In this study, we have revisited the isotropic and homogeneous cosmological model within the framework of f(Q) gravity, where Q denotes the non-metricity term. The solution of field equations are obtained by taking a time-dependent scale factor a(t)= (sinh (α t) )^1/n and specific form of f(Q) gravity f(Q)= Q + η Q^δ , where α , n, η and δ are arbitrary constants. We calculate all the parameters by using the estimated values obtained from observational Hubble dataset. We analyze various cosmic features of the derived model such as the deceleration parameter q, the energy density ρ , the pressure p, and the EoS parameter ω . The physical viability of the model is also examined through the analysis of energy conditions. We also investigate the speed of sound and jerk parameter associated with the proposal model that indicates the stability of a dark energy model. Our findings provide deeper insights into the dynamics of dark energy and its role in the accelerated expansion of the universe.
On 23 April 2023, the Earth experienced one of the most severe geomagnetic storms of the current solar cycle, triggered by a rapid coronal mass ejection (CME). Such events substantially disrupt the ionosphere, notably through the formation of Large-Scale Traveling Ionospheric Disturbances (LSTIDs), which can negatively affect satellite navigation and communication systems. In this paper, the morphology of LSTIDs was investigated over the Indian region during the storm event using detrended total electron content (dTEC) observations. To determine storm-time perturbations in Total Electron Content (TEC), data from a dense network of GNSS receivers (21 stations) in low and mid-latitudes in India were analyzed. Temporal variations of dTEC show specific properties of LSTIDs following the main phase of the storm with periodicities between 64 seconds to 512 seconds, a wavelength of 2293.3 km, a propagation velocity of about 956 m/sec (or) 3440.01 km/hr and a wave direction of 322.5º. The results add to the understanding of the low-latitude ionospheric responses to intense geomagnetic storms and provide much-needed understanding of improving space weather modeling and mitigation strategies on satellite navigation across the Indian subcontinent.
The main aim of this manuscript is to explore the generalized ghost dark energy in the context of f(𝒬,𝒞) theory, where 𝒬 is non-metricity scalar and 𝒞 represent boundary term. To achieve that aim, we take an isotropic and homogeneous with an ideal distribution of matter. Our analysis includes a case with interacting fluids, encompassing both dark matter and dark energy. Also, we rebuild a functional form of f(𝒬,𝒞) to examine how this modified model impacts cosmic evolution. The behavior of several cosmic parameters is examined for different parametric values. The stability of the generalized ghost dark energy model is assessed using the squared sound speed method, which confirms the rapid cosmic expansion. Moreover, we employ diagnostic tools to evaluate different phases of universe’s evolution. Our results are consistent with the latest empirical data, suggesting that the f(𝒬,𝒞) model provides an accurate description of both dark energy and the universe’s evolution. The cosmological parameters are constrained using observational data from cosmic chronometers.
Various lines of evidence are pointing to the possibility that the dynamo mechanism powering the solar magnetic activity cycle may be operating very close to, or even below, criticality. Such evidence includes the sudden drop in angular momentum loss in magnetized winds inferred in solar-type stars slighty older than the sun, and the ability of dynamos operating at or very close to criticality to generate solar-like patterns of long-term amplitude modulation when subjected to (relatively) weak stochastic forcing. After reviewing the notions of subcriticality, criticality and supercriticality in the context of the classical mean-field αΩ dynamo model, I review the broader extant literature on the topic and present a simple model based on energetics that provides upper bounds on the finite amplitude sustained by an otherwise subcritical large-scale “primary” dynamo in the presence of a secondary dynamo process. This allows new scenarios for intermittency, which are presented and discussed in qualitative terms, together with implications for our understanding of long-term solar cycle variability.
