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NASA NTRS1982

Examination of time series through randomly broken windows

The study of many astronomical problems involves a consideration of time series. The present investigation was prompted by a problem related to the determination of normal modes of oscillation of the sun. It is desirable to have an approach for obtaining an estimate of the accuracy with which oscillation modes may be determined when it appears possible to observe the sun over a long interval of time losing some blocks of time because of cloud cover. The present investigation has the objective to develop a model for addressing problems of this type. After presentation of a few general formulae, the problem is considerably simplified by supposing that observations are made over a large number N of equal time intervals, each of length t, so that the total time interval T is given by T = Nt. With certain additional simplifying assumptions, attention is given to the statistical properties of two functions which are representative of 'randomly broken' window functions.

Sturrock, P. A.↗

NASA NTRS1994

ASCA measurements of the grain-scattered X-ray halos of eclipsing massive X-ray binaries: Vela X-1 and Centaurus X-3

We have measured the decaying dust-scattered X-ray halo of Cen X-3 during its binary eclipse with the ASCA solid-state imaging spectrometer (SIS). The surface brightness profile (SBP) of the image in the low-energy band (0.5-3 keV) lies substantially above the point-spread function (PSF) of the X-ray telescope, while the SBP in the high-energy band (5-10 keV) exhibits no significant deviation. By contrast, the SBPs of Vela X-1 during its eclipse are consistent with the PSF in both the low- and high-energy bands -- strong evidence that a dust halo is indeed present in Cen X-3. Accordingly, we modeled the SBP of Cen X-3 taken from six consecutive time segments under the principal assumptions that the dust is distributed uniformly along a segment of the line of sight, the grains have a power-law size distribution, and the low-energy source flux was the same function of orbital phase before as during our observation. The best-fit set of parameters included a grain density value of 1.3 g/cu cm, substanially less than the density of 'astronomical silicate.' This result supports the idea that interstellar grains are 'fluffy' aggregates of smaller solid particles. We attribute the failure to detect a halo of Vela X-1 during its eclipse phase to extended strong circumsource absorption that probably occurred before the eclipse and allowed the halo to decay away before the observation began.

Woo, Jonathan W.↗

NASA NTRS1997

The Ambient and Perturbed Solar Wind: From the Sun to 1 AU

The overall objective of the proposed research was to use numerical solutions of the magnetohydrodynamic (MHD) equations along with comparisons of the computed results with observations to study the following topics: (1) ambient solar wind solutions that extend from the solar surface to 1 astronomical unit (AU), contain closed magnetic structures near the Sun, and are consistent with observed values; (2) magnetic and plasma structures in coronal mass ejections (CMES) as they propagate to the interplanetary medium; (3) relation of MHD shocks to CMEs in the interplanetary medium; (4) interaction of MHD shocks with structures (such as other shocks, corotating interaction regions, current sheets) in the interplanetary plasma; and (5) simulations of observed interplanetary structures. A planned close collaboration with data analysts served to make the model more relevant to the data. The outcome of this research program is an improved understanding of the physical processes occurring in solar-generated disturbances in the interplanetary medium between the Sun and 1 AU.

Steinolfson, R. S.↗

NASA NTRS2005

A GCM Recent History of Northern Martian Polar Layered Deposits: Contribution from Past Equatorial Ice Reservoirs

Polar layered deposits are exposed in the walls of the troughs cutting the north polar cap of Mars. They consist of alternating ice and dust layers or layers of an ice-dust mixture with varying proportions and are found throughout the cap. Layers thickness ranges from meters to several tens of meters with an approximately 30 meter dominant wavelength. Although their formation processes is not known, they are presumed to reflect changes in ice and dust stability over orbital and axial variations. Intensive 3-D LMD GCM simulations of the martian water cycle have been thus performed to determine the annual rates of exchange of surface ice between the northern cap and tropical areas for a wide range of obliquity and orbital parameters values.These rates have been employed to reconstruct an history of the northern cap and test simple models of dust-ice layers formation over the last 10 Ma orbital variations. We use the 3-D water cycle model simulated by the 3-D LMD GCM with an intermediate grid resolution (7.5 longitude x 5.625 latitude) and 25 vertical levels. The dust opacity is constant and set to 0,15. No exchange of ice with regolith is allowed. The evolution of the northern cap over obliquity and orbital changes (eccentricity, Longitude of perihelion) has been recently described with this model. High summer insolation favors transfer of ice from the northern pole to the Tharsis and Olympus Montes, while at low obliquity, unstable equatorial ice is redeposited in high-latitude and polar areas of both hemisphere. The disappearance of the equatorial ice reservoir leads to a poleward recession of icy high latitude reservoirs, providing an additional source for the cap accumulation during each obliquity or orbital cycle. Furthering the efforts, a quantitative evolution of ice reservoirs is here investigated for various astronomical conditions.

