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At least 793 records · Page 44
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

A disc-reflected component in the spectra of X-ray bursters

An argument is presented to the effect that a disk-reflected component should be present in the spectra of X-ray bursts and is best seen just after the burst peak when the reflected photons, delayed by their passage to the site of reflection, appear in stronger contrast to the fast-declining primary emission from the cooling neutron star. It is shown that disk reflection has an observable effect on the spectrum of the burst tail. The possible use of the concomitant absorption edge as a diagnostic of the accretion disk is discussed.

Day, C. S. R.↗
NASA NTRS1991

Observations of Scorpius X-1 with IUE - Ultraviolet results from a multiwavelength campaign

IUE UV results are presented for the low-mass X-ray binary Sco X-1. Models that predict UV continuum emission from the X-ray-heated surface from the companion star and from an X-ray illuminated accretion disk are adjusted for parameters intrinsic to Sco X-1, and fitted to the data. X-ray heating is found to be the dominant source of UV emission; the mass-accretion rate increases monotonically along the 'Z-shaped' curve in an X-ray color-color diagram. UV emission lines from He, C, N, O, and Si were detected; they all increase in intensity from the HB to the FB state. A model in which emission lines are due to outer-disk photoionization by the X-ray source is noted to give good agreement with line fluxes observed in each state.

Vrtilek, S. D.↗
NASA NTRS1989

No molecular gas disk in S106

The radio and optical bipolar H II region, S106, is bisected by a dark lane. The premain-sequence object, S106 IR, which is the source of a powerful ionized stellar wind, is the exciting source of this region and is found at the center of the equatorial emission gap. The existence of a massive, extended, molecular gas disk has previously been suggested as an explanation for the peculiar morphology of this source, and S106 has widely been quoted as the best example of theoretically posited accretion disks. The new, high-resolution, CS and (C-13)O observations presented show that the molecular emission, previously attributed to a disk structure, actually originates from distinct masses of molecular gas, swept up from the ambient cloud core by the ionized lobes.

Barsony, M.↗
NASA NTRS1990

Mass transport in a neutron star magnetosphere

The interaction between a thin Keplerian accretion disk and a magnetosphere surrounding a central object is investigated within the framework of an analytical description for the magnetic field configuration. The commonly held assumption that all accreting plasma flows from the magnetospheric boundary to the stellar surface is shown to be overly restrictive. If the magnetospheric boundary is defined as the distance where the rotation starts deviating significantly from the Kepler rate, it is found that there is an extensive region inside this boundary where gas, nearly corotating with the star, drifts inward across the field by an interchange instability. The linear analysis of this instability is presented. It is also found that gas tied to field lines can be in equilibrium at positions off the midplane, and that gas can plausibly flow from the midplane to these positions, in certain circumstances. The observational consequences of such a picture are briefly discussed.

Spruit, H. C.↗
NASA NTRS1993

Observations of the white dwarf in the U Geminorum system with the Hopkins Ultraviolet Telescope

The Hopkins Ultraviolet Telescope was used to obtain an FUV spectrum of U Gem about 10 d after the end of an outburst. Most of the FUV emission from U Gem appears to rise from the white dwarf (WD) in the system. Various possibilities to explain the hot component in the present spectrum are explored. It is most likely due to radiation from the boundary-layer region of the WD surface rather than from the accretion disk which exists in U Gem in quiescence. If the standard picture of boundary-layer emission is correct, then the hot component is not due to ongoing accretion onto the WD because the temperature observed is too low. Delayed emission from a viscously heated, differentially rotating atmosphere is argued to be a more valid explanation.

