The Earth’s atmosphere is incessantly bombarded by energetic charged particles called cosmic rays (CR) which are having either solar or non-solar origin. Analysis based on information theoretic estimators can be effectively employed as a potential technique to analyze the dynamical changes in cosmic ray intensity during different solar cycles. In the present study, dynamical complexity based analysis using Jensen-Shannon divergence (JSD) has been employed which reveals the existence of some peculiar fluctuation properties in CRI flux at Jung neutron monitor station. JSD based dynamical complexity analyses confirm the existence of difference in dynamical properties of CR flux during solar cycles 20-21 and 22-23.

The dust properties of the line-of-sight materials in neutron star low-mass X-ray binaries (LMXBs) can be probed by X-ray observations and laboratory experiments. We use a Markov chain Monte Carlo (MCMC) method to conduct a spectral analysis of Chandra ACIS-S/HETG archival data of a sample of LMXBs, including GX 5-1 and GX 13+1. Our MCMC-based analysis puts constraints on the Si K-edge dust properties of the outflowing disk winds in this sample. Further X-ray observations of other LMXBs will help us better understand the grain features of dense outflows and accretion flows in neutron star binary systems.

Computational heliophysics has shed light on the fundamental physical processes inside the Sun, such as the differential rotation, meridional circulation, and dynamo-generation of magnetic fields. However, despite the substantial advances, the current results of 3D MHD simulations are still far from reproducing helioseismic inferences and surface observations. The reason is the multi-scale nature of the solar dynamics, covering a vast range of scales, which cannot be solved with the current computational resources. In such a situation, significant progress has been achieved by the mean-field approach, based on the separation of small-scale turbulence and large-scale dynamics. The mean-field simulations can reproduce solar observations, qualitatively and quantitatively, and uncover new phenomena. However, they do not reveal the complex physics of large-scale convection, solar magnetic cycles, and the magnetic self-organization that causes sunspots and solar eruptions. Thus, developing a synergy of these approaches seems to be a necessary but very challenging task.

After 2 years of continuous observations, the eROSITA All-Sky Survey bears the potential to build a complete sample of X-ray dim isolated neutron stars (XDINS). Making use of their soft X-ray emission and large X-ray-to-optical flux ratios, we selected a sample of ∼100 candidates detected down to a limiting flux of ∼10-13 erg s-1cm-2(0.2-2 keV). Follow-up observations of the best candidates will rule out possible contaminants. Updated source catalogues and screening algorithms will further improve our efficiency to identify new XDINS.

One of the ways to understand the genesis and evolution of the universe is to know how galaxies have formed and evolved. In this regard, the study of star formation history (SFH) plays an important role in the accurate understanding of galaxies. In this paper, we used long-period variable stars (LPVs) for estimating the SFH in the Andromeda galaxy (M31). These cool stars reach their peak luminosity in the final stage of their evolution also their birth mass is directly related to their luminosity. Therefore, using stellar evolution models, we construct the mass function and hence the star formation history.

We study the long-term heating due to magnetic field decay in the core of neutron star. Two cases for the nucleonic core are considered: normal and strongly superconducting. We give simple scaling relations (depending on the internal stellar temperature and the averaged magnetic field in the core) to estimate the magnetic field decay rate for the most important dissipation processes. Comparison to properties of observed neutron stars suggests that heating due to the magnetic field decay is (at least partially) responsible for the thermal states of middle-aged magnetars and highly-magnetized isolated neutron stars with ages of 1 — 10 Myr.

The dynamics of massless planetesimals in the gravitational field of a star with a planet in a circular orbit is considered. The invariant of this problem is the Jacobi integral. Preserving the value of the Jacobi integral can be a test for numerical algorithms solving the equation of motion. The invariant changes for particles in the planetary chaotic zone due to numerical errors that occur during close encounters with the planet. The limiting distances from the planet, upon reaching which the value of the Jacobi integral changes, are determined for Bulirsch-Stoer algorithm. It is shown that violation of the Jacobi integral can be used to define the boundaries of the planetary chaotic zone.

