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At least 1,135 records · Page 63
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NASA NTRS1980

Simultaneous X-ray and optical observations of the 7.7 second X-ray pulsar 4U 1626-67

Simultaneous X-ray and optical observations of the 7.7-s X-ray pulsar 4U 1626-67, obtained during May 29-31, 1978, reveal the presence of (1) intense, correlated optical and X-ray flares and (2) 7.7-s optical pulsations which agree in phase with the 1-3 keV X-ray pulsations to within 0.5 s (two standard deviations) and which agree in period to within 0.001 s (one standard deviation). From these data it is concluded that a substantial fraction (a minimum of 8%) and possibly all of the optical emission is produced from within approximately 0.5 light sec or 1.5 x 10 to the 10th cm of the neutron star. Furthermore, the data rule out the possibility that this optical emission comes from the X-ray heated surface of a degenerate or nondegenerate dwarf companion. It most likely comes from an accretion disk.

Mcclintock, J. E.↗
NASA NTRS1980

The X-ray light curve of 4U 2129+47

The light curve of 4U 2129+47 is presented showing that it almost certainly has a period of approximately 0.22 days. This is the same period as the optical candidate proposed by Thorstensen et al. (1979). Binary X-ray source models, consisting of an M dwarf plus either a neutron star or a white dwarf, are briefly discussed.

Ulmer, M. P.↗
NASA NTRS1979

Sources of gravitational radiation; Proceedings of the Workshop, Seattle, Wash., July 24-August 4, 1978

Topics discussed are gravitational wave detection experiments, the theoretical foundations of gravitational wave physics, black holes, and neutron stars. Particular consideration is given to methods for circumventing the quantum limit for gravitational wave detectors, global problems in numerical relativity, massive black holes and gravitational radiation, the role of binaries in gravitational wave production, and astrophysical sources of gravitational radiation.

Smarr, L. L.↗
NASA NTRS1980

Discovery of the optical counterpart of the transient X-ray burster Centaurus X-4

The paper deals with the discovery and subsequent study of the optical counterpart to an X-ray nova which is almost certainly the historical transient Centaurus X-4, first discovered in 1969 and then dormant for the past decade. It is shown that Cen X-4 is a clear example of a soft, transient X-ray burster. The most important consequence of the connection between bursters and soft transients is the support it gives to the hypothesis that bursters are accreting neutron stars in binary systems. The observations support the hypothesis that at least some of the light comes from an accretion disk, and that X-ray heating plays an important role in the optical emission.

Canizares, C. R.↗
NASA NTRS1980

The Eddington limit and supercritical accretion. I - Time-independent calculations

Spherically symmetric, steady state accretion of an ionized hydrogen plasma onto a neutron star is considered for accretion rates which exceed a critical rate at which the Eddington limiting luminosity is produced. The coupled hydrodynamic and frequency integrated, radiative transfer equations are solved for accretion rates up to 10 times the nominal limit. Steady state solutions are presented that imply a multiplicity of different luminosity solutions for a single accretion rate in this 'supercritical' regime.

Burger, H. L.↗
NASA NTRS1979

Temporal studies of compact galactic X-ray sources

Advances in X-ray astronomy due to temporal studies are discussed. Attention is given to X-ray temporal variability categorized into periodic, quasi-periodic, and chaotic. Approximately 95% of all known X-ray sources exhibit chaotic variability, in which there is no apparent pattern or periodicities. The orbits of seven binary X-ray sources are presented, along with empirical findings of neutron star masses. In addition, the interior structure of a supergiant star, HD 77581, is investigated by attempting to measure its apsidal motion. Graphs are provided for the power density spectrum of 4U 1626-67, GX301-2 Doppler delay data, and empirically determined error contours for the mass and radius of four companions stars in X-ray binary systems.

