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At least 937 records · Page 52
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

A catalog of intracluster gas temperatures

We have searched the Einstein Monitor Proportional Counter (MPC) data base for observations of clusters of galaxies. By coadding the MPC spectra obtained during all pointed observations of clusters with IPC count rates greater than 0.1 counts/s, we have obtained sufficient photon statistics to estimate the X-ray temperature of 84 clusters. Combining the MPC results with EXOSAT and Ginga results reported in the literature yields a combined sample of 104 clusters with known X-ray temperatures. One of the best studied X-ray correlations between clusters is that between their X-ray luminosity and gas temperature. We show that the best-fit power-law relation for our combined cluster sample can be explained by the observed increase in the gas-to-stellar mass ratio between low- and high-temperature clusters. The statistical significance of any evolution in our combined X-ray sample has been examined and compared with the statistical properties of clusters culled from optical catalogs. We find that there is strong evidence for a decrease in the X-ray luminosity of optically rich clusters beyond z approximately about 0.06. This result is used to estimate the normalization of the primordial power spectrum of density fluctuations.

David, L. P.↗
NASA NTRS1983

Luminosity function and cosmological evolution of X-ray selected quasars

The preliminary analysis of a complete sample of 55 X-ray sources is presented as part of the Medium Sensitivity Survey of the Einstein Observatory. A pure luminosity evolutionary law is derived in order to determine the uniform distribution of the sources and the rates of evolution for Active Galactic Nuclei (AGNs) observed by X-ray and optical techniques are compared. A nonparametric representation of the luminosity function is fitted to the observational data. On the basis of the reduced data, it is determined that: (1) AGNs evolve cosmologically; (2) less evolution is required to explain the X-ray data than the optical data; (3) the high-luminosity portion of the X-ray luminosity can be described by a power-law with a slope of gamma = 3.6; and (4) the X-ray luminosity function flattens at low luminosities. Some of the implications of the results for conventional theoretical models of the evolution of quasars and Seyfert galaxies are discussed.

Maccacaro, T.↗
NASA NTRS1995

Variability of accretion disks surrounding black holes: The role of inertial-acoustic mode instabilities

The global nonlinear time-dependent evolution of the inertial-acoustic mode instability in accretion disks surrounding black holes has been investigated. The viscous stress is assumed to be proportional to the gas pressure only, i.e., tau = alphap(sub g). It is found that an oscillatory nonsteady behavior exists in the inner regions of disks (r is less than 10r(sub g) where r(sub g) is the Schwarzschild radius) for sufficiently large alpha(greater than or approximately equal to 0.2) and for mass accretion rates less than about 0.3 times the Eddington value. The variations of the integrated bolometric luminosity from the disk, Delta L/L, are less than 3%. A power spectrum analysis of these variations reveals a power spectrum which can be fitted to a power-law function of the frequency Pis proportional to f(exp -gamma), with index gamma = 1.4-2.3 and a low-frequency feature at about 4 Hz in one case. In addition, a narrow peak centered at a frequency corresponding to the maximum epicyclic frequency of the disk at approximately 100-130 Hz and its first harmonic is also seen. The low-frequency modulations are remarkably similar to those observed in black hole candidate systems. The possible existence of a scattering corona in the inner region of the disk and/or other processes contributing to the power at high frequencies in the inner region of the accretion disk may make the detection of the high-frequency component difficult.

Chen, Xingming↗
NASA NTRS1987

A scaling law for accretion zone sizes

Current theories of runaway planetary accretion require small random velocities of the accreted particles. Two body gravitational accretion cross sections which ignore tidal perturbations of the Sun are not valid for the slow encounters which occur at low relative velocities. Wetherill and Cox have studied accretion cross sections for rocky protoplanets orbiting at 1 AU. Using analytic methods based on Hill's lunar theory, one can scale these results for protoplanets that occupy the same fraction of their Hill sphere as does a rocky body at 1 AU. Generalization to bodies of different sizes is achieved here by numerical integrations of the three-body problem. Starting at initial positions far from the accreting body, test particles are allowed to encounter the body once, and the cross section is computed. A power law is found relating the cross section to the radius of the accreting body (of fixed mass).