The recurrent nova T Coronae Borealis (T CrB) has recorded eruptions in 1946 and 1866 and a periodicity of 80 years renders the next eruption due shortly. A search for the earlier records of eruption (Schaefer 2023a) provided one as early as 1217 and another in 1787. Here we report the results of search in astrolabes, whose star dials, as we show, serve as unconventional sources of sightings. The catalogues (Gunther 1932; Sarma 2023) list the stars on the dials of astrolabes and provide a rich source of such records, though some pointers are left unidentified and sometimes misidentified by later studies. The procedure adopted for identification is explained with the help of the astrolabe which has the supernova SN1604 recorded to emphasise the difficulty involved. We were able to pinpoint three astrolabes as possible records of eruption of T CrB. The difficulties associated with this procedure and overcoming the disadvantages of poor resolution are discussed. One eruption around 1450CE is identified in a Spanish astrolabe and another sometime around 1570CE. The third is from second half of seventeenth-century. A long-term recurrence period of 82 years seems to be valid in the last eight centuries, putting the next eruption to 2028.
We present Aditya-L1/VELC spectroscopic observations of 5303Å coronal emission line widths before and after coronal mass ejections (CMEs) which showed coronal dimming. The sit & stare mode of observations enabled us to study the changes in the line widths for the `limb' CMEs noticed on 16 July 2024 and 05 August 2024. The emission line widths during the pre-CME phase are higher than the thermal values in both the cases. After the onset of the CMEs, the widths increased further by ≈15% on 16 July 2024 and ≈7% on 05 August 2024. We find that the power spectral density (PSD) distributions of the line widths for the two events exhibits a power-law behavior. The PSD slopes, measured before and after the CMEs are nearly the same, and close to the Kolmogorov slope of -5/3. The results suggest that the observed larger than thermal width of the 5303Å emission line, before and after CMEs, is mostly due to turbulence. The additional increase in the line widths after the onset of the CMEs in both the cases are likely because of enhanced turbulence caused by the CME associated coronal dimmings and subsequent coronal magnetic field reconfiguration.
Aditya-L1 is the first Indian spacecraft placed in a halo-orbit around the first Lagrangian (L1) point to continuously observe the Sun. A fluxgate magnetometer (MAG) is also onboard with other payloads to measure the interplanetary magnetic field (IMF) coming from the Sun towards the Earth which is an important parameter in monitoring the near-Earth space weather. The MAG is a dual triaxial sensor set mounted on a 6 m long boom, deployed along the negative roll direction of Aditya-L1, with one set at the tip of boom and the other set at the centre of the boom around 3 m away from the spacecraft towards the boom-tip on the sun-viewing panel deck. The MAG boom was deployed (downwards from the spacecraft) on January 11, 2024 and since then MAG is measuring the IMF. MAG observations are found to match well with the magnetic field measurements of the Deep Space Climate Observatory (DSCOVR) from NOAA, USA. During its continuous operations since then MAG has observed several extreme solar transient events such as the Interplanetary Coronal Mass Ejections (ICMEs), Magnetic Clouds etc. In this paper we report MAG observations of passage of three ICMEs during September–October, 2024 which occurred due to extreme solar transient events. As the selected events led to severe geomagnetic storms Dst < − 100 nT, the impact of these events on the Earth’s magnetosphere are presented.
The first severe geomagnetic storm of Solar Cycle 25 occurred on 23-24 April 2023, reaching a minimum Dst of -213 nT. Utilizing the state-of-the-art observational and modeling techniques, we investigate the Sun-to-Earth evolution of the coronal mass ejection (CME) that caused this severe geomagnetic storm. We use multi-wavelength and multi-vantage point remote sensing observations to constrain the near-Sun CME properties, which serve as input for the interplanetary flux rope simulator (INFROS) model to simulate the magnetic vectors of the ICME at 1 AU. Utilizing multi-point in situ observations of the ICME detected by both STEREO-A and Wind, we validate the INFROS model results at each spacecraft. We further couple INFROS with the Drag-Based Model (DBM) and empirical Dst prediction models, presenting a space weather modeling framework to estimate the intensity of the associated geomagnetic storm. Based on the remote sensing observations, we find that the CME eruption was associated with the partial eruption of a pre-existing filament structure, which led to an underestimation of the poloidal flux when determined using the post-eruption arcade (PEA) method. In contrast, empirical model-based estimations provide more accurate results on constraining the poloidal flux of the CME. The INFROS model successfully captures the magnetic structure of the ICME observed near Earth but fails to reproduce the trailing part of the flux rope as observed at STEREO-A, likely due to distortion caused by a following interacting high-speed stream. Validation of the space weather modeling framework with the observed SYM/H index for this event shows good agreement between the observed and predicted SYM/H profiles. These results highlight that the INFROS-based space weather modeling framework could serve as an operational space weather forecasting tool for predicting the intensity of geomagnetic storms.
