Astrophysics and Space Science最新文献

The sample copula of order (m) provides an approximation to the copula that characterizes the dependence structure of a set of random variables. In this work, we first derive the sample copula of order (m) for a random vector (X = (X_{1},ldots ,X_{d})), with (d geq 2), by extending previously established results for the bivariate case. Based on the definition of a parametric copula with piecewise constant density, we show that the maximum likelihood estimation of the density parameters coincides with the elements employed in the definition of the sample copula of order (m), under the condition (2 le m le n), where (m) is an integer divisor of (n), and (n) denotes the given sample size. In the second part, we present an application of the sample copula of order (m) as a complementary alternative for estimating the cosmological parameters (H_{0}) and (Omega _{m0}), the current values of the Hubble constant and the matter density, respectively. This is carried out using a sample of observations of the redshift (z), the Hubble parameter (H), and its measurement error. To this end, several probability distributions, in addition to the Gaussian distribution, are proposed to model the observed error in the variable (H). Moreover, the applicability of this methodology is highlighted in the context of limited sample sizes.

This study investigates the (beta )-decay properties of (sd)-shell nuclei using the proton-neutron quasiparticle random phase approximation (pn-QRPA) model. The computed Gamow-Teller (GT) strength distributions show decent agreement with the measured data. The calculated (beta )-decay half-lives show good agreement with the previous shell model calculations. The computed log (textit{ft}) values align well with the available experimental data, validating the consistency of the theoretical approach. A key advancement of this work is the calculation of stellar weak interaction rates performed without assuming the Brink-Axel hypothesis for the estimation of GT distributions from parent excited states. The sum of (beta ^{-}) and positron capture ((beta ^{-}) + PC) rates were compared with earlier predictions from the shell model. The percentage contribution of (beta ^{-}) and PC is also investigated under stellar conditions. At low density and high temperature ((rho =10^{7}text{ g}/text{cm}^{3}), T = 30 GK) the pn-QRPA calculation compare well with the shell model and differs at most by a factor 10. Our findings may provide crucial and refined nuclear inputs for astrophysical simulations of (r)- and (s)-process nucleosynthesis.

The Gaia Alerts system is providing alerts on a variety of objects displaying a significant photometric change detected by the Gaia satellite from one passage to the next one over the same region of the sky. Among the ∼23,000 alerts published until the end of 2022, around 13% concern AGN or quasar candidates. At the same time, a different method to detect variations specifically in galactic nuclei was tested on Gaia data during a one year period only, leading to another set of candidates. We have embarked on a spectroscopic ground-based follow-up of some of those, to establish their true nature, and reveal the various phenomena leading to a change in magnitude of those AGN. The present paper deals with radio-quiet objects, while the radio-loud ones will be described in a companion paper. We confirm, on one hand, the AGN/quasar nature of 64 new candidates alerted by Gaia, and, on the other hand, obtained second-epoch spectra of over 200 known AGN also alerted for large photometric variations. The observed phenomena show a large variety described in this paper: from Flares to Tidal Disruption Events (TDEs) and a large number of Changing Look Quasars (CLQs, 56 secure ones, plus 23 probable ones), not forgetting some rarer events like SNe, microlensing events or Extreme Coronal Line Emitters. This sample shows that variability is an excellent tool to detect new quasars, especially radio-quiet ones that otherwise would be undetected, and that a significant fraction of variable AGN/quasars, around 10%, presents the CLQ phenomenon. Some of the new CLQs are followed-up to monitor further changes and measure time scales.

We present calculations concerning the surface gravitational redshift in neutron star at finite temperature using general relativity. The method is presented explicitly in detail. Numerical applications are shown and discussed. Confrontation with experiment and other calculations is performed with some success. In addition to use more complex cold or hot equation of state for the nuclear matter than the free neutron one to describe massive neutron stars, this work shows that we can also take into account the temperature.