Greenzweig, Yuval↗
NASA NTRS1976

Iron line emission from a high-temperature plasma in Cassiopeia A

The letter reports a measurement of the X-ray spectrum of Cas A with a proportional counter on board OSO-8. The high-energy data (greater than 7 keV) are found to agree equally well with a power law having a number index of about 4.5 and with a thermal spectrum having kT of about 3.9 keV. A narrow iron line predicted by the thermal model is also observed; this line is centered at about 6.67 keV and has an equivalent width of approximately 1270 eV. The low-energy data (below 6 keV) are not fully accounted for by the above thermal spectrum, but are fitted well by an additional low-temperature thermal component with kT not exceeding 0.7 keV and an atomic hydrogen column density of approximately 10 to the 22nd power per sq cm. The iron abundance in the plasma at an electron temperature of 40 keV is estimated from the observed line-to-continuum ratio, and it is shown that a normal cosmic iron abundance is adequate to explain the observation.

Pravdo, S. H.↗
NASA NTRS1985

Comparison of simulation results with sea-level experimental data on 10(14) - 10(16) air shower cores

Simulation predictions for the Leeds 35 sq m horizontal discharge chamber array for proton primaries with a approx. E sup 2.7 spectrum extrapolated from balloon data to 10 to the 16th power eV give power law rho (r)-spectra with constant slope approx. -2 consistent with the experimental data up to the point at which they steepen but overshooting them at higher densities, and at high shower sizes predicted cores which are significantly steeper than those observed. Further comparisons with results for heavy nuclei primaries (up to A = 56) point to the inadequacy of changes in primary composition to account for the observed density spectra and core flattening, and the shower size spectrum together, and point, therefore, to the failure of the scaling interaction model at approx. 10 to the 15th power eV primary energy.

Ash, A. G.↗
NASA NTRS2014

Nustar Reveals an Intrinsically X-ray Weak Broad Absorption Line Quasar in the Ultraluminous Infrared Galaxy Markarian 231

We present high-energy (3-30 keV) NuSTAR observations of the nearest quasar, the ultraluminous infrared galaxy (ULIRG) Markarian 231 (Mrk 231), supplemented with new and simultaneous low-energy (0.5-8 keV) data from Chandra. The source was detected, though at much fainter levels than previously reported, likely due to contamination in the large apertures of previous non-focusing hard X-ray telescopes. The full band (0.5-30 keV) X-ray spectrum suggests the active galactic nucleus (AGN) in Mrk 231 is absorbed by a patchy and Compton-thin N(sub H) approx. 1.2(sup +0.3) sub-0.3) x 10(exp 23) / sq cm) column. The intrinsic X-ray luminosity L(sub 0.5-30 Kev) approx. 1.0 x 10(exp 43) erg /s) is extremely weak relative to the bolometric luminosity where the 2-10 keV to bolometric luminosity ratio is approx. 0.03% compared to the typical values of 2-15%. Additionally, Mrk 231 has a low X-ray-to-optical power law slope alpha(sub 0X) approx. -1.7. It is a local example of a low-ionization broad absorption line (LoBAL) quasar that is intrinsically X-ray weak. The weak ionizing continuum may explain the lack of mid-infrared [O IV], [Ne V], and [Ne VI] fine-structure emission lines which are present in sources with otherwise similar AGN properties. We argue that the intrinsic X-ray weakness may be a result of the super-Eddington accretion occurring in the nucleus of this ULIRG, and may also be naturally related to the powerful wind event seen in Mrk 231, a merger remnant escaping from its dusty cocoon.

Reveals↗
NASA NTRS1981

Creep crack-growth: A new path-independent T sub o and computational studies

Two path independent integral parameters which show some degree of promise as fracture criteria are the C* and delta T sub c integrals. The mathematical aspects of these parameters are reviewed. This is accomplished by deriving generalized vector forms of the parameters using conservation laws which are valid for arbitrary, three dimensional, cracked bodies with crack surface tractions (or applied displacements), body forces, inertial effects and large deformations. Two principal conclusions are that delta T sub c is a valid crack tip parameter during nonsteady as well as steady state creep and that delta T sub c has an energy rate interpretation whereas C* does not. An efficient, small displacement, infinitestimal strain, displacement based finite element model is developed for general elastic/plastic material behavior. For the numerical studies, this model is specialized to two dimensional plane stress and plane strain and to power law creep constitutive relations.

