Macroscopic quantum behaviour is prominent in many physical systems, ranging from superfluid phases in ultra-cold atomic gases and heavy-ion collisions to superconducting transitions in metals and exotic quantum phases in dense nuclear matter as well as quark matter. With the interiors of neutron stars in mind, we consider the scenario of two coupled coexisting condensates, where one is...
The evolution of the magnetic field in NSs is strongly related to their internal structure. In the NS core there is a fluid mixture of neutrons, protons, and electrons (joined by other species at increasing densities) that scatter off each other through strong and electromagnetic interactions, causing effective friction forces, and can also convert into each other by weak interactions (‘‘Urca...
We present three-dimensional force-free electrodynamics simulations of magnetar magnetospheres with the Einstein Toolkit. Our recent work demonstrates the instability of certain degenerate, high energy equilibrium solutions of the Grad-Shafranov equation. This result indicates the existence of an unstable branch of twisted magnetospheric solutions and allows to formulate an instability...
We investigate the magnetic field evolution in the crust and the magnetosphere of a neutron star considering the feedback between the two regions. The crustal magnetic field evolves due to the Hall effect and the subdominant Ohmic dissipation. We explore three main cases: (i) A magnetic field fully confined in the crust. (ii) A magnetic field evolving in the crust coupled to a current-free...
We study the current closure problem for a neutron star with a force-free pulsar magnetosphere that develops a large-scale poloidal electric current circuit. The electric current closes through the interior of the neutron star where it provides the torque that spins-down the star. We study the internal electric current in an axisymmetric rotator and evaluate the path of the electric current by...
Among the most promising "Alternative Theories of Gravity”, one of the most studied class is that of “Scalar-Tensor Theories of Gravity”, because they are the most simple extensions of GR, they don’t lead to pathologies in the spacetime properties, and show behaviours that look promising in the context of cosmological constraints.
Some of these theories predict a phenomenon known as...
The extent of the magnetic field at the interior of a neutron star is mostly unknown from the observed radiation features as it can probe up to the outer stellar surface. Theoretical models on the interior magnetic field geometry are generally oversimplified to avoid the complexity and mostly based on the axisymmetric barotropic fluid system. But these static magnetic equilibrium...
In neutron stars, the magnetic field is believed to be mostly confined into the crust. Its topology strongly influences the surface temperature distribution, and hence the star observational properties. In this contribution, I will present some of the first simulations of the coupled crustal magneto-thermal evolution in three dimensions. In particular, I will discuss how the crust reacts to...
Black widow (BW) and redback (RB) systems are compact binaries in which the pulsar heats or ablates its low-mass companion by its intense wind of relativistic particles and emission. Radio, optical and X-ray follow-up of unidentified Fermi Large Area Telescope (LAT) sources has expanded the number of these systems from four to nearly 30. Orbital modulation in X-rays suggests that in many...
Clark et al. (in prep.) recently discovered a gamma-ray pulsar associated with the Fermi-LAT source 3FGL J2039.6-5618, now PSR J2039-5617, and they obtained an accurate pulsar ephemeris from the gamma-ray data. We processed observations with the Parkes Radio Telescope using the \gamma-ray ephemeris, and detected PSR J2039-5617 as a radio millisecond pulsar. The pulse shows a single broad...
The gamma-ray source 3FGL J2039.6-5618 contains a periodic optical and X-ray source that was predicted to be a ''redback'' millisecond pulsar binary. However, without the detection of pulsations, this identification remained inconclusive. Using new optical observations to refine the orbital ephemeris, we searched for gamma-ray pulsations with 10 years of Fermi-LAT data using the Einstein@Home...
The fast-spinning black-widow pulsar J1555-2908, recently discovered in radio, shows long-term variations in its spin frequency via gamma-ray timing analysis of Fermi-LAT data. If interpreted as a red timing noise process, these variations are much larger in amplitude than is observed from other millisecond pulsars. The frequency variations can also be explained by adding a second,...
Pulsating Ultra Luminous X-ray sources (PULXs) are thought to be X-ray bright, accreting, magnetized neutron stars. Their apparent X-ray luminosity, in the range 0.1keV to 10keV, can exceed the Eddington luminosity for a neutron star by a few orders of magnitude. In a magnetized neutron star the accretion flow is channeled onto the polar caps and this gives rise to the observed sinusoidal...
Double Neutron Stars (DNSs) have been observed as Galactic radio pulsars, gamma-ray bursts and gravitational-wave sources. They are believed to have experienced at least one common-envelope episode (CEE) during their prior evolution to DNS formation. In the last decades, there have been numerous efforts to understand the details of the common-envelope phase, but there is still no consensus. I...
In this talk we will present some very interesting solutions describing neutron stars in tensor-multi-scalar theories of gravity. It turns out that in certain subclasses of these theories, the spectrum of solutions can be very rich leading to interesting observational consequences. Taking into account that the scalar-tensor theories are ones of the few examples of mathematically well posed...
Scalarization is a very interesting nonlinear mechanism for developing of nontrivial scalar field that can have well pronounced observational signatures. If the scalar field is massive, the strong constraints on the theory coming from the binary pulsar observations, can be easily circumvented thus leading to large deviations from general relativity. In the talk we will discuss the...
Neutron star (NS) mergers are thought to be one of the primary sites of heavy element production in the Universe. In order to inform and predict future observations and meaningfully interpret the existing ones, we need first-principle models that describe the physics of the merger and its aftermath (e.g. kilonova, afterglow). Such models would allow us to predict the amount, composition, and...
Being able to determine the stationary structure of a neutron star allows to study its properties, like the parameter space of the equation of state, the mass-radius diagram, and the gravitational wave emission. Moreover, this stationary configuration can be used as initial condition for a much more resource demanding hydrodynamical simulation. A key approximation made for computing the...
The oscillations and instabilities of relativistic stars are studied by taking into account, for the first time, the contribution of a dynamic space-time. The study is based on the linearised version of Einstein’s equations and via this approach the oscillation frequencies, the damping and growth times as well as the critical values for the onset of the secular (CFS) instability are presented....
We report the existence of a gravitational-wave-driven secular instability in neutron star binaries, acting on the equilibrium tide. The instability is similar to the classic Chandrasekhar-Friedman-Schutz instability of normal modes and is active when the spin of the primary star exceeds the orbital frequency of the companion. Modeling the neutron star as a Newtonian n=1 polytrope, we...
Typically, self-gravitating objects are modeled through structure equations and assuming a polytropic equation of state that allows numerical determination of radial density profiles. This equation of state (EoS) is particularly interesting since it allows different astrophysical scenarios to be modeled by varying the polytropic index. In this work we determine the convective stability and the...
