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NASA NTRS1997

Energy Spectra and High Frequency Oscillations in 4U 0614+091

We investigate the behavior of the high frequency quasi-periodic oscillations (QPOs) in 4U 0614+091, combining timing and spectral analysis of RXTE (Rossi X-ray Timing Explorer) observations. The energy spectrum of the source can be described by a power law plus a blackbody component. The blackbody has a variable temperature (kT approximately 0.8 to 1.4 keV) and accounts for 10 to 25% of the total energy flux. The power law flux and photon index also vary (F approximately 0.8 to 1.6 x 10(exp -9) erg/sq cm.s and alpha approximately 2.0 to 2.8 respectively). We find a robust correlation of the frequency of the higher frequency QPO with the flux of the blackbody. The source follows the same relation even in observations separated by several months. The QPO frequency does not have a similarly unique correlation with the total flux or the flux of the power law component. The RMS amplitudes of the higher frequency QPO rise with energy but are consistent with a constant for the lower frequency QPO. These results may be interpreted in terms of a beat frequency model for the production of the high frequency QPOs.

Ford, E. C.↗

NASA NTRS2004

Small-Scale Variability of Large Cloud Drops

Cloud droplet size distribution is one of the most fundamental subjects in cloud physics. Understanding of spatial distribution and small-scale fluctuations of cloud droplets is essential for both cloud physics and atmospheric radiation. For cloud physics, it relates to the coalescence growth of raindrops while for radiation, it has a strong impact on a cloud's radiative properties. Most of the existing cloud radiation and precipitation formation models assume that the mean number of drops with a given radius varies proportionally to volume. The analysis of microphysical data on liquid water drop sizes shows that, for sufficiently small volumes, the number is proportional to the drop size dependent power of the volume. For abundant small drops present, the exponent is 1 as assumed in the conventional approach. However, for rarer large drops, the exponents fall below unity. At small scales, therefore, the mean number of large drops decreases with volume at a slower rate than the conventional approach assumes, suggesting more large drops at these scales than conventional models account for; their impact is consequently underestimated. Size dependent models of spatial distribution of cloud drops that simulate the observed power laws show strong drop clustering, the more so the larger the drops. The degree of clustering is determined by the observed exponents. The strong clustering of large drops arises naturally from the observed power-law statistics. Current theories of photon-cloud interaction and warm rain formation will need radical revision in order to produce these statistics; their underlying equations are unable to yield the observed power law.

Marshak, Alexander↗

NASA NTRS1996

Comparison of Non-Parabolic Hydrodynamic Simulations for Semiconductor Devices

Parabolic drift-diffusion simulators are common engineering level design tools for semiconductor devices. Hydrodynamic simulators, based on the parabolic band approximation, are becoming more prevalent as device dimensions shrink and energy transport effects begin to dominate device characteristic. However, band structure effects present in state-of-the-art devices necessitate relaxing the parabolic band approximation. This paper presents simulations of ballistic diodes, a benchmark device, of Si and GaAs using two different non-parabolic hydrodynamic formulations. The first formulation uses the Kane dispersion relationship in the derivation of the conservation equations. The second model uses a power law dispersion relation {(hk)(exp 2)/2m = xW(exp Y)}. Current-voltage relations show that for the ballistic diodes considered. the non-parabolic formulations predict less current than the parabolic case. Explanations of this will be provided by examination of velocity and energy profiles. At low bias, the simulations based on the Kane formulation predict greater current flow than the power law formulation. As the bias is increased this trend changes and the power law predicts greater current than the Kane formulation. It will be shown that the non-parabolicity and energy range of the hydrodynamic model based on the Kane dispersion relation are limited due to the binomial approximation which was utilized in the derivation.