Stonesifer, R. B.↗
NASA NTRS2007

The Jet/Disk Connection in AGN: Chandra and XMM-Newton Observations of Three Powerful Radio-Loud Quasars

The connection between the accretion process that powers AGN and the formation of jets is still poorly understood. Here we tackle this issue using new, deep Chandra and XMM-Newton observations of tlie cores of three powerful radio loud quasars: 1136-135, 1150+497 (Chandra), and 0723+679 (XMM-Newton), in the redshift range z=0.3-0.8. These sources are known from our previous Chandra siiapsliot survey to liave kpc-scale X-ray jets. In 1136-135 and 1150-1+497; evidence is found for the presence of diffuse thermal X-ray emission around the cores; on scales of 40-50 kpc and with luminosity L(sub 0.3-2 kev approx. 10(sup 43) erg per second, suggesting thermal emission from the host galaxy or a galaxy group. The X-ray continua of the cores in the three sources are described by an upward-curved (concave) broken power law, with photon indices GAMMA (sub soft) approx. 1.8 - 2.1 and GAMMA (sub hard) approx. 1.7 below and above approx. equal to 2 keV, respectively. There is evidence for an uiiresolved Fe K alpha line with EW approx. 70 eV in the three quasars. The Spectral Energy Distributions of the sources can be well described by a mix of jet and disk emission, with the jet dominating the radio and hard X-rays (via synchrotron and external Compton) and the disk dominating the optical/UV through soft X-rays. The ratio of the jet-to-disk powers is approx. 1, consistent with those derived for a number of gamma ray emitting blazars. This indicates that near equality of accretion and jet power may be common in powerful radio-loud AGN.

Sambruna, Rita↗
NASA NTRS2010

Messaging with Cost-Optimized Interstellar Beacons

On Earth, how would we build galactic-scale beacons to attract the attention of extraterrestrials, as some have suggested we should do? From the point of view of expense to a builder on Earth, experience shows an optimum trade-off. This emerges by minimizing the cost of producing a desired power density at long range, which determines the maximum range of detectability of a transmitted signal. We derive general relations for cost-optimal aperture and power. For linear dependence of capital cost on transmitter power and antenna area, minimum capital cost occurs when the cost is equally divided between antenna gain and radiated power. For nonlinear power-law dependence, a similar simple division occurs. This is validated in cost data for many systems; industry uses this cost optimum as a rule of thumb. Costs of pulsed cost-efficient transmitters are estimated from these relations by using current cost parameters ($/W, $/sq m) as a basis. We show the scaling and give examples of such beacons. Galactic-scale beacons can be built for a few billion dollars with our present technology. Such beacons have narrow "searchlight" beams and short "dwell times" when the beacon would be seen by an alien observer in their sky. More-powerful beacons are more efficient and have economies of scale: cost scales only linearly with range R, not as R(exp 2), so number of stars radiated to increases as the square of cost. On a cost basis, they will likely transmit at higher microwave frequencies, -10 GHz. The natural corridor to broadcast is along the galactic radius or along the local spiral galactic arm we are in. A companion paper asks "If someone like us were to produce a beacon, how should we look for it?"

Benford, James↗
DOE OSTI2025

Discovery of Powerful Multivelocity Ultrafast Outflows in the Starburst Merger Galaxy IRAS 05189–2524 with XRISM

We observed the X-ray-bright ultraluminous infrared galaxy IRAS 05189−2524 with XRISM during its performance verification phase. The unprecedented energy resolution of the onboard X-ray microcalorimeter revealed complex spectral features at ∼7–9 keV, which can be interpreted as blueshifted Fe XXV/XXVI absorption lines with various velocity dispersions, originating from ultrafast outflow (UFO) components with multiple bulk velocities of ∼0.076c, ∼0.101c, and ∼0.143c. In addition, a broad Fe–K emission line was detected around ∼7 keV, forming a P Cygni profile together with the absorption lines. The onboard X-ray CCD camera revealed a 0.4–12 keV broadband spectrum characterized by a neutrally absorbed power-law continuum with a photon index of ∼2.3 and intrinsic flare-like variability on timescales of ∼10 ks, both of which are likely associated with near-Eddington accretion. We also found potential variability of the UFO parameters on a timescale of ∼140 ks. Using these properties, we propose new constraints on the outflow structure and suggest the presence of multiple outflowing regions on scales of about tens to 100 Schwarzschild radii, located within roughly 2000 Schwarzschild radii. Since both the estimated momentum and energy outflow rates of the UFOs exceed those of galactic molecular outflows, our results indicate that powerful, multivelocity UFOs are already well developed during a short-lived evolutionary phase following a major galaxy merger, characterized by intense starburst activity and likely preceding the quasar phase. This system is expected to evolve into a quasar, sustaining strong UFO activity and suppressing star formation in the host galaxy.

