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At least 1,567 records · Page 87
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Cosmological gamma-ray bursts

The distribution in angle and flux of gamma-ray bursts indicates that the majority of gamma-ray bursters are at cosmological distances, i.e., at z of about 1. The rate is then about 10 exp -8/yr in a galaxy like the Milky Way, i.e., orders of magnitude lower than the estimated rate for collisions between neutron stars in close binary systems. The energy per burst is about 10 exp 51 ergs, assuming isotropic emission. The events appear to be less energetic and more frequent if their emission is strongly beamed. Some tests for the distance scale are discussed: a correlation between the burst's strength and its spectrum; the absorption by the Galactic gas below about 2 keV; the X-ray tails caused by forward scattering by the Galactic dust; about 1 month recurrence of some bursts caused by gravitational lensing by foreground galaxies; and a search for gamma-ray bursts in M31. The bursts appear to be a manifestation of something exotic, but conventional compact objects can provide an explanation. The best possibility is offered by a decay of a bindary composed of a spinning-stellar-mass black-hole primary and a neutron or a strange-quark star secondary. In the final phase the secondary is tidally disrupted, forms an accretion disk, and up to 10 exp 54 ergs are released. A very small fraction of this energy powers the gamma-ray burst.

Paczynski, Bohdan↗
DOE OSTI2025

Ab initio computations of the fourth-order charge density moments of 48 Ca and 208 Pb

Neutron skins of neutron-rich nuclei connect nuclei with the matter in neutron stars. High-precision measurements of nuclear charge densities to extract higher-order moments are proposed to be sensitive to neutron radii and skin thicknesses. We investigate the charge density of 48 Ca and 208 Pb, leading candidates for such studies, with ab initio nuclear structure calculations. We find strong correlations between the fourth-order charge density moment R$^{4}_{ch}$ and the charge and neutron radii, allowing us to predict R$^{4}_{ch}$ for 48 Ca and 208 Pb. We find a substantially weaker correlation between the fourth-order charge density moment and the neutron skin, limiting the ability of high-precision electron scattering to determine the neutron skin in a model-independent manner.

Ab initio nuclear theory↗
NASA NTRS1976

Effect of polarization on superfluidity in low density neutron matter

The singlet-state quasi-particle interaction in neutron matter is examined on the basis of results of a detailed evaluation of the Landau Fermi-liquid parameters for pure neutron effects, including polarization effects. This means that the interaction induced by exchange of density and spin-density excitations is taken into account. It is shown that polarization actually works to suppress the pairing matrix elements, owing to the spin dependence of the quasi-particle interaction and, ultimately, the balance of attraction, repulsion, and spin dependence in the fundamental two-neutron interaction. Since the isotropic energy gap and the condensation energy in low-density neutron-star matter are extremely sensitive functions of the pairing matrix elements, they will also be suppressed by the polarizability of the neutron medium.

Clark, J. W.↗
NASA NTRS1982

Discovery of 69 ms periodic X-ray pulsations in A0538-66

The detection of X ray pulsations with a period of 69.2126 ms from A0538-66 was done from the Einstein Observatory. A point source was located by the High Resolution Imager and a proportional counter registered a flux of 40 counts/sec over background. A column spectral density of 2 x 10 to the 23/sq cm identified the object as A0538-66. A persistent periodicity of 0.0692126 sec was observed throughout the event. The outbursts were not detected on three other observations of A0538-66, and a best fit to a change of rotation velocity indicated that the period observed was from a binary star with a Doppler shifted signal. The two stars are separated by less than 39 solar radii, with the larger star having a 40 solar radius. Consideration is given to the possibility that the X rays are due to accretion on a neutron star and that the two stars at times share a common envelope.

