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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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
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

Quasinormal g-modes of neutron stars with quarks

Quasinormal oscillation modes of neutron stars provide a means to probe their interior composition using gravitational wave astronomy. We compute the frequencies and damping times of composition-dependent core 𝑔-modes of neutron stars containing quark matter employing linearized perturbative equations of general relativity. We find that ignoring background metric perturbations due to the oscillating fluid, as in the Cowling approximation, underestimates the 𝑔-mode frequency by up to 10% for higher mass stars, depending on the parameters of the nuclear equation of state and how the mixed phase is constructed. The 𝑔-mode frequencies are well described by a linear scaling with the central lepton (or combined lepton and quark) fraction for nucleonic (hybrid) stars. Furthermore, our findings suggest that neutron stars with and without quarks are manifestly different with regards to their quasinormal 𝑔-mode spectrum, and may thus be distinguished from one another in future observations of gravitational waves from merging neutron stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Gravitational Wave Signal for Quark Matter with Realistic Phase Transition

The cores of neutron stars (NSs) near the maximum mass can realize a transitional change to quark matter (QM). Gravitational waves from binary NS mergers are expected to convey information about the equation of state (EOS) sensitive to the QM transition. Here, in this work, we present the first results of gravitational wave simulation with the realistic EOS that is consistent with ab initio approaches of χ⁢EFT and pQCD and is assumed to go through smooth crossover. We compare them to results obtained with another EOS with a first-order hadron-quark phase transition. Our results suggest that early collapse to a black hole in the post-merger stage after NS merger robustly signifies softening of the EOS associated with the QM onset in the crossover scenario. The nature of the hadron-quark phase transition can be further constrained by the condition that electromagnetic counterparts should be energized by the material left outside the remnant black hole.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2010

Highlights of GeV Gamma-Ray Astronomy

Because high-energy gamma rays are primarily produced by high-energy particle interactions, the gamma-ray survey of the sky by the Fermi Gamma-ray Space Telescope offers a view of sites of cosmic ray production and interactions. Gamma-ray bursts, pulsars, pulsar wind nebulae, binary sources, and Active Galactic Nuclei are all phenomena that reveal particle acceleration through their gamma-ray emission. Diffuse Galactic gamma radiation, Solar System gamma-ray sources, and energetic radiation from supernova remnants are likely tracers of high-energy particle interactions with matter and photon fields. This paper will present a broad overview of the constantly changing sky seen with the Large Area Telescope (LAT) on the Fermi spacecraft.

Thompson, David J.↗
DOE OSTI2024

A Theory for Neutron Star and Black Hole Kicks and Induced Spins

Abstract Using 20 long-term 3D core-collapse supernova simulations, we find that lower compactness progenitors that explode quasi-spherically due to the short delay to explosion experience smaller neutron star recoil kicks in the ∼100−200 km s −1 range, while higher compactness progenitors that explode later and more aspherically leave neutron stars with kicks in the ∼300−1000 km s −1 range. In addition, we find that these two classes are correlated with the gravitational mass of the neutron star. This correlation suggests that the survival of binary neutron star systems may in part be due to their lower kick speeds. We also find a correlation between the kick and both the mass dipole of the ejecta and the explosion energy. Furthermore, one channel of black hole birth leaves masses of ∼10 M ⊙ , is not accompanied by a neutrino-driven explosion, and experiences small kicks. A second channel is through a vigorous explosion that leaves behind a black hole with a mass of ∼3.0 M ⊙ kicked to high speeds. We find that the induced spins of nascent neutron stars range from seconds to ∼10 ms, but do not yet see a significant spin/kick correlation for pulsars. We suggest that if an initial spin biases the explosion direction, a spin/kick correlation would be a common byproduct of the neutrino mechanism of core-collapse supernovae. Finally, the induced spin in explosive black hole formation is likely large and in the collapsar range. This new 3D model suite provides a greatly expanded perspective and appears to explain some observed pulsar properties by default.

Astronomy & Astrophysics↗
NASA NTRS1971

X-ray astronomy from sounding rockets at MIT

The research in the program for designing and fabricating payloads for sounding rockets for use in X-ray astronomy is reported. Published papers concerning six Aerobee flights are presented. The results include: (1) the establishment of the radio galaxy M87 as an X-ray source; (2) the studies of the pulsar NPO532, including its X-ray pulse shape, the determination of the synchronism of the optical and X-ray pulses and the codiscovery of its existence as an X-ray pulsar; (3) the determination of many X-ray source positions, and (4) the precise position of Cyg X-1 which led directly to the identification of a ninth magnitude BO supergiant.

