Astrophysics and Space Science最新文献

We present a relativistic model of anisotropic compact objects with spherically symmetric matter distribution coupled with dark energy in the framework of general theory of relativity. We assumed an ansatz for the dark energy (DE) equation of state (EoS) in addition to the baryonic matter. The interior spacetime of the compact object is described by the Finch-Skea (FS) metric and a causal domain of DE with a coupling parameter (beta ). The stellar model is employed to investigate physical features such as energy density, radial pressure, transverse pressure, anisotropy, mass-radius relation, EoS, etc. for a known pulsar, PSR J0348+0432 ((M = 2.01 pm 0.04 M_{odot }) and (R = 12.072) km) taking different (beta ). We test the stability of the stellar models for various values of (beta ) in a relativistic stellar model. The EoS of the matter configuration inside the star is determined for different (beta ) which are non-linear. The EoS obtained varying the DE coupling parameter ((beta )) indicates that the matter inside the DE star is more stiff for less (beta ). The prescription for obtaining a realistic stellar model is used to employ for other pulsars. The equation of states for those pulsars and the model parameters are determined for their observed masses and radii. The Mass-Radius (M-R) relation is independent of the coupling parameter (beta ) and the M-R relation is consistent with the observational constraints.

In this work, we investigate the phenomenologically emergent dark energy (PEDE) model and its generalized form, namely the generalized emergent dark energy (GEDE) model, which introduces a free parameter (Delta ) that can discriminate between the (Lambda )CDM model and the PEDE model. Fitting the emergent dark energy (EDE) models with the observational datasets including the cosmology-independent gamma-ray bursts (GRBs) and the observational Hubble data (OHD) at intermediate redshift, we find a large value of (H_{0}) which is close to the results of local measurement of (H_{0}) from the SH0ES Collaboration in both EDE models. In order to refine our analysis and tighten the constraints on cosmological parameters, we combine mid-redshift observations GRBs and OHD with baryon acoustic oscillations (BAOs). Finally, we constrain DE models by using the simultaneous fitting method, in which the parameters of DE models and the relation parameters of GRBs are fitted simultaneously. Our results suggest that PEDE and GEDE models can be possible alternative to the standard cosmological model, pending further theoretical explorations and observational verifications.

We consider a recently modified version of holographic dark energy, known as Barrow holographic dark energy, in the framework of (f(R, T)) gravity and consider a flat Friedmann-Lemaitre-Robertson-Walker line element for our study. We solve the field equations of the model to obtain the values of Hubble parameter and scale factor of the universe. We obtain the values of deceleration parameter and effective equation of state to discuss the evolution of the universe. Further, we constrain the model parameters using various data sets like type Ia supernova, observational Hubble data, SH0ES data etc. We use the Monte Carlo Markov Chain method to obtain the best fit values of the model parameters. We observe that the best fit present values of Hubble parameter (H_{0}=67.764_{-1.354}^{+1.274}) and (H_{0}=70.440_{-0.869}^{+0.816}), obtained for two different combinations of observational data, are in agreement with recent observations. We also constrain the case in which our model converts to the Barrow holographic dark energy model in general relativity and compare the results of both models. We compare the results with (Lambda )-CDM model wherever required. We plot deceleration parameter against redshift parameter for best fit values of the model parameters. We observe a smooth phase transition of the universe from early time decelerated expansion to accelerated expansion which shows compatibility with recent observations. The values of equation of state parameter (omega _{h}) of Barrow holographic dark energy are found to be (-0.873_{-0.078}^{+0.115}) and (-0.866_{-0.130}^{+0.156}), and age of the universe are found to be 14.09 (Gyr) and 15.43 (Gyr) for two combination of data sets for Barrow holographic dark energy model in (f(R, T)) gravity. Furthermore, we apply statefinder and (Om) diagnostic to discriminate our model from existing dark energy models.

In this work, we aim to study the consequences of presently observed accelerated expansion of the universe by assuming power-law based Starobinsky type metric (f(R)) gravity as the theory of gravity. We, here, focused on evolution of the universe by studying different cosmological parameters like Hubble parameter, Deceleration parameter and Jerk parameter etc. From our analysis, we found that the phenomenological constant of Starobinsky model ((M^{2})) has a very small value (0.6538t_{0}^{-2}), where (t_{0}) is the present age of the universe. Again, from the nature of variation of deceleration parameter and jerk parameter, we concluded that the universe has been undergoing an accelerated phase of expansion from (0.711t_{0}) and this will continue even upto the distant future.

In this manuscript we pursue tidal disruption of magnetically confined clumps of matter as candidate to interpret the twin peak high-frequency quasiperiodic oscillations (HF QPOs) seen in low-mass x-ray binaries (LMXBs). In previews works we have proposed the upper HF QPO seen in neutron star (NS) LMXBs be linked to the energy released during tidal circularization of relativistic orbits. The lower HF QPO instead might come from the energy released during spiraling and tidal stretching of the clump. The observed behavior of the coherence (Q) of the lower HF QPO was related to both the tidal stretching time-scale of the clump along the orbit and the number of turns clumps make before reaching the innermost stable bound orbit. In this paper we focus on the maximum value of (Q) across LMXBs spanning two orders of magnitudes in luminosity. We show the (Q_{max}) behavior of the lower HF QPO seen across LMXBs luminosity might be related to tidal stretching time-scale as well, giving an estimate of the magnetic field in the plasma.

