Inferring neutron stars crust properties from quiescent thermal emission

The observation of thermal emission from isolated neutron stars and the modeling of the corresponding cooling curves has been very useful to get information on the properties of matter at very high densities. More recently, the detection of quiescent thermal emission from neutron stars in low mass X-ray binary systems after active periods opened a new window to the physics of matter at lower densities. Here we analyze a few sources that have been recently monitored and we show how the models can be used to establish constraints on the crust composition and their transport properties, depending on the astrophysical scenarios assumed.Comment: 4 pages, 2 figures, Proceedings of the conference "Compact Stars in the QCD Phase Diagram IV (CSQCD IV)" September 26-30, 2014, Prerow, German

Quiescent thermal emission from neutron stars in LMXBs

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). 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. 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, EXO 0748-676, XTE J1701-462 and IGR J17480-2446. 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.Comment: 19 pages, 32 figures, 2 Append., revised version accepted for publication in Astronomy & Astrophysic

Exploring jet-launching conditions for SFXTs

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 ($B \sim 10^8$ G). However, the most common manifestation of neutron stars are pulsars, whose magnetic field distribution peaks at $B \sim 10^{12}$ 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. 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. 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. 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 $\dot{M}\gtrsim10^{-10}$ M$_{\odot}$ yr$^{-1}$, surface magnetic fields can decay up to four orders of magnitude in $\sim$10$^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 strong wind-accreting high-mass binary systems like supergiant fast X-ray transients.Comment: 8 pages, 8 figures. Accepted for publication in A&

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 jets only if their surface magnetic field is weak enough (B ∼ 108 G). However, the most common manifestation of neutron stars are pulsars, whose magnetic field distribution peaks at B ∼ 1012 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 Ṁ ≥ 10-10 M⊙ yr-1, surface magnetic fields can decay up to four orders of magnitude in ∼107 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 strong wind-accreting high-mass binary systems like supergiant fast X-ray transients.Facultad de Ciencias Astronómicas y GeofísicasInstituto Argentino de Radioastronomí

Conditions for jet formation in accreting neutron stars: the magnetic field decay

Accreting neutron stars can produce jets only if they are weakly magnetized (B~10^8 G). On the other hand, neutron stars are compact objects born with strong surface magnetic fields (B~10^12 G). In this work we study the conditions for jet formation in a binary system formed by a neutron star and a massive donor star once the magnetic field has decayed due to accretion. We solve the induction equation for the magnetic field diffusion in a realistic neutron star crust and discuss the possibility of jet launching in systems like the recently detected Supergiant Fast X-ray Transients.Fil: García, Federico. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Instituto Argentino de Radioastronomía. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto Argentino de Radioastronomía; ArgentinaFil: Aguilera, Deborah N.. Ohio University; Estados UnidosFil: Romero, Gustavo Esteban. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Instituto Argentino de Radioastronomía. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto Argentino de Radioastronomía; ArgentinaInternational Astronomical Union SymposiumReino UnidoInternational Astronomical Unio

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 jets only if their surface magnetic field is weak enough (B ∼ 108 G). However, the most common manifestation of neutron stars are pulsars, whose magnetic field distribution peaks at B ∼ 1012 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 Ṁ ≥ 10-10 M⊙ yr-1, surface magnetic fields can decay up to four orders of magnitude in ∼107 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 strong wind-accreting high-mass binary systems like supergiant fast X-ray transients.Facultad de Ciencias Astronómicas y GeofísicasInstituto Argentino de Radioastronomí

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 (sPhs), can influence heat conduction in magnetized neutron stars. They can dominate the heat conduction transverse to magnetic field when the magnetic field B \gsim 10^{13} G. At density $\rho \simeq 10^{12}-10^{14}$ g/cm$^3$ the conductivity due to sPhs is significantly larger than that due to lattice phonons and is comparable to electron conductivity when temperature $\simeq 10^8$ 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.Comment: 4 pages, 3 figure