The tidal field of a black hole can turn a star into a gas stream whose orbit can precess, especially if the a black hole is rapidly spinning. In this work, we investigate the impact of precession on the light curves of tidal disruption events (TDE). To do so, we perform two-dimensional radiation-hydrodynamic simulations of the interaction of the TDE wind and luminosity with the precessed stream wrapped around the black hole. Our results show that in events with black holes of ∼ 10⁶ M_⊙ and no orbit-spin inclination, the line of sight has little effect on the light curves, since the stream covers a small fraction of the solid angle as the precession is confined to the orbital plane. In the case of black holes of ≳ 10⁷ M_⊙ and high inclination (𝑖 ∼ 90˚), the light curve peaks can be delayed by ∼100 days due to presence of the precessed stream blocking the radiation in the early phase of the event. We also discuss our efforts to model self-consistently the hydrodynamic evolution of a tidal stellar stream on curved spacetimes by the presence of a massive black hole.

Identifying the progenitors of thermonuclear supernovae (Type Ia supernovae; SNe Ia) remains a key objective in contemporary astronomy. The rare sub-class of SNe Ia-CSM that interacts with circumstellar material (CSM) allows for studies of the progenitor’s environment before explosion, and generally favours single-degenerate progenitor channels. The case of SN Ia-CSM PTF11kx clearly connected thermonuclear explosions with hydrogen-rich CSM-interacting events, and the more recent SN 2020eyj connected SNe Ia with helium-rich companion progenitors. Both of these objects displayed delayed CSM interaction which established their thermonuclear nature. Here we present a study of SN 2020aeuh, a Type Ia-CSM with delayed interaction. We analyse photometric and spectroscopic data that monitor the evolution of SN 2020aeuh and compare its properties with those of peculiar SNe Ia and core-collapse SNe. At early times, the evolution of SN 2020aeuh resembles a slightly overluminous SN Ia. Later, the interaction-dominated spectra develop the same pseudocontinuum seen in Type Ia-CSM PTF11kx and SN 2020eyj. However, the later-time spectra of SN 2020aeuh lack hydrogen and helium narrow lines. Instead, a few narrow lines could be attributed to carbon and oxygen. We fit the pseudobolometric light curve with a CSM-interaction model, yielding a CSM mass of 1 − 2 M_⊙. We propose that SN 2020aeuh was a Type Ia supernova that eventually interacted with a dense medium that was deficient in both hydrogen and helium. Whereas previous SNe Ia-CSM constitute our best evidence of non-degenerate companion progenitors, the CSM around SN 2020aeuh is more difficult to understand. We include a hydrodynamical simulation for a double-degenerate dynamical collision to showcase that such a progenitor scenario could produce significant amounts of hydrogen-poor CSM, although likely not as much as the inferred CSM mass around SN 2020aeuh. It is clear that SN 2020aeuh challenges current models of stellar evolution leading up to a SN Ia explosion.

The ejection of neutron-rich matter is one of the most important consequences of a neutron star merger. While the bulk of the matter is ejected at fast, but non-relativistic velocities (⁠∼0.2c⁠), a small amount of mildly relativistic dynamic ejecta have been seen in a number of numerical simulations. Such ejecta can have far-reaching observational consequences ranging from the shock breakout burst of gamma-rays promptly after the merger, to an early (⁠∼1 h post-merger) blue kilonova precursor signal, to synchrotron emission years after the merger (‘kilonova afterglow’). These all potentially carry the imprint of the binary system parameters and the equation of state. By analysing Lagrangian simulations in full general relativity, performed with the code sphincs_bssn, we identify two ejection mechanisms for fast ejecta: (i) about 30 per cent of the ejecta with v>0.4c are ‘sprayed out’ from the shear interface between the merging stars and escape along the orbital plane and (ii) the remaining ∼70 per cent of the fast ejecta result from the central object ‘bouncing back’ after strong, general-relativistic compression. This ‘bounce component’ is ejected in a rather isotropic way and reaches larger velocities (by ∼0.1c⁠) so that its faster parts can catch up with and shock slower parts of the spray ejecta. Even for a case that promptly collapses to a black hole, we find fast ejecta with similar properties to the non-collapsing case, while slower matter parts are swallowed by the forming black hole. We discuss observational implications of these fast ejecta, including shock breakout and kilonova afterglow.