This study focus on atypical Type II radio bursts observed in conjunction with three simultaneous coronal mass ejections (CMEs) on September 27, 2001. These CMEs originated from a single active region (AR) and were linked to relatively weak solar flares. Analysis of the CME sequences revealed distinct periods of interplanetary (IP) Type II radio emissions, characterized by pronounced increases in intensity. The first radio enhancement, lasting 20 minutes, exhibited very low density and frequency (1.65–1.5 MHz) at a height range of (7.8–8.2) solar radii (). Subsequently, the second radio signature persisted for 40 minutes with a frequency range of (900–700) kHz and a height range of (10.9–12.6) . The third radio signature spanned 1 hour and 20 minutes, featuring a frequency range of (660–390) kHz and a height range of (13.2–18.6) . The fourth enhancement extended over 3 hours, ranging from (550–250) kHz in frequency and (14.6–25.0) in height. We concluded that the initial low-density radio signature resulted from a shock wave generated by reconnection of magnetic field lines, without an intense flare or extreme ultraviolet imaging telescope (EIT) wave. This shock wave then accelerated subsequent CMEs. Alternatively, the radio burst could have formed in the wake of the initial slow CME, creating a low-density environment. The second radio enhancement coincided with the accelerated propagation of CME1’s core and was attributed to enhanced radio emission resulting from the Type II shock encountering filament material. The third radio enhancement aligned with the concept of a CME bow shock, indicating that the shock was positioned at the leading front of the CME. This enhancement occurred when the shock met remnant material from earlier CMEs, yet the shock continued propagating at a constant speed. The fourth enhancement progressed to higher frequencies due to the merging of CME1’s core with CME2, propagating along CME3’s path. This comprehensive analysis provides valuable insights into the complex dynamics and interactions associated with these unique Type II radio bursts and their correlation with coronal mass ejections.

For Galactic RRab stars 121 empirical and semiempirical metallicity–absolute magnitude relations known since 1990 are analyzed. For these variables, using their known empirical data, relations are determined between the mass, the radius, the effective surface temperature, the bolometric correction, the luminosity, the absolute magnitude, on the one hand, and the metallicity and the pulsation period, on the other hand. Using these relations, the empirical value of the Eddington pulsation constant is accurately determined for RRab stars for the first time.

X-ray binaries are known to exhibit different spectral states which are often associated with different black hole accretion modes. The exact geometry and properties of these accretion modes is still uncertain. Recent IXPE measurements of linear polarization of X-ray emission in canonical X-ray binary system Cygnus X-1 allow us to test models for the hard spectral state of accretion in a unique way. We show that general relativistic radiative magnetohydrodynamic (GRRMHD) simulations of accreting stellar black hole in a hard X-ray state may be consistent with the new observational information. In the presented framework, where first-principle models have limited number of free parameters, the polarimetric X-ray observations put constraints on the viewing angle of the inner hot accretion flow.

This work studies the motion around irregular elongated asteroids through two approaches. Firstly, it revisits the dipole-segment model, identifying families of periodic orbits for asymmetric mass distribution. Additionally, a new model incorporating variable density for elongated asteroids is introduced and compared to the dipole-segment model. Several families of periodic orbits have been found through continuation of planar orbits and out-of-plane bifurcation processes, obtaining results in agreement with previous studies about the dynamics around irregular asteroids. This highlights the relevance of simple mathematical models in studying asteroid dynamics and the importance of accounting for density and geometric properties. Although the families of periodic orbits studied in this work are not comprehensively sampled, they constitute an example of the variety of orbits that can be followed by a particle orbiting the asteroid, helping us to better understand the dynamics around these elongated bodies.

We utilized the Forbush decreases (magnitude (>1.5%)) detected in cosmic ray neutron monitor data during continuous five solar cycles, viz., 20, 21, 22, 23 and 24 (1965 to 2019) and subjected them to wavelet analysis in order to obtain the possible periodicities in their occurrence. We also studied the periodicities separately during the odd and even solar activity cycles. In addition to solar activity, the solar magnetic polarity and its extension into the interplanetary space makes significant difference in the cosmic ray modulation in the helisphere, we have also applied the wavelet analysis procedure separately during positive (A > 0) and negative (A < 0) polarity states of the heliospheric magnetic fields. Observed periodicities in Forbush decreases have been discussed and compared with earlier detected periodicities in solar and geomagnetic activity indices, e.g., sunspot numbers, sunspot areas, sunspot groups, solar flares, coronal mass ejections, and various geomagnetic activity indices. Significant short-term periodic behaviour detected in the occurrence of Forbush decreases, which in general, corroborates the observed behaviour in solar (in particular, solar eruptive activity) and geomagnetic activity. Understanding the quasi-periodic process in magnetic field emergence from solar active regions and solar eruptive activity, as well as solar-terrestrial coupling and space weather effects, requires comparing the quasi-periodic behaviour between parameters representing solar and geomagnetic activity along with cosmic ray variability.