Merger of binary neutron stars and black hole-neutron star binaries is the promising source of short-hard gamma-ray bursts, the most promising site for the r-process nucleosynthesis, and the source of kilonovae. To theoretically predict the merger and mass ejection processes and resulting electromagnetic emission, numerical simulation in full general relativity (numerical relativity) is the unique approach. We summarize our current understanding for the processes of neutron-star mergers and subsequent mass ejection based on the results of long-term numerical-relativity simulations. We pay particular attention to the electron fraction of the ejecta.

The entire southern sky (Declination, $\delta< 30^{\circ}$ ) has been observed using the Murchison Widefield Array (MWA), which provides radio imaging of $\sim$ 2 arcmin resolution at low frequencies (72–231 MHz). This is the GaLactic and Extragalactic All-sky MWA (GLEAM) Survey, and we have previously used a combination of visual inspection, cross-checks against the literature, and internal matching to identify the ‘brightest’ radio-sources ( $S_{\mathrm{151\,MHz}}>4$ Jy) in the extragalactic catalogue (Galactic latitude, $|b| >10^{\circ}$ ). We refer to these 1 863 sources as the GLEAM 4-Jy (G4Jy) Sample, and use radio images (of ${\leq}45$ arcsec resolution), and multi-wavelength information, to assess their morphology and identify the galaxy that is hosting the radio emission (where appropriate). Details of how to access all of the overlays used for this work are available at https://github.com/svw26/G4Jy. Alongside this we conduct further checks against the literature, which we document here for individual sources. Whilst the vast majority of the G4Jy Sample are active galactic nuclei with powerful radio-jets, we highlight that it also contains a nebula, two nearby, star-forming galaxies, a cluster relic, and a cluster halo. There are also three extended sources for which we are unable to infer the mechanism that gives rise to the low-frequency emission. In the G4Jy catalogue we provide mid-infrared identifications for 86% of the sources, and flag the remainder as: having an uncertain identification (129 sources), having a faint/uncharacterised mid-infrared host (126 sources), or it being inappropriate to specify a host (2 sources). For the subset of 129 sources, there is ambiguity concerning candidate host-galaxies, and this includes four sources (B0424–728, B0703–451, 3C 198, and 3C 403.1) where we question the existing identification.

PRESTALINE is a package allowing a user to simulate and analyse spectra of various astrophysical objects. The package is based on the numerical models PRESTA (Kochina & Wiebe (2017)) and RADEX (van der Tak et al. (2007)). PRESTALINE provides the direct comparison of theoretical models with observations and allows estimating physical conditions in a studied object, such as kinetic temperature and chemical composition. Here we present the results of applying PRESTALINE to the test object DR21(OH) and discuss possible applications and future extensions of the project.

The load imbalance and communication overhead of parallel computing are crucial bottlenecks for galaxy simulations. A successful way to improve the scalability of astronomical simulations is a Hamiltonian splitting method, which needs to identify such regions integrated with smaller timesteps than the global timestep for integrating the entire galaxy. In the case of galaxy simulations, the regions inside supernova (SN) shells require the smallest steps. We developed the deep learning model to forecast the region affected by the SN shell’s expansion during one global step. In addition, we identified the particles with small timesteps using image processing. We can identify target particles using our method with a higher identification rate (88 % to 98 % on average) and lower “non-target”-to-“target” fraction (6.4 to 5.5 on average) compared to the analytic approach with the Sedov-Taylor solution. Our method using Hamiltonian splitting and deep learning will improve the performance of extremely high-resolution galaxy simulations.

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.

GW170817, the merger of two neutron stars witnessed through both its gravitational wave siren and its glow at all wavelengths of light, represents the first multi-messenger detection of a compact binary merger. The association of the GW in-spiral signal from GW170817 with a γ-ray burst, a kilonova, and a non-thermal afterglow spanning all bands of the electromagnetic spectrum, has provided rich constraints on the physics and astrophysics of neutron stars. Starting from the example of GW170817, I briefly summarize recent results on observations of electromagnetic afterglows from gravitational wave triggers. In the light of these results, I highlight some key questions that are yet to be answered after the GW170817 discovery. I conclude by commenting briefly on some opportunities that lie in front of us, as improvements in ground-based gravitational wave detectors’ sensitivities will transform a trickle of multi-messenger discoveries into a flood, bringing the field of gravitational wave astronomy from its infancy to its maturity.

