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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Transition region fluxes in A-F Dwarfs: Basal fluxes and dynamo activity

The transition region spectra of 87 late A and early F dwarfs and subgiants were analyzed. The emission line fluxes are uniformly strong in the early F stars, and drop off rapidly among the late A stars. The basal flux level in the F stars is consistent with an extrapolation of that observed among the G stars, while the magnetic component displays the same flux-flux relations seen among solar-like stars. Despite the steep decrease in transition region emission flux for B-V less than 0.28, C II emission is detected in alpha Aql (B-V = 0.22). The dropoff in emission is inconsistent with models of the mechanically generated acoustic flux available. It is concluded that, although the nonmagnetic basal heating is an increasingly important source of atmospheric heating among the early F stars, magnetic heating occurs in any star which has a sufficiently thick convective zone to generate acoustic heating.

Walter, Frederick M.↗
NASA NTRS1995

Postaccretion magnetic field evolution of neutron stars

We present the first calculations of magnetic field evolution following accretion-induced field reduction which demonstrate the possibility of subsequent surface magnetic field growth. Assuming the preaccretion field is pushed down or advected below the accreting material, we show that significant diffusion of the suppressed field back to the surface can occur within a Galactic age approximately 10(exp 10) yr. This produces surface fields characteristic of standard pulsars if the total mass transferred is below a critical value delta M(sub c) approximately 0.04 solar masses. We then show how this provides a natural explanation for the existence of old pulsars with high inferred surface dipole fields which are still observable. Using our results in conjunction with a model for accretion-induced field reduction we predict the evolution of a binary pulsar's spin and magnetic field through its postaccretion lifetime. This analysis also provides a qualitative explanation for a recent suggestion that the magnetic fields and spins of recycled pulsars have a bimodal distribution.

Young, E. J.↗
DOE OSTI2015

Magnetic fields in mixed neutron-star-plus-wormhole systems

We consider mixed configurations consisting of a wormhole filled by a strongly magnetized isotropic or anisotropic neutron fluid. The nontrivial topology of the spacetime is allowed by the presence of exotic matter. By comparing these configurations with ordinary magnetized neutron stars, we clarify the question of how the presence of the nontrivial topology influences the magnetic field distribution inside the fluid. In the case of an anisotropic fluid, we find new solutions describing configurations, where the maximum of the fluid density is shifted from the center. A linear stability analysis shows that these mixed configurations are unstable.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

High-energy emission from a magnetar giant flare in the Sculptor galaxy

Magnetars are the most highly magnetized neutron stars in the cosmos (with magnetic field 10 13 –10 15 G). Giant flares from magnetars are rare, short-duration (about 0.1 s) bursts of hard X-rays and soft γ rays. Owing to the limited sensitivity and energy coverage of previous telescopes, no magnetar giant flare has been detected at gigaelectronvolt (GeV) energies. Here, we report the discovery of GeV emission from a magnetar giant flare on 15 April 2020 (refs. 3,4 and A. J. Castro-Tirado et al., manuscript in preparation). The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope detected GeV γ rays from 19 s until 284 s after the initial detection of a signal in the megaelectronvolt (MeV) band. Our analysis shows that these γ rays are spatially associated with the nearby (3.5 megaparsecs) Sculptor galaxy and are unlikely to originate from a cosmological γ-ray burst. Thus, we infer that the γ rays originated with the magnetar giant flare in Sculptor. Furthermore, we suggest that the GeV signal is generated by an ultra-relativistic outflow that first radiates the prompt MeV-band photons, and then deposits its energy far from the stellar magnetosphere. After a propagation delay, the outflow interacts with environmental gas and produces shock waves that accelerate electrons to very high energies; these electrons then emit GeV γ rays as optically thin synchrotron radiation. This observation implies that a relativistic outflow is associated with the magnetar giant flare, and suggests the possibility that magnetars can power some short γ-ray bursts.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Magnetic Reconnection in High-Energy Density Laboratory Plasmas

This project involved theoretical research on radiative magnetic reconnection in High-Energy Density (HED) plasmas, with applications to laboratory laser-plasma experiments [on facilities like OMEGA (LLE) and NIF (LLNL)] and to astrophysical systems, such as accreting black hole coronae and magnetospheres of strongly magnetized neutron stars (magnetars and pulsars). Relativistic magnetic reconnection is a leading candidate mechanism for powering high-energy flares and persistent emission in various astrophysical systems.The extremely high energy density found in many of these systems dictates that reconnection should proceed in the radiative regime, where radiation back reaction on the electrons (and, if present, positrons), manifested, e.g., as radiative cooling and radiative drag on reconnection-driven bulk flows, plays an important role. Moreover, under some conditions, the energy of the emitted photons is so high that they can readily produce electron-positron pairs, e.g., in black-hole accretion disk coronae and in pulsar and magnetar magnetospheres. Due to recent rapid developments of experimental capabilities in HED physics, we can now for the first time seriously contemplate studying these exciting physical processes in the laboratory. This project investigated fundamental physics of magnetic reconnection in this novel regime, by performing several computational [using radiative particle-in-cell (PIC) simulations] and theoretical studies.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1988