The merger origin long GRB 211211A was a class (re-)defining event. A precursor was identified with a  ~1s separation from the main burst, as well as a claimed candidate quasi-periodic oscillation (QPO) with a frequency ~20 Hz. Here, we explore the implications of the precursor, assuming the quasi-periodicity is real. The precursor variability time-scale requires relativistic motion with a Lorentz factor ⁠, and implies an engine-driven jetted outflow. The declining amplitude of the consecutive pulses requires an episodic engine with an ‘on/off’ cycle consistent with the QPO. For a black-hole central engine, the QPO can have its origin in Lense–Thirring precession of the inner disc at ~6−9 r_g (gravitational radii) for a mass  , and   for and dimensionless spin ⁠. Alternatively, at a disc density of ~10⁸⁻¹² g cm⁻³⁠, the required magnetic field strength for a QPO via magnetohydrodynamic effects will be of the order of B ~ 10¹²⁻¹⁴ G. If the central engine is a short-lived magnetar or hypermassive neutron star, then a low-frequency QPO can be produced via instabilities within the disc at a radius of  ~ 20–70 km, for a disc density ∼ 10⁹⁻¹² g cm⁻³ and magnetic field  G. The QPO cannot be coupled to the neutron star spin, as the co-rotation radius is beyond the scale of the disc. Neither engine can be ruled out – however, we favour an origin for the precursor candidate QPO as early jet–disc coupling for a neutron star–black hole merger remnant with mass ⁠.

The dynamics of a binary neutron stars merger is governed by physics under the most extreme conditions, including strong space–time curvature, ultrahigh matter densities, luminous neutrino emission, and the rapid amplification of the initial neutron star magnetic fields. Here, we systematically explore how sensitive the magnetic field evolution is to the total mass of the merging binary, to the mass ratio of its components, the stellar spins, and to the equation of state. For this purpose, we analyse 16 state-of-the-art general relativistic magnetohydrodynamics simulations that employ a subgrid-scale model to account for the unresolved small-scale turbulence. We find that strong and rapid amplification of the magnetic field to volume-averaged values of ∼10¹⁶ G in the high-density regions is a very robust outcome of a neutron star merger and this result is only marginally impacted by either mass, mass ratio, spin, or equation of state.

We present and explore a new shock-capturing particle hydrodynamics approach. Our starting point is a commonly used discretization of smoothed particle hydrodynamics. We enhance this discretization with Roe’s approximate Riemann solver, we identify its dissipative terms, and in these terms, we use slope-limited linear reconstruction. All gradients needed for our method are calculated with linearly reproducing kernels that are constructed to enforce the two lowest-order consistency relations. We scrutinize our reproducing kernel implementation carefully on a “glass-like” particle distribution, and we find that constant and linear functions are recovered to machine precision. We probe our method in a series of challenging 3D benchmark problems ranging from shocks over instabilities to Schulz-Rinne-type vorticity-creating shocks. All of our simulations show excellent agreement with analytic/reference solutions.

With ongoing advancements in nuclear theory and experimentation, together with a growing body of neutron star (NS) observations, a wealth of information on the equation of state (EOS) for matter at extreme densities has become accessible. Here, we utilize a hybrid EOS formulation that combines an empirical parametrization centered around the nuclear saturation density with a generic three-segment piecewise polytrope model at higher densities. We incorporate data derived from chiral effective field theory (LEFT), perturbative quantum chromodynamics (pQCD), and experiments such as PREX-II and CREX. Furthermore, we examine the influence of a total of 70 NS mass measurements up to April 2023, as well as simultaneous mass and radius measurements derived from the x-ray emission from surface hot spots on NSs. Additionally, we consider constraints on tidal properties inferred from the gravitational waves emitted by coalescing NS binaries. To integrate this extensive and varied array of constraints, we utilize a hierarchical Bayesian statistical framework to simultaneously deduce the EOS and the distribution of NS masses. We find that incorporating data from LEFT significantly tightens the constraints on the EOS of NSs near or below the nuclear saturation density. However, constraints derived from pQCD computations and nuclear experiments such as PREX-II and CREX have minimal impact. Taking into account all available data, we derive estimates for key parameters characterizing the EOS of dense nuclear matter. Specifically, we determine the slope (L) and curvature (Ksym) of the symmetry energy to be 54+10-10 and -158+73 90% credibility, respectively. Additionally, we infer the radius and tidal deformability of an NS with a mass of 1.4 solar masses (M circle dot) to be 12.34+0.43 maximum mass of a nonrotating NS to be 2.22+0.21 -0.19M circle dot with 90% credibility.