Levrard, B.↗

DOE OSTI2025

Enhancing Interpretability in Generative Modeling: Statistically Disentangled Latent Spaces Guided by Generative Factors in Scientific Datasets

This study addresses the challenge of statistically extracting generative factors from complex, high-dimensional datasets in unsupervised or semi-supervised settings. We investigate encoder-decoder-based generative models for nonlinear dimensionality reduction, focusing on disentangling low-dimensional latent variables corresponding to independent physical factors. Introducing Aux-VAE, a novel architecture within the classical Variational Autoencoder framework, we achieve disentanglement with minimal modifications to the standard VAE loss function by leveraging prior statistical knowledge through auxiliary variables. These variables guide the shaping of the latent space by aligning latent factors with learned auxiliary variables. We validate the efficacy of Aux-VAE through comparative assessments on multiple datasets, including astronomical simulations.

97 MATHEMATICS AND COMPUTING↗

NASA NTRS1992

The Hopkins Ultraviolet Telescope - Performance and calibration during the Astro-1 mission

Results are reported of spectrophotometric observations, made with the Hopkins Ultraviolet Telescope (HUT), of 77 astronomical sources throughout the far-UV (912-1850 A) at a resolution of about 3 A, and, for a small number of sources, in the extreme UV (415-912 A) beyond the Lyman limit at a resolution of about 1.5 A. The HUT instrument and its performance in orbit are described. A HUT observation of the DA white dwarf G191-B2B is presented, and the photometric calibration curve for the instrument is derived from a comparison of the observation with a model stellar atmosphere. The sensitivity reaches a maximum at 1050 A, where 1 photon/sq cm s A yields 9.5 counts/s A, and remains within a factor of 2 of this value from 912 to 1600 A. The instrumental dark count measured on orbit was less than 0.001 counts/s A.

Davidsen, Arthur F.↗

NASA NTRSDate not supplied

Compositional Analysis of Titan’s Atmosphere Using Spitzer Infrared Spectrograph Data

We present, for the first time, infrared spectra from the Spitzer Space Telescope’s Infrared Spectrograph (IRS) (2004-2008) of Titan in both the short wavelength-low resolution(SL, R=60~127,5.13-14.29 μm) and short wavelength-high resolution channels (SH, R=600, 9.89-19.51 μm) showing the emissions of CH4, C2H4, C2H2, C2H6, HCN, CO2, HC3N, C3H4, C4H2, and C3H8. Spitzer IRS data has been used to measure atmospheric composition of various Solar System bodies, including Neptune [1] and Uranus [2],[3]. Although Spitzer took multiple dedicated observations of Titan, none of the results have been modeled before. We conduct our own investigation of these datasets and search for new results. We retrieve temperature and gas composition profiles and compare the results obtained for Titan to those of the Cassini Composite Infrared Spectrometer (CIRS) and the Infrared Space Observatory Short Wavelength Spectrometer [4], and comment on the effect of spectral resolution on retrieved information content. We conclude by recommending gaps in current spectroscopic knowledge of molecular bands that could be addressed by theoretical and laboratory study to aid future astronomical studies of Titan, for example the James Webb Space Telescope (JWST) and the Stratospheric Observatory for Infrared Astronomy (SOFIA) Acknowledgments: Data analyzed in this project is public available online on the Spitzer Heritage Archive (sha.ipac.caltech.edu). This research has made use of the NASA/IPAC Infrared Science Archive, which is funded by the National Aeronautics and Space Administration and operated by the California Institute of Technology.

