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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 1,081 records · Page 60
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Heating mechanism(s) for transition layers in giants

The emission-line fluxes of lines originating in the lower parts of the transition layers between stellar chromospheres and coronas are studied. Simon and Drake (1989) suspect different heating mechanisms for 'hot' and cool stars. Changes in the flux ratios for the C IV to C II emission lines support this suspicion. Large C IV/C II line flux ratios appear to be indicative of magnetically controlled heating. A correlation between excess continuum flux around 1950 A and C II emission-line fluxes are confirmed for the cooler giants (late F and cooler). Excess continuum flux correlates positively with large C IV/C II line flux ratio. The excess continuum flux corresponds to an increase in temperature by several hundred degrees in layers with a mean optical depth of about 0.03. For chromospherically active stars these layers experience a mechanical flux deposition of the order of 1 percent of the total radiative flux. This flux is tentatively identified as an MHD wave flux similar to Alfven waves.

Bohm-Vitense, Erika↗
NASA NTRS1976

Hercules X-1: Spectral Variability of an X-Ray Pulsar in a Stellar Binary System

A cosmic X-ray spectroscopy experiment onboard the Orbiting Solar Observatory 8 (OSO-8), observed Her x-1 continuously for approximately 8 days. Spectral-temporal correlations of the X-ray emission were obtained. The major results concern observations of: (1) iron band emission, (2) spectral hardening (increase in effective x-ray temperature) within the X-ray pulse, and (3) a transition from an X-ray low state to a high state. The spectrum obtained prior to the high state can be interpreted as reflected emission from a hot coronal gas surrounding an accretion disk, which itself shields the primary X-ray source from the line of sight during the low state. The spectral hardening within the X-ray pulse was indicative of the beaming mechanism at the neutron star surface. The hardest spectrum by pulse phase was identified with the line of sight close to the Her x-1 magnetic dipole axis, and the X-ray pencil beam become harder with decreasing angle between the line of sight and the dipole axis.

Pravdo, S. H.↗
NASA NTRS1979

Degenerate dwarf X-ray sources

X-ray emission by accreting degenerate dwarfs is examined qualitatively, considering bremsstrahlung and cyclotron fluxes and cooling. Also discussed are strongly magnetic degenerate dwarfs, which are characterized by a blackbody-limited UV cyclotron component and a hard X-ray bremsstrahlung component, both produced by the hot emission region, and a hard UV or soft X-ray blackbody component produced by cyclotron and bremsstrahlung photons. With regard to nonmagnetic and weakly magnetic degenerate dwarfs, it is concluded that degenerate dwarfs can produce larger luminosities than earlier estimates indicated, that even high mass stars can produce low temperature (less than 10 keV) X-ray spectra, and, for moderate and high accretion rates and high (greater than 1.2 solar masses) stellar masses, the spectra exhibit a high energy tail. Individual sources of each type of dwarf are considered, such as the AM Her, DQ Her and SS Cyg classes as well as Cyg X-2 and the Sco X-1 class, and the number of such sources in the Galaxy is estimated.

Lamb, D. Q.↗
NASA NTRS1983

Microwave emission from the coronae of late-type dwarf stars

VLA microwave observations of 14 late-type dwarf and subgiant stars and binary systems are examined. In this extensive set of observations, four sources at 6 cm (Chi-1 Ori, UV Cet, YY Gem, and Wolf 630AB) were detected and low upper limits for the remaining stars were found. The microwave luminosities of the nondetected F-K dwarfs are as small as 0.01 those of the dMe stars. The detected emission is slowly variable in all cases and is consistent with gyroresonant emission from thermal electrons spiraling in magnetic fields of about 300 gauss if the source sizes are as large as R/R(asterisk) = 3-4. This would correspond to magnetic fields that are probably in the range 0.001-0.0001 gauss at the photospheric level. An alternative mechanism is gyrosynchrotron emission from a relatively small number of electrons with effective temperature.

