Deborah N. Aguilera

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.

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.

The Impact of Magnetic Field on the Thermal Evolution of Neutron Stars

The impact of strong magnetic fields B > 1013 G on the thermal evolution of neutron stars is investigated, including crustal heating by magnetic field decay. For this purpose, we perform 2D cooling simulations with anisotropic thermal conductivity considering all relevant neutrino emission processes for realistic neutron stars. The standard cooling models of neutron stars are called into question by showing that the magnetic field has relevant (and in many cases dominant) effects on the thermal evolution. The presence of the magnetic field significantly affects the thermal surface distribution and the cooling history of these objects during both the early neutrino cooling era and the late photon cooling era. The minimal cooling scenario is thus more complex than generally assumed. A consistent magnetothermal evolution of magnetized neutron stars is needed to explain the observations.

Superfluid Heat Conduction and the Cooling of Magnetized Neutron Stars

We report on a new mechanism for heat conduction in the neutron star crust. We find that collective modes of superfluid neutron matter, called superfluid phonons, can influence heat conduction in magnetized neutron stars. They can dominate the heat conduction transverse to the magnetic field when the magnetic field B> approximately 10(13) G. At a density of rho approximately 10(12)-10(14) g/cm3, the conductivity due to superfluid phonons is significantly larger than that due to lattice phonons and is comparable to electron conductivity when the temperature approximately 10(8) K. This new mode of heat conduction can limit the surface anisotropy in highly magnetized neutron stars. Cooling curves of magnetized neutron stars with and without superfluid heat conduction could show observationally discernible differences.

Quiescent thermal emission from neutron stars in low-mass X-ray binaries

Context. We monitored the quiescent thermal emission from neutron stars in low-mass X-ray binaries after active periods of intense activity in X-rays (outbursts). Aims. The theoretical modeling of the thermal relaxation of the neutron star crust may be used to establish constraints on the crust composition and transport properties, depending on the astrophysical scenarios assumed. Methods. We numerically simulated the thermal evolution of the neutron star crust and compared them with inferred surface temperatures for five sources: MXB 1659−29, KS 1731−260, XTE J1701−462, EXO 0748−676 and IGR J17480−2446. Results. We find that the evolution of MXB 1659−29, KS 1731−260 and EXO 0748−676 can be well described within a deep crustal cooling scenario. Conversely, we find that the other two sources can only be explained with models beyond crustal cooling. For the peculiar emission of XTE J1701−462 we propose alternative scenarios such as residual accretion during quiescence, additional heat sources in the outer crust, and/or thermal isolation of the inner crust due to a buried magnetic field. We also explain the very recent reported temperature of IGR J17480−2446 with an additional heat deposition in the outer crust from shallow sources.

QUIESCENT THERMAL EMISSION OF NEUTRON STARS IN LMXBs

Recent monitoring of the quiescent thermal emission from NSs in low mass X–ray binaries (LMXBs) after active periods (bursts) opened a new view to the physics of dense matter. Theoretical modeling of the thermal relaxation of the crust may be used to establish constraints on the thermal conductivity of matter, depending on the accretion rate. We present here cooling curves obtained from numerical simulations that fit the light curves for two sources (KS 1731-260, MXB 1659-29). We estimate the model parameters (accretion rate, thermal conductivity) that match the data and compare our results with previous constraints of neutron star crust properties in LMXBs.