Titan↗

DOE OSTI2023

Core collapse supernova gravitational wave emission for progenitors of 9.6, 15, and 25 M ⊙

Here, we present gravitational wave emission predictions based on three core collapse supernova simulations corresponding to three different progenitor masses. The masses span a large range, between 9.6 and 25M ⊙ , are all initially nonrotating, and are of two metallicities: zero and solar. We compute both the temporal evolution of the gravitational wave strains for both the plus and the cross polarizations, as well as their spectral decomposition and characteristic strains. The temporal evolution of our zero metallicity 9.6M ⊙ progenitor model is distinct from the temporal evolution of our solar metallicity 15M ⊙ progenitor model and our zero metallicity 25M ⊙ progenitor model. In the former case, the high-frequency gravitational wave emission is largely confined to a brief time period ~75 ms after bounce, whereas in the latter two cases high-frequency emission does not commence until ~125 ms after bounce or later. The excitation mechanisms of the high-frequency emission in all three cases correspond to proto-neutron star convection and accretion onto the proto-neutron star from the convective gain layer above it, with the former playing the dominant role for most of the evolution. The low-frequency emission in all three models exhibits very similar behavior. At frequencies below ~250 Hz, gravitational waves are emitted by neutrino-driven convection and the standing accretion shock instability (SASI). This emission extends throughout the simulations when a gain region is present. In all three models, explosion is observed at ~125, ~500, and ~250 ms after bounce in the 9.6, 15, and 25M ⊙ progenitor models, respectively. At these times, the low-frequency gravitational wave emission is joined by very low-frequency emission, below ~10 Hz. These very low-frequency episodes are the result of explosion and begin at the above designated explosion times in each of our models. Our characteristic strains tell us that, in principle, all three gravitational wave signals would be detectable by current-generation detectors for a supernova at a distance of 10 kpc. However, our 9.6M ⊙ progenitor model is a significantly weaker source of gravitational waves, with strain amplitudes approximately 5–10 times less than in our other two models. The characteristic strain for this model tells us that such a supernova would be detectable only within a much more narrow frequency range around the maximum sensitivity of today’s detectors. Finally, in our 9.6M ⊙ progenitor model, we see very high-frequency gravitational radiation, extending up to ~2000 Hz. This feature results from the interaction of shock- and deleptonization-induced convection with perturbations introduced in the progenitor by nuclear burning during core collapse. While unique to the 9.6M ⊙ progenitor model analyzed here, this very high-frequency emission may, in fact, be a generic feature of the predictions for the gravitational wave emission from all core collapse supernova models when simulations are performed with three-dimensional progenitors.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2022

Properties of High-redshift Gamma-Ray Bursts

Abstract The immense power of gamma-ray bursts (GRBs) makes them ideal probes of the early universe. By using absorption lines in the afterglows of high-redshift GRBs, astronomers can study the evolution of metals in the early universe. With an understanding of the nature of GRB progenitors, the rate and properties of GRBs observed at high redshift can probe the star formation history and the initial mass function of stars at high redshift. This paper presents a detailed study of the dependence on metallicity and mass of the properties of long-duration GRBs under the black hole accretion disk paradigm to predict the evolution of these properties with redshift. These models are calibrated on the current GRB observations and then used to make predictions for new observations and new missions (e.g., the proposed Gamow mission) studying high-redshift GRBs.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2023

Halted-pendulum Relaxation: Application to White Dwarf Binary Initial Data

Studying compact-star binaries and their mergers is integral to determining progenitors for observable transients. Today, compact-star mergers are typically studied via state-of-the-art computational fluid dynamics codes. One such numerical technique, smoothed particle hydrodynamics (SPH), is frequently chosen for its excellent mass, energy, and momentum conservation. The natural treatment of vacuum and the ability to represent highly irregular morphologies make SPH an excellent tool for the study of compact-star binaries and mergers. For many scenarios, including binary systems, the outcome of simulations is only as accurate as the initial conditions. For SPH, it is essential to ensure that the particles are distributed regularly, representing the initial density profile but without long-range correlations. Particle noise in the form of high-frequency local motion and low-frequency global dynamics must be damped out. Damping the latter can be as computationally intensive as the actual simulation. We discuss a new and straightforward relaxation method, halted-pendulum relaxation (HPR), to remove global oscillation modes of SPH particle configurations. In combination with effective external potentials representing gravitational and orbital forces, we show that HPR has an excellent performance in efficiently relaxing SPH particles to the desired density distribution and removing global oscillation modes. We compare the method to frequently used relaxation approaches and test it on a white dwarf binary model at its Roche-lobe overflow limit. We highlight the importance of our method in achieving accurate initial conditions and its effect on achieving circular orbits and realistic accretion rates when compared with other general relaxation methods.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2026