Linsky, J. L.↗
NASA NTRS1986

Magnetic nonequilibrium and current sheet formation

This paper investigates the equilibrium of a cluster of long, straight, twisted magnetic flux tubes extending in the z-direction in a highly conducting fluid, subject to the boundary condition that the cluster is surrounded by a uniform pressure P. It is shown that there is equilibrium only for axial symmetry, i.e., only for a single twisted flux tube. Any more complicated cluster of twisted tubes is subject to nonequilibrium reconnection of the transverse component of the field. The example provides another perspective on the general absence of equilibrium, and consequent rapid dissipation, of any magnetic field subject to random small-scale internal strains, such as the fields extending outward from the convective zones of stars.

Vainshtein, S. I.↗
NASA NTRS1987

Narrow polarized components in the OH 1612-MHz maser emission from supergiant OH-IR sources

High-resolution (300 Hz) OH 1612-MHz spectra of the supergiant OH-IR sources VY CMa, VX Sgr, IRC 10420, and NML Cyg are presented. Linewidths as small as 550 Hz (0.1 km/s) are found for narrow components in the spectra. The present results are consistent with current models for maser line-narrowing and for the physical properties in the OH maser regions. A significant degree of circular polarization is noted in many of the narrow components. The circular polarization suggests the presence of magnetic fields of about 1 mG in the circumstellar envelopes which would be strong enough to influence the outflow from the stars, and which may explain asymmetries found in the circumstellar envelopes.

Cohen, R. J.↗
NASA NTRS1992

Proton acceleration in neutron star magnetospheres

To explain the emission of TeV and PeV gamma rays from accreting X-ray binary sources, protons must be accelerated to several times the gamma-ray energy. It is shown here that at certain times, the plasma in the accretion column of the neutron star may form a deep enough pool that the top portion becomes unstable to convective motions in spite of the strong magnetic field. The resulting turbulence produces fluctuations in the strength of the magnetic field that travel up the accretion column, taking energy out to the region of the energetic protons. The protons resonantly absorb this energy and are accelerated to high energies. Including the synchrotron radiation losses of the protons, it is shown that they can be accelerated to energies that are high enough to explain the gamma-ray observations.

Smith, I. A.↗
NASA NTRS1995

Star formation in the Galactic Center GMC cores: Sagittarius B2 and the dust ridge

The total far-infrared luminosity and the ionizing flux inferred from radio continuum observations of the Galactic center region imply a rate of star formation per unit mass of molecular material comparable to that in the Galactic disk. However, H2O and OH masers commonly found in sites of high-mass star formation are relatively rare in the nuclear disk. Far-infrared studies suggest that the formation rate of stars with masses greater than approximately 20 Solar Mass is reduced in the central region compared to the Galactic disk. Star formation might be suppressed currently in the central region as a result of the different geometry and strength of the magnetic fields there, which arguably might tend to inhibit cloud collapse. High gas pressures implied by observations of the diffuse X-ray emission suggest that giant molecular clouds (GMCs) in the nuclear disk may be held together by external pressure rather than self-gravity. The gravitational collapse leading to the formation of high density cores may thus be suppressed in all but the most massive clouds.

Lis, D. C.↗
NASA NTRS2006

A Possible Magnetar Nature for IGR J16358-4726

We present detailed spectral and timing analysis of the hard x-ray transient IGR J16358-4726 using multi-satellite archival observations. A study of the source flux time history over 6 years, suggests that this transient outbursts can be occurring in intervals of at most 1 year. Joint spectral fits using simultaneous Chandra/ACIS and INTEGRAL/ISGRI data reveal a spectrum well described by an absorbed cut-off power law model plus an Fe line. We detected the pulsations initially reported using Chandra/ACIS also in the INTEGRAL/ISGRI light curve and in subsequent XMM-Newton observations. Using the INTEGRAL data we identified a pulse spin up of 94 s (P = 1.6 x 10(exp -4), which strongly points to a neutron star nature for IGR J16358-4726. Assuming that the spin up is due to disc accretion, we estimate that the source magnetic field ranges between 10(sup 13) approximately 10(sup 15) depending on its distance, possibly supporting a magnetar nature for IGR J16358-4726.