Skinner, G. K.↗
DOE OSTI2022

First Application of Mass Measurements with the Rare-RI Ring Reveals the Solar r -Process Abundance Trend at A = 122 and A = 123

The Rare-RI Ring (R3) is a recently commissioned cyclotronlike storage ring mass spectrometer dedicated to mass measurements of exotic nuclei far from stability at Radioactive Isotope Beam Factory (RIBF) in RIKEN. The first application of mass measurement using the R3 mass spectrometer at RIBF is reported. Rare isotopes produced at RIBF— 127 Sn, 126 In, 125 Cd, 124 Ag, 123 Pd—were injected in R3. Masses of 126 In, 125 Cd, and 123 Pd were measured whereby the mass uncertainty of 123 Pd was improved. This is the first reported measurement with a new storage ring mass spectrometry technique realized at a heavy-ion cyclotron and employing individual injection of the preidentified rare nuclei. The latter is essential for the future mass measurements of the rarest isotopes produced at RIBF. The impact of the new 123 Pd result on the solar r-process abundances in a neutron star merger event is investigated by performing reaction network calculations of 20 trajectories with varying electron fraction Y e . It is found that the neutron capture cross section on 123 Pd increases by a factor of 2.2 and β-delayed neutron emission probability, P 1n , of 123 Rh increases by 14%. The neutron capture cross section on 122 Pd decreases by a factor of 2.6 leading to pileup of material at A = 122, thus reproducing the trend of the solar r-process abundances. The trend of the two-neutron separation energies (S 2n ) was investigated for the Pd isotopic chain. The new mass measurement with improved uncertainty excludes large changes of the S 2n value at N = 77. Finally, such large increase of the S 2n values before N = 82 was proposed as an alternative to the quenching of the N = 82 shell gap to reproduce r-process abundances in the mass region of A = 112 – 124.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

Including Neutrino-driven Convection in the Force Explosion Condition to Predict Explodability of Multidimensional Core-collapse Supernovae (FEC+)

Abstract Most massive stars end their lives with core collapse. However, it is not clear which explode as a core-collapse supernova (CCSN), leaving behind a neutron star, and which collapse to a black hole, aborting the explosion. One path to predict explodability without expensive multidimensional simulations is to develop analytic explosion conditions. These analytic conditions also provide a deeper understanding of the explosion mechanism and they provide some insight into why some simulations explode and some do not. The analytic force explosion condition (FEC) reproduces the explosion conditions of spherically symmetric CCSN simulations. In this follow-up manuscript, we include the dominant multidimensional effect that aids explosion—neutrino-driven convection—in the FEC. This generalized critical condition (FEC+) is suitable for multidimensional simulations and has potential to accurately predict explosion conditions of two- and three-dimensional CCSN simulations. We show that adding neutrino-driven convection reduces the critical condition by ∼30%, which is consistent with previous multidimensional simulations.

Gogilashvili, Mariam (ORCID:0000000269448052)↗
NASA NTRS1983

The nautre of dwarf novae and related systems

The high energy emissions from cataclysmic variables (interacting binary systems containing an accreting white dwarf) and the relationship of the high energy emission of these stars to their optical properties were investigated. The relationship of the cataclysmic binaries to the strong X-ray emitting binaries that contain a neutron star as the accreting star was examined. A soft X-ray survey of cataclysmic variable stars was carried out. Approximately 75% of all the sources observed were detected in short (2000 sec) observations. Relationships between the X-ray and optical luminosity and the dependence of this quantity on the subclass of the variable were investigated and constraints on the nature of these systems were provided.

Lampton, M.↗
NASA NTRS1993

The progenitor of supernova 1993J - A stripped supergiant in a binary system?