We investigated the impact of pairing gaps on β -decay properties of neutron rich nuclei with neutron magic number (N = 50). The Gamow-Teller (GT) strength distributions, nuclear partition functions (NPFs) and electron capture cross-sections (ECCs) were analyzed for 78Ni, 82Ge, 86Kr, and 88Sr by utilizing the deformed proton-neutron quasiparticle random phase approximation (pn-QRPA) model with three different pairing gap schemes. The results show that a change in pairing gaps between like nucleons affects the GT strengths, NPFs and ECCs. The total GT strength, NPF and ECC changed as much as 62
Cosmic opacity may vary spatially due to the inhomogeneous distribution of dust, its grain properties, and the efficiency of photon attenuation. In this work, we present a model independent method to investigate the variation of cosmic opacity with redshift. Using strong gravitational lensing data we construct the opacity independent comoving distance function and we use latest supernovae type Ia (SNe Ia) Pantheon+ data to estimate the opacity dependent comoving distances. Using the distance duality equation, opacity parameter is constrained. Our analysis indicates a transparent Universe on average over the redshift range ( 0.01 ≤ z ≤ 2.26137 ) of Pantheon+ sample. However, if we split the dataset into subsamples with redshift bins of width △ z = 0.1 , we find appreciable deviation from the transparency in several redshift intervals. Particularly, in the redshift range 0.3 < z ≤ 0.4 , the opacity parameter is ϵ = -0.4283^+0.1914_-0.2027 . The current SNe Ia observations indicate the variation of opacity parameter with redshift. These results may have a significant impact on the values of the cosmological parameters deduced from the SNe Ia observations.
TIFR-ARIES near-infrared spectrometer (TANSPEC) is a spectrograph-cum-imager operating over the wavelength range 0.55-2.5 μ m. The instrument is mounted on the 3.6-m devasthal optical telescope (3.6-m DOT). It offers two resolution modes: low resolution (LR) and cross-dispersed (XD) via various slits of different widths (0.5”, 0.75”, 1.0”, 1.5”, 2.0” and 4.0”). The LR mode provides a resolving power (R) of ∼ 100-350 , while the XD mode achieves R∼ 2500 using the 0.5” slit. The previous version of the data reduction pipeline supported only wavelength-calibrated XD mode spectra and was limited to two slits (S-0.5 and S-1.0). In this work, we present an upgraded version of pyTANSPEC. The upgraded pipeline not only improves the data extraction algorithm but also introduces several new features for users. It now enables the reduction of spectra from all available slits for both LR and XD modes. The upgraded version also implements a template-matching method for more precise wavelength calibration. Additionally, a step for flux calibration is also included. Alongside pyTANSPEC, we upgraded HxRGproc, a Python package for cleaning and generating slope images from non-destructive readout frames taken with H1RG and H2RG detectors. The package performs non-linearity correction, flags saturated pixels, removes pink noise, and eliminates cosmic ray events. HxRGproc is updated to work for the H2RG detector of TANSPEC and is set up on the TANSPEC server, ensuring users receive data that are pre-cleaned and non-linearity corrected.
We present a semi-analytical model of magnetized accretion disks around neutron stars that extends previous disk-magnetosphere interaction frameworks by including Hall diffusion and disk-driven mass outflows. The modified induction and angular momentum equations are solved numerically to investigate the rotational structure of the disk. The results show that Hall diffusion significantly alters the angular momentum transport within the disk, broadening the transition region between Keplerian rotation and corotation with the neutron star. In contrast, the impact of disk outflows is twofold and critically depends on the specific angular momentum extracted from or delivered to the disk by the outflows, as parameterized by l . Outflows that remove angular momentum from the disk ( l < 0 ) reduce the fastness parameter and broaden the transition zone, while outflows that supply angular momentum to the disk ( l > 0 ) increase the fastness parameter and narrow the transition zone. When both effects of Hall diffusion and outflows operate together, they establish a competitive interplay that dictates the final disk configuration. Hall diffusion tends to expand the region of sub-Keplerian rotation, while outflows can either counteract or reinforce this broadening depending on the sign and magnitude of l . These findings highlight the importance of non-ideal MHD processes and outflows in determining the structure and dynamics of accretion disks around magnetized compact objects.