Opportunities in AI/ML for the Rubin LSST Dark Energy Science Collaboration

The Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) will produce unprecedented volumes of heterogeneous astronomical data (images, catalogs, and alerts) that challenge traditional analysis pipelines. The LSST Dark Energy Science Collaboration (DESC) aims to derive robust constraints on dark energy and dark matter from these data, requiring methods that are statistically powerful, scalable, and operationally reliable. Artificial intelligence and machine learning (AI/ML) are already embedded across DESC science workflows, from photometric redshifts and transient classification to weak lensing inference and cosmological simulations. Yet their utility for precision cosmology hinges on trustworthy uncertainty quantification, robustness to covariate shift and model misspecification, and reproducible integration within scientific pipelines. This white paper surveys the current landscape of AI/ML across DESC's primary cosmological probes and cross-cutting analyses, revealing that the same core methodologies and fundamental challenges recur across disparate science cases. Since progress on these cross-cutting challenges would benefit multiple probes simultaneously, we identify key methodological research priorities, including Bayesian inference at scale, physics-informed methods, validation frameworks, and active learning for discovery. With an eye on emerging techniques, we also explore the potential of the latest foundation model methodologies and LLM-driven agentic AI systems to reshape DESC workflows, provided their deployment is coupled with rigorous evaluation and governance. Finally, we discuss critical software, computing, data infrastructure, and human capital requirements for the successful deployment of these new methodologies, and consider associated risks and opportunities for broader coordination with external actors.

Aubourg, Eric [APC, Paris] (ORCID:000000025592023X↗

NASA NTRS1990

BATSE software for the analysis of the gamma ray burst spatial distribution

The Burst and Transient Source Experiment (BATSE) on the Gamma Ray Observatory (GRO) is designed to study astronomical gamma ray sources and to provide better positional, spectral, and time resolution about these objects than has previously been possible from one experiment. The procedure to be used in the analysis of the gamma ray burst spatial distribution is presented. Data is input from BATSE via the Gamma Ray Burst Catalog (listing individual burst positions, flux values, and associated errors) and the Sky Sensitivity Map (which summarizes observational selection effects in table format). A FORTRAN program generates Monte Carlo burst catalogs, which are models to be compared to the actual distribution. The Monte Carlo models are then filtered through the Sky Sensitivity Map so that they suffer from the same selection effects as the actual catalog data. Additionally, each burst position is converted into a probability distribution to mimic BATSE positional sensitivity. The Burst Catalog, Monte Carlo burst catalog, and Sky Sensitivity Map are then passed onto an IDL program that compares the catalogs for statistical significance. The Sky Sensitivity Map is used to estimate how often each sky area is observed above the minimum flux level in question. Each burst found in this sky area is then weighted according to the frequency with which this sky area is observed. The catalogs are then compared via tests of homogeneity (based on their radial distributions) and isotropy (based upon their angular distributions). The results of the statistical comparisons along with graphs and charts of the summaries, are output from the IDL program for study.

Hakkila, Jon↗

NASA NTRS1994

Topological analysis of chaotic orbits: Revisiting Hyperion

There is emerging interest in the possibility of chaotic evolution in astrophysical systems. To mention just one example, recent well-sampled ground-based observations of the Saturian satellite Hyperion strongly suggest that it is exhibiting chaotic behavior. We present a general technique, the method of close returns, for the analysis of data from astronomical objects believed to be exhibiting chaotic motion. The method is based on the extraction of pieces of the evolution that exhibit nearly periodic behavior-episodes during which the object stays near in phase space to some unstable periodic orbit. Such orbits generally act as skeletal features, tracing the topological organization of the manifold on which the chaotic dynamics takes place. This method does not require data sets as lengthy as other nonlinear analysis techniques do and is therefore well suited to many astronomical observing programs. Well sampled data covering between twenty and forty characteristic periods of the system have been found to be sufficient for the application of this technique. Additional strengths of this method are its robustness in the presence of noise and the ability for a user to clearly distinguish between periodic, random, and chaotic behavior by inspection of the resulting two-dimensional image. As an example of its power, we analyze close returns in a numerically generated data set, based on a model for Hyperion extensively studied in the literature, corresponding to nightly observations of the satellite. We show that with a small data set, embedded unstable periodic orbits can be extracted and that these orbits can be responsible for nearly periodic behavior lasting a substantial fraction of the observing run.