Coalescence of binary neutron stars gives rise to kilonova, thermal emission powered by radioactive decays of newly synthesized r-process nuclei. Observational properties of kilonova are largely affected by bound-bound opacities of r-process elements. It is, thus, important to understand atomic properties of heavy elements to link the observed signals with nucleosynthesis of neutron star mergers. In this paper, we introduce the latest status of kilonova modeling by focusing on the aspects of atomic physics. We perform systematic atomic structure calculations of r-process elements to understand element-to-element variation in the opacities. We demonstrate that the properties of the atomic structure of heavy elements are imprinted in the opacities of the neutron star merger ejecta and consequently in the kilonova light curves and spectra. Using this latest opacity dataset, we briefly discuss implications for GW170817, expected diversity of kilonova emission, and prospects for element identification in kilonova spectra.

The decay of the magnetic field in the interior of a magnetar may trigger electron captures by nuclei in the stellar crust, thus providing an internal source of heating. In turn, the onset of electron captures and the heat released are altered by the magnetic field due to the Landau–Rabi quantization of electron motion. The loss of magnetic pressure might also lead to pycnonuclear fusions of the lightest elements. The maximum amount of heat that can be possibly released by each reaction and their location are calculated using nuclear data from both experiments and theoretical predictions of the Brussels-Montreal models based on self-consistent Hartree-Fock-Bogoliubov calculations. Results are found to be consistent with those inferred empirically by comparing neutron-star cooling simulations with observed thermal luminosity of soft gamma-ray repeaters and anomalous X-ray pulsars.

Identification of the electromagnetic-wave (EM) counterparts of gravitational-wave (GW) sources can significantly broaden the research scope of GW astronomy, by pinpointing the exact locations of GW events and their environments, and using GW sources as standard sirens for cosmology. Yet, only one GW event has been found to be associated with an EM counterpart so far. Here, we outline the challenges of identifying EM counterparts of GW events, and describe our global network of telescopes that has been used to uncover GW EM counterparts. We also introduce a new facility in construction, the 7-dimensional telescope (7DT). Our GECKO observations have demonstrated that we can cover 50 deg2 within one hour to find kilonovae at a few hundred Mpc away. Furthermore, 7DT will produce a low resolution spectral map of the GW localization area, facilitating the EM counterpart search.

Binary neutron star (NS) mergers have been expected to synthesize r-process elements and cause electromagnetic radiation called kilonovae. Although r-process nucleosynthesis was confirmed by the observations of GW170817/AT2017gfo, individual elements have not been identified except for strontium. Toward identification of elements in kilonova spectra, we perform radiative transfer simulations in NS merger ejecta. We find that Sr II triplet lines appear in the spectrum, which is consistent with the absorption feature observed in GW170817/AT2017gfo. The synthetic spectrum also shows the strong Ca II triplet lines. Absence of the Ca II line features in GW170817/AT2017gfo implies that the Ca/Sr ratio is < 0.002 in mass fraction, which is consistent with nucleosynthesis for electron fraction ≥ 0.40 and entropy per nucleon (in units of Boltzmann constant) ≥ 25. Identification of absorption lines in near-infrared wavelengths which have not yet been decoded may lead to clarify the abundances synthesized in NS merger ejecta.

The formation of the first galaxies in the Universe is the new frontier of both galaxy formation and reionization studies. This creates a fierce new challenge, i.e. to simultaneously understand in a unique and coherent picture the processes of galaxy formation and reionization, and – crucially – their connection. To this end, we present the thesan suite of cosmological radiation-magneto-hydrodynamical simulations. They are unique since they: (i) cover a very broad range of spatial and temporal scales; (ii) include an unprecedentedly-broad range of physical processes for simulations of such scales and resolution; (iii) exploit knowledge accumulated at low redshift to minimize the number of free parameters in the physical model; (iv) use a variance-suppression technique in the production of initial conditions to increase their statistical fidelity. Finally, the thesan suite includes multiple runs of the same initial conditions, exploring current unknowns in the physics of dark matter and ionizing sources.

IXPE is a NASA/ASI Small Explorer Mission. It will probe the X-ray polarization properties of celestial sources. In particular, for Accretion-powered Millisecond Pulsars (AMPs), IXPE can provide us with unique information on their geometry. These information together with pulse shape modelling will strongly boost the achievable sensitivity on measuring the AMPs mass and the radius. As a case-study, we simulated an observation of SAXJ1808.4-3658 and studied the accuracy that can be achieved in the measured time-dependent Stokes profiles. From these data we estimated how well IXPE will be able to constraint NS geometrical parameters, such as the inclination and hot-spot co-latitude angles.