Supernova 1993J in the spiral galaxy M81 is the brightest supernova since SN1987A and, like the latter, appears to be another 'peculiar' type II supernova. The available photometry of the supernova region before the explosion requires the presence of at least two supergiants (one of early spectral type and the other of late type), but the actual progenitor has yet to be identified. We show that the explosion of a late-type supergiant can explain the initial sharp peak in the supernova light curve, provided that the star had lost almost all of its hydrogen-rich envelope before the explosion. In our model, the secondary brightening of the supernova, about 10 days later, is then a consequence of the radioactive decay of Ni-56 (and subsequently Co-56) produced in the explosion. The progenitor could have lost its hydrogen-rich envelope either in a strong stellar wind or, as seems more likely, through mass transfer to a companion star. In the latter case, the companion should reappear after the supernova photosphere has receded, the system having become a binary composed of a neutron star with a massive stellar companion.

Podsiaklowski, PH.↗
DOE OSTI2023

Constraining Nucleosythesis in Neutrino-Driven Winds Using the Impact of ({alpha}, xn) Reaction Rates

The lighter heavy elements of the first r-process peak, between strontium and silver, can be synthesized in the moderately neutron-rich neutrino-driven ejecta of either core-collapse supernovae or neutron star mergers via the weak r-process. This nucleosynthesis scenario exhibits uncertainties from the absence of experimental data from (alpha, xn) reactions on neutron-rich nuclei, which are currently based on statistical model estimates. We have performed a new impact study to identify the most important (alpha, xn) reactions that can affect the production of the lighter heavy elements under different astrophysical conditions using new, constrained (alpha, xn) reaction rates based on the Atomki-V2 alpha OMP. Our results show how when reducing the nuclear physics uncertainties, we can use abundance ratios to constrain the astrophysical conditions/environment. This can be achieved in the near future, when the key (alpha, xn) reaction rates will be measured experimentally in radioactive beam facilities.

Psaltis, Athanasios↗
NASA NTRS2008

An Eccentric Binary Millisecond Pulsar in the Galactic Plane

Binary pulsar systems are superb probes of stellar and binary evolution and the physics of extreme environments. In a survey with the Arecibo telescope, we have found PSR J1903+0327, a radio pulsar with a rotational period of 2.15 milliseconds in a highly eccentric (e = 0.44) 95-day orbit around a solar mass (M.) companion. Infrared observations identify a possible main-sequence companion star. Conventional binary stellar evolution models predict neither large orbital eccentricities nor main-sequence companions around millisecond pulsars. Alternative formation scenarios involve recycling a neutron star in a globular cluster, then ejecting it into the Galactic disk, or membership in a hierarchical triple system. A relativistic analysis of timing observations of the pulsar finds its mass to be 1.74 +/- 0.04 Solar Mass, an unusually high value.

Champion, David J.↗
NASA NTRS1972

On the physical parameters for Centaurus X-3 and Hercules X-1.

It is shown how upper and lower limits on the physical parameters of X-ray sources in Centaurus X-3 and Hercules X-1 may be determined from a reasonably simple and straightforward consideration. The basic assumption is that component A (the non-X-ray emitting component) is not a star collapsing toward its Schwartzschild radius (i.e., a black hole). This assumption appears reasonable since component A (the radius of the central occulting star) appears to physically occult component X. If component A is a 'normal' star, both observation and theory indicate that its mass is not greater than about 60 solar masses. The possibility in which component X is either a neutron star or a white dwarf is considered.

Mccluskey, G. E., Jr.↗
NASA NTRS1976

Evidence for the binary nature of A0535+26

The transient X-ray source A0535+26 was observed extensively with the SAS-3 satellite on two occasions. Sufficient timing data on the 104-s periodicity were obtained to indicate that the pulse period was changing during both of the observations. The possibility that these period changes are intrinsic to the compact star (e.g., due to accretion torques) cannot be completely excluded. However, it is demonstrated that all of the SAS-3 timing data can be explained by orbital motion of the X-ray star about a companion. Constraints are then placed on the orbital elements of the system. The results indicate a model for this source that consists of a neutron star in a long-period orbit (period of at least 17 days) about an OB star with a variable stellar wind.