Smith, A. W.↗

NASA NTRS1987

CCD photometry of globular cluster core structure. I - NGC 6388, NGC 6624, and M15: One flat core and two cusps

The first results of a major survey of globular cluster core structure are reported. Surface brightness profiles for three clusters are presented: one with a normal flat core profile (NGC 6388), and two with central cusps (NGC 6624 and M15). The profiles are fitted with both seeing-convolved King models and seeing-convolved power laws. A King model provides a reasonable fit to the entire profile of NGC 6388, but there are no acceptable King profiles for NGC 6624 and M15. For these two clusters, a seeing-convolved power law provides a good fit to the inner part of the profile, while a King model gives a good fit to the outer part of the profile. The interpretation of the measured power-law slopes of the central surface brightness cusps of NGC 6624 and M15 are discussed in terms of models for core collapse in a multicomponent cluster.

Lugger, Phyllis M.↗

NASA NTRS2013

The Importance of Physical Models for Deriving Dust Masses and Grain Size Distributions in Supernova Ejecta. I. Radiatively Heated Dust in the Crab Nebula

Recent far-infrared (IR) observations of supernova remnants (SNRs) have revealed significantly large amounts of newly condensed dust in their ejecta, comparable to the total mass of available refractory elements. The dust masses derived from these observations assume that all the grains of a given species radiate at the same temperature, regardless of the dust heating mechanism or grain radius. In this paper, we derive the dust mass in the ejecta of the Crab Nebula, using a physical model for the heating and radiation from the dust. We adopt a power-law distribution of grain sizes and two different dust compositions (silicates and amorphous carbon), and calculate the heating rate of each dust grain by the radiation from the pulsar wind nebula. We find that the grains attain a continuous range of temperatures, depending on their size and composition. The total mass derived from the best-fit models to the observed IR spectrum is 0.019-0.13 Solar Mass, depending on the assumed grain composition. We find that the power-law size distribution of dust grains is characterized by a power-law index of 3.5-4.0 and a maximum grain size larger than 0.1 micron. The grain sizes and composition are consistent with what is expected for dust grains formed in a Type IIP supernova (SN). Our derived dust mass is at least a factor of two less than the mass reported in previous studies of the Crab Nebula that assumed more simplified two-temperature models. These models also require a larger mass of refractory elements to be locked up in dust than was likely available in the ejecta. The results of this study show that a physical model resulting in a realistic distribution of dust temperatures can constrain the dust properties and affect the derived dust masses. Our study may also have important implications for deriving grain properties and mass estimates in other SNRs and for the ultimate question of whether SNe are major sources of dust in the Galactic interstellar medium and in external galaxies.

Importance↗

NASA NTRS1991

On the reconciliation of simultaneous microwave imaging and hard X-ray observations of a solar flare

Microwave imaging data for a small flare with simultaneous hard X-ray spectral observations are compared. The X-ray data suggest that the power-law index delta of the energy distribution of the radiating electrons is 5.3 (thick-target) which differs significantly from the estimate (delta = 1.4) from a homogeneous optically-thin gyrosynchrotron model which fits the radio observations well. In order to reconcile these results, a double power-law energy spectrum is investigated for the energetic electrons in the flare, as assumed by other authors: the power law is steep at low energies and much flatter at the higher energies which produce the bulk of the microwaves. The emission of soft photons by the flat tail strongly contributes to the observed hard X-ray range and would flatten the spectrum there. A thin-target model for the X-ray emission is also inconsistent with radio data. An inhomogenous gyrosychrotron model with a number of free parameters and containing an electron distribution given by the thick-target X-ray model could be made to fit the radio data.

Nitta, N.↗

NASA NTRS1991

Compton reflection in active galactic nuclei and the cosmic X-ray background

This paper discusses further the model of Fabian et al. (1990) for the cosmic X-ray background (XRB) spectrum, in which the emission is due to AGNs at large redshifts. Each AGN emits a power-law spectrum modified by Compton reflection from cold gas. It is found that the model provides an excellent fit to the XRB from 3 to 500 keV if the sources radiate from a maximum redshift of at least 3.75, if their comoving emissivity varies like (1 + z) exp beta, with beta between 1.8 and 3.3, and if the intrinsic power-law energy index is between 0.9 and 1.2. About 90 percent of the power-law radiation in each AGN must be processed by Compton reflection. The model predicts that the intensity of the XRB below 3 keV is significantly above that extrapolated from observations above 3 keV, without requiring contributions from other types of source.