The purpose of the present work is a detailed investigation of the dynamical evolution of Collinder 135 and UBC 7 star clusters. We present a set of dynamical numerical simulations using realistic star cluster -body modeling technique with the forward integration of the star-by-star cluster models to the present day, based on best-available 3D coordinates and velocities obtained from the latest Gaia EDR3 data release. We have established that Collinder 135 and UBC 7 are probably a binary star cluster and have common origin. We carried out a full star-by-star N-body simulation of the stellar population of both clusters using the new algorithm of Single Stellar Evolution and performed a comparison of the results obtained in the observational data (like cumulative number counts), which showed a fairly good agreement.

The convection-enhanced neutrino-driven supernova engine’s success in explaining a myriad of supernova properties has set it as the standard engine behind supernova. However, due to the success of rotationally-powered engines in explaining astrophysical transients like gamma-ray bursts, these engines have been revived as possible drivers of normal supernovae, competing with this standard engine. In this paper, these competing engines, and the constraints placed by compact remnant observations on these engines, are reviewed. We find that, with these constraints, such rotationally-powered engines can explain less than 1% of the current supernova remnants. In addition, we find that the remnant mass distribution can be used to constrain properties of the convection-enhanced neutrino-driven engine, helping astronomers understand the nature of convection in this engine.

Five long gamma-ray bursts (GRBs) have been found to have very high energy (VHE, > 100GeV) counterparts. Interestingly, more than one emission mechanism has been invoked to explain the VHE counterpart from different events. As a result of this discovery, it has become apparent that we have been missing half of the energy produced in the afterglow of GRBs. We have been studying the radio afterglows in order to investigate whether these VHE GRBs have unusual jet properties. Studying these events in the radio waveband is advantageous as the emission at lower frequencies is brighter for longer enabling detailed, long term study of the jet evolution. The jet properties and environments of these GRBs vary hugely in a similar manner to that seen in the ‘regular’ long GRB population with evidence of bright reverse shock emission and multiple jet components. This work is presented on behalf of a much larger collaboration.

In the present work, we have developed a two-dimensional gravitational model of barred galaxies to analyse the fate of escaping stars from the central barred region. For that, the model has been analysed for two different bar profiles viz. strong and weak. Here the phenomena of stellar escape from the central barred region have been studied from the perspective of an open Hamiltonian dynamical system. We observed that the escape routes correspond to the escape basins of the two index-1 saddle points. Our results show that the formation of spiral arms is encouraged for the strong bars. Also, the formation of grand design spirals is more likely for strong bars if they host central super massive black holes (SMBHs). In the absence of central SMBHs, the formation of less-prominent spiral arms is more likely. Again, for weak bars, the formation of inner disc rings is more probable.

T Tauri Stars (TTSs) offer a unique chance to study the physics of non-relativistic accretion engines. In this invited talk, the current status of the field is presented with special emphasis on the predictive power of the numerical simulation of magnetospheric accretion and close binary systems and its impact on astronomical observations.

The observational properties of isolated NSs are shaped by their magnetic field and surface temperature. They evolve in a strongly coupled fashion, and modelling them is key in understanding the emission properties of NSs. Much effort was put in tackling this problem in the past but only recently a suitable 3D numerical framework was developed. We present a set of 3D simulations addressing both the long-term evolution (≈ 104–106 yrs) and short-lived outbursts (≲ 1 yr). Not only a 3D approach allows one to test complex field geometries, but it is absolutely key to model magnetar outbursts, which observations associate to the appearance of small, inherently asymmetric hot regions. Even though the mechanism that triggers these phenomena is not completely understood, following the evolution of a localised heat injection in the crust serves as a model to study the unfolding of the event.

We report the initial results of deep eROSITA monitoring of the magnificent seven isolated neutron stars (INSs). Thanks to a combination of high count statistics and good energy resolution, the eROSITA datasets unveil the increasingly complex energy distribution of these presumably simple thermal emitters. For three targets, we report the detection of multiple (in some cases, phase-dependent) spectral absorption features and deviations from the dominant thermal continuum. Unexpected long-term changes of spectral state and timing behaviour have additionally been observed for two INSs. The results pose challenging theoretical questions on the nature of the variations and absorption features and ultimately impact the modeling of the atmosphere and cooling of highly magnetised neutron stars.