Kick velocities produced during accretion induced collapse (AIC) can substantially alter the dynamics of binary pulsars. Here we examine how these kicks reshape post-AIC orbits and identify the conditions under which binaries survive this event. We modeled the orbital response by requiring that the companion’s location at the moment of collapse be consistent with both the pre- and post-kick trajectories. Monte Carlo simulations of 10000 binaries show that modest kicks of 10 to 100 km/s reproduce the observed population spread in the orbitals of close binaries. Post-AIC periods show that the ratio of the two depends sensitively on kick amplitude and the induced eccentricity. Roughly 23 ≤ 50 km/s remain bound with high probability, whereas stronger kicks disrupt a large fraction of binaries. significant orbital evolution driven by the AIC process and its role in shaping the population of short-period binary pulsars. These results clarify the dynamical pathways through which AIC contributes to neutron star formation and the diversity of binary pulsar orbits.
The observed late-time acceleration of the universe remains one of the most profound puzzles in modern cosmology. While the standard approach attributes this acceleration to an unknown dark energy component within the framework of General Relativity, modified gravity theories-particularly f(R) gravity-have emerged as compelling alternatives that do not invoke exotic matter fields. In this work, we propose a new f(R) gravity model of the form f(R)=R+γR^1/n/1+μR^1/n where γ, μ and n are model parameters. For small curvature, this model effectively behaves as f(R)≈ R+γR^1/n , allowing the curvature modification to mimic a dynamically evolving dark energy component with ρ_DE∝a^-2/np , which drives late-time acceleration, particularly ρ_DE∝a^-1 for np=2 , by assuming a power-law behaviour for scale factor. At high curvature, the correction term is suppressed, ensuring the model closely approximates General Relativity and enables a chameleon-like screening mechanism that satisfies local gravity constraints. We derive the modified Friedmann equations in a spatially flat Friedmann-Lemaître-Robertson-Walker (FLRW) background and analyse the cosmological dynamics driven by the proposed model. Through analytical techniques, we investigate the conditions under which the model produces late cosmic acceleration without introducing a cosmological constant. We also examine the behaviour of the effective equation of state and demonstrate the physical viability of the model by comparing its predictions with current observational data and applying various stability tests, including a thermodynamic analysis. Our findings suggest that the proposed model is a viable alternative to dark energy, consistent with both cosmological and solar system constraints.
This research investigates the complex gravitational dynamics of the Kepler-1625 exoplanetary system by modeling the interactions among the host star, a massive exoplanet, its candidate Neptune-sized exomoon, and a test particle within an extended restricted four-body framework. Building upon the generalized restricted four-body problem, we analytically derive and numerically locate quasi-Lagrangian points (QLPs) that extend classical equilibrium concepts to this more intricate setting. Utilizing canonical normalization and system-specific parameterization, we perform comprehensive numerical simulations encompassing trajectory tracking, spectral and resonance analyses, gravitational potential characterization, and tidal force computations. Our results identify distinct stable and unstable QLPs, reveal underlying resonance structures and chaotic behaviors, and quantify tidal stresses that potentially impact exomoon orbital evolution and internal heating. This integrative approach advances theoretical understanding and provides practical insights for future observational and mission endeavors aimed at detecting and characterizing exomoons in complex multi-body systems like Kepler-1625.
This study presents exact solution to the Einstein-Maxwell field equations for charged anisotropic stars. By employing the embedding condition and a Chaplygin equation of state, we generate a charged star model which is physically and astrophysical significant. A nonlinear differential equation and field equations are transformed using the Durgapal and Bannerji transformations to simplify the analysis. This enables the specification of metric function on physical grounds. The analysis indicates that both gravitational potentials and matter variables are well behaved, and the model satisfies important physical conditions such as stability, equilibrium, energy conditions, the mass-radius relationship, compactness, measure of anisotropy, and matching.