Differential rotation in neutron stars allows for significantly larger masses than rigid rotation. Some of those hypermassive objects are, however, unstable and collapse to a black hole immediately after formation. Yet, the exact threshold of dynamical stability is still unknown.

In our work we explore the limits on masses of neutron stars with various degrees of differential rotation which could be stable against a prompt collapse to a black hole by using turning-point (j-constant) criterion. We considered both spheroidal and quasi-toroidal differentially rotating neutron stars described by the polytropic equation of state. We find that massive configurations could be temporarily stabilized by differential rotation. Such objects are important sources of gravitational waves. Our results are a starting point for more detailed studies of stability using hydrodynamical codes.

We present a new algorithm to solve the equations of radiation hydrodynamics (RHD) in a frequency-integrated, two-moment formulation. Novel features of the algorithm include i) the adoption of a non-local Variable Eddington Tensor (VET) closure for the radiation moment equations, computed with a ray-tracing method, ii) support for adaptive mesh refinement (AMR), iii) use of a time-implicit Godunov method for the hyperbolic transport of radiation, and iv) a fixed-point Picard iteration scheme to accurately handle the stiff nonlinear gas-radiation energy exchange. Tests demonstrate that our scheme works correctly, yields accurate rates of energy and momentum transfer between gas and radiation, and obtains the correct radiation field distribution even in situations where more commonly used – but less accurate – closure relations like the Flux-limited Diffusion and Moment-1 approximations fail. Our scheme presents an important step towards performing RHD simulations with increasing spatial and directional accuracy, effectively improving their predictive capabilities.

Astrophysical systems possess various sites of particle acceleration, which gives rise to the observed non-thermal spectra. Diffusive shock acceleration (DSA) and stochastic turbulent acceleration (STA) are the candidates for producing very high energy particles in weakly magnetized regions. While DSA is a systematic acceleration process, STA is a random energization process, usually modelled as a biased random walk in energy space with a Fokker-Planck equation. In astrophysical systems, different acceleration processes work in an integrated manner along with various energy losses.

Here we study the interplay of both STA and DSA in addition to various energy losses, in a simulated RMHD jet cocoon. Further, we consider a phenomenologically motivated STA timescale and discuss its effect on the emission profile of the RMHD jet. A parametric study on the turbulent acceleration timescale is also conducted to showcase the effect of turbulence damping on the emission structure of the simulated jet.

In this paper I present a method to enhance the search sensitivity for long transient Gravitational Waves produced by Neutron Star instabilities. This method consists in a selective image filter, called Triangular Filter, to be applied to data spectrograms. It is shown that thanks to this implementation a ∼20% gain in sensitivity is achievable.

We present a mesh-free, neutrino transport approximation called Advanced Spectral Leakage, designed as a powerful tool for simulations of neutrino-driven winds in binary neutron star mergers with the Smoothed-Particle Hydrodynamics. We post-process a number of snapshots and compare relevant neutrino quantities with respect to computationally more expensive transport approaches. We find that the scheme recovers neutrino luminosities and mean energies within 25% accuracy and is computationally more efficient.

We present an a posteriori shock-capturing finite volume method algorithm called GP-MOOD. The method solves a compressible hyperbolic conservative system at high-order solution accuracy in multiple spatial dimensions. The core design principle in GP-MOOD is to combine two recent numerical methods, the polynomial-free spatial reconstruction methods of GP (Gaussian Process) and the a posteriori detection algorithms of MOOD (Multidimensional Optimal Order Detection). We focus on extending GP’s flexible variability of spatial accuracy to an a posteriori detection formalism based on the MOOD approach. The resulting GP-MOOD method is a positivity-preserving method that delivers its solutions at high-order accuracy, selectable among three accuracy choices, including third-order, fifth-order, and seventh-order.

To form stars in hydrodynamical simulations, we introduce the grouped star formation prescription to convert the grouped sink particles into stars that follow the IMF. We show that this method is robust in different physical scales. Such methods to form stars are likely to become more important as galactic or even cosmological scale simulations begin to probe sub-parsec scales.

GW190425 is the second gravitational wave (GW) event caused by a binary neutron star (BNS) merger. We report the result of the follow-up observation of GW190425 by the Gravitational-wave Electromagnetic-wave (EM) Counterpart Korean Observatory (GECKO). Our observation demonstrates that GECKO can detect EM counterpart of a GW170817-like event.

