José A. Pons

Evolution of Proto-Neutron Stars

We study the thermal and chemical evolution during the Kelvin-Helmholtz phase of the birth of a neutron star, employing neutrino opacities that are consistently calculated with the underlying equation of state (EOS). Expressions for the diffusion coefficients appropriate for general relativistic neutrino transport in the equilibrium diffusion approximation are derived. The diffusion coefficients are evaluated using a field-theoretical finite-temperature EOS that includes the possible presence of hyperons. The variation of the diffusion coefficients is studied as a function of EOS and compositional parameters. We present results from numerical simulations of proto-neutron star cooling for internal stellar properties as well as emitted neutrino energies and luminosities. We discuss the influence of the initial stellar model, the total mass, the underlying EOS, and the addition of hyperons on the evolution of the proto-neutron star and on the expected signal in terrestrial detectors. We find that the differences in predicted luminosities and emitted neutrino energies do not depend much upon the details of the initial models or the underlying high-density EOS for early times (t<10 s), provided that opacities are calculated consistently with the EOS. The same holds true for models that allow for the presence of hyperons, except when the initial mass is significantly larger than the maximum mass for cold, catalyzed matter. For times larger than about 10 s, and prior to the occurrence of neutrino transparency, the neutrino luminosities decay exponentially with a time constant that is sensitive to the high-density properties of matter. We also find the average emitted neutrino energy increases during the first 5 s of evolution and then decreases nearly linearly with time. In general, increasing the proto-neutron star mass increases the average energy and the luminosity of neutrinos, as well as the overall evolutionary timescale. The influence of hyperons or variations in the dense matter EOS is increasingly important at later times. Metastable stars, those with hyperons that are unstable to collapse upon deleptonization, have relatively long evolution times, which increase the nearer the mass is to the maximum mass supportable by a cold, deleptonized star.

Unifying the observational diversity of isolated neutron stars via magneto-thermal evolution models

Observations of magnetars and some of the high magnetic field pulsars have shown that their thermal luminosity is systematically higher than that of classical radiopulsars, thus confirming the idea that magnetic fields are involved in their X-ray emission. Here we present the results of 2D simulations of the fully-coupled evolution of temperature and magnetic field in neutron stars, including the state-of-the-art kinetic coefficients and, for the first time, the important effect of the Hall term. After gathering and thoroughly re-analysing in a consistent way all the best available data on isolated, thermally emitting neutron stars, we compare our theoretical models to a data sample of 40 sources. We find that our evolutionary models can explain the phenomenological diversity of magnetars, high-B radio-pulsars, and isolated nearby neutron stars by only varying their initial magnetic field, mass and envelope composition. Nearly all sources appear to follow the expectations of the standard theoretical models. Finally, we discuss the expected outburst rates and the evolutionary links between different classes. Our results constitute a major step towards the grand unification of the isolated neutron star zoo.

TOWARD A MASS AND RADIUS DETERMINATION OF THE NEARBY ISOLATED NEUTRON STAR RX J185635(3754

We discuss efforts to determine the mass, radius, and surface composition of the nearby compact object RX J185635-3754 from its multiwavelength spectral energy distribution. We compute nonmagnetized model atmospheres and emergent spectra for selected compositions and gravities and discuss efforts to fit existing and new observational data from ROSAT, the Extreme Ultraviolet Explorer, and the Hubble Space Telescope. The spectral energy distribution matches that expected from a heavy-element-dominated atmosphere, but not from a uniform-temperature blackbody. Nonmagnetic light-element atmospheres cannot be simultaneously reconciled with the optical and X-ray data. We extend previous studies, which were limited to one fixed neutron star mass and radius. For uniform-temperature models dominated by heavy elements, the redshift z is constrained to be 0.3 z 0.4 and the best-fit mass and radius are M ≈ 0.9 M☉ and R ≈ 6 km (for a 61 pc distance). These values for M and R together are not permitted for any plausible equation of state, including that of a self-bound strange quark star. A simplified two-temperature model allows masses and radii up to about 50% larger, or a factor of 2 in the case of a blackbody. The observed luminosity is consistent with the thermal emission of an isolated neutron star no older than about 106 yr, the age inferred from available proper motion and parallax information.