Field-aligned irregularities (FAI) are a persistent feature of the equatorial ionosphere and can significantly impact satellite-based communication and navigation systems. Despite extensive documentation of their large-scale occurrence patterns, there is still a lack of understanding regarding their short-term temporal variability and detection uncertainty. To address this, a statistical framework based on Kernel Density Estimation (KDE) was developed to investigate the temporal characteristics of FAI events with higher resolution. Data from the Equatorial Atmosphere Radar (EAR) in West Sumatra, Indonesia, were analyzed, focusing on two years: 2016 (a leap year) and 2017 (a non-leap year). KDE was applied to generate smoothed daily probability estimates of FAI occurrence, along with associated confidence intervals, allowing the temporal evolution of FAI activity to be visualized more clearly. To examine short-term variability, FAI events are grouped into 1-day, 2-day, and 3-day sequential patterns. Results show consistent seasonal signatures across both years, suggesting stable ionospheric behaviours despite differences in calendar structure. The KDE approach captures fluctuations more clearly than standard methods and highlights subtle patterns in event occurrences. This method offers a reproducible way to study FAI dynamics and can be extended to multi-year or multi-site analyses. It supports a more complex conception of equatorial ionospheric variability and its relevance to space weather monitoring and forecasting, where precise characterization of ionospheric disturbances is essential.

Hot stars born as rapid rotators are expected to induce meridional currents that mix hydrogen from the envelope into the core and return CNO-cycle processed material to the envelope, which should enhance the N at the surface at the expense of C and possibly also O depending on the ambient conditions. But the photospheric C and N abundances could also be influenced by mass transfer in a close binary system which spins up the mass gainer and deposits either processed or unprocessed material to its surface depending on just how much material has been peeled off the mass donor. We focus on the chemical composition of Be star photospheres to infer the present and past evolution of rapidly rotating early B stars. To mitigate the effects of gravity darkening and photospheric line blending on the abundances, we chose 8 Be stars with low (vsin i) that have good high-resolution FUV spectra in the IUE archive. We carried out a conventional NLTE abundance analysis of selected N iii, N i, and C iii lines in the FUV spectral region. We find clear evidence that the C iii 1176 Å multiplet is weak in the core region in most program stars, suggesting CNO processing. However, in all cases we infer a N abundance that is solar or less, raising a conundrum as to what happened to the “missing C.” Since a similar pattern of weak C yet normal N is also found in the mass gainer in some Algol binaries, there appears to be an emerging challenge to explain this apparent abundance anomaly. We speculate that the excess N from CNO processing might be converted into O (and perhaps on to Ne) by fusion with He in the hot but low-density regions either in the trail of ashes just outside the receding carbon-fusing core, or in He-shell flash regions, of a highly evolved mass loser in its final stage of mass transfer.

We used CORSIKA to simulate extensive air showers initiated by primary protons and iron nuclei at an energy of 30 TeV and a zenith angle of 0^∘. By analyzing secondary particles within 0-600 m from the shower axis, we extracted statistical-moment features for the electromagnetic and muonic components, and fitted the radial distributions of both components with the NKG function to obtain the age parameters and particle-number parameters (Georgios in EPJ Web Conf. 137:13001, 2017). We first constructed a stacking ensemble model (with XGBoost, Random Forest (RF), and Support Vector Machines (SVMs) as base learners, and Logistic Regression as the meta-learner) for proton-iron classification, and then introduced single classifiers (SVM, XGBoost, and Random Forest) as benchmarks to validate the reliability of the stacking framework. The area under the ROC curve (AUC) and the Q-factor were used as evaluation metrics for composition discrimination. The results show that, under this idealized proton-iron setting, the single classifiers already reach near-saturated performance (AUC close to 1). The stacking model achieves an AUC of 0.995 on an independent test set, with a maximum Q ≈1 × 10(mathtt{^{4}}) in the threshold scan. The comparable performance between stacking and the single models indicates that the stacking framework is stable and reliable for this task; the lack of a significant performance gain is mainly due to the idealized physical setup and the strong class separability, which leave limited room for improvement beyond the single-model ceiling. This workflow can serve as a baseline for future composition-discrimination studies that incorporate detector response, noise/systematic uncertainties, and broader energy and zenith-angle ranges.