Context. The reported discovery of a cold (∼104 K) disc-like structure within the central 5 × 10-3 pc around the super-massive black hole at the centre of the Milk Way, Sagittarius A∗ (Sgr A∗), has challenged our understanding of the gas dynamics and thermodynamic state of the plasma in its immediate vicinity. State-of-the-art simulations do not agree on whether or not such a disc can indeed be a product of the multiple stellar wind interactions of the mass-losing stars in the region. Aims. The aims of this study are to constrain the conditions for the formation of a cold disc as a natural outcome of the system of the mass-losing stars orbiting around Sgr A∗, to investigate whether the disc is a transient or long-lasting structure, and to assess the validity of the model through direct comparisons with observations. Methods. We performed a set of hydrodynamic simulations of the observed Wolf-Rayet (WR) stars feeding Sgr A∗ using the finite- volume adaptive mesh refinement code Ramses. We focus, for the first time, on the impact of the chemical composition of the plasma emanating from the WR stars. Results. The simulations show that the chemical composition of the plasma affects the radiative cooling to a sufficient degree to impact the properties of the medium, such as density and temperature, and, as a consequence, the rate at which the material inflows onto Sgr A∗. We demonstrate that the formation of a cold disc from the stellar winds is possible for certain chemical compositions that are consistent with the current observational constraints. However, even in such cases, it is not possible to reproduce the reported properties of the observed disc-like structure, namely its inclination and the fluxes of its hydrogen recombination lines. Conclusions. We conclude that the stellar winds alone are not sufficient to form the cold disc around Sgr A∗ inferred from observations. Either relevant ingredients are still missing in the model, or the interpretation of the observed data needs to be revised.

Context. Several enigmatic dusty sources have been detected in the central parsec of the Galactic Center. Among them is X7, located at only ~0.02 pc from the central supermassive black hole, Sagittarius A∗ (Sgr A∗). Recent observations have shown that X7 is becoming elongated due to the tidal forces of Sgr A∗. X7 is expected to be fully disrupted during its pericenter passage around 2035, which might impact the accretion rate of Sgr A∗. However, its origin and nature are still unknown. Aims. We investigated the tidal interaction of X7 with Sgr A∗ in order to constrain its origin. We tested the hypothesis that X7 was produced by one of the observed stars with constrained dynamical properties in the vicinity of Sgr A∗. Methods. We employed a set of test-particle simulations to reproduce the observed structure and dynamics of X7. The initial conditions of the models were obtained by extrapolating the observationally constrained orbits of X7 and the known stars into the past, making it possible to find the time and source of origin by minimizing the three-dimensional separation and velocity difference between them. Results. Our results show that ejecta from the star S33/S0-30, launched in ~1950, can to a large extent replicate the observed dynamics and structure of X7, provided that it is initially elongated with a velocity gradient across it, and with an initial maximum speed of ~600 km s-1. Conclusions. Our results show that a grazing collision between the star S33/S0-30 and a field object such as a stellar-mass black hole or a Jupiter-mass object is a viable scenario to explain the origin of X7. Despite the uncertainties in the rate of these encounters, recent estimations show that it is plausible for such a scenario to have occurred recently.

A major ingredient for kilonova lightcurves is the radioactive heating rate and its dependence on the electron fraction and velocity of the ejecta and, in principle, on the nuclear mass formula. Heating-rate formulae commonly used as the basis for kilonova models previously employed in the literature produce substantially different outputs for high electron fractions (Y e ≳ 0.3) and at late times (t ≳ 1 day) compared to newer prescriptions. Here, we employ standard semianalytical models for kilonovae with better heating rate prescriptions valid for the full parameter space of kilonova velocities and electron fractions to explore the impact of the heating rate on kilonova lightcurves. We show the dangers of using inappropriate heating rate estimates by simulating realistic observations and inferring the kilonova parameters via a misspecified heating-rate prescription. While providing great fits to the photometry, an incorrect heating-rate prescription fails to recover the input ejecta masses with a bias significantly larger than the typical statistical uncertainty. This bias from an incorrect prescription has significant consequences for interpreting kilonovae, their use as additional components in gamma-ray burst afterglows, and understanding their role in cosmic chemical evolution or for multimessenger constraints on the nuclear equation of state. We showcase a framework and tool to better determine the impact of different modeling assumptions and uncertainties on inferences into kilonova properties.