Magnetized Proto-neutron Stars Research Articles

Three-dimensional General-relativistic Simulations of Neutrino-driven Winds from Magnetized Proto–Neutron Stars

Formed in the aftermath of a core-collapse supernova or neutron star merger, a hot proto–neutron star (PNS) launches an outflow driven by neutrino heating lasting for up to tens of seconds. Though such winds are considered potential sites for the nucleosynthesis of heavy elements via the rapid neutron capture process (r-process), previous work has shown that unmagnetized PNS winds fail to achieve the necessary combination of high entropy and/or short dynamical timescale in the seed nucleus formation region. We present three-dimensional general-relativistic magnetohydrodynamical simulations of PNS winds which include the effects of a dynamically strong (B ≳ 1015 G) dipole magnetic field. After initializing the magnetic field, the wind quickly develops a helmet-streamer configuration, characterized by outflows along open polar magnetic field lines and a “closed” zone of trapped plasma at lower latitudes. Neutrino heating within the closed zone causes the thermal pressure of the trapped material to rise in time compared to the polar outflow regions, ultimately leading to the expulsion of this matter from the closed zone on a timescale of ∼60 ms, consistent with the predictions of Thompson. The high entropies of these transient ejecta are still growing at the end of our simulations and are sufficient to enable a successful second-peak r-process in at least a modest ≳1% of the equatorial wind ejecta.

Proto-magnetar jets as central engines for broad-lined Type Ic supernovae

ABSTRACT A subset of Type Ic supernovae (SNe Ic), broad-lined SNe Ic (SNe Ic-bl), show unusually high kinetic energies (∼1052 erg) that cannot be explained by the energy supplied by neutrinos alone. Many SNe Ic-bl have been observed in coincidence with long gamma-ray bursts (GRBs) that suggests a connection between SNe and GRBs. A small fraction of core-collapse supernovae form a rapidly rotating and strongly magnetized protoneutron star (PNS), a proto-magnetar. Jets from such magnetars can provide the high kinetic energies observed in SNe Ic-bl and also provide the connection to GRBs. In this work, we use the jetted outflow produced in a 3D general relativistic magnetohydrodynamic CCSN simulation from a consistently formed proto-magnetar as the central engine for full-star explosion simulations. We extract a range of central engine parameters and find that the extracted engine energy is in the range of 6.231 × 1051−1.725 × 1052 erg, the engine time-scale in the range of 0.479−1.159 s and the engine half-opening angle in the range of ∼9°−19°. Using these as central engines, we perform 2D special relativistic (SR) hydrodynamic (HD) and radiation transfer simulations to calculate the corresponding light curves and spectra. We find that these central engine parameters successfully produce SNe Ic-bl that demonstrates that jets from proto-magnetars can be viable engines for SNe Ic-bl. We also find that only the central engines with smaller opening angles (∼11°) form a GRB implying that GRB formation is likely associated with narrower jet outflows and Ic-bl’s without GRBs may be associated with wider outflows.

Effects of Fallback Accretion on Protomagnetar Outflows in Gamma-Ray Bursts and Superluminous Supernovae

Abstract Rapidly spinning, strongly magnetized protoneutron stars (“millisecond protomagnetars”) are candidate central engines of long-duration gamma-ray bursts (GRBs), superluminous supernovae (SLSNe), and binary neutron star mergers. Magnetar birth may be accompanied by the fallback of stellar debris, lasting for seconds or longer following the explosion. Accretion alters the magnetar evolution by (1) providing an additional source of rotational energy (or a potential sink, if the propeller mechanism operates), (2) enhancing the spin-down luminosity above the dipole rate by compressing the magnetosphere and expanding the polar cap region of open magnetic field lines, and (3) supplying an additional accretion-powered neutrino luminosity that sustains the wind baryon loading, even after the magnetar’s internal neutrino luminosity has subsided. The more complex evolution of the jet power and magnetization of an accreting magnetar more readily accounts for the high 56Ni yields of GRB SNe and the irregular time evolution of some GRB light curves (e.g., bursts with precursors followed by a long quiescent interval before the main emission episode). Additional baryon loading from accretion-powered neutrino irradiation of the polar cap lengthens the time frame over which the jet magnetization is in the requisite range σ ≲ 103 for efficient gamma-ray emission, thereby accommodating GRBs with ultralong durations. Though accretion does not significantly raise the maximum energy budget from the limit of ≲ few × 1052 erg for an isolated magnetar, it greatly expands the range of magnetic field strengths and birth spin periods capable of powering GRB jets, reducing the differences between the magnetar properties normally invoked to explain GRBs versus SLSNe.

