Describing the most compact objects in the Universe requires the use of Einstein's general relativity and solving its field equations. In the presence of matter, these equations must at least be coupled with the equations of general-relativistic hydrodynamics, and the potential inclusion of magnetic fields or neutrino radiation further increases the complexity.
While analytic solutions exist...
Evaluating joint models for the electromagnetic and gravitational signals received from the 2017 neutron-star merger has proven both challenging and insightful. By incorporating further model assumptions or different data sources, we can obtain even tighter constraints on related fields. In particular, such mergers offer a natural connection to both nuclear properties through the dense-matter...
The Hubble constant is a crucial cosmological parameter that is a measure of the rate of change of the cosmic scale factor per unit cosmic scale factor i.e. a_dot / a. There is a considerable discrepancy between the measurements of the Hubble constant from standard candle observations and those from cosmic microwave background (CMB) observations. Data from gravitational wave (GW) events can...
In this presentation I briefly discuss the role of the symmetry energy on low mass twin compact stars under the framework of the state-of-the-art compact star measurements. In particular, I review the conjecture of the universal symmetry energy contribution to the compact star equation of state that follows from the direct Urca constraint for the cooling of low mass compact stars. Implications...
Exercises for the lectures of Prof. T. Dietrich.
The crust of a neutron star is important for many astrophysical phenomena such as the cooling of the star and its transport properties. I will discuss results for neutron-star-crust elastic properties, namely the shear modulus, and their associated uncertainties, obtained within a bayesian analysis.
Identifying electromagnetic counterparts to neutron star mergers (Kilonovae) is crucial for multi-messenger astronomy but is made difficult by the presence of contaminants. In this work, we present an analysis of AT2025ulz: an optical candidate identified within the localization of the gravitational wave event S250818k. Despite initial spatio-temporal consistency, deep Hubble Space Telescope...
We investigate the equation of state (EOS) and macroscopic properties of neutron stars (NSs) and hyperonic stars within an Ab initio approach extended to include interacting Λ hyperons. The ΛN and ΛΛ interactions are described by realistic spin- and parity-dependent potentials fitted to hypernuclear data. Cold, charge-neutral, and β-equilibrated matter composed of neutrons, protons, electrons,...
The equation of state (EoS) for dense, strongly interacting matter serves as the central input in astrophysical simulations involving isolated compact objects and binary systems across various scenarios. While numerous models exist to describe the composition of cold neutron stars, the range narrows considerably when considering EoS that encompass varying temperatures, densities, and electron...
Gravitational wave astronomy provides a vital tool for probing extreme matter. This study investigates $f$-mode oscillations of cold, catalyzed Neutron stars as well as protoneutron stars with different evolutionary phases. We analyze the collective impact of nucleons, hyperons, phase transition to the quark matter, and dark matter admixtures on these oscillations employing full General...
In contrast to symmetric nuclear matter, which has been extensively studied in laboratory experiments, the matter inside neutron stars is highly isospin-asymmetric. We investigate the properties of strongly interacting matter under both symmetric and neutron-star-like conditions to determine how electric charge neutrality and beta equilibrium influence the emergence of quark matter. In...
The nuclear matter equation of state (EOS) is poorly known at the limit of high density and temperature. While this limit is not currently reached in laboratory settings, it is in neutron star interiors. In this talk, I will discuss an approach to probing the EOS using neutron star measurements. I will describe how neutron star mass and radius inferences from the NICER telescope and tidal...
We investigate the impact of a stiff dark matter equation of state (EoS) on the structure and stability of neutron stars. For dark matter, we use bosonic, self-interacting scalar fields that generate ultra-compact boson stars with compactness exceeding 1/3. Varying the dark matter particle mass and stiffness shifts stellar configurations across distinct regions of the mass–radius diagram,...
Spin-induced deformations of compact binary components can be probed through their gravitational-wave signals and are characterized by a parameter called kappa (κ), which equals 1 for black holes. Measuring its symmetric combination, κs, enables tests of the black hole nature of binary constituents. While previous studies focused on circular binaries, we investigate eccentric systems and...
The neo-neutron star (neo-NS) phase directly follows the proto-neutron star (PNS) phase. During the PNS phase, the core of the star forms and acquires its final properties, such as composition and size, whereas the crust forms later during the neo-NS phase. This work is a further development of previous neo-NS studies carried out by one of the authors.
I will present our preliminary results...
