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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 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
NASA NTRS2011

Fermi/GBM Results of Magnetars

Magnetars are magnetically powered rotating neutron stars with extreme magnetic fields (over 10(exp 14) Gauss). They were discovered in the X- and gamma-rays where they predominantly emit their radiation. Very few sources (roughly 18) have been found since their discovery in 1987. NASA's Fermi Gamma-ray Space Telescope was launched June 11,2009; since then the Fermi Gamma-ray Burst Monitor (GBM) recorded emission from four magnetar sources. Two of these were brand new sources, SGR J0501 +4516, discovered with Swift and extensively monitored with Swift and GBM, SGR J0418+5729, discovered with GBM and the Interplanetary Network (IPN). A third was SGR Jl550-5418, a source originally classified as an Anomalous X-ray Pulsar (AXP IEI547.0-5408), but exhibiting a very prolific outburst with over 400 events recorded in January 2009. In my talk I will give a short history of magnetars and describe how this, once relatively esoteric field, has emerged as a link between several astrophysical areas including Gamma-Ray Bursts. Finally, I will describe the exciting new results of Fermi in this field and the current status of our knowledge of the magnetar population properties and magnetic fields.

Kouveliotou, chryssa↗
NASA NTRS2012

Three Decades of High Energy Transients

Gamma-Ray Bursts are the most brilliant explosions in space. The first GRB was discovered on 1967, just over 40 years ago. It took several years and multiple generations of space and ground instruments to unravel some of the mysteries of this phenomenon. However, many questions remain open today. I will discuss the history, evolution and current status of the GRB field and its contributions in our understanding of the transient high energy sky. Finally, I will describe how GRBs can be utilized in future missions as tools, to probe the cosmic chemical evolution of the Universe Magnetars are magnetically powered rotating neutron stars with extreme magnetic fields (over 10(exp 14) Gauss). They were discovered in the X- and gamma-rays where they predominantly emit their radiation. Very few sources (roughly 24) have been found since their discovery in 1987. NASA's Fermi Gamma-ray Space Telescope was launched June 11, 2009; since then the Fermi Gamma-ray Burst Monitor (GBM) recorded emission from several magnetar sources. In total, six new sources were discovered between 2008 and 2011, with a synergy between Swift, RXTE, Fermi and the Interplanetary Network (IPN). I will give a short history of magnetars and describe how this, once relatively esoteric field, has emerged as a link between several astrophysical areas including Gamma-Ray Bursts.

Kouveliotou, Chryssa↗
NASA NTRS1994

A Possible Cyclotron Line Signature from Quiescent Gamma-Ray Burst Counterparts

If gamma-ray bursts are associated with isolated, magnetized neutron stars in the Galaxy, they may be accreting directly from the interstellar medium after the burst event. In addition to a soft thermal component (T(sub e) approx. 100 eV), we predict that the faint quiescent emission spectra from these sources will possess a unique nonthermal signature: for magnetic field strengths B approx. equal to(0.7-7) x 10(exp 12) G, approx. 0.5-5% of the total luminosity could be emitted in a narrow (E/Delta E approx. 2-4) cyclotron emission line which peaks between approx. 5-20 keV below the cyclotron resonance energy (E(sub B) = 11.6(B/10(exp 12) G) keV). Unlike the soft thermal emission, this hard cyclotron component will not be strongly absorbed by the intervening H I gas. For stars with B approx. 10(exp 12) G, this nonthermal feature should be detectable by the instruments onboard ASCA and on the next generation of X-ray satellites (e.g., ASTRO-E). We therefore propose that deep X-ray images be taken on gamma-ray burst sources with excellent positions (error boxes less than or approx. equal to l') such as is being planned for GRB920501 to look for this feature. A positive detection would strongly suggest that at least some of the gamma-ray bursters are associated with isolated, magnetized neutron stars in the Galaxy.

Wang, John C. L.↗
NASA NTRS1987

Stellar winds and magnetic fields in the helium peculiar stars

This note discusses the consequences of the interaction of radiatively driven mass loss with magnetic fields in upper-main-sequence helium-peculiar stars. The field constrains mass loss to occur in narrow cones above the magnetic poles. These jets can explain the observed behavior of UV resonance lines in the helium-peculiar stars, especially the sn and helium-rich stars. Suppression of mass outflow at the magnetic equator produces a region in which radiatively driven diffusive mass transport can levitate helium in the hottest stars, producing helium enrichment of the atmosphere. In the cooler stars, the surface becomes progressively more helium poor.

Shore, Steven N.↗
NASA NTRS1990

Spectroscopic measurements of magnetic fields on solar-like stars - Techniques and results

This paper reviews the basic requirements for, and difficulties in, measuring magnetic fields on cool stars. The main types of the analysis methods are described, including methods that employ Fourier transforms; line profile models; the Mathys and Solanki (1989) method based on the difference between Fm and Fq (where Fm is the profile in nonmagnetic regions and Fq is the profile in magnetic regions); and the Basri and Marcy (1991) method based on the fact that, for optically thick lines, Zeeman splitting will increase a line's equivalent width of the Fq. The assumptions made in each case are examined and the results obtained are summarized.