Astronomical interferometry on the Moon

Optical interferometric arrays are particularly attractive candidates for a manned lunar base. A permanent lunar can provide support for a variety of astronomical investigations. An optical interferometric array, perhaps of the general form of the VLA but designed for optical instead of radio wavelengths, would lead to a qualitative advance in the understanding of the universe. A wide variety of scientific problems could be addressed by such an instrument. The stellar analogs of the solar cycle, the behavior of sunspots on other stars, the magnetic field configurations of other stars, and the behavior of dynamic plasma phenomena such as flares and winds are examples of star related problems that ultimately would lead to both increased understanding of our Sun and fundamental knowledge of the manner in which stars form and evolve.

Burke, Bernard F.↗
NASA NTRS1991

High-energy transients

The observational characteristics of X-ray bursts are reviewed and the thermonuclear-flash model is examined to assess the suitability of the model and the sources of X- and gamma-ray bursts. The profiles of X-ray bursts from five different sources are delineated, and the global analysis of nuclear burning is discussed to review th range possible X-ray burst behavior. The analysis by Fushiki and Lamb (1987) is noted for the description of regions which lie away from the steady-state cut. The discussion of cyclotron lines in gamma-ray bursts emphasizes the source GB880205, and it is shown that Raman and cyclotron resonant scattering can explain the observed positions, strengths, and widths of the dips in the spectrum. X-ray bursts are theorized to emanate from weakly magnetic neutron stars, and gamma-ray bursts are shown to be the products of strongly magnetic neutron stars.

Lamb, D. Q.↗
DOE OSTI2014

Upper limits to the magnetic field in central stars of planetary nebulae

More than about 20 central stars of planetary nebulae (CSPNs) have been observed spectropolarimetrically, yet no clear, unambiguous signal of the presence of a magnetic field in these objects has been found. We perform a statistical (Bayesian) analysis of all the available spectropolarimetric observations of CSPN to constrain the magnetic fields in these objects. Assuming that the stellar field is dipolar and that the dipole axis of the objects is oriented randomly (isotropically), we find that the dipole magnetic field strength is smaller than 400 G with 95% probability using all available observations. The analysis introduced allows integration of future observations to further constrain the parameters of the distribution, and it is general, so that it can be easily applied to other classes of magnetic objects. We propose several ways to improve the upper limits found here.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Secular outflows from 3D MHD hypermassive neutron star accretion disc systems

Magnetized hypermassive neutron stars (HMNSs) have been proposed as a way for neutron star mergers to produce high electron fraction, high-velocity ejecta, as required by kilonova models to explain the observed light curve of GW170817. The HMNS drives outflows through neutrino energy deposition and mechanical oscillations, and raises the electron fraction of outflows through neutrino interactions before collapsing to a black hole (BH). Here, in this study, we perform 3D numerical simulations of HMNS–torus systems in ideal magnetohydrodynamics, using a leakage/absorption scheme for neutrino transport, the nuclear APR equation of state, and Newtonian self-gravity, with a pseudo-Newtonian potential added after BH formation. Due to the uncertainty in the HMNS collapse time, we choose two different parametrized times to induce collapse. We also explore two initial magnetic field geometries in the torus, and evolve the systems until the outflows diminish significantly ($\sim\!\! 1\!\! - \!\!2\ \mathrm{s}$). We find bluer, faster outflows as compared to equivalent BH–torus systems, producing M ∼ 10 −3 M ⊙ of ejecta with Y e ≥ 0.25 and v ≥ 0.25c by the simulation end. Approximately half the outflows are launched in disc winds at times $t\lesssim 500 \ \mathrm{ms}$, with a broad distribution of electron fractions and velocities, depending on the initial condition. The remaining outflows are thermally driven, characterized by lower velocities and electron fractions. Nucleosynthesis with tracer particles shows patterns resembling solar abundances in all models. Although outflows from our simulations do not match those inferred from two-component modelling of the GW170817 kilonova, self-consistent multidimensional detailed kilonova models are required to determine whether our outflows can power the blue kilonova.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1975