Boyd, Patricia T.↗

NASA NTRS2003

A Very Energetic Supernova Associated with the Gamma Ray Burst of 29 March 2003

Over the past five years evidence has mounted that long-duration (greater than 2s) gamma-ray bursts (GRBs), the most brilliant of all astronomical explosions, signal the collapse of massive stars in our Universe. This evidence, originally based on the probable association of one unusual GRB with a supernova, now includes the association of GRBs with regions of massive star-formation in distant galaxies, tantalizing evidence of supernova-like light-curve 'bumps' in the optical afterglows of several bursts, and lines of freshly synthesized elements in the spectra of a few X-ray afterglows. These observations support, but do not yet conclusively validate, models based upon the deaths of massive stars, presumably associated with core collapse. Here we report evidence for a very energetic supernova (a hypernova), temporally and spatially coincident with a GRB at redshift z=0.1685. The timing of the supernova indicates that it exploded within a few days of the GRB, strongly suggesting that core-collapse events can give rise to GRBs. Amongst the GRB central engine models proposed to-date, the properties of this supernova thus favour the collapsar model.

Hjorth, Jens↗

DOE OSTI2021

$\mathcal{g}$ modes of neutron stars with hadron-to-quark crossover transitions

Here we perform the first study of the principal core g-mode oscillation in hybrid stars containing quark matter, utilizing a crossover model for the hadron-to-quark transition inspired by lattice QCD. The ensuing results are compared with our recent findings of g-mode frequencies in hybrid stars with a first-order phase transition using Gibbs constructions. We find that models using Gibbs construction yield g-mode amplitudes and the associated gravitational energy radiated that dominate over those of the chosen crossover model owing to the distinct behaviors of the equilibrium and adiabatic sound speeds in the various models. Based on our results, we conclude that were g modes to be detected in upgraded LIGO and Virgo detectors it would indicate a first-order phase transition akin to a Gibbs construction.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2022

Composition design of high-entropy alloys with deep sets learning

High entropy alloys (HEAs) are an important material class in the development of next-generation structural materials, but the astronomically large composition space cannot be efficiently explored by experiments or first-principles calculations. Machine learning (ML) methods might address this challenge, but ML of HEAs has been hindered by the scarcity of HEA property data. In this work, the EMTO-CPA method was used to generate a large HEA dataset (spanning a composition space of 14 elements) containing 7086 cubic HEA structures with structural properties, 1911 of which have the complete elastic tensor calculated. The elastic property dataset was used to train a ML model with the Deep Sets architecture. The Deep Sets model has better predictive performance and generalizability compared to other ML models. Association rule mining was applied to the model predictions to describe the compositional dependence of HEA elastic properties and to demonstrate the potential for data-driven alloy design.

36 MATERIALS SCIENCE↗

NASA NTRS1978

Gravitational experiments on solar probe

A covariance analysis was performed for a solar probe trajectory which encounters the sun at four solar radii. The unknown parameters in the analysis are the six initial cartesian coordinates for the probe, six initial cartesian coordinates for the earth, the astronomical unit, the solar gravitational quadrupole coefficient and two post Newtonian meters (beta, gamma). Errors in the unknown parameters were computed as a function of standard errors on the radio tracking data and on the nongravitational forces which act on the probe. Results were obtained for several tracking geometries and for several orbital inclinations to the ecliptic. The analysis shows that the principal scientific result from the radio tracking of a solar probe would be the determination of the quadrupole moment, which would place a constraint on models of the solar interior.

Anderson, J. D.↗

NASA NTRS1987

The multichannel astrometric photometer and atmospheric limitations in the measurement of relative positions

The operational Multichannel Astrometric Photometer (MAP) now in use in the Allegheny Observatory astrometric program is the culmination of a decade of design and development effort. A detailed description of the system and its related software is followed by analysis of data acquired in four stellar regions. The study indicates an accuracy (in the sense of conformity to the best model), per night, for stars of the eighth magnitude or brighter, of 0.003 arcsec or better. These data points each have approximately twice the precision of the annual normal points obtained in our photographic program. Accuracy is shown to depend on: (1) the photon-count rate of the target star (it follows that the number of photons from the reference frame is also in important factor), (2) the duration of the observation, (3) the angular size of the reference frame, and (4) the quality of the astronomical seeing. Since (4) and, to a lesser extent, (1) involve the atmospheric characteristics at the time of observation, the probable performance at more favorable sites is discussed briefly.

Gatewood, George D.↗