The interior of a neutron star is expected to contain at least three distinct regions: (i) an outer crust made of exotic nuclei coexisting with a degenerate electron gas, (ii) an inner crust where neutron-proton clusters are immersed in a sea of free neutrons in addition to electrons, and (iii) a liquid core made of neutrons, protons, and leptons. In this contribution, we will present our...
I will present recent progress on our long-term study of the thermal response of neutron stars to long phase of accretion in low-mass X-ray binaries. During the accretion phase, the crust of the neutron star is strongly heated and most of this heat flows into the core. During the quiescence phase, the star relaxes back to thermal equilibrium and observation of this phase allows us to map the...
The properties of slowly rotating proto-neutron stars and merger remnants are studied using finite-temperature equation of state models derived from the covariant density functional theory. In addition to the whole baryonic octet we account for Delta-isobars, as particle degrees of freedom. Wide ranges of entropy per baryon, lepton fraction and baryonic mass are considered. We investigate the...
In the near future, the large amount of new data that will be made available by SKA will allow us to determine neutron star properties with much smaller uncertainties and set strong constraints on the equation of state of stellar matter. Neutron stars will, as a consequence, become a real laboratory to test the nuclear force under extreme conditions of density, proton-neutron asymmetry and...
The measurement of tidal deformability from GW170817 and the existence of pulsars with $\sim 2 M_\odot$ pose great challenges to the usual way of understanding the equation of state (EOS) of dense nuclear matter. We have studied a large set of relativistic mean field EOSs and found that only few can survive these constraints which predict a stiff overall equation of state but with a soft...
Thermal X-ray radiation of neutron stars gives a chance to study their fundamental characteristics such as radii, masses, effective temperatures, and chemical composition of the surfaces. The X-ray emergent spectra form in the gaseous envelopes of the neutron stars and can be computed together with the structures of the upper envelope layers which can be named atmospheres. Comparison of the...
We will present an iterative hybrid approach that self-consistently combines ideal force-free electrodynamics in the bulk of the magnetosphere with particle acceleration along the equatorial current sheet. We derive analytic approximations for the orbits of the particles, and obtain the structure of the magnetosphere for various values of the pair-formation multiplicity parameter. We show that...
I report on the recent progress in understanding the physics of pair formation in pulsar polar caps. I discuss how much pair plasma can be produced in polar cap cascades and what it means for the physics of pulsar magnetospheres and PWNs. Relativistic particles accelerated in pair formation zones heat the NS surface, I demonstrate that the temperatures of pulsar polar caps predicted in the...
Due to their huge rotational energy and large magnetic fields, pulsars have been proposed as candidate sources of high-energy cosmic rays. However, a precise description of the acceleration processes at play is still to be established.
Using particle-in-cell simulations, we study proton acceleration in axisymmetric pulsar magnetosphere. In these numerical experiments, electrons and protons...
When subject to the rotationally induced electric field of pulsar polar caps, electrons and positrons are accelerated along the magnetic field, producing gamma-ray curvature radiation. The emitted gamma-rays, in turn, are absorbed by the magnetic field, converting to new electron-positron pairs. The repetition of this process leads to a cascade of elementary particles that are the source of...
The partially screened vacuum gap (PSG) in the inner acceleration region, a variant of the pure vacuum gap model, has been developed to account for the observed thermal X-ray emission from the polar cap as well as subpulse drifting timescales in normal radio pulsars. We have used this model to explain the presence of death lines in pulsar population and in particular understand the location of...
Complex distortions of polarization angle curve in radio pulsars are mainly caused by superposition of radiation in two orthogonal polarization modes. The resulting polarization depends on several factors, such as the relative amount of the modes, their ellipticity, the statistical spread of their polarization state and on how precisely the modes are orthogonal. Moreover, the observed...
After almost two decades from the discovery of the first accreting millisecond X-ray pulsar (AMXP) SAX J1808.4-3658, the sample of accreting rapidly-rotating neutron stars harboured in low mass X-ray binary systems has increased in number up to 22. The extremely short spin periods shown by the accreting millisecond X-ray pulsars are the result of long-lasting mass transfer from low mass...
In continuation of our earlier work on the accretion/propeller transition of accreting neutron stars, we have investigated torque and luminosity variations during the spin-up/spin-down transitions of these systems. Our analytical model includes the critical conditions for transitions from the strong propeller to the weak propeller and to the spin-up phase together with the accompanying X-ray...
Transitional millisecond pulsars can swing between a radio
pulsar behaviour and a regime characterized by the presence of an accretion disk. The observed multi-wavelength properties of the disk state are enigmatic. Most of the models proposed involve some sort of ejection of plasma, but the driving physical mechanism has not yet been firmly singled out. The recent discovery that in one of...
Transitional millisecond pulsars (tMSPs) are a class of neutron star binary where the system is observed to transition between a rotation-powered pulsar state and an accretion-powered low-mass X-ray binary (LMXB) state, each lasting for several years. As such, tMSPs present a unique opportunity to unveil the mechanics of binary evolution, linking the two distinct populations of binary MSPs and...
The recent discovery of three millisecond pulsars able to alternate a state powered by the rotation of the neutron star magnetic field and a state characterized by the presence of an accretion disk has revealed the existence of an extremely peculiar phase in the evolution of binary pulsars. These pulsars are known as transitional millisecond pulsars. I will present the results of systematic...
The multi-band variability of transitional millisecond pulsar binaries and redbacks during active radio pulsar state will be presented, comparing their properties over a wide energy range to understand whether all, if any, redback is prone to make transition to an accretion powered state.
I will present a summary of interesting phenomena related to the observed spin-down history of pulsars, with emphasis on those that depart from the canonical behaviour. In doing so, I will pose questions regarding how well we are estimating ages and magnetic fields, and point to observations and simulations that provide answers.
The rotation of the Vela pulsar was regularly interrupted by large glitches 17 times during the last 50 years. In contrast, only 3 small glitches (sizes < 10 uHz) have been reported for the same time period. There is general agreement that all these glitches distribute normally around 20 uHz, with a standard deviation of close to 10 uHz. However, the completeness of this sample is unclear. We...
High resolution, pulse to pulse observation of the 2016 Vela glitch and its relaxation provided us an opportunity to probe the neutron star internal structure and dynamics with unprecedented detail. Glitch spin up timescale is constrained below 12.6 seconds, which put stringent limits to the efficiency of angular momentum exchange between crustal superfluid and observed crust. Observed...
During the spin-up phase of a large pulsar glitch - a sudden decrease of the rotational period of a neutron star - the angular velocity of the star may overshoot, namely reach values greater than that observed for the new post-glitch equilibrium. These transient phenomena are expected on the basis of theoretical models for pulsar internal dynamics and their observation has the potential to...
We present the results of an optical timing analysis of the Vela pulsar and the transitional millisecond pulsar PSR J1023+0038. The Vela pulsar was observed in 2009 with the fast photometer Iqueye mounted at the ESO 3.5 m New Technology Telescope (Chile). We determined an independent optical timing solution and the most detailed optical pulse profile of this pulsar available to date. The...
Measuring the braking indices gives the opportunity to search out the braking mechanism of pulsars and evolutionary links between the population. Such measurements should be though rigorous in some cases due to the short and intermediate term effects, most notably timing noise and glitches which superimpose the long-term behaviour of the spindown rate. In particular, various interglitch...
