Astrophysics and Space Science最新文献

Radiative gravitational collapse is an important and much studied phenomenon in astrophysics. Einstein’s theory of general relativity (GR) is well suited to describing such processes provided closure of the system of nonlinear differential equations is achieved. Within a perturbative scheme, the property of vanishing complexity factor is used in order to complete the description of the radiative, self-gravitating system. We show that a physically viable model may be obtained which reflects the absence of energy inhomogeneities for lower density systems, in contrast to what might be expected for more aggressive collapse processes.

Generalized restricted three body problems consist of adding some extra hypotheses to the Restricted three body problem (RTBP) in order to have a new problem, not very different of the original RTBP. However, not any additional hypothesis is allowed; it must satisfy the laws of Physics. Among the several generalizations found in literature, we prove that at least there are two hypotheses that cannot be used, namely: 1) Perturbation in Coriolis and/or centrifugal forces, and 2) primaries are spheroids moving on elliptical orbits.

In 1953 I heard of an experiment in 1925 by Bengt Strömgren where he observed transit times with the meridian circle at the Copenhagen University Observatory measuring the current in a photocell behind slits when a star was crossing. In 1954 just 22 years old I was given the task as a student to make first test observations with a new meridian circle of the observatory. I became fascinated by the instrument and by the importance of astrometry for astronomy. Work at four meridian circles, two in Denmark, one in Hamburg, one in Lund, and Pierre Lacroute’s vision of space astrometry in France had by 1973 created the foundation for development of the Hipparcos satellite, and Gaia followed. In 2013 I proposed a successor satellite which has gained momentum especially thanks to the efforts of David Hobbs and it has a good chance to be launched by ESA about 2045. – But 70 years ago, optical astrometry was considered a dying branch of astronomy, unattractive compared with astrophysics. The following growth built on the still active interest in astrometry in Europe in those years and it was supported by ESA, the European Space Agency. – This review is only about astrometry where I was personally involved.

We have carried out a detailed study of the single pulse of PSR J1946 + 1805 with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). We reassessed the nulling fraction (NF) to be 57.5%. We also reverted the multiple drifting modes and obtained their drifting parameters. It is found that, for this pulsar, there exists the dwarf pulse at the beginning or end of the burst state, and the duration of dwarf pulse usually is one to two pulse periods. Additionally, we observed that there are scattered dwarf pulses detected in the null state. Detailed analysis of the fraction of linear polarization for all dwarf pulse is presented. It is worth noting that relative to normal pulses these dwarf pulses have a very high fraction of linear polarization, even more than 90%. We proposed that the dwarf pulses appearing at the beginning or end of the burst state may be a common behavior, which is important to understand the physical mechanism of the process of switching between two states.

We study the magnetosonic waves (MWs) propagating in magnetized plasmas comprising light warm ions, cold heavy ions and hot electrons at the Earth’s ionosphere. The dispersion relation is given and the effects of different physical parameters on the frequency of fast and slow MSWs are shown. We derive a KdV equation for the MSWs in multi-ion plasma and study the two-soliton and three-soliton overtaking collision of KdV MSWs by Hirota’s bilinear method. We show the trajectories and amplitude of solitons in this process, and the phase shift after collision. It is found that when solitons merge, it will form a new soliton whose amplitude first decreases then increases. The effects of physical parameters on the phase shift of multi-soliton collision are presented. It is worth noting that the solitons’ interaction can result in the redistribution of momentum and energy in the plasmas.