Patel, S.↗
NASA NTRS2010

AXTAR: Mission Design Concept

The Advanced X-ray Timing Array (AXTAR) is a mission concept for X-ray timing of compact objects that combines very large collecting area, broadband spectral coverage, high time resolution, highly flexible scheduling, and an ability to respond promptly to time-critical targets of opportunity. It is optimized for sub-millisecond timing of bright Galactic X-ray sources in order to study phenomena at the natural time scales of neutron star surfaces and black hole event horizons, thus probing the physics of ultra-dense matter, strongly curved spacetimes, and intense magnetic fields. AXTAR s main instrument, the Large Area Timing Array (LATA) is a collimated instrument with 2 50 keV coverage and over 3 square meters effective area. The LATA is made up of an array of super-modules that house 2-mm thick silicon pixel detectors. AXTAR will provide a significant improvement in effective area (a factor of 7 at 4 keV and a factor of 36 at 30 keV) over the RXTE PCA. AXTAR will also carry a sensitive Sky Monitor (SM) that acts as a trigger for pointed observations of X-ray transients in addition to providing high duty cycle monitoring of the X-ray sky. We review the science goals and technical concept for AXTAR and present results from a preliminary mission design study

Ray, Paul S.↗
NASA NTRS2013

High-Energy X-rays from J174545.5-285829, the Cannonball: a Candidate Pulsar Wind Nebula Associated with Sgr a East

We report the unambiguous detection of non-thermal X-ray emission up to 30 keV from the Cannonball, a few arcsecond long diffuse X-ray feature near the Galactic Center, using the NuSTAR X-ray observatory. The Cannonball is a high-velocity (v(proj) approximately 500 km s(exp −1)) pulsar candidate with a cometary pulsar wind nebula (PWN) located approximately 2' north-east from Sgr A*, just outside the radio shell of the supernova remnant Sagittarius A (Sgr A) East. Its non-thermal X-ray spectrum, measured up to 30 keV, is well characterized by a Gamma is approximately 1.6 power law, typical of a PWN, and has an X-ray luminosity of L(3-30 keV) = 1.3 × 10(exp 34) erg s(exp −1). The spectral and spatial results derived from X-ray and radio data strongly suggest a runaway neutron star born in the Sgr A East supernova event. We do not find any pulsed signal from the Cannonball. The NuSTAR observations allow us to deduce the PWN magnetic field and show that it is consistent with the lower limit obtained from radio observations.

stars: neutron↗
NASA NTRS1986

Definition and empirical structure of the range of stellar chromospheres-coronae across the H-R diagram: Cool stars

Major advances in our understanding of non-radiative heating and other activity in stars cooler than T sub eff = 10,000K has occured in the last few years. This observational evidence is reviewed and the trends that are now becoming apparent are discussed. The evidence for non-radiatively heated outer atmospheric layers (chromospheres, transition regions, and coronae) in dwarf stars cooler than spectral type A7, in F and G giants, pre-main sequence stars, and close bindary systems is unambiguous, as is the evidence for chromospheres in the K and M giants and supergiants. The existence of non-radiative heating in the outer layers of the A stars remains undetermined despite repeated searches at all wavelengths. Two important trends in the data are the decrease in plasma emission measure with age on the main sequence and decreasing rotational velocity. Variability and atmospheric inhomogeneity are commonly seen, and there is considerable evidence that magnetic fields define the geometry and control the energy balance in the outer atmospheric layers. In addition, the microwave observations imply that non-thermal electrons are confined in coronal magnetic flux tubes in at least the cool dwarfs and RS CVn systems. The chromospheres in the K and M giants and supergiants are geometrically extended, as are the coronae in the RS CVn systems and probably also in other stars.