Rappaport, S.↗
DOE OSTI2023

Metal Mixing in the r -process Enhanced Ultrafaint Dwarf Galaxy Reticulum II

The ultrafaint dwarf galaxy Reticulum II was enriched by a single rare and prolific r-process event. The r-process content of Reticulum II thus provides a unique opportunity to study metal mixing in a relic first galaxy. Using multi-object high-resolution spectroscopy with VLT/GIRAFFE and Magellan/M2FS, we identify 32 clear spectroscopic member stars and measure abundances of Mg, Ca, Fe, and Ba where possible. We find $72^{+10}_{-12}$ of the stars are r-process-enhanced, with a mean $\langle$[Ba / H]$\rangle$ = -1.68 ± 0.07 and unresolved intrinsic dispersion σ [Ba/H] <0.20. The homogeneous r-process abundances imply that Ret II’s metals are well mixed by the time the r enhanced stars form, which simulations have shown requires at least 100 Myr of metal mixing in between bursts of star formation to homogenize. This is the first direct evidence of bursty star formation in an ultrafaint dwarf galaxy. The homogeneous dilution prefers a prompt and high-yield r-process site, such as collapsar disk winds or prompt neutron star mergers. We also find evidence from [Ba/H] and [Mg/Ca] that the r-enhanced stars in Ret II formed in the absence of substantial pristine gas accretion, perhaps indicating that ≈70% of Ret II stars formed after reionization.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1984

Magnetorotational iron core collapse

During its final evolutionary stages, a massive star, as considered in current astrophysical theory, undergoes rapid collapse, thereby triggering a sequence of a catastrophic event which results in a Type II supernova explosion. A remnant neutron star or a black hole is left after the explosion. Stellar collapse occurs, when thermonuclear fusion has consumed the lighter elements present. At this stage, the core consists of iron. Difficulties arise regarding an appropriate model with respect to the core collapse. The present investigation is concerned with the evolution of a Type II supernova core including the effects of rotation and magnetic fields. A simple neutrino model is developed which reproduced the spherically symmetric results of Bowers and Wilson (1982). Several two-dimensional computational models of stellar collapse are studied, taking into account a case in which a 15 solar masses iron core was artificially given rotational and magnetic energy.

Symbalisty, E. M. D.↗
NASA NTRS2024

TIC 290061484: A Triply Eclipsing Triple System With the Shortest Known Outer Period of 24.5 Days

We have discovered a triply eclipsing triple star system, TIC 290061484, with the shortest known outer period, $P_{\rm out}$, of only 24.5 days. This `eclipses' the previous record set by $\lambda$ Tauri at 33.02 days which held for 68 years \citep{1956ApJ...124..507E}. The inner binary, with an orbital period of $P_{\rm in}=1.8$ days, produces primary and secondary eclipses, and exhibits prominent eclipse timing variations with the same periodicity as the outer orbit. The tertiary star eclipses, and is eclipsed by, the inner binary with pronounced asymmetric profiles. The inclinations of both orbits evolve on observable timescales such that the third-body eclipses exhibit dramatic depth variations in TESS data. A photodynamical model provides a complete solution for all orbital and physical parameters of the triple system, showing that the three stars have masses of \MAa M$_\odot$, \MAb M$_\odot$, and \MB M$_\odot$, radii near those corresponding to the main sequence, and $T_{\rm eff}$ in the range of 21,000 K - 23,700 K. Remarkably, the model shows that the triple is in fact a subsystem of a hierarchical 2+1+1 quadruple with a distant fourth star. The outermost star has a period of $\sim3,200$ days and a mass comparable to the stars in the inner triple. In $\sim20$ Myr, all three components of the triple subsystem will merge, undergo a type II supernova explosion, and leave a single remnant neutron star. At the time of writing, TIC 290061484 is the most compact triple system, and one of the tighter known compact triples (i.e., $P_{\rm out}/P_{\rm in} = 13.7$).

