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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 649 records · Page 36
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

On magnetohydrodynamic thermal instabilities in magnetic flux tubes

The stability of current-driven filamentary modes in magnetic flux tubes embedded in a plane-parallel atmosphere in LTE and in hydrostatic equilibrium is discussed. Within the tube, energy transport by radiation only is considered. The dominant contribution to the opacity is due to H- ions and H atoms (in the Paschen continuum). A region in the parameter space of the equilibrium configuration in which the instability is effective is delimited, and the relevance of this process for the formation of structured coronae in late-type stars and accretion disks is discussed.

Massaglia, S.↗
NASA NTRS1989

The behavior of beams of relativistic non-thermal electrons under the influence of collisions and synchrotron losses

For many astrophysical situations, such as in solar flares or cosmic gamma-ray bursts, continuum gamma rays with energies up to hundreds of MeV were observed, and can be interpreted to be due to bremsstrahlung radiation by relativistic electrons. The region of acceleration for these particles is not necessarily the same as the region in which the radiation is produced, and the effects of the transport of the electrons must be included in the general problem. Hence it is necessary to solve the kinetic equation for relativistic electrons, including all the interactions and loss mechanisms relevant at such energies. The resulting kinetic equation for non-thermal electrons, including the effects of Coulomb collisions and losses due to synchrotron emission, was solved analytically in some simple limiting cases, and numerically for the general cases including constant and varying background plasma density and magnetic field. New approximate analytic solutions are presented for collision dominated cases, for small pitch angles and all energies, synchrotron dominated cases, both steady-state and time dependent, for all pitch angles and energies, and for cases when both synchrotron and collisional energy losses are important, but for relativistic electrons. These analytic solutions are compared to the full numerical results in the proper limits. These results will be useful for calculation of spectra and angular distribution of the radiation (x rays, gamma-rays, and microwaves) emitted via synchrotron or bremsstrahlung processes by the electrons. These properties and their relevance to observations will be observed in subsequent papers.

Mctiernan, James M.↗
NASA NTRS1989

Comparison of backgrounds in OSO-7 and SMM spectrometers and short-term activation in SMM

The backgrounds in the OSO-7 Gamma-Ray Monitor and the Solar Maximum Mission Gamma-Ray Spectrometer are compared. After scaling to the same volume, the background spectra agree to within 30 percent. This shows that analyses which successfully describe the background in one detector can be applied to similar detectors of different sizes and on different platforms. The background produced in the SMM spectrometer by a single trapped-radiation belt passage is also studied. This background is found to be dominated by a positron-annihilation line and a continuum spectrum with a high energy cutoff at 5 MeV.

Dunphy, P. P.↗
NASA NTRS1990

The behavior of beams of relativistic nonthermal electrons under the influence of collisions and synchrotron losses

For many astrophysical situations, such as in solar flares or cosmic gamma-ray bursts, continuum gamma rays with energies up to hundreds of MeV were observed, and can be interpreted to be due to bremsstrahlung radiation by relativistic electrons. The region of acceleration for these particles is not necessarily the same as the region in which the radiation is produced, and the effects of the transport of the electrons must be included in the general problem. Hence it is necessary to solve the kinetic equation for relativistic electrons, including all the interactions and loss mechanisms relevant at such energies. The resulting kinetic equation for non-thermal electrons, including the effects of Coulomb collisions and losses due to synchrotron emission, was solved analytically in some simple limiting cases, and numerically for the general cases including constant and varying background plasma density and magnetic field. New approximate analytic solutions are presented for collision dominated cases, for small pitch angles and all energies, synchrotron dominated cases, both steady-state and time dependent, for all pitch angles and energies, and for cases when both synchrotron and collisional energy losses are important, but for relativistic electrons. These analytic solutions are compared to the full numerical results in the proper limits. These results will be useful for calculation of spectra and angular distribution of the radiation (X rays, gamma-rays, and microwaves) emitted via synchrotron or bremsstrahlung processes by the electrons. These properties and their relevance to observations will be observed in subsequent papers.

