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.

So far detached compact binaries containing neutron stars have been observed either at intermediate stages of the evolution by radio telescopes or at merger by ground-based gravitational wave detectors. Sensitive to gravitational waves from binaries millions to thousands years prior to the merger, the future Laser Interferometer Space Antenna (LISA) will be crucial for bridging the gap between the currently accessible regimes. Depending on the binary type, LISA could potentially discover from a few to several hundreds in the entirely new regime throughout the Milky Way. Here we provide a concise summary of the current expectation for the detection of Galactic binaries containing neutron stars with LISA, focusing on double neutron stars and neutron star - white dwarf binaries. We outline a few examples of science investigations that LISA data will enable for these binaries.

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.

We join gravitational-wave and electromagnetic data to implement a combined simultaneous fit of the GW170817 event. The LIGO-Virgo analysis includes the estimation of the inclination, the angle of the binary with respect to the gravitationa-wave detector network line of sight. From the observations of the afterglow, instead, we can recover the viewing angle. The inclination and the viewing angle are supplementary angles, and can be treated as a single parameter. The value of the inclination that we recover from the fit is in agreement with the LIGO-Virgo previous works, with an uncertainty that is 10-fold smaller, thanks to contribution of the electromagnetic data. Moreover, with the inclusion of the gravitational-wave data, the degeneracy between the viewing angle and the jet opening angle is broken. This procedure is useful not only for analyzing GW170817, but any gravitational-wave event with an electromagnetic counterpart.

The binary neutron star merger gravitational-wave event GW170817 and observations of the subsequent electromagnetic signals at different wavelengths have helped better understand the outflows that follow these mergers. In particular, the off-axis afterglow of the jetted ejecta has allowed to probe the lateral structure of such jets, especially thanks to VLBI imagery of the source. In this work, we model this afterglow including a decelerating jet with lateral structure, while synchrotron emission and synchrotron self-Compton scatterings power the jet radiation. In particular, we extend our analysis to very high energies and predict the light curve in the energy range of H.E.S.S. and the CTA. We finally discuss how future detections of afterglows by these observatories can help break the degeneracies in some key physical parameter measurements, and allow to probe efficiently a sub-population of fast-merging binaries.

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.

We introduce the HandOff, a set of computational tools to transfer general relativistic magnetohydrodynamic and spacetime data from the numerical code IllinoisGRMHD to Harm3d. While the former simulates binary neutron star (BNS) mergers in full dynamical General Relativity, the latter specializes in modeling accretion disks around black holes (BH) over long timescales. The HandOff, then, enables us to transfer the outcome of BNS mergers after BH formation from IllinoisGRMHD to Harm3d and to evolve the post-merger system efficiently over long timescales. We show our latest results in this respect, for matter modeled as a magnetized ideal-fluid, and discuss our future plans which involve incorporating advanced equations of state and neutrino physics into BNS simulations using the HandOff approach.

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.

In the neutron-star mergers, the radioactive decay of freshly synthesized heavy elements produces emissions in the ultraviolet-optical-infrared range, producing a transient called kilonova. The observational properties of the kilonova depend on the bound-bound opacity of the heavy elements, which was largely unavailable for the conditionssuitable at an early time (t < day). In this article, I share some of our recent progress on modeling the early kilonova light curve, focusing on the atomic opacity calculation.

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 detection of an electromagnetic counterpart to the gravitational-wave source GW 170817 marked year zero of the multi-messenger gravitational-wave era. This event was generated by the merger of two neutron stars and gave rise to an electromagnetic transient, dubbed a “kilonova”. In this proceeding article, I will show how radiative transfer simulations can illuminate neutron star mergers and provide a connection between numerical models and observational data. I will present viewing-angle dependent kilonova predictions made with the Monte Carlo radiative transfer code POSSIS and show how these can be used to interpret data, place constraints on models and guide future follow-up campaigns of gravitational-wave triggers.

In the first observed neutron star merger, GW170817, two dynamical components, mildly- and ultra-relativistic outflows were detected independently. The first component triggered a rapidly evolving thermal transient named macronova (kilonova), while the second caused an observed short GRB where the early gamma-ray signal was followed by a multi-wavelength afterglow. These two distinct components are typically modelled independently and the observational consequences of their interplay are hardly explored. Here we summarize the results of 3D special-relativistic simulations that we have used to investigate the consequences of jet propagation through a realistic environment. We show how the presence of a jet can lead to the macronova being brighter and bluer for on-axis observers in the first few days. Then we show the consequences on the interaction on the shape of the emerging jet. Finally, we will discuss how small scale features in the emerging jet structure can impact the best-fit afterglow parameters.