V B Kostov↗
NASA NTRS1993

On the mass of the compact object in the black hole binary A0620-00

Multicolor orbital light curves of the black hole candidate binary A0620-00 are presented. The light curves exhibit ellipsoidal variations and a grazing eclipse of the mass donor companion star by the accretion disk. Synthetic light curves were generated using realistic mass donor star fluxes and an isothermal blackbody disk. For mass ratios of q = M sub 1/M sub 2 = 5.0, 10.6, and 15.0 systematic searches were executed in parameter space for synthetic light curves that fit the observations. For each mass ratio, acceptable fits were found only for a small range of orbital inclinations. It is argued that the mass ratio is unlikely to exceed q = 10.6, and an upper limit of 0.8 solar masses is placed on the mass of the companion star. These constraints imply 4.16 +/- 0.1 to 5.55 +/- 0.15 solar masses. The lower limit on M sub 1 is more than 4-sigma above the mass of a maximally rotating neutron star, and constitutes further strong evidence in favor of a black hole primary in this system.

Haswell, Carole A.↗
NASA NTRS1994

Discovery of Be/x-ray stars in two supernova remnants in the Small Magellanic Cloud

We present ROSAT high resolution x-ray images of two previously cataloged supernova remnants in the Small Magellanic Cloud (SMC): 0101-72.4 and 0104-72.3. These remnants were known to show optical, x-ray, and radio emission based on earlier observations: the present data show the first evidence for arcsecond-scale x-ray structure. There is no diffuse x-ray emission associated with the optically emitting shell in 0101-72.4; we set a 3 sigma upper limit of 7 x 10(exp 34) erg s(exp -1) on the 0.02-2 keV luminosity from the region. The x-ray emission comes instead from a weak pointlike object near the limb of the remnant. Optical observations of this source reveal a m(sub v) = 14.8 blue star with H alpha and H beta in emission; we identify this as a Be star in the SMC. No evidence for variability down to time scales of about 1 s was found in the ROSAT data; however, a comparison of the ROSAT and Einstein fluxes indicates possible long term variability by a factor of approximately 2 over several years. The other SNR, 0104-72.3, also contains a pointlike x-ray source with a blue optical counterpart (m(sub v) = 16.7) and H alpha emission. We tentatively identify this as a Be star as well. In addition to the point source there is weak diffuse x-ray emission from 0104-72.3 (L(sub x) approximately 1.4 x 10(exp 35) erg s(exp -1)), but the remnant's appearance in the x-ray band is considerably different from that in either the radio or optical band. We argue for a physical association between the supernova remnants (SNRs) and Be/x-ray stars. A large space velocity (greater than or approximately 100 km s(exp -1)) for the Be/x-ray binaries is required if the explosions that produced the remnants also formed the neutron stars in the binaries. Alternatively, the associations could be the result of common membership in OB associations in the SMC.

Hughes, John P.↗
NASA NTRS2002

Nonthermal X-Ray Emission from the Shell-Type Supernova Remnant G347.3-0.5

Recent Advanced Spacecraft for Cosmology Astrophysics (ASCA) observations of G347.3-0.5, a supernova remnant (SNR) discovered in the ROSAT All-Sky Survey, reveal nonthermal emission from a region along the northwestern shell. Here we report on new pointed ASCA observations of G347.3-0.5 that confirm this result for all the bright shell regions and also reveal similar emission, although with slightly different spectral properties, from the remainder of the SNR. Curiously, no thermal X-ray emission is detected anywhere in the remnant. We derive limits on the amount of thermal emitting material present in G347.3-0.5 and present new radio continuum, CO, and infrared results that indicate that the remnant is distant and of moderate age. We show that our observations are broadly consistent with a scenario that has most of the supernova remnant shock wave still within the stellar wind bubble of its progenitor star, while part of it appears to be interacting with denser material. A point source at the center of the remnant has spectral properties similar to those expected for a neutron star and may represent the compact relic of the supernova progenitor.

Slane, Patrick O.↗