Mctiernan, James M.↗
DOE OSTI2020

Outside the Lyman-break box: detecting Lyman continuum emitters at 3.5 < z < 5.1 with CLAUDS

ABSTRACT Identifying non-contaminated sample of high-redshift galaxies with escaping Lyman continuum (LyC) flux is important for understanding the sources and evolution of cosmic reionization. We present CLAUDS (CFHT Large Area u-band deep survey) u-band photometry of the COSMOS field to probe LyC radiation from spectroscopically confirmed galaxies at $z$ ≥ 3.5 and outside the standard Lyman-break galaxy colour-selection expectations. Complementary to the CLAUDS data, we use Subaru multifilter photometry, Hubble Space Telescope (HST) multifilter imaging, and the spectroscopic surveys D10K, VUDS, and 3D-HST. We present a sample of Lyman continuum galaxy (LCG) candidates in the redshift range 3.5 ≲ $z$ ≲ 5.1. Here, we introduce 5 LCG candidates, where two are flagged quality 1 and three quality 2. The estimated $f_{\rm esc}^{\rm abs}$ for quality 1 candidates are in the range $\sim 5 - 73{{\ \rm per\ cent}}$ and $\sim 30 - 93{{\ \rm per\ cent}}$. These estimates are based on our derived parameters from individual galaxies as inputs to a range of BPASS models as well as mean intergalactic medium (IGM) and maximal intergalactic and circumgalactic media (IGM+CGM) transmission. We conclude that our search for LCGs is most likely biased to lines of sight with low H i densities or free from Lyman limit systems. Our two best LCG candidates have EW (Lyα) ≤ 50 Å and we find no correlation or anticorrelation between EW (Lyα), $f_{\rm esc}^{\rm abs}$, and Robs, the ratio of ionizing to non-ionizing observed flux in the measured passbands. Stacking candidates without solid LyC detections (S/N < 3) results in an estimated $f_{\rm esc}^{\rm abs}$ from galaxies not greater than $1{{\ \rm per\ cent}}$.

Meštrić, U.↗
DOE OSTI2022

β - p and β - α decay of the 11 Be neutron halo ground state

Beta-delayed proton emission from the neutron halo ground state of 11 Be raised much attention due to the unusually high decay rate. It was argued that this may be due to the existence of a resonance just above the proton decay threshold. In this letter, we use the lenses of real-energy continuum shell model to describe several observables including the Gamow–Teller rates for the β - -delayed α and proton decays, and argue that, within our model, the large β - p branching ratio cannot be reconciled with other data.

11Be↗
NASA NTRS1978

The structure and spectrum of a colliding-cloud system and its possible relationship to QSOs

A collision between two gas clouds with initial densities of approximately 10 million per cu cm, velocities of about 1000 km/s, and radii of approximately 1 pc is investigated quantitatively by coupling a calculation of the radiation spectrum resulting from the anticipated shock fronts with a computation for the conversion of this high-energy radiation into optical emission in adjacent photoionized regions. The detailed structure of the colliding clouds is discussed, and the effects of an ambient magnetic field are considered. The combined emission-line spectrum is presented along with continuum emission estimates for thermal, synchrotron, and very-high-energy bremsstrahlung mechanisms. It is shown that significant continua can be produced over the range from 300 microns to 3 keV, including a blackbody contribution from a high-density neutral region between the shock fronts, free-free and free-bound radiation from the cooling zones directly behind the shocks, and free-free, free-bound, and two-photon radiation from the photoionized regions immediately ahead of and behind the cooling zones. The theoretical spectrum of the structure resulting from the collision is found to be similar in general and in some details to those observed for typical quasars.

Daltabuit, E.↗
DOE OSTI2024

Autoionizing polaritons with the Jaynes-Cummings model

Intense laser pulses have the capability to couple resonances in the continuum, leading to the formation of a split pair of autoionizing polaritons. These polaritons can exhibit extended lifetimes due to interference between radiative and Auger decay channels. In this work we show how an extension of the Jaynes-Cummings model to autoionizing states quantitatively reproduces the observed phenomenology. Furthermore, this extended model allows us to study how the dressing laser parameters can be tuned to control the ionization rate of the polariton multiplet.