High-entropy ejections from magnetized proto-neutron star winds: implications for heavy element nucleosynthesis

Although initially thought to be promising for production of the r-process nuclei, standard models of neutrino-heated winds from proto-neutron stars (PNSs) do not reach the requisite neutron-to-seed ratio for production of the lanthanides and actinides. However, the abundance distribution created by the r-, rp-, or νp-processes in PNS winds depends sensitively on the entropy and dynamical expansion time-scale of the flow, which may be strongly affected by high magnetic fields. Here, we present results from magnetohydrodynamic simulations of non-rotating neutrino-heated PNS winds with strong dipole magnetic fields from 1014 to 1016 G, and assess their role in altering the conditions for nucleosynthesis. The strong field forms a closed zone and helmet streamer configuration at the equator, with episodic dynamical mass ejections in toroidal plasmoids. We find dramatically enhanced entropy in these regions and conditions favourable for third-peak r-process nucleosynthesis if the wind is neutron-rich. If instead the wind is proton-rich, the conditions will affect the abundances from the νp-process. We quantify the distribution of ejected matter in entropy and dynamical expansion time-scale, and the critical magnetic field strength required to affect the entropy. For B ≳1015 G, we find that ≳10−6 M⊙ and up to ∼10−5 M⊙ of high-entropy material is ejected per highly magnetized neutron star birth in the wind phase, providing a mechanism for prompt heavy element enrichment of the universe. Former binary companions identified within (magnetar-hosting) supernova remnants, the remnants themselves, and runaway stars may exhibit overabundances. We provide a comparison with a semi-analytic model of plasmoid eruption and discuss implications and extensions.

Neutrino-heated winds from millisecond protomagnetars as sources of the weak r-process

We explore heavy element nucleosynthesis in neutrino-driven winds from rapidly-rotating, strongly magnetized proto-neutron stars for which the magnetic dipole is aligned with the rotation axis, and the field is assumed to be a static force-free configuration. We process the proto-magnetar wind trajectories calculated by Vlasov et al 2014 through the r-process nuclear reaction network SkyNet using contemporary models for the evolution of the wind electron fraction during the proto-neutron star cooling phase. Although we do not find a successful second or third peak r-process for any rotation period P, we show that proto-magnetars with P around 1-5 ms produce heavy element abundance distributions that extend to higher nuclear mass number than from otherwise equivalent spherical winds (with the mass fractions of some elements enhanced by factors of 100-1000). The heaviest elements are synthesized by outflows emerging along flux tubes which graze the closed zone and pass near the equatorial plane outside the light cylinder. Due to dependence of the nucleosynthesis pattern on the magnetic field strength and rotation rate of the proto-neutron star, natural variations in these quantities between core collapse events could contribute to the observed diversity of the abundances of weak r-process nuclei in metal-poor stars. Further diversity, including possibly even a successful third-peak r-process, could be achieved for misaligned rotators with non-zero magnetic inclination with respect to the rotation axis. If proto-magnetars are central engines for GRBs, their relativistic jets should contain a high mass fraction of heavy nuclei of characteristic average mass number A of order 100, providing a possible source for ultra-high energy cosmic rays comprised of heavy nuclei with an energy spectrum that extends beyond the nominal GZK cut-off for protons or iron nuclei.

A central compact object in Kes 79: the hypercritical regime and neutrino expectation

We present magnetohydrodynamical simulations of a strong accretion onto magnetized proto-neutron stars for the Kesteven 79 (Kes 79) scenario. The supernova remnant Kes 79, observed with the Chandra ACIS-I instrument during approximately 8.3 h, is located in the constellation Aquila at a distance of 7.1 kpc in the galactic plane. It is a galactic and a very young object with an estimate age of 6 kyr. The Chandra image has revealed, for the first time, a point-like source at the center of the remnant. The Kes 79 compact remnant belongs to a special class of objects, the so-called Central Compact Objects, which exhibits no evidence for a surrounding pulsar wind nebula. In this work we show that the submergence of the magnetic field during the hypercritical phase can explain such behavior for Kes 79 and others CCOs. The simulations of such regime were carried out with the adaptive-mesh-refinement code FLASH in two spatial dimensions, including radiative loss by neutrinos and an adequate equation of state for such regime. From the simulations, we estimate that the number of thermal neutrinos expected on the Hyper-Kamiokande Experiment is 733$\pm$364. In addition, we compute the flavor ratio on Earth for a progenitor model.