We present an autonomous model to simulate the daytime variation of sub-ionospheric Very Low Frequency (VLF) signal amplitude, beginning with the computation of the D-region electron density by numerically solving the electron continuity equation (ECE). From the resulting altitude-dependent electron density profile ((N_{e})), we extract Wait’s ionospheric parameters ((h^{prime }) and (beta )) using a log-linear fitting method. The study focuses on two VLF propagation paths (one short and one medium in length) in India, originating from the VTX / 18.2 kHz transmitter. The model effectively employs the Long Wave Propagation Capability (LWPC) framework to reproduce the daytime VLF signal amplitude profile. It accurately captures the daytime variations observed at the Bengaluru (BAN) station, where the ground wave component is dominant, as well as at Khukurdaha, WB (KHK). A quantitative comparison between the simulated ((A_{sim})) and ((A_{obs})) observed amplitudes shows justified agreement, validating the physical consistency and possible predictive capability of the proposed approach.

Type I superluminous supernovae (SLSNe) are a diverse class of exceptionally bright massive star explosions, which typically exhibit absorption from ionised oxygen in their early spectra. While their photometric properties (luminosity and duration) both span an order of magnitude, population studies suggest that these distributions are continuous. However, spectroscopic samples have shown some indications of distinct sub-types, either through similarity to certain prototype objects, or in terms of their velocity evolution. Here we show that a well-observed SLSN, PTF12dam, completely changes its O II absorption profile as it rises to maximum light, moving from one proposed sub-type to another. This supports an interpretation where spectroscopic diversity is driven by the ejecta temperature at maximum light, rather than fundamental differences in the explosion or progenitor. Motivated by this, we develop a new diagnostic, the Brightness-Timescale-Temperature-Radius diagram, and a simple toy model for the evolution of the photospheric velocity, to show that diversity in the light curve rise time (likely due to differences in ejected mass) naturally explains why SLSNe with broader light curves generally have weaker O II lines, lower photospheric velocities after maximum, and slower changes in photospheric velocity over time. We show that the velocity distribution of the known SLSN population favours a relatively flat ejecta density profile, consistent with a hot bubble inflated by a central engine.

Astrophysical plasma environments and quantum-gravity-inspired spacetime regularization collectively modify photon dynamics near compact objects, yet their coupled impact on photon sphere structure remains unquantified. This study examines the photon sphere radius ((r_{mathrm{ph}})) for static Hayward regular black holes immersed in a cold, non-magnetized plasma with a power-law density profile ((omega _{p}^{2} propto r^{-k})). The modified condition for circular photon orbits, incorporating the regularization parameter (l) and plasma strength (K), was numerically solved across physical parameter ranges. Results demonstrate a consistent inward shift of (r_{mathrm{ph}}) relative to the Schwarzschild vacuum case, with individual increases in (l) (to (1.0M)) or (K) (to 0.5) reducing (r_{mathrm{ph}}) by up to (12.7%) and over (18%), respectively. Crucially, combined effects exhibit nonlinear synergy: the contraction exceeds additive expectations, peaking at (l approx 0.8M) and (K approx 0.3) with a residual shift (Delta r_{mathrm{ph}}/M approx -0.32) and a total shift (delta r_{mathrm{ph}}^{mathrm{total}}/M approx -0.32) for typical accretion parameters ((k = 1.5)). This synergy, arising from the combined amplification by plasma gradients and quantum-corrected curvature, enhances sensitivity to plasma strength by (60%) compared to singular geometries. The corresponding 4–(12%) reduction in black hole shadow diameter underscores the significance of these interactions for interpreting next-generation interferometric observations of strong-gravity lensing features.