Magneto-thermal evolution of neutron stars

Context. The presence of magnetic fields in the crust of neutron stars c auses a non-spherically symmetric temperature distribution. The strong temperature dependence of the magnetic diffusivity and thermal conductivity, together with the heat generated by magnetic dissipation, couple the magnetic and thermal evolution of NSs, that cannot be formulated as separated one‐dimensional problems. Aims. We study the mutual influence of thermal and magnetic evoluti on in a neutron star’s crust in axial symmetry. Taking into account realistic microphysical inputs, we find the heat rel eased by Joule effect consistent with the circulation of currents in the crust , and we incorporate its effects in 2‐dimensional cooling calculations. Methods. We solve the induction equation numerically using a hybrid method (spectral in angles, but a finite‐di fferences scheme in the radial direction), coupled to the thermal diffusion equation. To improve the boundary conditions, we also revisit the envelope stationary solutions updating the well known Tb− Ts‐relations to include the effect of 2‐D heat transfer calculations and new microphysical inputs. Results. We present the first long term 2‐dimensional simulations of t he coupled magneto-thermal evolution of neutron stars. This substantially improves previous works in which a very crude approximation in at least one of the parts (thermal or magnetic diffusion) has been adopted. Our results show that the feedback between Joule heating and magnetic diffusion is strong, resulting in a faster dissipation of the stronger fields during the first 10 5 − 10 6 years of a NS’s life. As a consequence, all neutron stars born with fields larger than a critical value (> 5×10 13 G) reach similar field strengths (≈ 2−3×10 13 G) at late times. Irrespectively of the initial magnetic field strength, after 10 6 years the temperature becomes so low that the magnetic diffusion timescale becomes longer than the typical ages of radio‐pulsars, thus resulting in apparently no diss ipation of the field in old NS. We also confirm the strong correl ation between the magnetic field and the surface temperature of relatively young NSs discussed in preliminary works. The effective temperature of models with strong internal toroidal components are systematically higher than those of models with purely poloidal fie lds, due to the additional energy reservoir stored in the toroidal field tha t is gradually released as the field dissipates.

Numerical 3+1 General Relativistic Magnetohydrodynamics: A Local Characteristic Approach

We present a general procedure to solve numerically the general relativistic magnetohydrodynamics (GRMHD) equations within the framework of the 3+1 formalism. The work reported here extends our previous investigation in general relativistic hydrodynamics (Banyuls et al. 1997) where magnetic fields were not considered. The GRMHD equations are written in conservative form to exploit their hyperbolic character in the solution procedure. All theoretical ingredients necessary to build up high-resolution shock-capturing schemes based on the solution of local Riemann problems (i.e., Godunov-type schemes) are described. In particular, we use a renormalized set of regular eigenvectors of the flux Jacobians of the relativistic MHD equations. In addition, the paper describes a procedure based on the equivalence principle of general relativity that allows the use of Riemann solvers designed for special relativistic MHD in GRMHD. Our formulation and numerical methodology are assessed by performing various test simulations recently considered by different authors. These include magnetized shock tubes, spherical accretion onto a Schwarzschild black hole, equatorial accretion onto a Kerr black hole, and magnetized thick disks accreting onto a black hole and subject to the magnetorotational instability.

A New Low Magnetic Field Magnetar: The 2011 Outburst of Swift J1822.3-1606

We report on the long-term X-ray monitoring withSwift,RXTE,Suzaku,Chandra, andXMM-Newton of the outburst of the newly discovered magnetar Swift J1822.3−1606 (SGR 1822−1606), from the first observations soon after the detection of the short X-ray bursts which led to its discovery, through the first stages of its outburst decay (covering the time span from 2011 July until the end of 2012 April). We also report on archival ROSAT observations which detected the source during its likely quiescent state, and on upper limits on Swift J1822.3−1606’s radiopulsed and optical emission during outburst, with the Green Bank Telescope and the Gran Telescopio Canarias, respectively. Our X-ray timing analysis finds the source rotating with a period of P = 8.43772016(2) s and a period derivative ˙ P = 8.3(2) × 10 −14 ss −1 , which implies an inferred dipolar surface magnetic field of B � 2.7 × 10 13 G at the equator. This measurement makes Swift J1822.3−1606 the second lowest magnetic field magnetar (after SGR 0418+5729). Following the flux and spectral evolution from the beginning of the outburst, we find that the flux decreased by about an order of magnitude, with a subtle softening of the spectrum, both typical of the outburst decay of magnetars. By modeling the secular thermal evolution of Swift J1822.3−1606, we find that the observed timing properties of the source, as well as its quiescent X-ray luminosity, can be reproduced if it was born with a poloidal and crustal toroidal fields of Bp ∼ 1.5 × 10 14 G and Btor ∼ 7 × 10 14 G, respectively, and if its current age