Long, Knox S.↗
NASA NTRS1993

31 Cygni: The B star and the wind

I have followed photospheric and wind absorptions in International Ultraviolet Explorer (IUE) spectra over much of the orbit of this K4 Ib+B3-4 binary system, using interstellar lines to transform all the velocities to a scale with the interstellar medium at -10.1 km/s. Results are as follows: Interstellar Fe II lines give NL (Fe(+)) = 1-2 x 10(exp 14)/sq cm with a Doppler width of 7 km/s; depletions of heavy elements along the line of sight to this star are similar to those for other stars with low interstellar densities. Photospheric lines yield a new velocity amplitude for the B star, K(sub B) = 23.2 +/- 1.0 km/s, which gives the mass ratio M(sub K)/M(sub B) = 1.66 and masses M(sub B) = 7.1 solar mass and M(sub K) = 11.7 solar mass. Existing photometry gives R(sub K) = 197 solar radius, R(sub B) = 5.2 solar radius, and i = 87.19 deg. Breadths of photospheric lines imply the rotational velocity of the B star is V sin i = 70-80 km/s. Most of the wind features are formed in a shell much larger than the binary system for which the terminal velocity seems to be 73 +/- 2 km/s, but it may be as high as approximately 90 km/s. The Doppler width (approximately 13 km/s) of Fe II shell lines likely results from acceleration of the wind, although it could reflect orbital motion of the K supergiant. Shell lines of Mg II and Si II are roughly consistent with Fe II, although their strengths are phase dependent, while S II requires a somewhat thicker shell in S(+) than in Fe(+) with a greater spread in expansion velocity. Shell lines of O I and N I are abnormally weak, if detected at all, and this implies that hydrogen is ionized throughout much of the wind. Lines of highly ionized species (C IV, Si Iv, and N V) appear sporadically in the spectrum and have inverse P Cyg profiles implying collapse (probably of an accretion disk) onto the B star. They do not seem to be isolated at particular phases as might be expected for formation in a conical shock front around an accretion column. I argue that the wind is roughly isotopic and is accelerated slowly, in agreement with existing ideas about winds.

Eaton, Joel A.↗
NASA NTRS2000

The 35 Day Evolution of the Hercules X-1 Pulse Profile: Evidence for a Resolved Inner Disk Occultation of the Neutron Star

Ginga and Rossi X-Ray Timing Explorer observations have allowed an unprecedented view of the recurrent systematic pulse shape changes associated with the 35 day cycle of Hercules X-1, a phenomenon currently unique among the known accretion-powered pulsars. We present observations of the pulse shape evolution. An explanation for the pulse evolution in terms of a freely precessing neutron star is reviewed and shown to have several major difficulties in explaining the observed pulse evolution pattern. Instead, we propose a phenomenological model for the pulse evolution based on an occultation of the pulse-emitting region by the tilted, inner edge of a precessing accretion disk. The systematic and repeating pulse shape changes require a resolved occultation of the pulse emission region. The observed pulse profile motivates the need for a pulsar beam consisting of a composite coaxial pencil and fan beam, but the observed evolution pattern requires the fan beam to be focused around the neutron star and beamed in the antipodal direction. The spectral hardness of the pencil beam component suggests an origin at the magnetic polar cap, with the relatively softer fan beam emission produced by backscattering from within the accretion column, qualitatively consistent with several theoretical models for X-ray emission from the accretion column of an accreting neutron star.

Scott, D. Matthew↗
NASA NTRS1985

A model for the UHE gamma-rays from Hercules X-1

An outburst of gamma rays with energies E gamma 10 to the 12th power eV was recently detected from the X-ray pulsar Hercules X-1. The outburst had a 3 minute duration and occurred at a time during the 35 day X-ray modulation that is associated with X-ray turnon. The gamma rays also have the same 1.24 second modulation that is observed at X-ray energies. Subsequently a 40 minute outburst was detected at E gamma 10 to the 14th power eV. The interaction of ultrahigh energy particles with a precessing accretion disk explain the observed gamma ray light curve. The constraints one can place on acceleration mechanisms and the possibility that the UHE particles are accelerated by shocks in an accretion flow are explained.

Eichler, D.↗
NASA NTRS1985

On the nature of the UX Ursa Majoris-type nova-like variables - CPD-48 deg 1577

A series of low-dispersion spectra and one high-dispersion spectrum have been obtained of the catacylsmic variable CPD-48 deg 1577. Like other UX UMa stars, this variable exhibits the flat continuum of a luminous, thick, steady state accretion disk dominating its light from the far UV to the infrared. A wind mass loss rate of less than about 10 to the -9th solar mass/yr and an accretion rate of roughly 5 x 10 to the -9th solar mass/yr is roughly consistent with the range which characterizes other UX UMa stars. An SWP echelle spectrum reveals detailed line profile information on the N V, Si IV, and C IV resonance lines. He II 1640 A is remarkably weak and even absent in two of the spectra. The known properties and evolutionary status of CPD-48 deg 1577 and the other UX UMa stars are summarized.