Exploring jet-launching conditions for supergiant fast X-ray transients

Context. In the magneto-centrifugal mechanism for jet formation, accreting neutron stars are assumed to produce relativistic j ets only if their surface magnetic field is weak enough ( B∼ 10 8 G). However, the most common manifestation of neutron stars are pulsars, whose magnetic field distribution peaks at B∼ 10 12 G. If the neutron star magnetic field has at least this strengt h at birth, it must decay considerably before jets can be launched in binary systems. Aims. We study the magnetic field evolution of a neutron star that ac cretes matter from the wind of a high-mass stellar companion so that we can constrain the accretion rate and the impurities i n the crust, which are necessary conditions for jet formation. Methods. We solved the induction equation for the diffusion and convection of the neutron star magnetic field confin ed to the crust, assuming spherical accretion in a simpliflied one-dimensio nal treatment. We incorporated state-of-the-art microphysics, including consistent thermal evolution profiles, and assumed two di fferent neutron star cooling scenarios based on the superfluid ity conditions at the core. Results. We find that in this scenario, magnetic field decay at long time scales is governed mainly by the accretion rate, while the impurity content and thermal evolution of the neutron star play a secondary role. For accretion rates ˙ M& 10 −10 M⊙ yr −1 , surface magnetic fields can decay up to four orders of magnitude in ∼10 7 yr, which is the timescale imposed by the evolution of the high-mass stellar companion in these systems. Based on these results, we discuss the possibility of transient jet-launching in st rong windaccreting high-mass binary systems like supergiant fast X-ray transients.Context. In the magneto-centrifugal mechanism for jet formation, accreting neutron stars are assumed to produce relativistic jets only if their surface magnetic field is weak enough (B ∼ 10 8 G). However, the most common manifestation of neutron stars are pulsars, whose magnetic field distribution peaks at B ∼ 10 12 G. If the neutron star magnetic field has at least this strength at birth, it must decay considerably before jets can be launched in binary systems. Aims. We study the magnetic field evolution of a neutron star that accretes matter from the wind of a high-mass stellar companion so that we can constrain the accretion rate and the impurities in the crust, which are necessary conditions for jet formation. Methods. We solved the induction equation for the diffusion and convection of the neutron star magnetic field confined to the crust, assuming spherical accretion in a simpliflied one-dimensional treatment. We incorporated state-of-the-art microphysics, including consistent thermal evolution profiles, and assumed two different neutron star cooling scenarios based on the superfluidity conditions at the core. Results. We find that in this scenario, magnetic field decay at long timescales is governed mainly by the accretion rate, while the impurity content and thermal evolution of the neutron star play a secondary role. For accretion rates u �

Exploring jet-launching conditions for SFXTs

Context. In the magneto-centrifugal mechanism for jet formation, accreting neutron stars are assumed to produce relativistic j ets only if their surface magnetic field is weak enough ( B∼ 10 8 G). However, the most common manifestation of neutron stars are pulsars, whose magnetic field distribution peaks at B∼ 10 12 G. If the neutron star magnetic field has at least this strengt h at birth, it must decay considerably before jets can be launched in binary systems. Aims. We study the magnetic field evolution of a neutron star that ac cretes matter from the wind of a high-mass stellar companion so that we can constrain the accretion rate and the impurities i n the crust, which are necessary conditions for jet formation. Methods. We solved the induction equation for the diffusion and convection of the neutron star magnetic field confin ed to the crust, assuming spherical accretion in a simpliflied one-dimensio nal treatment. We incorporated state-of-the-art microphysics, including consistent thermal evolution profiles, and assumed two di fferent neutron star cooling scenarios based on the superfluid ity conditions at the core. Results. We find that in this scenario, magnetic field decay at long time scales is governed mainly by the accretion rate, while the impurity content and thermal evolution of the neutron star play a secondary role. For accretion rates ˙ M& 10 −10 M⊙ yr −1 , surface magnetic fields can decay up to four orders of magnitude in ∼10 7 yr, which is the timescale imposed by the evolution of the high-mass stellar companion in these systems. Based on these results, we discuss the possibility of transient jet-launching in st rong windaccreting high-mass binary systems like supergiant fast X-ray transients.Context. In the magneto-centrifugal mechanism for jet formation, accreting neutron stars are assumed to produce relativistic jets only if their surface magnetic field is weak enough (B ∼ 10 8 G). However, the most common manifestation of neutron stars are pulsars, whose magnetic field distribution peaks at B ∼ 10 12 G. If the neutron star magnetic field has at least this strength at birth, it must decay considerably before jets can be launched in binary systems. Aims. We study the magnetic field evolution of a neutron star that accretes matter from the wind of a high-mass stellar companion so that we can constrain the accretion rate and the impurities in the crust, which are necessary conditions for jet formation. Methods. We solved the induction equation for the diffusion and convection of the neutron star magnetic field confined to the crust, assuming spherical accretion in a simpliflied one-dimensional treatment. We incorporated state-of-the-art microphysics, including consistent thermal evolution profiles, and assumed two different neutron star cooling scenarios based on the superfluidity conditions at the core. Results. We find that in this scenario, magnetic field decay at long timescales is governed mainly by the accretion rate, while the impurity content and thermal evolution of the neutron star play a secondary role. For accretion rates u �

