Continuous gravitational Waves (CWs) are a very promising, not yet detected, and interesting class of persistent and semi-periodic signals. They are emitted mainly by rapidly rotating asymmetric neutron stars, with frequencies that are well covered by the [10-3 000] Hz range of the advanced LIGO-Virgo detectors. Due to the expected small degree of asymmetry of a neutron star, the search for this kind of signals is extremely challenging, and can be very computationally expensive when the source parameters are not known or not well constrained. CW detection from a spinning neutron star will allow us to characterize its structure and properties, making this source an unparalleled laboratory for studying several key issues in fundamental physics and relativistic astrophysics, in conditions that cannot be reproduced on Earth. The most recent methodologies used in CW searches will be discussed, and the latest results from the third advanced LIGO-Virgo observational run will be presented. A summary of future prospects to feasibly detect such feeble signals as the detector performance improves, and ever-more-sensitive and robust data-analysis algorithms are implemented, will be also outlined.

The dense matter equation of state (EoS), describing the state of matter under the extreme conditions found in neutron stars, is not accurately known. However, significant process has been made in recent years through the emergence of new observational avenues of neutron stars. Firstly, the X-ray timing telescope NICER has delivered two joint mass-radius measurements, for pulsars PSR J0030+0451 and PSR J0740+6620, using pulse profile modeling. Secondly, gravitational wave detections of binary neutron star (BNS) mergers allow for a measurement of the EoS-dependent tidal deformability, as demonstrated in the first detected BNS merger GW170817. Additionally, electromagnetic radiation from the subsequent ultraviolet-optical-infrared transient (the kilonova) originating from the ejected material in GW170817 further probes the binary system and the EoS. We demonstrate how the joint analysis of these multi-messenger observations of neutron stars significantly constrains the dense matter EoS. We then describe, in more detail, a framework to jointly analyse a gravitational wave signal and the accompanying kilonova light curves, focusing on possible future black hole–neutron star (BHNS) mergers. We highlight the potential for multimessenger BHNS to constrain the tidal deformability of the neutron star, thereby increasing our understanding of the dense matter EoS.

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.

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.

The hosts of binary neutron star (BNS) mergers and hence short GRBs are not only galaxies with old stellar populations, infact most short GRB hosts are at least mildly star-forming galaxies. According to theoretical studies of merger populations, both short and long merging time-scales are expected. The immediate environments of BNS mergers are not as directly related to the property of the progenitor system as for long GRBs, since the system usually travelled a significant distance from their birth place. However, studying the stellar population properties across the host can still give us vital information on the contribution of formation channels and on merger timescales. Here we review the properties of NS merger hosts in emission using integrated-light and resolved observations. The afterglows of short GRBs furthermore serve to study the interstellar medium in their host galaxies in absorption. We present our best example to date, GRB 160410A at z = 1.7, one of the highest redshift short GRBs.

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.

Situation with highly magnetized neutron stars in binary systems is not yet certain. On the one hand, all best studied magnetars seem to be isolated objects. On the other, there are many claims based on model-dependent analysis of spin properties or/and luminosity of neutron stars in X-ray binaries in favour of large fields. In addition, there are a few results suggesting a magnetar-like activity of neutron stars in close binary systems. Most of theoretical considerations do not favour even existence, not speaking about active decay, of magnetar-scale fields in neutron stars older than ∼106 yrs. However, alternative scenarios of the field evolution exist. I provide a brief review of theoretical and observational results related to the presence of neutron stars with large magnetic field in binaries and discuss perspectives of future studies.

Fast Radio Bursts (FRBs) are millisecond-long bursts of radio emission of extragalactic origin. The nature or FRBs is still unknown. Whether all FRBs are representatives of the same source population, or whether multiple underlying populations exist, is also unknown. One class that stands out is that of the “repeaters”, i.e. FRBs from which multiple bursts have been detected. In these cases, appropriate models should be non-cataclysmic but yet being able to create powerful coherent radio emission. Magnetars are among those source types that are considered as possible explanation for (repeating) FRBs. This review will summarise the basic properties of FRBs and those of magnetars to provide a critical assessment of the possible physical connection between these classes of sources. We conclude that while magnetars may indeed be related to the FRB phenomenon, it is unlikely that they explain all FRBs, i.e. at least two classes of FRBs exist.

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.

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.

GW170817/GRB 170817A has had a huge impact on our understanding of gamma-ray burst (GRB) afterglows, and has prompted a huge sustained effort at modeling the details of the geometry and emission from GRB jets. While no additional electro-magnetic counterparts have been detected to gravitational wave emission from neutron star mergers so far, it is certainly reasonable to expect further detections in the future. Whether these will be very similar in nature to GRB 170817A or instead will provide us with samplings of afterglow model parameters across a wide parameter space remains an open question. In this presentation I will survey some of the work done by the various active groups worldwide in theoretical modeling and understanding afterglows post 170817A.