Rogers, R. D.↗

DOE OSTI2024

Investigating the mixing between two black hole populations in LIGO-Virgo-KAGRA GWTC-3

Here, we introduce a population model to analyze the mixing between hypothesised power-law and ~ 35M ⊙ Gaussian bump black hole populations in the latest gravitational wave catalog, GWTC 3, estimating their co-location and separation. We find a relatively low level of mixing, $3.1^{+5.0}_{-3.1}$%, between the power-law and Gaussian populations, compared to the percentage of mergers containing two Gaussian bump black holes, $5.0^{+3.2}_{-1.7}$%. Our analysis indicates that black holes within the Gaussian bump are generally separate from the power-law population, with only a minor fraction engaging in mixing and contributing to the $\mathcal{M}$ ~ 14M ⊙ peak in the chirp mass. This leads us to identify a distinct population of Binary Gaussian Black Holes (BGBHs) that arise from mergers within the Gaussian bump. We suggest that current theories for the formation of the massive 35M ⊙ Gaussian bump population may need to reevaluate the underlying mechanisms that drive the preference for BGBHs.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1996

Stratospheric Horizontal Wavenumber Spectra of Winds, Potential Temperature, and Atmospheric Tracers Observed by High-Altitude Aircraft

Horizontal wavenumber power spectra of vertical and horizontal wind velocities, potential temperatures, and ozone and N(2)O mixing ratios, as measured in the mid-stratosphere during 73 ER-2 flights (altitude approx. 20km) are presented. The velocity and potential temperature spectra in the 100 to 1-km wavelength range deviate significantly from the uniform -5/3 power law expected for the inverse energy-cascade regime of two-dimensional turbulence and also for inertial-range, three-dimensional turbulence. Instead, steeper spectra approximately consistent with a -3 power law are observed at horizontal scales smaller than 3 km for all velocity components as well as potential temperature. Shallower spectra are observed at scales longer than 6 km. For horizontal velocity and potential temperature the spectral indices at longer scales are between -1.5 and -2.0. For vertical velocity the spectrum at longer scales become flat. It is argued that the observed velocity and potential temperature spectra are consistent with gravity waves. At smaller scales, the shapes are also superficially consistent with a Lumley-Shur-Weinstock buoyant subrange of turbulence and/or nonlinear gravity waves. Contemporaneous spectra of ozone and N(sub 2)O mixing ratio in the 100 to 1-km wavelength range do conform to an approximately uniform -5/3 power law. It is argued that this may reflect interactions between gravity wave air-parcel displacements and laminar or filamentary structures in the trace gas mixing ratio field produced by enstropy-cascading two-dimensional turbulence.

TRACE GAS MIXING RATIO FIELDS↗

NASA NTRS1996

X-Ray Spectral Variability in NGC 7469

We present analyses of two Ginga observations and two observations from the ROSAT database of NGC 7469, focusing on the spectral variability observed on timescales of days and longer. During the 1988 Ginga observation, the hardness ratio (8-21 keV/3.4-5.7 keV) increased significantly as the total flux decreased by 30%. As the spectrum is well fit by the reflection model and since the spectra variability dominates the higher energy band, this could be explained by either a variation in the power law index or in the effective covering fraction of the reflecting material. This ambiguity is inherent in reflection modeling of Ginga spectra from moderate flux Seyfert 1 galaxies. Assuming that the power law index did not change, we find that the reflected flux is consistent with being constant, suggesting that much of the reflecting material may be located more than 3 light-days from the continuum source with the molecular torus being a plausible site. This scenario is also supported by the report of a narrow rather than broad iron K-alpha line in the ASCA data by Guainazzi et al. NGC 7469 was faint during the 1989 Ginga observation, but variability was observed with doubling timescale of 5 hr, and the spectrum was harder. A reflection component could not be constrained, and the change in the spectrum could be explained by an increase in neutral absorption. The brighter of two ROSAT spectra was significantly softer, and in both spectra there was evidence of spectral complexity, as has been previously reported by Turner, George, & Mushotzky and Brandt et al. The spectrum could be fit by a variety of two-component models, including a warm absorber model, an ionized disk model, and a thermal model with single-component blackbody spectrum, but joint fitting of the 1988 average Ginga spectrum and the nonsimultaneous ROSAT spectra favored thermal models, and other models required an anomalously high reflection ratio. This model is supported by the observation of a soft excess component and the lack of ionized absorption edges in the ASCA spectrum by Guainazzi et al. The long-term spectral variability could be explained by relative variability between the power-law and soft excess component normalizations, perhaps implying that hard X-ray reprocessing in thermal material does not dominate on long timescales.