Saar, Steven H.↗
NASA NTRS1992

Starspots - The zebra effect

Recent observations of brightness variations on the sun during the solar cycle have motivated us to re-examine the widely held view that cool, dark starspots, covering a significant fraction of the star, are the centers of magnetic activity on BY Dra stars. We propose that the magnetic regions are better described by a bright facular network, and that the dark areas which give rise to photometric rotational modulation are actually regions where the underlying quiet photosphere is seen. This interpretation is consistent with recent observations of late-type stars that show that bright areas covering much of the star have magnetic fields with strengths of several thousand gauss. It resolves several problems with the current model, including the size, location, and stability of the starspots required to match photometric and Doppler-imaging observations. It also has interesting observational implications for the correlation of photometric rotational modulation and long term brightness variations with other surface activity, and for the positions of magnetically active stars in the H-R diagram.

Pettersen, Bjorn R.↗
NASA NTRS1995

Geometry and pulse profiles of x-ray pulsars: Asymmetric relativistic fits to 4U1538 - 52 and vela x-1

Model calculation of inhomogenuous magnetized neutron star atmospheres are used to analyze the phase-dependent spectra of 4U 1538-52 and Vela X-1 taking into account general relativistic effects. A chi (exp 2) fitting procedure is used on the Ginga data for these objects to determine the geometry and the magnetic structure of the polar caps, as well as their size and location on the surface of the stars. Other parameters determined by the fit are the masses and radii of neutron stars, the accretion rates, and the rotation and magnetic inclination angles including an azimuthal offset to reproduce the asymmetry of the pulse shapes. We find that general relativistic effects play a significant role in determining the derived cap sizes, the fluxes, and the observational appearance of the pulses. There is evidence that the polar caps are unequal and nonantipodal, suggesting either an off-center or bent magnetic axis, or a strong nondipole component. The magnetic field structure is approximated by a two-component model. The rotation axes are inferred to be moderately close to the mean magnetic axes and seen at large inclinations with respect to the line of sight.

Bulik, T.↗
NASA NTRS1996

Toward an Astrophysical Theory of Chondrites

Sunlike stars are born with disks. Based on our recently developed model to understand how a magnetized new star interacts with its surrounding accretion disk, we advanced an astrophysical theory for the early solar system. The aerodynamic drag of a magnetocentrifugally driven wind out of the inner edge of a shaded disk could expose solid bodies lifted into the heat of direct sunlight, when material is still accreting onto the protosun. Chondrules, calcium-aluminum-rich inclusions (CAI's), and rims could form along the flight for typical self-consistent parameters of the outflow in different stages of star formation. The process gives a natural sorting mechanism that explains the size distribution of CAI's and chondrules, as well as their associated rims. Chondritic bodies then subsequently form by compaction of the processed solids with the ambient nebular dust comprising the matrices after their reentry at great distances from the original launch radius.

Shang, Hsien↗
DOE OSTI2026

Magnetic flux emergence in binary neutron star remnants

Using high-resolution AthenaK simulations of a twisted toroidal flux tube, we study the flux emergence of magnetic structures in the shear layer of a hot massive neutron star typical of a binary neutron star remnant. High-resolution simulations demonstrate that magnetic buoyant instabilities allow for emergence only for extremely large magnetic fields significantly exceeding 10 17 G, and more typical fields around 10 16 G are instead dominated by hydrodynamic effects. Because merger remnants tend to be stable against hydrodynamic convection, our work places strong limitations on the mechanisms by which massive binary neutron star remnants can produce the magnetically-driven outflows needed to power jets.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1984

Physics of gamma-ray bursts

Attention is given to the accumulating evidence for the view that gamma-ray bursts come from strongly magnetic neutron stars, discussing the physical properties of the emission region and the radiation processes expected in strong magnetic fields, and emphasizing that the observed burst spectra require that the emission region be optically thin. This entails that the energy of the emitting plasma and/or the plasma itself be continuously replenished during a burst, and that the cooling time scale of the emitting plasma be much shorter than the observed duration of the bursts. This characteristic of the cooling time scale implies that the burst intensity and spectrum can vary on extremely short time scales, and that the burst duration must have a separate explanation. It is emphasized that synchrotron emission is favored as the gamma-ray production mechanism; it is the only mechanism capable of satisfying the optical thinness constraint while producing the observed luminosity.

Lamb, D. Q.↗
NASA NTRS1993

Relativistic electron-positron beams in gamma-ray bursters

Beams of relativistic electrons and/or positrons leaving the surface of a strongly magnetized neutron star may give rise to gamma-ray bursts. The beams could be accelerated by strong, magnetically aligned electric fields that are produced by oscillations of the stellar surface. Here we investigate the particle acceleration in these electric fields, the resulting electron-positron pair cascade, and the gamma-ray emission. We find that beams of electrons and positrons moving parallel to the magnetic field are generated, with a reported differential energy distribution. These beams produce the bulk of the gamma-ray burst radiation below about 1 MeV by the resonant Compton scattering of thermal photons emitted from the stellar surface. The escaping synchrotron radiation from the cascade dominates the radiation spectrum above about 1 MeV.