The spectacular detection of the first electromagnetic counterpart of a gravitational wave event detected by the LIGO/Virgo interferometers and originated by the coalescence of a double neutron star (NS) system (GW 170817) marked the dawn of a new era for astronomy. The short GRB 170817A associated to the gravitational wave event provided the long-sought evidence that at least a fraction of short GRBs are originated by NS-NS merging and suggested the intriguing possibility that relativistic jets can be launched in the process of a NS-NS merger. The wealth of data collected provided the first compelling observational evidence for the existence of kilonovae, i.e. the emission due to radioactive decay of heavy nuclei produced through rapid neutron capture. Besides the remarkable event associated to GW 170817, kilonova signatures have been identified in a few short GRBs light curves, supporting a scenario where kilonovae are ubiquitous and can probe neutron star mergers well beyond the horizon of the gravitational wave detectors. In this paper I will review the situation and perspectives of our understanding of short GRBs progenitors and kilonovae in the multi-messenger era.

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.

Scientific synergies between Athena and some of the key multi-messenger facilities that should be operative concurrently with Athena are presented. These facilities include LIGO A+, Advanced Virgo+ and future detectors for ground-based observation of gravitational waves (GW), LISA for space-based observations of GW, IceCube and KM3NeT for neutrino observations, CTA for very high energy observations. Multimessenger synergy science themes discussed here include pressing issues in the field of Astrophysics, Cosmology and Fundamental physics such as: the central engine and jet physics in compact binary mergers, accretion processes and jet physics in SMBBHs and in compact stellar binaries, the equation of state in neutron stars, cosmic accelerators and the origin of cosmic rays, the origin of intermediate and high-Z elements in the Universe, the Cosmic distance scale and tests of General Relativity and Standard Model. Observational strategies for implementing the identified science topics are also discussed.

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.

The evolution of the magnetic field in neutron star crusts because of the Hall effect has received significant attention over the last two decades, which is strongly justified because of the dominance of this effect in highly magnetised neutron stars. However, the applicability of the Hall effect is based on the assumption that the crust does not fail and sustains its rigidity. This assumption can be violated for substantially strong magnetic fields. If this is the case, the evolution of the magnetic field is described by a different set of equations, which include the effects of a non-rigid crust. In this talk, after a brief review of the main characteristic of the Hall evolution, I will discuss the impact a plastic flow of the crust has on the magnetic field, studying axisymmetric models. Moreover, the way the crust fails impacts the overall evolution, with major differences appearing if the failure is local, intermediate or global. Quite remarkably, crustal failure and plasticity do not annul the Hall effect, and under certain circumstances they may even lead to a more dramatic evolution. I will discuss the impact of these effects in the context of neutron star timing behaviour, with special focus on timing noise, outbursts and glitches.

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.

Magnetars are neutron stars with exceptionally strong dipole magnetic fields which are observed to display a range of x-ray flaring behavior, but the flaring mechanism is not well understood. The third observing run of Advanced LIGO and Virgo extended from April 1, 2019 to March 27, 2020, and contained x-ray flares from known magnetar SGR 1935+2154, as well as the newly-discovered magnetar, Swift J1818-1607. We search for gravitational waves coincident with these magnetar flares with minimally modeled, coherent searches which specifically target both short-duration gravitational waves produced by excited f-modes in the magnetar’s core, as well as long-duration gravitational waves motivated by the Quasi-Periodic Oscillations observed in the tails of giant flares. In this talk, we report on the methods and sensitivity estimates of there searches, and the astrophysical implications.

During the late stages of a neutron star binary inspiral finite-size effects come into play, with the tidal deformability of the supranuclear density matter leaving an imprint on the gravitational-wave signal. As demonstrated in the case of GW170817—the first direct detection of gravitational waves from a neutron star binary—this can lead to strong constraints on the neutron star equation of state. As detectors become more sensitive, effects which may have a smaller influence on the neutron star tidal deformability need to be taken into consideration. Dynamical effects, such as oscillation mode resonances triggered by the orbital motion, have been shown to contribute to the tidal deformability, especially close to the neutron star coalesence. We calculate the contribution of the various stellar oscillation modes to the tidal deformability and demonstrate the (anticipated) dominance of the fundamental mode, showing what the impact of the matter composition is on the tidal deformability.

A detailed description of the properties of dense matter in extreme conditions, as those within Neutron Star cores, is still an open problem, whose solution is hampered by both the lack of empirical data, and by the difficulties in developing a suitable theoretical framework for the microscopic nuclear dynamics in such regimes.

We report here the results of a study aimed at inferring the properties of the repulsive three-nucleon interaction, driving the stiffness of the equation of state at high densities, by performing bayesian inference on current and future astrophysical observations.