Astronomy and AstroPhysics↗
NASA NTRS1999

Statistical Properties of SGR 1900+14 Bursts

We study the statistics of soft gamma repeater (SGR) bursts using a database of 187 events detected with BATSE and 837 events detected with the Rossi X-Ray Timing Explorer Proportional Counter Array: all events are from SGR 1900+14 during its 1998-1999 active phase. We find that the fluence or energy distribution of bursts is consistent with a power law of index 1.66, over 4 orders of magnitude. This scale-free distribution resembles the Gutenberg-Richter law for earthquakes and gives evidence for self-organized criticality in SGRS. The distribution of time intervals between successive bursts from SGR 1900+14 is consistent with a lognormal distribution. There is no correlation between burst intensity and the waiting times till the next burst, but there is some evidence for a correlation between burst intensity and the time elapsed since the previous burst. We also find a correlation between the duration and the energy of the bursts, but with significant scatter. In all these statistical properties, SGR bursts resemble earthquakes and solar flares more closely than they resemble any known accretion-powered or nuclear-powered phenomena. Thus, our analysis lends support to the hypothesis that the energy source for SGR bursts is internal to the neutron star and plausibly magnetic.

Gogus, Ersin↗
DOE OSTI2024

Plasma Dynamics and Nonthermal Particle Acceleration in 3D Nonrelativistic Magnetic Reconnection

Understanding plasma dynamics and nonthermal particle acceleration in 3D magnetic reconnection has been a long-standing challenge. In this paper, we explore these problems by performing large-scale fully kinetic simulations of multi-X-line plasmoid reconnection with various parameters in both the weak- and strong-guide-field regimes. In each regime, we have identified its unique 3D dynamics that lead to field-line chaos and efficient acceleration, and we have achieved nonthermal acceleration of both electrons and protons into power-law spectra. The spectral indices agree well with a simple Fermi acceleration theory that includes guide-field dependence. In the low-guide-field regime, the flux rope kink instability governs the 3D dynamics for efficient acceleration. The weak dependence of the spectra on the ion-to-electron mass ratio and β ($\ll$1) implies that the particles are sufficiently magnetized for Fermi acceleration in our simulations. While both electrons and protons are injected at reconnection exhausts, protons are primarily injected by perpendicular electric fields through Fermi reflections and electrons are injected by a combination of perpendicular and parallel electric fields. The magnetic power spectra agree with in situ magnetotail observations, and the spectral index may reflect a reconnection-driven size distribution of plasmoids instead of the Goldreich–Sridhar vortex cascade. As the guide field becomes stronger, the oblique flux ropes of large sizes capture the main 3D dynamics for efficient acceleration. Intriguingly, the oblique flux ropes can also experience flux rope kink instability, to drive extra 3D dynamics. This work has broad implications for 3D reconnection dynamics and particle acceleration in heliophysics and astrophysics.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1988

Creep behavior of tungsten/niobium and tungsten/niobium-1 percent zirconium composites

The creep behavior and microstructural stability of tungsten fiber reinforced niobium and niobium 1 percent zirconium was determined at 1400 and 1500 K in order to assess the potential of this material for use in advanced space power systems. The creep behavior of the composite materials could be described by a power law creep equation. A linear relationship was found to exist between the minimum creep rate of the composite and the inverse of the composite creep rupture life. The composite materials had an order of magnitude increase in stress to achieve 1 percent creep strain and in rupture strength at test temperatures of 1400 and 1500 K compared to unreinforced material. The composite materials were also stronger than the unreinforced materials by an order of magnitude when density was taken into consideration. Results obtained on the creep behavior and microstructural stability of the composites show significant potential improvement in high temperature properties and mass reduction for space power system components.