Source record↗
NASA NTRS1976

The structure and content of the galaxy and galactic gamma rays

Papers are presented dealing with galactic structure drawing on all branches of galactic astronomy with emphasis on the implications of the new gamma ray observations. Topics discussed include: (1) results from the COS-B gamma ray satellite; (2) results from SAS-2 on gamma ray pulsar, Cygnus X-3, and maps of the galactic diffuse flux; (3) recent data from CO surveys of the galaxy; (4) high resolution radio surveys of external galaxies; (5) results on the galactic distribution of pulsars; and (6) theoretical work on galactic gamma ray emission.

Fichtel, C. E.↗
DOE OSTI2023

Addition of tabulated equation of state and neutrino leakage support to illinoisgrmhd

Here we have added support for realistic, microphysical, finite-temperature equations of state (EOS) and neutrino physics via a leakage scheme to illinoisgrmhd, an open-source GRMHD code for dynamical spacetimes in the einstein toolkit. These new features are provided by two new, nrpy+-based codes: nrpyeos, which performs highly efficient EOS table lookups and interpolations, and nrpyleakage, which implements a new, adaptive mesh refinement (AMR)-capable neutrino leakage scheme in the einstein toolkit. We have performed a series of strenuous validation tests that demonstrate the robustness of these new codes, particularly on the Cartesian AMR grids provided by carpet. Furthermore, we show results from fully dynamical GRMHD simulations of single unmagnetized neutron stars, and magnetized binary neutron star mergers. This new version of illinoisgrmhd, as well as nrpyeos and nrpyleakage, is pedagogically documented in jupyter notebooks and fully open source. The codes will be proposed for inclusion in an upcoming version of the einstein toolkit.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Framework for phase transitions between the Maxwell and Gibbs constructions

By taking the nucleon-to-quark phase transition within a neutron star as an example, we present a thermodynamically consistent method to calculate the equation of state of ambient matter so that transitions that are intermediate to those of the familiar Maxwell and Gibbs constructions can be described. This method does not address the poorly known surface tension between the two phases microscopically (as, for example, in the calculation of the core pasta phases via the Wigner-Seitz approximation) but instead combines the local and global charge neutrality conditions characteristic of the Maxwell and Gibbs constructions, respectively. Overall charge neutrality is achieved by dividing the leptons to those that obey local charge neutrality (Maxwell) and those that maintain global charge neutrality (Gibbs). The equation of state is obtained by using equilibrium constraints derived from minimizing the total energy density. The results of this minimization are then used to calculate neutron star mass-radius curves, tidal deformabilities, equilibrium and adiabatic sound speeds, and nonradial g-mode oscillation frequencies for several intermediate constructions. Various quantities of interest transform smoothly from their Gibbs structures to those of Maxwell as the local-to-total electron ratio η, introduced to mimic the hadron-to-quark interface tension from 0 (Gibbs) to ∞ (Maxwell), is raised from 0 to 1. As a result, a notable exception is the g-mode frequency for the specific case of η = 1 for which a gap appears between the quark and hadronic branches.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2025

Cocoon shock breakout emission from binary neutron star mergers

Shock breakout emission is among the first observable signals in a wide variety of astrophysical phenomena, including neutron star (NS) mergers, and it can be the dominant component in low-luminosity short γ-ray bursts, as exemplified by GRB 170817A. In this work, we investigate the cocoon shock breakout emission in NS mergers and how its signal depends on the outermost layers of the ejecta profile, which we derive from general relativistic radiation hydrodynamic simulations. We study the formation of the cocoon as a consequence of a relativistic jet propagating through the ejecta. To explore the influence of the outermost layers of the ejecta on the breakout emission, we explore cases where the ejecta has a sharp cutoff or an extended smooth tail. We find that the shock breakout emission is strongly influenced by the shape of the ejecta outer layers, with extended tails yielding results consistent with the observed properties of GRB 170817A, whereas sharp cutoffs overestimate the radiated energy. Using a Bayesian analysis, we estimate the best fit parameters of the central engine, considering both accreting black hole and magnetized neutron star scenarios. Our findings indicate a slight preference for the scenarios where the engine is a black hole. Furthermore, our work probes the nature of neutron star mergers and highlights the importance of the shape of the ejecta profile in modeling early electromagnetic counterparts to these mergers.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

Neutrino Trapping and Out-of-Equilibrium Effects in Binary Neutron-Star Merger Remnants