Smith, I. A.↗
NASA NTRS1992

The Zeeman effect in astrophysical water masers and the observation of strong magnetic fields in regions of star formation

The present study solves the transfer equations for the polarized radiation of astrophysical 22-GHz water masers in the presence of a magnetic field which causes a Zeeman splitting that is much smaller than the spectral line breadth. The emphasis is placed on the relationship between the recently detected circular polarization in this maser radiation and the strength of the magnetic field. When the observed spectral line breadth is smaller than about 0.8 km/s (FWHM), it is calculated that the uncertainty is less than a factor of about 2. The accuracy is improved significantly when the angle between the line of sight and the direction of the magnetic field does not exceed about 45 deg. Uncertainty in the strength of the magnetic field due to lack of knowledge about which hyperfine transition is the source of the 22-GHz masers is removed. The 22-GHz maser feature is found to be the result of a merger of the three strongest hyperfine components.

Nedoluha, Gerald E.↗
DOE OSTI2017

EQUATION OF STATE FOR NUCLEONIC AND HYPERONIC NEUTRON STARS WITH MASS AND RADIUS CONSTRAINTS

We obtain a new equation of state for the nucleonic and hyperonic inner core of neutron stars that fulfils the 2 M {sub ⊙} observations as well as the recent determinations of stellar radii below 13 km. The nucleonic equation of state is obtained from a new parameterization of the FSU2 relativistic mean-field functional that satisfies these latest astrophysical constraints and, at the same time, reproduces the properties of nuclear matter and finite nuclei while fulfilling the restrictions on high-density matter deduced from heavy-ion collisions. On the one hand, the equation of state of neutron star matter is softened around saturation density, which increases the compactness of canonical neutron stars leading to stellar radii below 13 km. On the other hand, the equation of state is stiff enough at higher densities to fulfil the 2 M {sub ⊙} limit. By a slight modification of the parameterization, we also find that the constraints of 2 M {sub ⊙} neutron stars with radii around 13 km are satisfied when hyperons are considered. The inclusion of the high magnetic fields present in magnetars further stiffens the equation of state. Hyperonic magnetars with magnetic fields in the surface of ∼10{sup 15} G and with values of ∼10{sup 18} G in the interior can reach maximum masses of 2 M {sub ⊙} with radii in the 12–13 km range.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1985

Ultra high energy gamma rays, cosmic rays and neutrinos from accreting degenerate stars

Super-Eddington accretion for a recently proposed unipolar induction model of cosmic ray acceleration in accreting binary star systems containing magnetic white dwarfs or neutron stars is considered. For sufficiently high accretion rates and low magnetic fields, the model can account for: (1) acceleration of cosmic ray nuclei up to energies of 10 to the 19th power eV; (2) production of more or less normal solar cosmic ray composition; (3) the bulk of cosmic rays observed with energies above 1 TeV, and probably even down to somewhat lower energies as well; and (4) possibly the observed antiproton cosmic ray flux. It can also account for the high ultra high energy (UHE) gamma ray flux observed from several accreting binary systems (including Cygnus X-3), while allowing the possibility of an even higher neutrino flux from these sources, with L sub nu/L sub gamma is approximately 100.

Brecher, K.↗
NASA NTRS1992

On the evolution of slowly accreting neutron stars

In the light of recent calculations of pycnonuclear reaction rates for light elements, we consider the problem of slow interstellar accretion onto old, possibly magnetized neutron stars. We argue that accretion will occur at the Hoyle-Lyttleton rate after the star has spun down in less than about 10 exp 9 yr. A deep ocean of liquid hydrogen and helium, extending down to depths about 100 m, will cover the surface of the star once it has accreted about 10 exp 25 g of gas. Beneath the ocean will be a layer of almost pure solid O-16 which undergoes two-stage electron capture to C-16 above a pressure 2.7 x 10 exp 28 dyne/sq cm, corresponding to an accreted mass of about 10 exp 27 g. Taking into account the presence of multiple layers of distinct chemical composition, we conclude that the crust will be stable to small perturbations under the conditions envisaged for instellar accretion. A thick layer of up to 10 exp 27 g of metastable C-16 will then accumulate. We discuss the implications of these results to old Galactic neutron stars as sources of gamma-ray bursts.

Blaes, O. M.↗
NASA NTRS1978

Potential drops above pulsar polar caps - Ultrarelativistic particle acceleration along the curved magnetic field

The paper calculates the acceleration of a nonneutral stream of charged particles from the surface of an isolated rotating magnetized neutron star in the case where the particle stream flows along the open field lines of a curved poloidal magnetic field. It is shown that very large electrostatic potential drops can occur in steady unidirectional space-charge-limited flow along the curved open field lines. The explicit incorporation of magnetic field line curvature is the essential element leading to this new result, assuming that all the charges are supplied by emission from the surface. It is emphasized that the current flow and charge density are unique if all of the assumptions are met, with the result that the accelerating electric field near the stellar surface is quite small compared to the vacuum field.

Scharlemann, E. T.↗