Petrasek, Donald W.↗
NASA NTRS1999

Statistical Properties of SGR 1900+14 Bursts

We study the statistics of soft gamma repeater (SGR) bursts, using a data base of 187 events detected with BATSE and 837 events detected with RXTE PCA, all from SGR 1900+14 during its 1998-1999 active phase. we find that the fluence or energy distribution of bursts is consistent with a power law of index 1.66, over 4 orders of magnitude. This scale-free distribution resembles the Gutenberg-Richter Law for earthquakes, and gives evidence for self-organized criticality in SGRS. The distribution of time intervals between successive bursts from SGR 1900+14 is consistent with a log-normal distribution. There is no correlation between burst intensity and the waiting times till the next burst, but there is some evidence for a correlation between burst intensity and the time elapsed since the previous burst. We also find a correlation between the duration and the energy of the bursts, but with significant scatter. In all these statistical properties, SGR bursts resemble earthquakes and solar flares more closely than they resemble any known accretion-powered or nuclear-powered phenomena. Thus our analysis lends support to the hypothesis that the energy source for SGR bursts is internal to the neutron star, and plausibly magnetic.

Gogus, Ersin↗
NASA NTRS1995

A Simple Theory of Capillary-Gravity Wave Turbulence

Employing a recently proposed 'multi-wave interaction' theory, inertial spectra of capillary gravity waves are derived. This case is characterized by a rather high degree of nonlinearity and a complicated dispersion law. The absence of scale invariance makes this and some other problems of wave turbulence (e.g., nonlinear inertia gravity waves) intractable by small-perturbation techniques, even in the weak-turbulence limit. The analytical solution obtained in the present work for an arbitrary degree of nonlinearity is shown to be in reasonable agreement with experimental data. The theory explains the dependence of the wave spectrum on wind input and describes the accelerated roll-off of the spectral density function in the narrow sub-range separating scale-invariant regimes of purely gravity and capillary waves, while the appropriate (long- and short-wave) limits yield power laws corresponding to the Zakharov-Filonenko and Phillips spectra.

Glazman, Roman E.↗
DOE OSTI2021

The Highest Energy HAWC Sources are Likely Leptonic and Powered by Pulsars

The HAWC Collaboration has observed gamma rays at energies above 56 TeV from a collection of nine sources. It has been suggested that this emission could be hadronic in nature, requiring that these systems accelerate cosmic-ray protons or nuclei up to PeV-scale energies. In this paper, we instead show that the spectra of these objects favor a leptonic (inverse Compton) origin for their emission. More specifically, the gamma-ray emission from these objects can be straightforwardly accommodated within a model in which $\sim \mathcal{O}(10\%)$ of the host pulsar's spindown power is transferred into the acceleration of electrons and positrons with a power-law spectrum that extends to several hundred TeV or higher. The spectral break that is observed among these sources is naturally explained within the context of this simple model, and occurs at the energy where the timescale for energy losses matches the age of the pulsar. In contrast, this spectral feature cannot be straightforwardly accommodated in hadronic scenarios. Furthermore, hadronic models predict that these sources should produce more emission at GeV-scale energies than is observed. In light of these considerations, we conclude that HAWC's highest energy sources should be interpreted as TeV halos or pulsar wind nebulae, which produce their emission through inverse Compton scattering, and are powered by the rotational kinetic energy of their host pulsar.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2021

Time–connectivity superposition and the gel/glass duality of weak colloidal gels

Colloidal gels result from the aggregation of Brownian particles suspended in a solvent. Gelation is induced by attractive interactions between individual particles that drive the formation of clusters, which in turn aggregate to form a space-spanning structure. We study this process in aluminosilicate colloidal gels through time-resolved structural and mechanical spectroscopy. Using the time–connectivity superposition principle a series of rapidly acquired linear viscoelastic spectra, measured throughout the gelation process by applying an exponential chirp protocol, are rescaled onto a universal master curve that spans over eight orders of magnitude in reduced frequency. This analysis reveals that the underlying relaxation time spectrum of the colloidal gel is symmetric in time with power-law tails characterized by a single exponent that is set at the gel point. The microstructural mechanical network has a dual character; at short length scales and fast times it appears glassy, whereas at longer times and larger scales it is gel-like. These results can be captured by a simple three-parameter constitutive model and demonstrate that the microstructure of a mature colloidal gel bears the residual skeleton of the original sample-spanning network that is created at the gel point. Our conclusions are confirmed by applying the same technique to another well-known colloidal gel system composed of attractive silica nanoparticles. The results illustrate the power of the time–connectivity superposition principle for this class of soft glassy materials and provide a compact description for the dichotomous viscoelastic nature of weak colloidal gels.

36 MATERIALS SCIENCE↗