Linsky, J. L.↗
NASA NTRS2016

Discovery of Coherent Pulsations from the Ultraluminous X-Ray Source NGC 7793 P13

We report the detection of coherent pulsations from the ultraluminous X-ray source (ULX) NGC 7793P13. The approx. =0.42 s nearly sinusoidal pulsations were initially discovered in broadband X-ray observations using XMM-Newton and NuSTAR taken in 2016. We subsequently also found pulsations in archival XMM-Newton data taken in 2013 and 2014. The significant (>>5(sigma)) detection of coherent pulsations demonstrates that the compact object in P13 is a neutron star, and given the observed peak luminosity of 10(exp 40) erg/ s (assuming isotropy), it is well above the Eddington limit for a 1.4 Stellar Mass accretor. This makes P13 the second ULX known to be powered by an accreting neutron star. The pulse period varies between epochs, with a slow but persistent spin-up over the 2013-2016 period. This spin-up indicates a magnetic field of B1.51012 G, typical of many Galactic accreting pulsars. The most likely explanation for the extreme luminosity is a high degree of beaming; however, this is difficult to reconcile with the sinusoidal pulse profile.

accretion disks – pulsars: individual (NGC 7793 ↗
NASA NTRS2002

The Sun: Our Nearest Star

We have in our celestial backyard, a prime example of a variable star. The Sun, long thought to be "perfect" and unvarying, began to reveal its cycles in the early 1600s as Galileo Galilei and Christoph Scheiner used a telescope to study sunspots. For the past four hundred years, scientists have accumulated data, showing a magnetic cycle that repeats, on average, every eleven (or twenty-two) years. In addition, modern satellites have shown that the energy output at radio and x-ray wavelengths also varies with this cycle. This talk will showcase the Sun as a star and discuss how solar studies may be used to understand other stars.

Adams, M. L.↗
NASA NTRS1988

Semi-empirical models of the wind in cool supergiant stars

A self-consistent semi-empirical model for the wind of the supergiant in zeta Aurigae type systems is proposed. The damping of the Alfven waves which are assumed to drive the wind is derived from the observed velocity profile. Solution of the ionization balance and energy equation gives the temperature structure for given stellar magnetic field and wave flux. Physically acceptable solutions of the temperature structure place limits on the stellar magnetic field. A crude formula for a critical mass loss rate is derived. For a mass loss rate below the critical value the wind cannot be cool. Comparison between the observed and the critical mass loss rate suggests that the proposed theory may provide an explanation for the coronal dividing line in the Hertzsprung-Russell diagram. The physical explanation may be that the atmosphere has a cool wind, unless it is physically impossible to have one. Stars which cannot have a cool wind release their nonthermal energy in an outer atmosphere at coronal temperatures. It is possible that in the absence of a substantial stellar wind the magnetic field has less incentive to extend radially outward, and coronal loop structures may become more dominant.

Kuin, N. P. M.↗
NASA NTRS1987

Are the Galactic-bulge X-ray sources magnetized?

This paper attempts to demonstrate that a better understanding of Galactic-bulge X-ray sources can be achieved if their magnetic moments are assumed to have the same values as those of young pulsars. It is argued that most of the matter leaving the inner edge of the accretion disk can reach the neutron star's surface in the form of massive clumps in quasi-Keplerian orbits. As a result, most of the accretion flow covers a broad equatorial belt rather than the polar caps, and the star shines as an almost unpulsed source. The liberation of half of the accretion power before the surface is reached can lead to the reported UHE pulses and bright infrared bursts. Spasmodic accretion is discussed as a model for gamma-ray bursts, and the observed low-energy X-ray absorption features are considered as an indication of strong magnetic fields shifted to lower energies during super-Eddington outbursts.

Kundt, W.↗
NASA NTRS1982

Protostellar mass and angular momentum loss

Recent radio observations have indicated that appreciable, continuing mass loss is occurring from regions of star formation. The conditions under which massive protostellar objects may exhibit strong, rotationally driven winds are investigated. For plausible rotational velocities and magnetic field strengths, ejection of about 10,000 solar masses a year at speeds of 10-100 km/s can be maintained for time scales of about 0.0001 yr; these values are insensitive to the protostellar luminosity. An evolutionary scenario is sketched which suggests that protostellar clouds may be expected to be rapidly rotating and to possess substantial magnetic fields when the clouds have contracted to radii of about 10 to the 14th cm. The centrifugally driven winds proposed present an alternative to fragmentation for providing the angular momentum loss required in the phase of rapid cloud contraction.

Hartmann, L.↗