Asymmetric neutrino production in strongly magnetized proto-neutron stars

We calculate the neutrino production cross-section through the direct URCA process in proto-neutron star matter in the presence of a strong magnetic field. We assume isoentropic conditions and introduce a new equation of state parameter-set in the relativistic mean-field approach that can reproduce neutron stars with $M > 1.96$ M$_\odot$ as required by observations. We find that the production process increases the flux of emitted neutrinos along the direction parallel to the magnetic field and decreases the flux in the opposite direction. This means that the neutrino flux asymmetry due to the neutrino absorption and scattering processes in a magnetic field becomes larger by the inclusion of the neutrino production process.

Quasithermal neutrinos from rotating protoneutron stars born during core collapse of massive stars

Rotating and magnetized protoneutron stars (PNSs) may drive relativistic magneto-centrifugally accelerated winds as they cool immediately after core collapse. The wind fluid near the star is composed of neutrons and protons, and the neutrons become relativistic while collisionally coupled with the ions. Here, we argue that the neutrons in the flow eventually undergo inelastic collisions around the termination shock inside the stellar material, producing ~0.1-1 GeV neutrinos, without relying on cosmic-ray acceleration mechanisms. Even higher-energy neutrinos may be produced via particle acceleration mechanisms. We show that PINGU and Hyper-Kamiokande can detect such neutrinos from nearby core-collapse supernovae, by reducing the atmospheric neutrino background via coincident detection of MeV neutrinos or gravitational waves and optical observations. Detection of these GeV and/or higher-energy neutrinos would provide important clues to the physics of magnetic acceleration, nucleosynthesis, the relation between supernovae and gamma-ray bursts, and the properties of newly born neutron stars.

Relativistic mean-field treatment of pulsar kicks from neutrino propagation in magnetized proto-neutron stars

We make a perturbative calculation of neutrino scattering and absorption in hot and dense hyperonic neutron-star matter in the presence of a strong magnetic field. We calculate that the absorption cross-sections in a fully relativistic mean-field theory. We find that there is a remarkable angular dependence, i.e., the neutrino absorption strength is reduced in a direction parallel to the magnetic field and enhanced in the opposite direction. This asymmetry in the neutrino absorption is estimated to be as much as 2.2% of the entire neutrino momentum for an interior magnetic field of $\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}{10}^{17}\text{ }\text{ }\mathrm{G}$. The pulsar kick velocities associated with this asymmetry are shown to be comparable to observed velocities.

Neutrino spectra evolution during protoneutron star deleptonization

The neutrino-driven wind, which occurs after the onset of a core-collapse supernova explosion, has long been considered as the possible site for the synthesis of heavy $r$-process elements in the Universe. Only recently, it has been possible to simulate supernova explosions up to $\ensuremath{\sim}10\text{ }\text{ }\mathrm{seconds}$, based on three-flavor Boltzmann neutrino transport. These simulations show that the neutrino luminosities and spectra of all flavors are very similar and their difference even decreases during the deleptonization of the proto-neutron star. As a consequence, the ejecta are always proton rich which rules out the possible production of heavy $r$-process elements ($Z>56$). We perform a detailed analysis of the different weak processes that determine the neutrino spectra. Nonelectron flavor (anti)neutrinos are produced and interact only via neutral-current processes, while electron (anti)neutrinos have additional contributions from charge-current processes. The latter are dominated by ${\ensuremath{\nu}}_{e}$-absorption on neutrons and ${\overline{\ensuremath{\nu}}}_{e}$-absorption on protons. At early times, charge-current processes are responsible for spectral differences between ${\ensuremath{\nu}}_{e}$, ${\overline{\ensuremath{\nu}}}_{e}$ and ${\ensuremath{\nu}}_{\ensuremath{\mu}/\ensuremath{\tau}}$. However, as the region of neutrino decoupling moves to higher densities during deleptonization, charge-current reactions are suppressed by final state Pauli blocking. ${\overline{\ensuremath{\nu}}}_{e}$ absorption on protons is suppressed due to the continuously increasing chemical potential of the neutrons. ${\ensuremath{\nu}}_{e}$ absorption on neutrons is blocked by the increasing degeneracy of the electrons. These effects result in negligible contributions from charge-current reactions on time scales on the order of tens of seconds, depending on the progenitor star. Hence, the neutrino spectra are mainly determined from neutral-current processes which do not distinguish between the different flavors and result in the convergence of the spectra. These findings are independent of the charge-current reaction rates used. It rules out the possibility of neutron-rich ejecta at late times and the production of heavy $r$-process elements from nonrotating and not magnetized proto-neutron stars.