74 ATOMIC AND MOLECULAR PHYSICS↗
NASA NTRS1972

Discovery of a hectometric solar continuum

Solar continuum emission was detected in the hectometric range for the first time by RAE-1 satellite. Hectometric continuum is quite similar to decametric continuum, showing that the phenomenon extends out in the corona to at least 50 solar radii, or 36.4 gigameters(0.25 AU). An interesting point is that the amplitude distribution and the bandwidth of type 3 bursts associated with hectometric continuum are the same as those of type 1 bursts associated with the metric noise storm. It is believed that the storm radiation all the way from metric through hectometric can be accounted for by plasma wave radiation. This means that streams of superthermal electrons are continuously coming out of active regions and as they travel outward through the interplanetary medium, they produce plasma waves that result in metric, decametric, hectrometric, and probably kilometric storms.

Malitson, H.↗
NASA NTRS1985

Photodissociation regions. I - Basic model. II - A model for the Orion photodissociation region

A theoretical parameter study of the temperature and chemical structure of dense photodissociation regions and their resultant spectrum is presented. Models are discussed which are relevant not only to the dust and gas between molecular clouds and H II regions, but also apply to any neutral cloud illuminated by intense FUV fluxes. The models relate observed line and continuum emission from these regions to physical parameters such as the gas density and temperature, the elemental and chemical abundances, the local radiation field, and the grain properties. The results are applied to observational data from the OMC-1 region. The model shows that the observed high brightness temperature of the C I 609 microns line can be explained by emission from the C(+)/C/CO transition region. This difference with previous chemical models is due to a higher gas phase elemental abundance of carbon, to the charge exchange reactions of C(+) with S and SiO, and to carbon self-shielding.

Tielens, A. G. G. M.↗
NASA NTRS2007

Global Properties of X-Ray Flashes and X-Ray-Rich GRBs Observed by Swift

We describe and discuss the spectral and temporal characteristics of the prompt emission and X-ray afterglow emission of X-ray flashes (XRFs) detected and observed by Swift between December 2005 and September 2006. We compare these characteristics to a sample of X-ray rich gamma-ray bursts (XRRs) and conventional classical gamma-ray bursts (C-GRBs)observed during the same period. We confirm the correlation between Epeak and fluence noted by others and find further evidence that XRFs and C-GRBs form a continuum. We also confirmed that our known redshift samples are consistent with the correlation between the peak energy (Epeak) and the isotropic radiated energy (Eiso), so called the Epeak-Eiso relation. The spectral properties of X-ray afterglows are similar to those of gamma-ray burst afterglows, but the temporal properties of the two classes are quite different. We found that the light curves of C-GRBs afterglow show a break to steeper indices (shallow-to-steep break) at much earlier times than do XRF afterglows. Moreover, the overall luminosity of X-ray afterglows of XRFs are systematically smaller by a factor of two or more compared with that of C-GRBs. These distinct differences in the X-ray afterglow between XRFs and C-GRBs are key to understanding not only a mysterious shallow-to-steep phase in the X-ray afterglow but also the unique nature of XRFs.

Sakamoto, T.↗
NASA NTRS2000

Two-Dimensional Array Beam Scanning Via Externally and Mutually Injection Locked Coupled Oscillators

Some years ago, Stephan proposed an approach to one dimensional (linear) phased array beam steering which requires only a single phase shifter. This involves the use of a linear array of voltage-controlled electronic oscillators coupled to nearest neighbors. The oscillators are mutually injection locked by controlling their coupling and tuning appropriately. Stephan's approach consists of deriving two signals from a master oscillator, one signal phase shifted with respect to the other by means of a single phase shifter. These two signals are injected into the end oscillators of the array. The result is a linear phase progression across the oscillator array. Thus, if radiating elements are connected to each oscillator and spaced uniformly along a line, they will radiate a beam at an angle to that line determined by the phase gradient which is, in turn, determined by the phase difference between the injection signals.The beam direction is therefore controlled by adjusting this phase difference. Recently, Pogorzelski and York presented a formulation which facilitates theoretical analysis of the above beam steering technique. This was subsequently applied by Pogorzelski in analysis of two dimensional beam steering using perimeter detuning of a coupled oscillator array. The formulation is based on a continuum model in which the oscillator phases are represented by a continuous function satisfying a partial differential equation of diffusion type. This equation can be solved via the Laplace transform and the resulting solution exhibits the dynamic behavior of the array as the beam is steered. Stephan's beam steering technique can be similarly generalized to two-dimensional arrays in which the beam control signals are applied to the oscillators on the perimeter of the array. In this paper the continuum model for this two-dimensional case is developed and the dynamic solution for the corresponding aperture phase function is obtained. The corresponding behavior of the resulting far-zone radiation pattern is displayed as well.

