Neutron stars have shown diverse characteristics, leading us to classify them into different classes. In this proceeding, I review the observational properties of isolated neutron stars: from magnetars, the strongest magnets we know of, to central compact objects, the so-called anti-magnetars, stopping by the rotation-powered pulsars and X-ray dim isolated neutron stars. Finally, I highlight a few sources that have exhibited features straddling those of different groups, blurring the apparent diversity of the neutron star zoo.

Galactic winds probe how feedback regulates the mass and metallicity of galaxies. Galactic winds have cold gas, which is mainly observable with absorption and emission lines. Theoretically studying how absorption lines are produced requires numerical simulations and realistic starburst UV backgrounds. We use outputs from a suite of 3D PLUTO simulations of wind-cloud interactions to first estimate column densities and temperatures. Then, to create synthetic spectra, we developed a python interface to link our PLUTO simulations to TRIDENT via the YT-package infrastructure. We produce UV backgrounds accounting for the star formation rate of starbursts. For this purpose, we use fluxes generated by STARBURST99, which are then processed through CLOUDY to create customised ion tables. Such tables are subsequently read into TRIDENT to generate absorption spectra. We explain how the various packages and tools communicate with each other to create ion spectra consistent with spectral energy distributions of starburst systems.

To study the dynamics of relativistic flows in astrophysical objects such as radio jets, we have developed a new special relativistic hydrodynamic (RHD) code based on the weighted essentially non-oscillatory (WENO) scheme, a high-order finite difference scheme. The code includes different WENO versions, and high-order time integration methods such as the 4th-order accurate Runge-Kutta (RK4) and strong stability preserving RK (SSPRK), as well as the equations of state (EOSs) that closely approximate the EOS of the single-component perfect gas in relativistic regime. Additionally, it is optimized for the reproduction of complex structures in multi-dimensional flows, and implements a modification of eigenvalues for the acoustic modes to effectively control carbuncle instability. As the first application of the code, we have simulated ultra-relativistic jets of FR II radio galaxies, and studied the nonlinear flow structures, such as shocks, velocity shear, and turbulence, through large-scale.

In this work we applied the previously developed self-consistent 1D model of hydrogen-helium atmosphere with suprathermal electrons to close-in hot neptune GJ 436 b. The obtained height profile of density shows the two-scale structure of the planetary atmosphere. The mass-loss rate is found to be about .

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.

On August 17, 2017, the LIGO/VIRGO collaboration detected the first gravitational wave signal coming from the merger of two neutron stars. This groundbreaking discovery, referred to as GW170817, revealed to us how heavy elements, such as gold and platinum, are synthesized through a mechanism known as rapid neutron capture (r-process). In order to fully understand these signals, we need to simulate the resulting accretion disk around a black hole, and its outflows. This task requires efficient computing codes that include general relativity magnetohydrodynamics (GRMHD), neutrino physics, and a model for matter at high densities. We present the implementation of a tabulated equation of state that takes care of matter at high densities and a neutrino leakage scheme that considers the impact of neutrinos into HARM3D, a GRMHD parallelized code. We also apply the tools to a magnetized torus.

Recent observations of the various phases in the lives of neutron star binaries across different messengers have led to major discoveries and raised new exciting questions about the environments of these systems. We will overview the current observational constraints on the formation rates and environments of neutron star binaries, as well as theoretical predictions that will be confronted with future observations.

Transit observations have revealed the existence of atmospheric escape in several hot Jupiters. High energy photons from the host star heat the upper atmosphere and drive the hydrodynamic escape. The escaping atmosphere can interact with the stellar wind from the host star. We run radiation hydrodynamics simulations with non-equilibrium chemistry to investigate the wind effects on the escape and the transit signature. Our simulations follow the planetary outflow driven by the photoionization heating and the wind interaction in a dynamically coupled, self-consistent manner. We show that the planetary mass-loss rate is almost independent of the wind strength, which however affects the Ly-α transit depth considerably. But the Hα transit depth is almost independent of the wind strength because it is largely caused by the lower hot layer. We argue that observations of both lines can solve the degeneracy between the EUV flux from the host and the wind strength.