We study out-of-thermodynamic-equilibrium effects in neutron-star mergers with 3D general-relativistic neutrino-radiation large-eddy simulations. During mergers, the cores of the neutron stars remain cold (T ~ a few MeV) and out of thermodynamic equilibrium with trapped neutrinos originating from the hot collisional interface between the stars. However, within ~2 to 3 ms matter and neutrinos reach equilibrium everywhere in the remnant massive neutron star. Furthermore, our results show that dissipative effects, such as bulk viscosity, if present, are only active for a short window of time after the merger.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1984

Variation of the pulse profile of Hercules X-1

The X-ray pulsar Her X-1 was observed in an on-state during its 35th cycle of activity in May, 1983 using the gas scintillation proportional counter (GSPC) array of the Tenma X-ray astronomy satellite. The outstanding features observed during the declining phase of the on-state included: a sharp decrease in the main X-ray pulse amplitude; and a steady increase in the column density of cool matter. On the basis of the spectral shape of the pulses, it is suggested that the main phase was attenuated due to electron scattering of the X-ray beam in a highly ionized medium located 3 x 10 to the 8th cm from the neutron star. Near the end of the on-state, the main pulse totally disappeared and a plain sinusoidal profile was observed. The observed pulse profiles are reproduced in graphic form.

Ohashi, T.↗
NASA NTRS1995

Gamma Ray Astronomy; E2 Symposium of COSPAR Scientific Commission E, COSPAR Scientific Assembly, 30th, Hamburg, Germany, July 11-21, 1994

This issue on gamma ray astronomy presents many results from observations made with the Compton Gamma Ray Observatory (CGRO) and other spaceborne experiments and includes studies of Seyfert galaxies, X-ray binaries, neutron stars, diffuse cosmic background radiation, pulsars, blazars, gamma ray bursts, the galactic center, and the distribution of Aluminum-26 in the galaxy. The issue begins with a survey of Compton Telescope (COMPTEL) results and concludes with future prospects for the field.

Gehrels, N.↗
NASA NTRS1979

GSFC Contributions to the NATO X-ray Astronomy Institute, Erice, July 1979

An overview of X-ray astronomical spectroscopy in general is presented and results obtained by HEAO 1 and 2 as well as earlier spacecraft are examined. Particular emphasis is given to the spectra of supernova remnants; galactic binary X-ray sources, cataclysmic variables, bulges, pulsars, and stars; the active nuclei of Seyfert 1 galaxy, BL Lac, and quasars; the diffuse X-ray background; and galactic clusters.

Holt, S. S.↗
DOE OSTI2022

Constraining the local pulsar population with the cosmic-ray positron fraction

Observations of the TeV halos associated with nearby pulsars indicate that these objects inject significant fluxes of very high-energy electron-positrons pairs into the interstellar medium (ISM), thereby likely providing the dominant contribution to the cosmic-ray positron flux. In this paper, we use the cosmic-ray positron fraction as measured by the AMS-02 Collaboration to constrain the characteristics of the local pulsar population. For reasonable model parameters, we find that we can obtain good agreement with the measured positron fraction up to energies of E e ~ 300 GeV. At higher energies, the positron fraction is dominated by a small number of pulsars, making it difficult to reliably predict the shape of the expected positron fraction. The low-energy positron spectrum supports the conclusion that pulsars typically transfer approximately η ~ 5–20% of their total spindown power into the production of very high-energy electron-positron pairs, producing a spectrum of such particles with a hard spectral index, α ~ 1.5–1.7. Such pulsars typically spindown on a timescale on the order of τ ~ 10 4 years. Our best fits were obtained for models in which the radio and gamma-ray beams from pulsars are detectable to 28% and 62% of surrounding observers, respectively.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2023

Nuclear and hybrid equations of state in light of the low-mass compact star in HESS J1731-347

Here, we sample over 500 relativistic mean-field theories constrained by chiral effective-field theory and properties of isospin-symmetric nuclear matter and test them against known stellar structure constraints. This includes a recent mass and radius measurement of a compact object in supernova remnant HESS J1731-347, with an unusually low mass of M = 0.77$^{+0.20}_{–0.17}$M ⊙ and a compact radius of R = 10.4$^{+0.86}_{–0.78}$ km. We show that none of the sampled nuclear models meet all constraints at the 68% credibility level, but that hybrid equations of state with a quark-matter inner core and nuclear outer core easily can. This indicates a tension between astrophysical constraints and low-energy nuclear theory.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