Asymmetric Neutrino Reaction in Magnetized Proto-Neutron Stars in Fully Relativistic Approach

We calculate asymmetric neutrino absorption and scattering cross sections on hot and dense magnetized neutron-star matter including hyperons in fully relativistic mean-field theory. The absorption/scattering cross sections are suppressed/enhanced incoherently in the direction of the magnetic field B = Bẑ . The asymmetry is 2–4% at the matter density ρ0 ≤ ρB ≤ 3ρ0 and temperature T ≤ 40MeV for B = 2 × 1017 G. Then we solve the Boltzmann equation for the neutrino transport in 1D attenuation approximation, and get the result that the kick velocity becomes about 300 km/s for the proto-neutron star with 168 solar mass at T = 20MeV.

Nonaxisymmetric instabilities of neutron star with toroidal magnetic fields

Context.

Asymmetric neutrino emission from magnetized proto-neutron star matter including hyperons in relativistic mean field theory

We calculate asymmetric neutrino absorption and scattering cross sections on hot and dense magnetized neutron star matter, including hyperons in fully relativistic mean-field theory. The absorption/scattering cross sections are suppressed/enhanced incoherently in the direction of the magnetic field $\mathbf{B}=B\stackrel{^}{z}$. The asymmetry is 2--4% at the matter density ${\ensuremath{\rho}}_{0}\ensuremath{\le}{\ensuremath{\rho}}_{B}\ensuremath{\le}3{\ensuremath{\rho}}_{0}$ and temperature $T\ensuremath{\le}40\text{ }\text{ }\mathrm{MeV}$ for $B=2\ifmmode\times\else\texttimes\fi{}{10}^{17}\text{ }\text{ }\mathrm{G}$. This asymmetry is comparable to the effects owing to parity violation or asymmetric magnetic field topology proposed for the origin of pulsar kicks.

The protomagnetar model for gamma-ray bursts

Long duration Gamma-Ray Bursts (GRBs) originate from the core collapse of massive stars, but the identity of the central engine remains elusive. Previous work has shown that rapidly spinning, strongly magnetized proto-neutron stars (`millisecond proto-magnetars') produce outflows with energies, timescales, and magnetizations sigma_0 (maximum Lorentz factor) that are consistent with those required to produce long GRBs. Here we extend this work in order to construct a self-consistent model that directly connects the properties of the central engine to the observed prompt emission. Just after the launch of the supernova shock, a wind heated by neutrinos is driven from the proto-magnetar. The outflow is collimated into a bipolar jet by its interaction with the star. As the magnetar cools, the wind becomes ultra-relativistic and Poynting-flux dominated (sigma_0 >> 1) on a timescale comparable to that required for the jet to clear a cavity through the star. Although the site and mechanism of the prompt emission are debated, we calculate the emission predicted by two models: magnetic dissipation and internal shocks. Our results favor the magnetic dissipation model in part because it predicts a relatively constant `Band' spectral peak energy E_peak with time during the GRB. The jet baryon loading decreases abruptly when the neutron star becomes transparent to neutrinos at t ~ 10-100 seconds. Jets with ultra-high magnetization cannot effectively accelerate and dissipate their energy, suggesting this transition ends the prompt emission and may explain the steep decay phase that follows. We assess several phenomena potentially related to magnetar birth, including low luminosity GRBs, thermal-rich GRBs/X-ray Flashes, very luminous supernovae, and short duration GRBs with extended emission.

Magnetorotational instability in proto-neutron stars

Magnetorotational instability (MRI) has been suggested to lead a rapid growth of the magnetic field in core collapse supernovae and produce departures from spherical syymmetry that can be important in determining the explosion mechanism. We address the problem of stability in differentially rotating magnetized proto-neutron stars at the beginning of their evolution. Criteria for MRI in proto-neutron stars are derived without simplying assumptions about a weak magnetic field and are substantially different from the standard condition. If the magnetic field is strong, MRI can occur only in the neighbourhood of the region where the spherical radial component of the magnetic field vanishes. The growth rate of MRI is relatively low except for perturbations with very small scales which usually are not detected in numerical simulations. We find that MRI in proto-neutron stars grows more slowly than than the double diffusive instability analogous the Goldreich-Schubert-Fricke instability in ordinary stars.