Leighly, Karen↗

NASA NTRS2018

Seeing Double: ASASSN-18bt Exhibits a Two-component Rise in the Early-time K2 Light Curve

On 2018 February 4.41, the All-Sky Automated Survey for SuperNovae (ASAS-SN) discovered ASASSN-18bt in the K2 Campaign 16 field. With a redshift of z=0.01098 and a peak apparent magnitude of B(max)=14.31, ASASSN-18bt is the nearest and brightest SNe Ia yet observed by the Kepler spacecraft. Here we present the discovery of ASASSN-18bt, the K2 light curve, and prediscovery data from ASAS-SN and the Asteroid Terrestrial-impact Last Alert System. The K2 early-time light curve has an unprecedented 30-minute cadence and photometric precision for an SN Ia light curve, and it unambiguously shows a ∼4 day nearly linear phase followed by a steeper rise. Thus, ASASSN-18bt joins a growing list of SNe Ia whose early light curves are not well described by a single power law. We show that a double-power-law model fits the data reasonably well, hinting that two physical processes must be responsible for the observed rise. However, we find that current models of the interaction with a nondegenerate companion predict an abrupt rise and cannot adequately explain the initial, slower linear phase. Instead, we find that existing published models with shallow 56Ni are able to span the observed behavior and, with tuning, may be able to reproduce the ASASSN-18bt light curve. Regardless, more theoretical work is needed to satisfactorily model this and other early-time SNe Ia light curves. Finally, we use Swift X-ray nondetections to constrain the presence of circumstellar material (CSM) at much larger distances and lower densities than possible with the optical light curve. For a constant-density CSM, these nondetections constrain ρ<4.5×10(exp 5)per cu.cm at a radius of 4×10(exp 15) cm from the progenitor star. Assuming a wind-like environment, we place mass loss limits of M˙ < 8 x 10(exp -6) M(ʘ)per yr for v(w)=100 km/s, ruling out some symbiotic progenitor systems. This work highlights the power of well-sampled early-time data and the need for immediate multiband, high-cadence follow-up for progress in understanding SNe Ia.

B. J. Shappee↗

NASA NTRS1987

A fractal interpretation of topography and geoid spectra on the earth, moon, Venus, and Mars

Global spectra are available for topography and geoid on the earth, Venus, Mars, and the moon. If the spectral energy density has a power law dependence on wave number, a fractal is defined. The topography spectrum for the earth is a well-defined fractal with D = 1.5; this corresponds to Brown noise with the amplitude proportional to the wavelength. Although there is more scatter for the other planetary bodies, the data for Mars and the moon correlate well with the data for the earth. Venus topography also exhibits a Brown noise behavior but with a smaller amplitude. The power law dependence of the earth's geoid is known as Kaula's law. It is shown that uncompensated Brown topography gives a geoid with a power law dependence that is in quite good agreement with Kaula's law. However, the required amplitude is only 8 percent of the observed topography. A similar result is found for the other bodies, with the ratio of the amplitude of topography required to explain the geoid to the observed topography increasing to 72 percent for the moon.

