The birth of a neutron star with an extremely strong magnetic field, called a magnetar, has emerged as a promising scenario to power a variety of outstanding explosive events. This includes gamma-ray bursts, supernovae with extreme kinetic energies called hypernovae and super-luminous supernovae. The origin of these extreme magnetic fields (of the order of 10^15 Gauss) is not fully understood...
The magnetorotational instability (MRI) is considered to be a promising mechanism to amplify the magnetic field in fast-rotating protoneutron stars. Many local studies have shown that the magnetic field could be amplified on small scales. However, the efficiency of the MRI at generating a large-scale field similar to the dipolar magnetic field of magnetars ($10^{14}-10^{15}$ G) is still...
Magnetars are isolated neutron stars characterized by a variable X-ray activity powered by the dissipation of strong magnetic fields. Their spin-inferred dipole field strengths range from 100 to 1000 times those of radio pulsars. For many decades now, understanding the origin of these objects has been a theoretical challenge. Thanks to the first 3D MHD direct numerical simulations of thermal...
Using 3D MHD code, we explore magnetic field configurations with different contributions of the toroidal component. We solve coupled magneto-thermal equations in the NS crust on Myr timescale for magnetic fields of 1e14 G. In this research, we confirm previous findings that a large fraction of the toroidal magnetic field leads to the formation of small magnetic spots.
In general, we see a...
We present recent X-ray timing and spectral results on isolated pulsars, including the X-ray and radio mode switching PSR B0943+10, and PSR J0726-2612, which is an old radio pulsar sharing several of the properties of the XDINSs. Our analysis properly accounts for the effects of the (relatively) high magnetic field on the surface emission properties.
Previous studies of PSR B0943+10 showed...
Neutron stars are incredibly dense compact objects having the strongest magnetic field in the universe known to date. The exact configuration of the field is not known and the simplest model that is often considered is that of a dipole. Such a dipolar poloidal field is however known to be unstable and the equilibrium configuration is an open problem of great astrophysical relevance. In order...
Neutron stars in binary systems may accrete matter from their companion star ; so far, only the case of a crust fully replaced by accreted matter has been considered in detailed calculations. However if the star has only accreted a small amount of matter, the crust is not fully but only partially accreted. This could be for example the case of IGR J17480−2446. The observed decrease of its...
Neutron stars (NSs) in low-mass X-ray binaries have an accreted crust, whose equation of state and composition differs from that in isolated NSs. To determine it, one usually makes a number of simplifying assumptions regarding both thermodynamics and kinetics of crust matter. We critically revise some of these assumptions and propose new thermodynamically consistent derivation of the crust...
Transiently accreting neutron stars in low mass X-ray binaries are generally believed to be heated up by nuclear reactions in accreted matter during hydrostatic compression. Detailed modeling of these reactions is required for the correct interpretation of observations. We construct a simplified reaction network, which can be easily implemented and depends mainly on atomic mass tables as...
We investigate how deep carbon can survive in the envelope of very young (a few minutes after birth) and very hot neo-neutron stars. The question is motivated by the existence of at least two neutron stars which are best described with a carbon atmosphere model. Such models unfortunately do not answer the question of how deep a carbon layer can be (a few centimeters are sufficient to form a...
The maximum mass of a neutron star has important implications across multiple research fields, including astrophysics, nuclear physics and gravitational wave astronomy. Compact binary millisecond pulsars are key to constraining such maximum mass observationally. Applying a new method to measure the velocity of both sides of the companion star, we previously found that the compact binary...
A numerical rotating neutron star solver is used to study the temporal evolution of accreting neutron stars using a multi-polytrope model for the nuclear equation of state named ACB5. The solver is based on a quadrupole expansion of the metric, but confirms the results of previous works, revealing the possibility of an abrupt transition of a neutron star from a purely hadronic branch to a...
The gravitational wave signal from the merger of two neutron stars cannot be easily distinguished
from the signal produced by a comparable-mass mixed binary, in which one of the component is a
black hole. Although the existence of low-mass black holes ( $<5M_{\odot}$) is astrophysically
disfavoured, their formation may be of primordial origin or as the outcome of the interaction...
We investigate binary neutron star mergers employing state-of-the-art microscopic equations of state, considering both zero-temperature and finite temperature extensions of the same. I will discuss the results we have achieved in this context, with respect to the thermodynamic conditions and the gravitational wave emission.
Neutron-star mergers provide unique environments for mass accretion, ejection, and r-process nucleosynthesis. Theoretically, however, simulating such systems are challenging, especially within the assumed equation of state (EOS) of the post-merger material. Although the ideal gas EOS, commonly used in simulations of post-merger systems, is a good approximation, a realistic EOS can account for...
Gross properties of merger components and remnant in GW170817 are investigated using equations of state (EoSs) within the finite temperature field theoretical models. Tidal deformabilities and radii of merger components are estimated in light of GW170817. An analytical expression for the radius of a merger component is derived in terms of the combined tidal deformability for binary neutron...
The detection of the binary neutron star merger (GW170817) marked the first multi-wavelength light and gravitational waves detection of a neutron star merger. While gravitational waves can provide us information about the mass and spin of the pre-merger system, the resulting merger and accretion disk, created by a combination of tidally disrupted material and the dynamical ejecta are...
The modelling of ejected matter, its dynamics and thermodynamic properties, is of fundamental importance in the study of binary neutron star mergers (BNS); it serves as a starting point to investigate the gamma ray burst emission, r-process nucleosynthesis and kilonova signal. While processes such as neutrino transport, magnetic fields, viscous effects and relativistic gravity are usually...
Making use of Big Data techniques and High Performance Computing (HPC) we explored high-energy data archives in new ways, extracting new information buried in the vast volume of high-energy astrophysical data. These efforts of mixed Data Mining and HPC approaches allowed us to uncover a new population of Extragalactic Neutron Stars (NS), and in particular showed that probably most of the Ultra...
I will discuss the results from a recent simultaneous observing campaign involving FAST and LOFAR to study PSR J0250+5854, the slowest known pulsar with a period of 23.54 seconds, across a wide range of frequencies. This will be one of the early science results from the currently-being-commissioned Five-hundred-metre Aperture Spherical Telescope (FAST) in Guizhou, China. FAST is the largest...
Neutron star (NS) matter is typically modelled as a superconductor, and as a result numerical simulations, by and large, evolve the ideal MHD equations. There is reason to believe that during events such as NS mergers or accretion on to black holes, however, resistive effects may become important and significantly change the structure of the magnetic fields and the dynamics of ejecta. In this...
At the dawn of multi-messenger astrophysics with gravitational wave sources, numerical relativity simulations of compact binary mergers involving black holes and neutron stars play, more than ever, a central role. An accurate representation of these systems requires solving Einstein’s equations on dynamical spacetimes, coupled with the general relativistic magnetohydrodynamic (GRMHD)...
Pulsars in relativistic binary systems are excellent probes of fundamental physics and binary evolution. Long term measurements of pulse arrival times from such pulsars enable theory-independent measurements of relativistic parameters that can then be used for testing different theories of gravity such as General Relativity and scalar-tensor theories of gravity. Assuming a theory of gravity,...
Due to the high compactness of neutron stars, signatures of relativistic effects are expected in their vicinity, effects that will affect, among other things, the trajectory of photons produced inside their magnetosphere. We have plotted light curves and sky maps for radio and high energy photons, taken into account light bending and Shapiro delay within the Scharwzschild metric, and compared...