We report on our observing campaign of the compact binary merger GW190814, detected by the Advanced LIGO and Advanced Virgo detectors on August 14th, 2019. This signal has the best localisation of any observed gravitational wave (GW) source, with a 90% probability area of 18.5 deg2, and an estimated distance of ≈240 Mpc. We obtained wide-field observations with the Deca-Degree Optical Transient Imager (DDOTI) covering 88% of the probability area down to a limiting magnitude of w = 19.9 AB. Nearby galaxies within the high probability region were targeted with the Lowell Discovery Telescope (LDT), whereas promising candidate counterparts were characterized through multi-colour photometry with the Reionization and Transients InfraRed (RATIR) and spectroscopy with the Gran Telescopio de Canarias (GTC). We use our optical and near-infrared limits in conjunction with the upper limits obtained by the community to constrain the possible electromagnetic counterparts associated with the merger. A gamma-ray burst seen along its jet’s axis is disfavoured by the multi-wavelength dataset, whereas the presence of a burst seen at larger viewing angles is not well constrained. Although our observations are not sensitive to a kilonova similar to AT2017gfo, we can rule out high-mass (> 0.1 M⊙) fast-moving (mean velocity ≥ 0.3c) wind ejecta for a possible kilonova associated with this merger.

Computational fluid dynamics is a crucial tool to theoretically explore the cosmos. In the last decade, we have seen a substantial methodological diversification with a number of cross-fertilizations between originally different methods. Here we focus on recent developments related to the Smoothed Particle Hydrodynamics (SPH) method. We briefly summarize recent technical improvements in the SPH-approach itself, including smoothing kernels, gradient calculations and dissipation steering. These elements have been implemented in the Newtonian high-accuracy SPH code MAGMA2 and we demonstrate its performance in a number of challenging benchmark tests. Taking it one step further, we have used these new ingredients also in the first particle-based, general-relativistic fluid dynamics code that solves the full set of Einstein equations, SPHINCS_BSSN. We present the basic ideas and equations and demonstrate the code performance at examples of relativistic neutron stars that are evolved self-consistently together with the spacetime.

The Laser Interferometer Space Antenna (LISA) mission will observe from space gravitational waves emitted by neutron stars and white dwarfs within galactic binaries. These compact stars can have intense magnetic fields. Therefore, the impact of the magnetic fields on the orbital and the spins evolution of binary systems can potentially be detected by LISA through the GW’s strain. Within the magnetic dipole-dipole approximation, we found that magnetism generates a secular drift of the mean longitude which, in turn, shifts all the frequencies contained in the GW signal. For a quasi-circular orbit, the signal is mainly monochromatic and the magnetic shift is proportional to the product of the magnetic moments and is inversely proportional to the 7/2 power of the semi-major axis. Hence, for a highly magnetic binary system in compact orbit, a non-negligible amount of the frequency measured by LISA might have a magnetic origin.

Vortices are patches of fluid revolving around a central axis. They are ubiquitous in fluid dynamics. To the human eye, detecting vortices is a trivial task thanks to our inherent ability to identify patterns. To solve this task automatically, we developed the Vortector pipeline which was used to identify and characterize vortices in around one million snapshots of planet-disk interaction simulations in the context of planet formation. From the emergence of two regimes of vortex lifetime, one of which shows very long-lived vortices, we conclude that future resolved disk observations will predominantly detect vortices in the outer parts of protoplanetary disks.

In recent years, an increasing amount of attention is being paid to the gravitational few-body problem and its applications to astrophysical scenarios. Among the main reasons for this renewed interest there is large number of newly discovered exoplanets and the detection of gravitational waves. Here, we present two numerical codes to model three- and few-body systems, called tsunami and okinami. The tsunami code is a direct few-body code with algorithmic regularization, tidal forces and post-Newtonian corrections. okinami is a secular, double-averaged code for stable hierarchical triples. We describe the main methods implemented in our codes, and review our recent results and applications to gravitational-wave astronomy, planetary science and statistical escape theories.