Turcotte, Donald L.↗

NASA NTRS2007

Quasi-Periodic Variability in NGC 5408 X-1

We report the discovery with XMM-Newton of quasiperiodic variability in the 0.2 - 10 keV X-ray flux from the ultraluminous X-ray source NGC 5408 X-1. The average power spectrum of all EPIC-pn data reveals a strong 20 mHz QPO with an average amplitude (rms) of 9%, and a coherence, Q identical with nu(sub 0)/sigma approximately equal to 6. In a 33 ksec time interval when the 20 mHz QPO is strongest we also find evidence for a 2nd QPO peak at 15 mHz, the first indication for a close pair of QPOs in a ULX source. Interestingly, the frequency ratio of this QPO pair is inconsistent with 3:2 at the 3 sigma level, but is consistent with a 4:3 ratio. A powerlaw noise component with slope near 1.5 is also present below 0.1 Hz with evidence for a break to a flatter slope at about 3 mHz. The source shows substantial broadband variability, with a total amplitude (rms) of about 30% in the 0.1 - 100 mHz frequency band, and there is strong energy dependence to the variability. The power spectrum of hard X-ray photons (greater than 2 keV) shows a "classic" flat-topped continuum breaking to a power law with index 1.5 - 2. Both the break and 20 mHz QPO are detected in the hard band, and the 20 mHz QPO is essentially at the break. The QPO is both strong and narrow in this band, having an amplitude (rms) of 15%, and Q approx. equal to 25. The energy spectrum is well fit by three components, a "cool" disk with kT = 0.15 keV, a steep power law with index 2.56, and a thermal plasma at kT = 0.87 keV. The disk, power law, and thermal plasma components contribute 35, 60, and 5% of the 0.3 - 10 keV flux, respectively. Both the timing and spectral properties of NGC 5408 X-1 are strikingly reminiscent of Galactic black hole systems at high inferred accretion rates, but with its characteristic frequencies (QPO and break frequencies) scaled down by a factor of 10 - 100. We discuss the implications of these findings in the context of models for ULXs, and their implications for the object's mass.

Strohmayer, Tod E.↗

DOE OSTI2021

Test of model of equivalence of tree height growth and transpiration rates in percolation-based phenomenology for root-soil interaction

We recently introduced a phenomenological model to represent root-soil interactions that accounts for variability in tree growth on local scales and within species (Hunt et al., 2020). Here, using publicly available datasets, we examine the developed model to provide additional empirical evidence, which also gives novel insight into two particular tree species, Eucalyptus regnans and Eucalyptus sieberri. In particular, we discovered two novel aspects of the model: a power-law time dependence with a different power-law exponent than usually heretofore applied, and apparently a direct equivalence of results for transpiration and growth rates. Also, although our original expectation was that growth and transpiration were merely proportional, comparison of the entire BAAD data set (Falster et al., 2015) for tree height with our model (Hunt et al., 2020; Fig. 1) already suggested approximate equality of these disparate measurements (volume/area, height). In this present manuscript we find further evidence for equivalence of the two measurements and provide additional theoretical basis for a model assumption regarding constancy of sap flow rates.

54 ENVIRONMENTAL SCIENCES↗

NASA NTRS1980

The volume emissivity of type III radio bursts

The volume emissivity has been calculated for thirty-six type III solar radio bursts obtained from approximately 6.5 years of Imp 8 and ISEE 1 satellite data. Although the emissivities for these events vary over a large range, all the emissivities decrease rapidly with increasing heliocentric radial distance. The best fit power law for the emissivity, using the average power law index for all events analyzed, is J = J(0)R(-6.0), with J0 = 1.5 x 10 to the -24th W/cu m sr. This best fit emissivity is used to estimate the expected radial variation of the plasma oscillations responsible for the type III radio emissions.

Tokar, R. L.↗

NASA NTRS1990

Gamma radiation from the Crab and Vela pulsars

The young pulsars in Crab and Vela were observed as very efficient emitters of high energy gamma radiation. While their radiation in the radio, optical, and x ray range was always known to differ considerably, the gamma ray emission on a superficial level appears quite similar: lightcurves with two narrow peaks, separated by 141 deg (Crab) and 153 deg (Vela) and photon energies in excess of 1 GeV with spectra that can be described by a power-law for Crab and a broken power-law for Vela. The detailed observations of these sources with the COS-B instrument, extending over nearly seven years, have revealed significant differences in the characteristics of the pulsars in the gamma-ray domain. Secular changes in the temporal (Crab) and spectral (Vela) properties above 50 MeV were found. These tantalizing signatures of the pulsar emission processes must now be explored in more detail and over a larger spectral range with the GRO (Gamma Ray Observatory) instruments in order to gain a deeper understanding of the physics of young neutron stars.