Magnetic fields and dense chromospheres in dMe stars

The hypothesis is investigated that dense chromospheres of dMe stars are heated by dissipation of hydromagnetic waves which may be generated in active regions where the nonspot magnetic field strength can be as large as 5 to 10 kG. It is proposed that dMe stars are a set of magnetic stars on the lower main sequence which have strong fields generated by dynamo action in deep convective envelopes, while dM stars are nonmagnetic or weakly magnetic stars having no starspots on their surfaces. The combination of magnetic fields and dense chromospheres in dMe stars is shown to provide consistent evidence for several conclusions, including: (1) the dMe stars which are most likely to be flare stars are those with hydrogen emission lines and (2) propagation of flare-initiated coronal waves can trigger sympathetic stellar flares. It is suggested that grain formation occurs in starspots of dMe stars and that such grains in a circumstellar shell are responsible for the systematic IR excesses of dMe stars relative to dM stars.

Mullan, D. J.↗
NASA NTRS2002

Unveiling the Nature of Soft Gamma Repeaters and Magnetars: Scientists Measure the Most Powerful Magnet Known

This newsletter from NASA Goddard Space Flight Center (GSFC) announces measurements of the magnetic field of a magnetar. The magnetic field was approx. 10(exp 15) gauss, up to 10 times more powerful than previous estimates. The newsletter also describes how the star's magnetic field slows its rotation, and how starquakes emit protons, which are trapped in this neutron star's magnetic field, and make it a soft gamma repeater (SGR).

Swank, Jean↗
DOE OSTI2014

Magnetic fields and massive star formation

Massive stars (M > 8 M {sub ☉}) typically form in parsec-scale molecular clumps that collapse and fragment, leading to the birth of a cluster of stellar objects. We investigate the role of magnetic fields in this process through dust polarization at 870 μm obtained with the Submillimeter Array (SMA). The SMA observations reveal polarization at scales of ≲0.1 pc. The polarization pattern in these objects ranges from ordered hour-glass configurations to more chaotic distributions. By comparing the SMA data with the single dish data at parsec scales, we found that magnetic fields at dense core scales are either aligned within 40° of or perpendicular to the parsec-scale magnetic fields. This finding indicates that magnetic fields play an important role during the collapse and fragmentation of massive molecular clumps and the formation of dense cores. We further compare magnetic fields in dense cores with the major axis of molecular outflows. Despite a limited number of outflows, we found that the outflow axis appears to be randomly oriented with respect to the magnetic field in the core. This result suggests that at the scale of accretion disks (≲ 10{sup 3} AU), angular momentum and dynamic interactions possibly due to close binary or multiple systems dominate over magnetic fields. With this unprecedentedly large sample of massive clumps, we argue on a statistical basis that magnetic fields play an important role during the formation of dense cores at spatial scales of 0.01-0.1 pc in the context of massive star and cluster star formation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

TESTING THE MAGNETAR MODEL VIA LATE-TIME RADIO OBSERVATIONS OF TWO MACRONOVA CANDIDATES

Compact binary mergers may have already been observed as they are the leading model for short gamma-ray bursts (sGRBs). Radioactive decay within the ejecta from these mergers is expected to produce an infrared flare, dubbed macronova (or kilonova), on a timescale of a week. Recently, two such macronova candidates were identified in followup observations of sGRBs, strengthening the possibility that those indeed arise from mergers. The same ejecta will also produce long-term (months to years) radio emission due to its interaction with the surrounding interstellar medium. In the search for this emission, we observed the two macronova candidates, GRB 130603B and GRB 060614, with the Jansky Very Large Array (VLA) and the Australia Telescope Compact Array (ATCA). Our observations resulted in null-detections, putting strong upper limits on the kinetic energy and mass of the ejecta. A possible outcome of a merger is a highly magnetized neutron star (a magnetar), which has been suggested as the central engine for GRBs. Such a magnetar will deposit a significant fraction of its energy into the ejecta leading to a brighter radio flare. Our results, therefore, rule out magnetars in these two events.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

MODELING THE RISE OF FIBRIL MAGNETIC FIELDS IN FULLY CONVECTIVE STARS

Many fully convective stars exhibit a wide variety of surface magnetism, including starspots and chromospheric activity. The manner by which bundles of magnetic field traverse portions of the convection zone to emerge at the stellar surface is not especially well understood. In the solar context, some insight into this process has been gleaned by regarding the magnetism as consisting partly of idealized thin flux tubes (TFTs). Here we present the results of a large set of TFT simulations in a rotating spherical domain of convective flows representative of a 0.3 M {sub ⊙} main-sequence star. This is the first study to investigate how individual flux tubes in such a star might rise under the combined influence of buoyancy, convection, and differential rotation. A time-dependent hydrodynamic convective flow field, taken from separate 3D simulations calculated with the anelastic equations, impacts the flux tube as it rises. Convective motions modulate the shape of the initially buoyant flux ring, promoting localized rising loops. Flux tubes in fully convective stars have a tendency to rise nearly parallel to the rotation axis. However, the presence of strong differential rotation allows some initially low-latitude flux tubes of moderate strength to develop rising loops that emerge in the near-equatorial region. Magnetic pumping suppresses the global rise of the flux tube most efficiently in the deeper interior and at lower latitudes. The results of these simulations aim to provide a link between dynamo-generated magnetic fields, fluid motions, and observations of starspots for fully convective stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