Sion, E. M.↗
NASA NTRS1987

Neutron tori and the origin of r-process elements

It is shown here that if an accretion disk gets hotter than a few MeV, nuclei in the infalling matter are dissociated into their constituent neutrons and protons. Neutrons released by dissociation of matter falling at high accretion rates into a black hole or neutron star accumulate in a dense 'neutron torus'. The occurrence of the classical rapid or r-process nucleosynthesis in such a torus is investigated.

Hogan, C. J.↗
NASA NTRS1984

The solar nebula and the planetesimal disk

Two popular theories of solar system formation are briefly reviewed, then used as background in an examination of several new developments related to planetary ring dynamics that promise to have great impact on future research. Most important are the incorporation of accretion disk and density wave theories into cosmogonic theory. A successful integration of these mechanisms may significantly constrain evolutionary models of the early solar system and also provide new insight into the mechanisms themselves.

Ward, W. R.↗
DOE OSTI2023

Striped Jets in Post–Neutron Star Merger Systems

Abstract Models invoking magnetic reconnection as the particle acceleration mechanism within relativistic jets often adopt a gradual energy dissipation profile within the jet. However, such a profile has yet to be reproduced in first-principles simulations. Here we perform a suite of 3D general relativistic magnetohydrodynamic simulations of post–neutron star merger disks with an initially purely toroidal magnetic field. We explore the variations in both the microphysics (e.g., nuclear recombination, neutrino emission) and system parameters (e.g, disk mass). In all of our simulations, we find the formation of magnetically striped jets. The stripes result from the reversals in the poloidal magnetic flux polarity generated in the accretion disk. The simulations display large variations in the distributions of stripe duration, τ , and power, 〈 P Φ 〉. We find that more massive disks produce more powerful stripes, the most powerful of which reaches 〈 P Φ 〉 ∼ 10 49 erg s −1 at τ ∼ 20 ms. The power and variability that result from the magnetic reconnection of the stripes agree with those inferred in short-duration gamma-ray bursts. We find that the dissipation profile of the cumulative energy is roughly a power law in both radial distance, z , and τ , with a slope in the range of ∼1.7–3; more massive disks display larger slopes.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2021

Modeling Magnetic Disk Wind State Transitions in Black Hole X-Ray Binaries

We analyze three prototypical black hole X-ray binaries, 4U 1630–472, GRO J1655–40, and H1743–322, in an effort to systematically understand the intrinsic state transition of the observed accretion disk winds between wind-on and wind-off states by utilizing state-of-the-art Chandra/HETGS archival data from multi-epoch observations. We apply our magnetically driven wind models in the context of magnetohydrodynamic(MHD)calculations to constrain(1)their global density slope(p),(2)their density (n17) at the foot point of the inner most launching radius, and(3)the abundances of heavier elements(AFe,S,Si). Incorporating the MHD winds into xstar photoionization calculations in a self-consistent manner, we create a library of synthetic absorption spectra given the observed X-ray continua. Our analysis clearly indicates a characteristic bimodal transition of multi-ion X-ray winds; i.e., the wind density gradient is found to steepen(fro mp∼1.2–1.4 to∼1.4–1.5)while its density normalization declines as the source transitions from the wind-onto the wind-off state. The model implies that the ionized wind remains physically present even in the wind-off state, despite its apparent absence in the observed spectra. Super solar abundances for heavier elements are also favored. Our global multi-ion wind models, taking into account soft X-ray ions as well as Fe K absorbers, show that the internal wind condition plays an important role in wind transitions besides photoionization changes. Simulated XRISM/Resolve and Athena/X-IFU spectra are presented to demonstrate a high fidelity of the multi-ion wind model for a better understanding of these powerful ionized winds in the coming decades

Keigo Fukumura↗
DOE OSTI2024

Magnetic Field Strength Effects on Nucleosynthesis from Neutron Star Merger Outflows