Kanbach, Gottfried↗

NASA NTRS1999

Reprocessing in Luminous Disks

We develop and investigate a procedure that accounts for disk reprocessing of photons that originate in the disk itself. Surface temperatures and simple, black body spectral energy distributions (SEDs) of protostellar disks are calculated. In disks that flare with radius, reprocessing of stellar photons results in temperature profiles considerably shallower than r(sup -3/4). Including the disk as a radiation source (as in the case of actively secreting disks) along with the stellar source further flattens the temperature profile. Disks that flare strongly near the star and then smoothly curve over and become shadowed at some distance ("decreasing curvature" disks) exhibit nearly power-law temperature profiles which result in power-law infrared SEDs with slopes in agreement with typical observations of young stellar objects. Disk models in which the photospheric thickness is controlled by the local opacity and in which the temperature decreases with radius naturally show this shape. Uniformly flaring models do not match observations as well; progressively stronger reprocessing at larger radii leads to SEDs that flatten toward the infrared or even have a second peak at the wavelength corresponding (through the Wien law) to the temperature of the outer edge of the disk. In FU Orionis outbursting systems, the dominant source of energy is the disk itself. The details of the reprocessing depend sensitively on the assumed disk shape and emitted temperature profile. The thermal instability outburst models of Bell Lin reproduce trends in the observed SEDs of Fuors with T varies as r(sup -3/4) in the inner disk (r approx. less than 0.25au corresponding to lambda approx. less than 10 microns) and T varies as r(sup -1/2) in the outer disk. Surface irradiation during outburst and quiescence is compared in the region of planet formation (1 - 10 au). The contrast between the two phases is diminished by the importance of the reprocessing of photons from the relatively high mass flux outer disk (dot-M = 10(exp -5) solar mass/yr) which is present during both outburst and quiescence.

Bell, K. Robbins↗

NASA NTRS2004

Multi-Epoch XMM Observations of NGC4258

The goal of this project was detection of variability in the X-ray absorption column of the AGN in NGC4258 through monitoring with the XMM satellite. We have accomplished this goal and submitted the results to ApJ for publication in a paper entitled, "X-ray Luminosity and Absorption Column Fluctuations in the H2O Maser Galaxy NGC4258 from Weeks to Years," by Fruscione, A., Greenhill, L.J., Filippedco, A.V., Moran, J.M., Hermstein, J.R., and Galle, E. We have received a favorable referee report and expect the article will appear in 2005. To complete the project, we reduced our four epochs of XMM data for NGC4258, one archival XMM observation, and all existing Chandra datasets for NGC4258 (with the latest calibration tables and a grid of corrections for pileup). Self-consistent reduction of all these data permitted detailed comparison that could not have been accomplished simply by taking published model fits that appear in the literature. To accumulate a broader monitoring record, we combined the Chandra and XMM results with those published for SAX and ASCA. We modeled the Chandra and XMM data self-consistently with partially absorbed, hard power-law, soft thermal plasmas, and soft power-law components. Over nine years, the photo-electric absorbing column exhibited a 40% drop between two ASCA epochs separated by 3 years and a 60% rise between two XMM epochs separated by just 5 months. In contract, uncorrelated factor of of 2-3 changes were seen in absorbed flux on te timescale of years, which suggests intrinsic variability of the central engine. The warped disk that is a known source of H2O maser emission in 4258 is believed to cross the line of sight to the central engine. We have proposed that the variations in absorbing column arise from inhomogeneities in the rotating disk, as they sweep across the line of sight. We estimate from the XMM data that the inhomogeneities are about 1E+15 cm in size at radii greater than 0.27 pc. This is consistent with the estimated radius of the disk crossing estimated entirely independently, and it solidifies evidence that the warped accretion disk is the absorber in this (and possible other) AGN.

Greenhill, Lincoln J.↗