Identifying the electromagnetic counterparts to gravitational wave sources is vital to enabling the myriad of investigations possible with multi-messenger astronomy. However, locating faint, fast-varying transients in large sky localisations, given the intrinsic uncertainty in their detailed properties, is challenging. Here we investigate the expected gravitational wave localisations for a...
A significant fraction of neutron stars (NSs) rotate, and they can rotate very fast (e.g., millisecond pulsar). An isolated rotating NS or an NS that has permanently stopped accreting from its companion gradually loses its rotational energy and spins down. The most common cause for this is magnetic braking. As the star spins down, the centrifugal force decreases, the star shrinks, and its...
We compute the thermal conductivity and thermoelectric power
(thermopower) of the inner crust of compact stars across a broad
temperature–density domain relevant for proto–neutron stars, binary
neutron-star mergers, and accreting neutron stars. The analysis
covers the transition from a semi-degenerate to a highly degenerate
electron gas and assumes temperatures above the melting...
We study the influence of positrons on the outer crusts of neutron
stars and the interiors of white dwarfs, introducing them as a novel
component in both the composition of matter and in transport
processes. We solve a system of coupled Boltzmann kinetic equations
for the electron and positron distribution functions in the
relaxation-time approximation, taking into account...
One of the key factors influencing the structure and evolution of neutron stars is differential rotation. It plays a crucial role in understanding the behavior of matter in extreme conditions and affects observable properties like gravitational wave signals from Core-collapse supernovae or binary merger events. However, a major uncertainty exists surrounding the equation of state (EoS) used...
We explore the role of color superconductivity in quarkyonic matter under the conditions of color and electric neutrality at $\beta$- and strong equilibrium, as relevant for neutron stars. By explicitly incorporating the color-superconducting pairing gap into the phenomenological model of a smooth transition from hadron to quark matter, we extend the known quarkyonic framework to include this...
The hot quark matter created in heavy-ion collision experiments can
exhibit strong temperature and chemical-potential gradients, which in turn can generate electric fields through thermoelectric
effects. In this work, we investigate two relevant thermoelectric
coefficients—the thermopower (Seebeck coefficient) and the Thomson
coefficient—of two-flavor quark matter using the Kubo...
At high densities and low temperatures, hadronic matter is expected to undergo a first-order phase transition into a color-superconducting state. While such conditions occur in neutron stars, studies focusing only on cold neutron stars are not fully conclusive because they neglect the evolutionary processes that may influence the appearance of color-superconducting phases. A proto–neutron...
The measurement of the Hubble constant has became a critical effort given the discrepancy between the early and late universe measurements, dubbed as the Hubble tension. Since the discovery of gravitational waves (GWs) in 2015, compact binary mergers are used as an independent probe to infer cosmological parameters, since they may allow for joint measurements of their luminosity distance and...
Active galactic nuclei (AGNs) are among the most energetic objects in the Universe, with relativistic jets launched by accreting supermassive black holes. Blazars, a subclass of AGNs with jets aligned close to our line of sight, often exhibit strong variability across the electromagnetic spectrum. Quasi-periodic oscillations (QPOs) in blazar light curves can provide important clues about the...
The nuclear matter equation of state (EOS) is poorly known at the limit of high density and temperature. While this limit is not currently reached in laboratory settings, it is in neutron star interiors. In this talk, I will discuss an approach to probing the EOS using neutron star measurements. I will describe how neutron star mass and radius inferences from the NICER telescope and tidal...
Compact binary black hole systems are a primary source of gravitational wave detection through LIGO-Virgo-KAGRA detectors. Recent improvements to the detectors have led to improved sensitivity, leading to the detection of more gravitational wave signals. Recent studies have suggested the possibility of observing binary systems in the sub-solar mass range. In third-generation detectors, the...
The merging of binary black hole (BBH) systems produces gravitational wave (GW) events. We can use their direct measurement of luminosity distance (dL) in combination with the redshift of the merging system to place cosmological constraints. BBH mergers are not expected to produce any electromagnetic counterpart; thus, to obtain the redshift of the source event, we rely on a statistical...
Neutron stars provide a unique laboratory to probe dark matter (DM) through its gravitational imprint on stellar evolution. We study proto-neutron stars in a two-fluid, quasi-static framework with non-annihilating asymmetric DM (fermionic or bosonic) that interacts with ordinary matter only via gravity, and follow the Kelvin–Helmholtz cooling phase. We find a clear thermal signature: compact...