The Murchison Widefield Array (MWA) has observed the entire southern sky (Declination, $\delta< 30^{\circ}$ ) at low radio frequencies, over the range 72–231MHz. These observations constitute the GaLactic and Extragalactic All-sky MWA (GLEAM) Survey, and we use the extragalactic catalogue (EGC) (Galactic latitude, $|b| >10^{\circ}$ ) to define the GLEAM 4-Jy (G4Jy) Sample. This is a complete sample of the ‘brightest’ radio sources ( $S_{\textrm{151\,MHz}}>4\,\text{Jy}$ ), the majority of which are active galactic nuclei with powerful radio jets. Crucially, low-frequency observations allow the selection of such sources in an orientation-independent way (i.e. minimising the bias caused by Doppler boosting, inherent in high-frequency surveys). We then use higher-resolution radio images, and information at other wavelengths, to morphologically classify the brightest components in GLEAM. We also conduct cross-checks against the literature and perform internal matching, in order to improve sample completeness (which is estimated to be $>95.5$ %). This results in a catalogue of 1863 sources, making the G4Jy Sample over 10 times larger than that of the revised Third Cambridge Catalogue of Radio Sources (3CRR; $S_{\textrm{178\,MHz}}>10.9\,\text{Jy}$ ). Of these G4Jy sources, 78 are resolved by the MWA (Phase-I) synthesised beam ( $\sim2$ arcmin at 200MHz), and we label 67% of the sample as ‘single’, 26% as ‘double’, 4% as ‘triple’, and 3% as having ‘complex’ morphology at $\sim1\,\text{GHz}$ (45 arcsec resolution). We characterise the spectral behaviour of these objects in the radio and find that the median spectral index is $\alpha=-0.740 \pm 0.012$ between 151 and 843MHz, and $\alpha=-0.786 \pm 0.006$ between 151MHz and 1400MHz (assuming a power-law description, $S_{\nu} \propto \nu^{\alpha}$ ), compared to $\alpha=-0.829 \pm 0.006$ within the GLEAM band. Alongside this, our value-added catalogue provides mid-infrared source associations (subject to 6” resolution at 3.4 $\mu$ m) for the radio emission, as identified through visual inspection and thorough checks against the literature. As such, the G4Jy Sample can be used as a reliable training set for cross-identification via machine-learning algorithms. We also estimate the angular size of the sources, based on their associated components at $\sim1\,\text{GHz}$ , and perform a flux density comparison for 67 G4Jy sources that overlap with 3CRR. Analysis of multi-wavelength data, and spectral curvature between 72MHz and 20GHz, will be presented in subsequent papers, and details for accessing all G4Jy overlays are provided at https://github.com/svw26/G4Jy.

Einstein Telescope (ET), a the future third generation gravitational wave detector will be sensitive to gravitational wave signal down to 1 Hz. We present an algorithm to estimate the parameters of the low mass merging compact binary systems such as localisation, chirp mass, redshift, mass ratios and total mass of the source which are crucial in order to estimate the capability of ET to study various compact binary populations. We analysed the compact binary populations of (i) Pop I and Pop II, (ii) Pop III, and (iii) globular clusters, with single ET.

Accretion discs appear in many astrophysical systems. In most cases, these discs are probably not completely axisymmetric. Discs in binary systems are often found to be misaligned with respect to the binary orbit. In this case, the gravitational torque from a companion induces nodal precession in misaligned rings of gas. We first calculate whether this precession is strong enough to overcome the internal disc torques communicating angular momentum. For typical parameters, precession torque wins. To check this result, we perform numerical simulations using the Smoothed Particle Hydrodynamics code, PHANTOM, and confirm that sufficiently thin and sufficiently inclined discs can break into distinct planes that precess effectively independently. Disc tearing is widespread and severely changes the disc structure. It enhances dissipation and promotes stronger accretion onto the central object. We also perform a stability analysis on isolated warped discs to understand the physics of disc breaking and tearing observed in numerical simulations. The instability appears in the form of viscous anti-diffusion of the warp amplitude and the surface density. The discovery of disc breaking and tearing has revealed new physical processes that dramatically change the evolution of accretion discs, with obvious implications for observed systems.