Pogorzelski, Ronald J.↗
DOE OSTI2022

Superradiance in alpha clustered mirror nuclei

Resonances in unstable quantum systems are radiating states that despite decaying overall normalization have a well-defined structure which is being balanced by outgoing radiation. Such an interplay between outgoing wave and internal quantum many-body dynamics leads to several unique effects. One of those is known as superradiance, or alignment, where due to decay or virtual coupling to the continuum the states undergo restructuring so that their wave functions align towards the decay channels thus facilitating the decay. This effect is well understood theoretically and is closely related to the fundamental properties of reaction physics. Direct observation of superradiance in open quantum many-body systems is difficult because it is hard to find identical complex quantum systems that are different only in their coupling to the continuum of reaction states describing the decay. Here we report this phenomenon in alpha cluster decays of mirror nuclei 18 O and 18 Ne.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
NASA NTRS1978

Ionization of interstellar H2 clouds by supernovae

Two destruction mechanisms for molecular hydrogen in the H I regions of the interstellar medium were given by Stecher and Williams (1967). The first is dominant in weak radiation fields and involves photon excitation (at about 1000 A) to the first excited electronic state with a subsequent decay into the vibrational continuum of the ground state. The second destruction mechanism is dominant in the strong photon fields that exist when a supernova explosion occurs. It involves photoionization (at about 1000 A) from metastable high vibrational levels produced by photon excitation (in the same wavelength range) to an excited electronic state and subsequent decay to bound vibrational levels. It is shown that ionization of molecular hydrogen can occur at considerable distances from such supernova events while the atomic hydrogen remains neutral. The effect should be observable in the dispersion measures of new pulsars.

Stecher, T. P.↗
NASA NTRS1980

Oscillator strengths of ultraviolet Ni I lines from hook-method and absorption measurements in a furnace

Measurements of the oscillator strengths of the ultraviolet lines of neutral nickel obtained by the use of the combined hook and absorption technique are reported. A total of 221 transitions in the range 1964-4094 A was measured for nickel atoms from a high-temperature graphite furnace (2000-2500 K) using a continuum background source, a Mach-Zehnder interferometer and a 3-m Czerny-Turner spectrograph. Hook and absorption measurements are presented, and radiative lifetimes are derived from log gf values. Comparison of the present values with previous results indicates only those of Bell et al. (1966) and Lennard et al. (1975) to consistently agree with the data presented, although the reliability laser-excitation technique of lifetime measurement is supported over that of Hanle methods.

Huber, M. C. E.↗
NASA NTRS1982

High resolution far-infrared observations of the evolved H II region M16

A balloon-borne telescope was used to map NGC 6611, and three far-IR sources embedded in an extended ridge of emission were detected. It is found that Source I is an unresolved far-IR source, embedded in a molecular cloud, which despite the association of an H2O maser with its source does not exhibit radio continuum emissions. Far-IR sources II and III are associated with ionized gas-molecular cloud interfaces, with the far-IR radiation arising from dust at the boundary heated by the OB cluster. These observations are consistent with models of the later stages of evolution of an H II region.

Mcbreen, B.↗
NASA NTRS1985

Transition probabilities of the B-prime 1Sigma(u)(+) to X 1Sigma(g)(+) system of molecular hydrogen

From published potential energy curves and transition dipole moments, there are obtained by numerical integration of the equations of nuclear motion the vibrational eigenfunctions of the X 1Sigma(g)(+) and B-prime 1Sigma(u)(+) states of H2. The probabilities of radiative transitions from the discrete vibrational levels of the excited B-prime 1Sigma(u)(+) electronic state of H2 to the discrete and continuum vibrational levels of the ground X 1Sigma(g)(+) electronic state are calculated. The Franck-Condon factors are also presented.

Kwok, T. L.↗