The LIGO-Virgo gravitational wave detectors have confidently observed 4 events involving neutron stars: two binary neutron star (BNS) mergers (GW170817 and GW190425), and two neutron star-black hole mergers (GW200105 and GW200115). However, our theoretical understanding of the remnant properties of such systems is incomplete due to the complexities related to the modeling of matter effects and the very high computational cost of corresponding numerical relativity simulations. An important such property is the recoil velocity, which is imparted onto the remnant due to the anisotropic emission of gravitational radiation and the dynamical ejection of matter in the kilonova. In this work, we combine gravitational radiation as well as dynamical ejecta distributions, computed by the Computational Relativity numerical simulations, to get accurate estimates for BNS remnant recoil velocities. We find that recoils due to ejection of matter dominate those caused by gravitational wave emission. Knowledge of BNS remnant recoil velocities is important in determining if the remnant is retained by its environment for future hierarchical mergers which, in turn, can form binaries with black holes in the so-called lower mass gap of ∼ 3 – 5M⊙.

The nuclear equation-of-state (EOS) describing newly formed proto-neutron stars (PNSs) in core-collapse supernovae (CCSNe) is yet uncertain, and varying its prescription affects multimessenger signatures in CCSN simulations. Focusing on the gravitational wave (GW) signal, we demonstrate the effect of varying parameter values in the EOS. We conclude that an especially important parameter is the effective mass of nucleons which affect thermal properties and subsequently the PNS compactness, regulating the GW signal in both amplitude and frequency. By radially decomposing the GW emission, we show where in the PNS the GWs originate from.

The INTEGRAL satellite, in orbit since October 2002, has significantly contributed to the study of magnetars and, thanks to its unique capabilities for the study of transient gamma-ray phenomena, it is now playing an important role in multimessenger astrophysics. The most recent results include the discovery of a peculiar burst from SGR J1935+2154, which gave the first observational evidence for the connection between magnetars and fast radio bursts, and extensive searches for bursting activity in peculiar sources, such as the repeating FRB 20200120E in M81 and the ultra-long period magnetar candidate GLEAMX J162759.5–523504.3.

Pulsating Ultra Luminous X-ray sources (PULXs) are thought to be X-ray bright, accreting, magnetized neutron stars, and could be the first and only evidence for the existence of magnetars in binary systems. Their apparent soft (< 20 keV) X-ray luminosity can exceed the Eddington luminosity for a neutron star (NS) by a few orders of magnitude. Although several scenarios have been proposed to explain the different components observed in the X-ray spectra and the characteristics of the X-ray lightcurve of these system, detailed quantitative calculations are still missing. In particular, the observed soft X-ray lightcurves are almost sinuosidal and show an increase in the pulsed fraction (from 8% up to even 30%) with increasing energy. Here, we present how emission originating from an optically thick envelope, expected to be formed during super-Eddington accretion, can result in pulsed fractions similar to observations.

Broadband X-ray data of five magnetars show that their hard X-ray pulses suffer periodic phase modulations, at a period ∼ 104 times their pulse period. The phenomenon is interpreted as a result of free precession of neutron stars that are prolately deformed to an asphericity of ∼ 10−4, by the magnetic stress of toroidal fields reaching ∼ 1016 G. The behavior is absent in their soft X-ray pulses, probably due to a higher emission symmetry. The ultra-high toroidal fields, considered common to magnetars, may persist longer than their dipole fields.

Boasting supreme magnetic strengths, magnetars are among the prime candidates to generate fast radio bursts. Several theories have been proposed for the formation mechanism of magnetars, but have not yet been fully tested. As different magnetar formation theories expect distinct magnetar space velocity distributions, high-precision astrometry of Galactic magnetars can serve as a probe for the formation theories. In addition, magnetar astrometry can refine the understanding of the distribution of Galactic magnetars. This distribution can be compared against fast radio bursts (FRBs) localized in spiral galaxies, in order to test the link between FRBs and magnetars. Swift J1818.0–1607 is the hitherto fastest-spinning magnetar and the fifth discovered radio magnetar. In an ongoing astrometric campaign, we have observed Swift J1818.0–1607 for one year using the Very Long Baseline Array, and have determined a precise proper motion as well as a tentative parallax for the magnetar.

Anomalous X-ray pulsars (AXPs) and Soft gamma repeaters (SGRs) form together a single class of astrophysical sources, commonly associated to magnetars. New-generation X-ray polarimeters will play a key role in assessing the nature of these sources by directly probing the star magnetic field. In the highly magnetized environment radiation is expected to be strongly polarized and such a measure will be easily within reach of IXPE and eXTP. Polarization measurements will eventually confirm the presence of ultra-strong magnetic fields, probing the magnetar scenario. In this work we will discuss theoretical expectations for the polarization signature of AXPs and SGRs and present numerical simulations for the detector response of the polarimeters currently under construction. We will also show how these sources can be used to test vacuum birefringence, a QED effect predicted by Heisemberg and Euler in the Thirties and not experimentally verified as yet.

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.