Evolution of neutron stars with toroidal magnetic fields: Axisymmetric simulation in full general relativity

We study the stability of neutron stars with toroidal magnetic fields by magnetohydrodynamic simulation in full general relativity under the assumption of axial symmetry. Nonrotating and rigidly rotating neutron stars are prepared for a variety of magnetic field configuration. For modeling the neutron stars, the polytropic equation of state with the adiabatic index $\ensuremath{\Gamma}=2$ is used for simplicity. It is found that nonrotating neutron stars are dynamically unstable for the case where toroidal magnetic field strength varies $\ensuremath{\propto}{\ensuremath{\varpi}}^{2k\ensuremath{-}1}$ with $k\ensuremath{\ge}2$ (here $\ensuremath{\varpi}$ is the cylindrical radius), whereas for $k=1$ the neutron stars are stable. After the onset of the instability, unstable modes grow approximately in the Alfv\'en time scale and, as a result, a convective motion is excited to change the magnetic field profile until a new state, which is stable against axisymmetric perturbation, is reached. We also find that rotation plays a role in stabilization, although the instability still occurs in the Alfv\'en time scale when the ratio of magnetic energy to rotational kinetic energy is larger than a critical value $\ensuremath{\sim}0.2$. Implication for the evolution of magnetized protoneutron stars is discussed.

Magnetorotational collapse of massive stellar cores to neutron stars: Simulations in full general relativity

We study magnetohydrodynamic (MHD) effects arising in the collapse of magnetized, rotating, massive stellar cores to proto-neutron stars (PNSs). We perform axisymmetric numerical simulations in full general relativity with a hybrid equation of state. The formation and early evolution of a PNS are followed with a grid of $2500\ifmmode\times\else\texttimes\fi{}2500$ zones, which provides better resolution than in previous (Newtonian) studies. We confirm that significant differential rotation results even when the rotation of the progenitor is initially uniform. Consequently, the magnetic field is amplified both by magnetic winding and the magnetorotational instability (MRI). Even if the magnetic energy ${E}_{\mathrm{EM}}$ is much smaller than the rotational kinetic energy ${T}_{\mathrm{rot}}$ at the time of PNS formation, the ratio ${E}_{\mathrm{EM}}/{T}_{\mathrm{rot}}$ increases to 0.1\char21{}0.2 by the magnetic winding. Following PNS formation, MHD outflows lead to losses of rest mass, energy, and angular momentum from the system. The earliest outflow is produced primarily by the increasing magnetic stress caused by magnetic winding. The MRI amplifies the poloidal field and increases the magnetic stress, causing further angular momentum transport and helping to drive the outflow. After the magnetic field saturates, a nearly stationary, collimated magnetic field forms near the rotation axis and a Blandford-Payne\char21{}type outflow develops along the field lines. These outflows remove angular momentum from the PNS at a rate given by $\stackrel{\ifmmode \dot{}\else \textperiodcentered \fi{}}{J}\ensuremath{\sim}\ensuremath{\eta}{E}_{\mathrm{EM}}{C}_{B}$, where $\ensuremath{\eta}$ is a constant of order $\ensuremath{\sim}0.1$ and ${C}_{B}$ is a typical ratio of poloidal to toroidal field strength. As a result, the rotation period quickly increases for a strongly magnetized PNS until the degree of differential rotation decreases. Our simulations suggest that rapidly rotating, magnetized PNSs may not give rise to rapidly rotating neutron stars.

Pulsar kicks induced by spin flavour oscillations of neutrinos in gravitational fields

The origin of pulsar kicks is reviewed in the framework of the spin-flip conversion of neutrinos propagating in the gravitational field of a magnetized protoneutron star. We find that for a mass in rotation with angular velocity w, the spin connections entering in the Dirac equation give rise to the coupling term ω . p, where p is the neutrino momentum. Such a coupling can be responsible for pulsar kicks owing to the neutrino emission asymmetry generated by the relative orientation of p with respect to w. For our estimations, the large non-standard neutrino magnetic momentum, μ ν ≤ 10 -11 μ B , is considered.

Active-sterile neutrino oscillations and pulsar kicks

We develop a thorough description of neutrino oscillations in a magnetized protoneutron star, based on a resonance layer for neutrinos with different momentum directions. We apply our approach to the calculation of the asymmetry in the neutrino emission during the birth of a neutron star and the pulsar acceleration in the case of an active-sterile neutrino resonant conversion. The observed velocities can be obtained with the magnetic fields expected in the interior of a protoneutron star, for sterile neutrino masses of the order of KeV and small mixing angles.

The influence of asymmetry on a magnetized proto-neutron star

Using the Relativistic Mean Field Theory (RMF) it is shown that different proton fraction which is directly connected with the neutron excess and with the asymmetry of the system affects proto-neutron stars parameters and changes their composition. The obtained form of the equation of state allows to construct the mass–radius relations and shows that the increasing asymmetry creates more compact stars. The inclusion of δ meson together with nonlinear vector meson interaction terms and magnetic field make this effect even stronger.