In this paper, we have explored the relativistic stellar model considering pressure anisotropy. An anisotropic solution of the Einstein Field Equations (henceforth EFE) has been presented for charged strange quark stars considering the interior space-time geometry described by the Durgapal (IV^{th}) metric. In the case of a strange quark star, if the equation of state (p_{r}=frac{1}{3}(rho -4B_{g})) as prescribed in the MIT bag model is applicable, then at the surface, one may consider that the surface energy density (rho _{s}=4B_{g}). Imposing the constraint value of (B_{g}) within the range of 57.55 - (95.11~MeV/fm^{3}) required for stable quark matter with respect to a neutron when the external pressure is zero, we have determined the maximum mass and radius of the strange quark star and other relevant properties. It is noted that the maximum mass and radius for (B_{g}=57.55~MeV/fm^{3}) are (M_{max}=2.92M_{odot }) and (b_{max}=13.749~km), respectively, whereas those for (B_{g}=95.11~MeV/fm^{3}), are (M_{max}=2.27M_{odot }) and (b_{max}=10.695~km) for an isotropic uncharged star. In the presence of pressure anisotropy and charge, the value of maximum mass increases. We have predicted the radii of a few recently observed pulsars from our model and found that the radii agree with the predictions from observations. Furthermore, stability and energy conditions are also satisfied in the present model.

We study the multi-wavelength spectral properties of the black hole X-ray binary MAXI J1348–630 using quasi-simultaneous ALMA, NICER, and Swift observations during the decay phase of the January 2019 outburst. In millimeter wavelengths, radio continuum emissions in the frequency range of 89.56–351.44 GHz are measured. We found that the flux densities at millimeter wavelength varied between 12.18 mJy and 18.47 mJy with spectral index ((alpha )) of (0.28pm 0.02). The broadband spectrum suggests that the source was accompanied by weak synchrotron emission from the compact jets. Broadband spectral study indicates that MAXI J1348–630 falls in the regime of “radio-quiet” during the decay phase of the outburst. The NICER spectrum is fitted by a combined model of disk blackbody component (diskbb) along with a comptonization component (simpl) which explains the power-law continuum caused by the thermal Comptonisation of soft disk photons in a hot gas of electrons. The NICER spectrum is dominated by the comptonised components during the decay phase of the outburst close to the hard state of the source. We have investigated the correlation between X-ray and radio luminosity using quasi-simultaneous ALMA and NICER data to understand the source nature by locating the source in the (L_{X})-(L_{R}) diagram. The correlation study of radio/X-ray luminosity suggests that MAXI J1348–630 did not follow the well-known track for black holes and it is a new member of a restricted group of sources.

Measurements of the current expansion rate of the Universe, (H_{0}), using standard candles, disagree with those derived from observations of the Cosmic Microwave Background (CMB). This discrepancy, known as the Hubble tension, is substantial and suggests the possibility of revisions to the standard cosmological model (Cosmological constant (Lambda ) and cold dark matter – (Lambda CDM)). Dynamic dark energy (DE) models that introduce deviations in the expansion history relative to (Lambda CDM) could potentially explain this tension. We used Type Ia supernovae (SNe) data to test a dynamic DE model consisting of an equation of state that varies linearly with the cosmological scale factor (a). To evaluate this model, we developed a new statistic (the (T_{alpha }) statistic) used in conjunction with an optimization code that minimizes its value to obtain model parameters. The (T_{alpha }) statistic reduces bias errors (in comparison to the (chi ^{2}) statistic) because it retains the sign of the residuals, which is meaningful in testing the dynamic DE model as the deviations in the expansion history introduced by this model act asymmetrically in redshift space. The DE model fits the SNe data reasonably well, but the available SNe data lacks the statistical power to discriminate between (Lambda CDM) and alternative models. To further assess the model using CMB data, we computed the distance to the last scattering surface and compared the results with that derived from the Planck observations. Although the simple dynamic DE model tested does not completely resolve the tension, it is not ruled out by the data and could still play a role alongside other physical effects.

Neutron star low-mass X-ray binaries accrete via Roche-lobe overflow from a stellar companion that is ≲ 1 M_⊙. The accretion disk in these systems can be externally illuminated by X-rays that are reprocessed by the accreting material into an emergent reflection spectrum comprised of emission lines superimposed onto the reprocessed continuum. Due to proximity to the compact object, strong gravity effects are imparted to the reflection spectrum that can be modeled to infer properties of the NS itself and other aspects of the accreting system. This short review discusses the field of reflection modeling in neutron star low-mass X-ray binary systems with the intention to highlight the work that was awarded the 2023 AAS Newton Lacy Pierce Prize, but also to consolidate key information as a reference for those entering this subfield.