MAGNETIC GAMES BETWEEN A PLANET AND ITS HOST STAR: THE KEY ROLE OF TOPOLOGY

Magnetic interactions between a star and a close-in planet are postulated to be a source of enhanced emissions and to play a role in the secular evolution of the orbital system. Close-in planets generally orbit in the sub-alfvénic region of the stellar wind, which leads to efficient transfers of energy and angular momentum between the star and the planet. We model the magnetic interactions occurring in close-in star–planet systems with three-dimensional, global, compressible magnetohydrodynamic numerical simulations of a planet orbiting in a self-consistent stellar wind. We focus on the cases of magnetized planets and explore three representative magnetic configurations. The Poynting flux originating from the magnetic interactions is an energy source for enhanced emissions in star–planet systems. Our results suggest a simple geometrical explanation for ubiquitous on/off enhanced emissions associated with close-in planets, and confirm that the Poynting fluxes can reach powers of the order of 10{sup 19} W. Close-in planets are also shown to migrate due to magnetic torques for sufficiently strong stellar wind magnetic fields. The topology of the interaction significantly modifies the shape of the magnetic obstacle that leads to magnetic torques. As a consequence, the torques can vary by at least an order of magnitude as the magnetic topology of the interaction varies.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1981

X-ray astronomy from Uhuru to HEAO-1

The nature of galactic and extragalactic X-ray sources is investigated using observations made with nine satellites and several rockets. The question of X-ray pulsars being neutron stars or white dwarfs is considered, as is the nature of Population II and low-luminosity X-ray stars, the diffuse X-ray emission from clusters of galaxies, the unidentified high-galactic-latitude (UHGL) sources, and the unresolved soft X-ray background. The types of sources examined include binary pulsars, Population II X-ray stars (both nonbursters and bursters) inside and outside globular clusters, coronal X-ray emitters, and active galactic nuclei. It is concluded that: (1) X-ray pulsars are strongly magnetized neutron stars formed in the evolution of massive close binaries; (2) all Population II X-ray stars are weakly magnetized or nonmagnetic neutron stars accreting from low-mass companions in close binary systems; (3) the diffuse emission from clusters is thermal bremsstrahlung of hot matter processed in stars and swept out by ram pressure exerted by the intergalactic gas; (4) most or all of the UHGL sources are active galactic nuclei; and (5) the soft X-ray background is emission from a hot component of the interstellar medium.

Clark, G. W.↗
NASA NTRS1991

Synchronization of the magnetic Nova V1500 Cygni

The study addresses a program of polarimetric monitoring of V1500 Cyg and a detection of spin-down of the magnetic primary star. Data on the spectral shape of polarized cyclotron flux is presented, and a lower limit to the field strength on the magnetic primary star is derived. The implications which the field strength and observed rate of period change have on locking scenarios are discussed, including the likelihood of identifying other AM Her systems in the process of formation. It is noted that two-color polarimetry of V1500 Cyg indicates a polar magnetic-field strength consistent with those measured in the synchronized AM Her variables; however, the symmetry and long-term repeatability of the polarized flux curves indicate that accretion occurs onto large regions of the white-dwarf surface.

Schmidt, Gary D.↗
NASA NTRS1984

Physical processes in the strong magnetic fields of accreting neutron stars

Analytical formulae are fitted to observational data on physical processes occurring in strong magnetic fields surrounding accreting neutron stars. The propagation of normal modes in the presence of a quantizing magnetic field is discussed in terms of a wave equation in Fourier space, quantum electrodynamic effects, polarization and mode ellipticity. The results are applied to calculating the Thomson scattering, bremsstrahlung and Compton scattering cross-sections, which are a function of the frequency, angle and polarization of the magnetic field. Numerical procedures are explored for solving the radiative transfer equations. When applied to modeling X ray pulsars, a problem arises in the necessity to couple the magnetic angle and frequency dependence of the cross-sections with the hydrodynamic equations. The use of time-dependent averaging and approximation techniques is indicated.

Meszaros, P.↗