The six-body problem, a specific instance of the general N-body problem in celestial mechanics, presents one of the most complex and chaotic dynamics in the study of gravitational systems. Rooted in Newton’s law of universal gravitation, the problem extends to a system of 36 coupled first-order differential equations requiring intricate initial conditions for solutions. Despite advances in understanding smaller systems, such as the three-body problem and its association with chaos (notably established by Poincaré), the restricted six-body problem remains a frontier of computational and theoretical challenge. This paper investigates the chaotic behavior of the six-body problem through rigorous analysis of its equations of motion, Lyapunov exponents, and energy dynamics. Numerical simulations are employed to visualize the intricate chaos and uncover the underlying patterns, while advanced techniques such as ergodic theory, probability distributions, and OGY control methods are applied to rationalize and mitigate the chaotic nature of the system. Symmetry reductions are explored as a means to simplify the complexity, making the problem more tractable without losing its core dynamics. The study also provides a detailed exploration of libration points and their stability, contributing to the ongoing discourse on chaotic behavior in multi-body gravitational interactions. The results highlight the interplay between chaos and order, offering new insights into the rationalization of complex dynamical systems. Numerical experiments, visualizations, and theoretical discussions together illuminate potential pathways to control and predict such intricate systems, advancing our understanding of chaotic dynamics in celestial mechanics.
Photometric classification of Type Ia supernovae is essential for modern time-domain surveys, where spectroscopic confirmation is not always feasible for the full transient sample. In this work, we investigate a compact and physically interpretable feature representation derived from multi-band light curves and evaluate its performance using gradient-boosted decision trees on the Supernova Photometric Classification Challenge (SPCC) dataset. The compact representation is derived from an initial pool of 31 light-curve features, reduced to 30 after removing redundant variables, and further optimized to a 16-feature model using systematic ablation and performance analysis. The final compact model achieves an F1–score of 0.844 on the held-out test set. This is consistent with k-fold cross-validation results (0.841 ± 0.006). The precision–recall area under the curve (PR-AUC) is 0.928, with similarly low variance across folds. Relative to our earlier 31-feature optimized XGBoost model for the same SPCC classification task (F1 ≈ 0.923), the compact 16-feature representation retains strong classification performance (F1 = 0.844) while substantially improving interpretability and reducing feature-space complexity. The ablation results show that temporal evolution provides the dominant classification signal, while brightness, color, and variability features supply complementary information. A reduced core of approximately ten physically meaningful features retains a large fraction of the performance of the compact model, with only a small decrease in F1-score, indicating that reliable classification does not require large high-dimensional feature spaces. These results demonstrate that interpretable feature-based models can capture the essential astrophysical information needed for Type Ia photometric classification, with direct implications for survey cadence, filter coverage, and the design of transparent and efficient machine learning pipelines for future time-domain surveys.
The response of topside ionosphere over low and midlatitudes to the severe geomagnetic storm of 23–24 April 2023 are studied using in-situ plasma measurements made by the Swarm satellite constellation at altitudes of 470 and 510 km. The local time coverage during the period was on dawn and dusk sectors. The geomagnetic storm is a two-step storm reaching a minimum SYM-H index magnitude of −233 nT. Observations reveal positive ionospheric storm with plasma density enhancements during sunlit periods of the main phase of the geomagnetic storm. This enhancement is significant over the Southern hemisphere. A clear increase in the equatorial electric field due to prompt penetration field is inferred through the increased separation of EIA crests and plasma density enhancements. This event showcased a longlasting prompt penetration electric field covering both the main phases. Interestingly, during the temporary recovery phase between the first and second main phases, the ionospheric observations are similar to those in the main phases with no indication of an overshielding electric field. During predawn hours of temporary recovery phase, a negative ionospheric storm is observed only over low latitudes. During the recovery phase, disturbed dynamo effects are also observed. However, for the first five hours in the early recovery phase, the effects were still similar to that of main phase. While the ring current recovery indicated by SYM-H indices took more than 36 hours, the disturbance dynamo effects were seen in the ionosphere only for about 11 hours, and the ionosphere was behaving similar to the quiet periods after 16 hours of start of the recovery phase. The observations are explained by means of undershielding prompt penetration electric fields during the main phases and development of disturbed dynamo effect well into the recovery phase.