Abstract Magnetohydrodynamic turbulence drives the central engine of post-merger remnants, potentially powering both a nucleosynthetically active disk wind and the relativistic jet behind a short gamma-ray burst. We explore the impact of the magnetic field on this engine by simulating three post-merger black hole accretion disks using general relativistic magnetohydrodynamics with Monte Carlo neutrino transport, in each case varying the initial magnetic field strength. We find increasing ejecta masses associated with increasing magnetic field strength. We find that a fairly robust main r -process pattern is produced in all three cases, scaled by the ejected mass. Changing the initial magnetic field strength has a considerable effect on the geometry of the outflow and hints at complex central engine dynamics influencing lanthanide outflows. We find that actinide production is especially sensitive to magnetic field strength, with the overall actinide mass fraction calculated at 1 Gyr post-merger increasing by more than a factor of 6 with a tenfold increase in magnetic field strength. This hints at a possible connection to the variability in actinide enhancements exhibited by metal-poor, r -process-enhanced stars.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1980

Sweeping of the Jovian resonances and the evolution of the asteroids

During the formation of the solar system the variation of the gravitational field produced by removal of a nebula with its moderately massive accretion disk led to sweeping of the Jovian commensurability resonances through the asteroid zone. This process produced increased eccentricities and random velocities of the early planetesimals which resulted in collisional comminution rather than accretion. The existence of the asteroids, their low mass density, and their high relative velocities are interpreted as due to disruption of the accretion processes of the terrestrial planets by the influences of Jupiter.

Torbett, M.↗
NASA NTRS1991

Optical emission enhancements in Be stars

Be stars show enhancement of optical emission up to a few magnitudes. Optical continuum enhancements due to (1) conversion of Lyman continuum photons of the Be star to visual photons in the envelope of the Be star, (2) emission from the H II region formed by X-ray and EUV emission from a compact object in binary motion around the Be star, and (3) emission from an accretion disk around the compact object, are considered. These considerations indicate that large increases of optical emission (above 0.5 mag) would need an ionizing source other than the Be star (for example, an accreting compact object emitting radiation). Specific examples of large increases of optical emission as in the sources Gamma Cas and 4U 0115 + 63 are discussed.

Apparao, Krishna M. V.↗
NASA NTRS2016

NuSTAR Observations of Water Megamaser AGN

Aims. We study the connection between the masing disk and obscuring torus in Seyfert 2 galaxies. Methods. We present a uniform X-ray spectral analysis of the high energy properties of 14 nearby megamaser active galactic nuclei observed by NuSTAR. We use a simple analytical model to localize the maser disk and understand its connection with the torus by combining NuSTAR spectral parameters with the available physical quantities from VLBI mapping.Results. Most of the sources that we analyzed are heavily obscured, showing a column density in excess of approx.10(exp 23) cm(exp -2); in particular, 79% are Compton-thick [NH is greater than 1.5 x 10(exp 24) cm(exp -2)]. When using column densities measured by NuSTAR with the assumption that the torus is the extension of the maser disk, and further assuming a reasonable density profile, we can predict the torus dimensions. They are found to be consistent with mid-IR interferometry parsec-scale observations of Circinus and NGC 1068. In this picture, the maser disk is intimately connected to the inner part of the torus. It is probably made of a large number of molecular clouds that connect the torus and the outer part of the accretion disk, giving rise to a thin disk rotating in most cases in Keplerian or sub-Keplerian motion. This toy model explains the established close connection between water megamaser emission and nuclear obscuration as a geometric effect.

masers / galaxies: active / galaxies: Seyfert↗
NASA NTRS2004

Molecular Hydrogen in the Quiescent Disk of SW UMA

The FUSE observation has been reduced and a paper is in progress. The analysis has been slow because of the very noisy quality of the data. We anticipate submitting a paper in 2 or 3 months. The primary result is that there is no H_2 fluorescent emission. However, there seems to be no accretion disk. The upper limit to the continuum level is well below that observed by IUE, and while O VI lines are present, they are extremely faint and much narrower that the published and archival H alpha profiles. It appears that the accretion has essentially shut off entirely, leaving only very faint O VI and C III emission from the white dwarf. We are considering a magnetic model analogous to that for AE Aqr.

Raymond, John C.↗