A numerical nonstationary two-dimensional MHD model of a protoplanetary disk near T Tauri star with a jet is developed. The model assumes consideration of pure gas ionization, optically thin cooling, anisotropic thermal conductivity and viscosity. The relaxation of gas-dynamic flows is analyzed. Based on the evolution of plasma flows, profiles of hydrogen spectral lines have been obtained.

While predominantly dipolar, large-scale magnetic fields are usually assumed in most studies involving neutron stars, there are multiple observational, theoretical hints and numerical simulations highlightening the importance of non-dipolar components. I review here the most important observational facts and numerical studies pointing towards the existence of magnetospheric currents and internal small-scale structures, arising from multipoles of poloidal and toroidal fields. This holds for all neutron star stages: at birth, during their lifetime and after a merger.

We argue that measurements of X-ray polarization using the recently launched Imaging X-ray Polarimetry Explorer will answer many open questions about magnetars in particular the physical state of their surfaces, whether vacuum birefringence exists, and the nature of the hard X-ray emission from these objects. We outline the capabilities of the instrument, specific models and the results of simulations for the magnetar 4U 0142+61.

We present our findings on magnetar spectral line analysis in the context of upcoming high resolution, high effective area, high throughput X-ray telescopes for two cases: persistent magnetar emission and magnetar bursts. For magnetars in quiescence, we present our preliminary work on modelling for phase-resolved emission. Our results reveal the necessity of constraining line depth and width concurrently with line energy to conclusively determine hotspot emission and corresponding magnetic field geometry. We then present the results of our simulations using effective area and response of various current and upcoming X-ray telescopes for magnetar spectral line detection and expand on the exciting opportunities upcoming telescopes provide to probe quiescent and burst emission region geometry and propagation in the extreme magnetic field of a magnetar.

Neutron stars are known to host extremely powerful magnetic fields. Among other effects, one of the consequences of harboring such fields is the deformation of the neutron star structure, leading, together with rotation, to the emission of continuous gravitational waves (CGWs). We present an extensive numerical study of magnetized neutron stars in GR with a large variety of different Equations of State (EoSs) and show that it is possible to find simple relations between the magnetic deformation of a neutron star, its mass and radius, that are mostly independent on the EoS or magnetic configuration. We discuss how these relations can be used in conjunction with possible future CGWs detection to set constrains on the EoS and magnetic configurations of NSs (e.g. the presence of a superconducting phase). By carrying out a population synthesis, we estimate the possible CGWs detectability of galactic millisecond pulsars, with third generation GW detectors.

We summarize a series of numerical experiments of collisional dynamics in dense stellar systems such as globular clusters (GCs) and in weakly collisional plasmas using a novel simulation technique, the so-calledMulti-particle collision (MPC) method, alternative to Fokker-Planck and Monte Carlo approaches. MPC is related to particle-mesh approaches for the computation of self consistent long-range fields, ensuring that simulation time scales with N log N in the number of particles, as opposed to N2 for direct N-body. The collisional relaxation effects are modelled by computing particle interactions based on a collision operator approach that ensures rigorous conservation of energy and momenta and depends only on particles velocities and cell-based integrated quantities.

Several populations of neutron stars have surface magnetic fields above the critical strength of 4.4 × 1013 G where the electron cyclotron energy equals its rest mass. These include high-field rotation-powered pulsars, X-ray dim isolated neutron stars (XDIN), and magnetars. In such ultra-strong fields, quantum effects in physical processes as well as additional exotic Quantum Electrodynamic processes only occurring at these high field strengths have a significant influence on the emitted radiation. Although very strong magnetic fields play a critical role both inside and outside of neutron stars, I will review primarily processes that operate in the neutron star magnetospheres and how they influence the observed radiation.