)) are imposed using DESI BAO Measurements, Pantheon+ (without SHOES), and cosmic chronometer datasets. Results indicate that the model effectively captures the universe’s expansion history, including the deceleration–acceleration transition, consistent with observations. This framework provides an alternative explanation for late-time cosmic acceleration without invoking a cosmological constant.