Exploring jet launching conditions for SFXTs transients

Context. In the magneto-centrifugal mechanism for jet formation, accreting neutron stars are assumed to produce relativistic j ets only if their surface magnetic field is weak enough ( B∼ 10 8 G). However, the most common manifestation of neutron stars are pulsars, whose magnetic field distribution peaks at B∼ 10 12 G. If the neutron star magnetic field has at least this strengt h at birth, it must decay considerably before jets can be launched in binary systems. Aims. We study the magnetic field evolution of a neutron star that ac cretes matter from the wind of a high-mass stellar companion so that we can constrain the accretion rate and the impurities i n the crust, which are necessary conditions for jet formation. Methods. We solved the induction equation for the diffusion and convection of the neutron star magnetic field confin ed to the crust, assuming spherical accretion in a simpliflied one-dimensio nal treatment. We incorporated state-of-the-art microphysics, including consistent thermal evolution profiles, and assumed two di fferent neutron star cooling scenarios based on the superfluid ity conditions at the core. Results. We find that in this scenario, magnetic field decay at long time scales is governed mainly by the accretion rate, while the impurity content and thermal evolution of the neutron star play a secondary role. For accretion rates ˙ M& 10 −10 M⊙ yr −1 , surface magnetic fields can decay up to four orders of magnitude in ∼10 7 yr, which is the timescale imposed by the evolution of the high-mass stellar companion in these systems. Based on these results, we discuss the possibility of transient jet-launching in st rong windaccreting high-mass binary systems like supergiant fast X-ray transients.Context. In the magneto-centrifugal mechanism for jet formation, accreting neutron stars are assumed to produce relativistic jets only if their surface magnetic field is weak enough (B ∼ 10 8 G). However, the most common manifestation of neutron stars are pulsars, whose magnetic field distribution peaks at B ∼ 10 12 G. If the neutron star magnetic field has at least this strength at birth, it must decay considerably before jets can be launched in binary systems. Aims. We study the magnetic field evolution of a neutron star that accretes matter from the wind of a high-mass stellar companion so that we can constrain the accretion rate and the impurities in the crust, which are necessary conditions for jet formation. Methods. We solved the induction equation for the diffusion and convection of the neutron star magnetic field confined to the crust, assuming spherical accretion in a simpliflied one-dimensional treatment. We incorporated state-of-the-art microphysics, including consistent thermal evolution profiles, and assumed two different neutron star cooling scenarios based on the superfluidity conditions at the core. Results. We find that in this scenario, magnetic field decay at long timescales is governed mainly by the accretion rate, while the impurity content and thermal evolution of the neutron star play a secondary role. For accretion rates u �

JOULE HEATING IN THE COOLING OF MAGNETIZED NEUTRON STARS

We present 2D simulations of the cooling of neutron stars with strong magnetic fields (B \geq 10^{13} G). We solve the diffusion equation in axial symmetry including the state of the art microphysics that controls the cooling such as slow/fast neutrino processes, superfluidity, as well as possible heating mechanisms. We study how the cooling curves depend on the the magnetic field strength and geometry. Special attention is given to discuss the influence of magnetic field decay. We show that Joule heating effects are very large and in some cases control the thermal evolution. We characterize the temperature anisotropy induced by the magnetic field for the early and late stages of the evolution of isolated neutron stars.