Magnetic field dissipation in neutron star crusts : from magnetars to isolated neutron stars

Context. We study the non-linear evolution of magnetic fields in neutron star crusts with special attention to the influence of the Hall drift. Aims. Our goal is to understand the conditions for fast dissipation due to the Hall term in the induction equation. We study the interplay of Ohmic dissipation and Hall drift in order to find a timescale for the overall crustal field decay. Methods. We solve the Hall induction equation numerically by means of a hybrid method (spectral in angles but finite differences in the radial coordinate). The microphysical input consists of the most modern available crustal equation of state, composition, and electrical conductivities. Results. We present the first long-term simulations of the non-linear magnetic field evolution in realistic neutron star crusts with a stratified electron number density and temperature dependent conductivity. We show that Ohmic dissipation influenced by Hall drift takes place in neutron star crusts on a timescale of 10 6 years. When the initial magnetic field has magnetar strength, the fast Hall drift results in an initial rapid dissipation stage that lasts ∼10 4 years. The interplay of the Hall drift with the temporal variation and spatial gradient of conductivity tends to favor the displacement of toroidal fields toward the inner crust, where stable configurations can last for ∼ 10 6 years. We show that the thermally emitting, isolated neutron stars, such as the Magnificent Seven, are very likely descendants of neutron stars born as magnetars.

2D Cooling of magnetized neutron stars

Context. Many thermally emitting, isolated neutron stars have magnetic fields that are larger than 10 13 G. A realistic cooling model that includes the presence of high magnetic fields should be r econsidered. Aims. We investigate the effects of an anisotropic temperature distribution and Joule heating on the cooling of magnetized neutron stars. Methods. The 2D heat transfer equation with anisotropic thermal conductivity tensor and including all relevant neutrino emissi on processes is solved for realistic models of the neutron star interior and crust. Results. The presence of the magnetic field a ffects significantly the thermal surface distribution and the cooling history during both, the early neutrino cooling era and the late photon cooling era. Conclusions. There is a large effect of Joule heating on the thermal evolution of strongly magnetized neutron stars. Both magnetic fields and Joule heating play an important role in keeping mag netars warm for a long time. Moreover, this effect is important for intermediate field neutron stars and should be considered in radio‐quiet isolated neutron stars or high magnetic field ra dio‐pulsars.

Relativistic models of magnetars: the twisted-torus magnetic field configuration

We find general relativistic solutions of equilibrium magnetic field configurations in magnetars, extending previous results of Colaiuda et al. Our method is based on the solution of the relativistic Grad-Shafranov equation, to which Maxwells equations can be reduced. We obtain equilibrium solutions with the toroidal magnetic field component confined into a finite region inside the star, and the poloidal component extending to the exterior. These so-called twisted torus configurations have been found to be the final outcome of dynamical simulations in the framework of Newtonian gravity, and appear to be more stable than other configurations. The solutions include higher-order multipoles, which are coupled to the dominant dipolar field. We use arguments of minimal energy to constrain the ratio of the toroidal to the poloidal field.

THE OUTBURST DECAY OF THE LOW MAGNETIC FIELD MAGNETAR SGR 0418+5729

N.R. is supported by a Ramon y Cajal Research Fellowship, and by grants AYA2009-07391, AYA2012-39303, SGR2009-811, TW2010005, and iLINK 2011-0303. J.A.P. and D.V. acknowledge support from the grants AYA 2010-21097-C03-02 and Prometeo/2009/103. R.T. and S.M. are partially funded through an INAF 2011 PRIN grant. A.P. is supported by a JAE-Doc CSIC fellowship co-funded with the European Social Fund under the program “Junta para la Ampliacion de Estudios,” by the Spanish MICINN grant AYA2011-30228-C03-02 (co-funded with FEDER funds), and by the AGAUR grant 2009SGR1172 (Catalonia).