Rappaport, S.↗
NASA NTRS1979

X-ray pulsar spectroscopy - Observational prospects

Three experimental concepts have been suggested to advance the study of X-ray pulsar spectroscopy. They are: (1) a higher bit rate for pulse phase spectroscopy; (2) an emphasis on the 10 keV 'pulsar surface' emission, and (3) an improved resolution to understand the nature of broadened iron lines in pulsars. It is shown that for conventional X-ray spectroscopy at least 5 KB/s are needed to substantially improve pulse phase spectroscopy; the broadened iron line emission in the pulsar spectra will require improved spectral resolution at 6.7 keV to discriminate among the models for this emission. It is concluded that this work will elucidate the physics of neutron star surface and circumstellar X-ray emission.

Pravdo, S. H.↗
NASA NTRS1979

Prospects for experimental research on black holes in binary systems

Cygnus X-1, the single widely accepted example of a black hole in a binary system, is characterized by unusual X-ray properties. The X-ray spectrum of Cygnus X-1 is not cut off above 20 keV, as in most strong X-ray sources. Recent scintillation counter measurements reveal a power law spectrum extending from 40 to 200 keV with a photon spectral index of approximately 2.2. However, it is not clear that these and other X-ray properties of the system are related to the black-hole nature of Cygnus X-1. It is suggested that without a direct test to show that the mass of the compact object in other systems similar to Cygnus X-1 (Circinus X-1 and GX339-4) exceeds the limit of the neutron star mass, a better understanding of the accretion disk phenomenon must be achieved to demonstrate how the properties peculiar to these systems are related to the black hole nature of the compact object. Current accretion disk models are examined, including the alpha-accretion disk and two-temperature accretion disk models.

Long, K. S.↗
NASA NTRS1979

High-resolution X-ray observations of the Cassiopeia A supernova remnant with the Einstein Observatory

Cas A was observed with the imaging detectors on the Einstein X-ray Observatory. The mass of the expanding shell due to the interaction of the blast wave and the ambient medium is determined. This indicates that Cas A is probably not in the free expansion phase, but has not yet reached the adiabatic phase of expansion. Detailed comparison of X-ray, optical, and radio images shows various degrees of correlation, indicating that several processes for emission are present. An upper limit of 1,500,000 K is established for any stellar remnant that is below the expected temperature of a neutron star by a factor of about 5. The total mass of the SNR is found to be 10-30 solar masses

Murray, S. S.↗
NASA NTRS1980

Pair annihilation in superstrong magnetic fields

The kinematical and dynamical aspects of the annihilation processes in superstrong magnetic fields are studied. The feasibility and potential significance of detecting from magnetic neutron stars are discussed. The discussion proceeds from the derivation of the fully relativistic differential cross sections and annihilation rates for both one- and two-photon emission from a ground-state gas of electrons and positrons in a static, uniform magnetic field.

Daugherty, J. K.↗
NASA NTRS1980

A determination of the orbit of GX 301-2

The pulse phase of GX 301-2(4U 1223-62) was tracked for 30 days with the SAS 3 satellite during 1979 January and February. It is suggested that most of the observed changes in pulse period are the result of Doppler shifts in a binary orbit, as opposed to changes in the intrinsic pulse period alone. The SAS 3 data allow orbital periods P(orb) equal to or greater than 23 days when a constant rate of change in the intrinsic pulse period is allowed as a free parameter in the orbital fits. For each trial orbital period the other orbital elements of the binary system are well determined. The SAS 3 data is combined with the Ariel 5 pulse arrival-time data to further restrict the allowed orbits. In both data sets a sharp minimum is observed in the Doppler delays of the pulse arrival times. Evidence is presented that the correct orbit is most likely the one with P(orb) = 35.0d, a projected semimajor axis for the neutron star of 304 light-seconds, and an eccentricity of 0.44. The relation of this system to the six X-ray binaries whose orbits have been determined previously is also discussed.

Kelley, R.↗
NASA NTRS1980

Flux distributions and colors of accretion disks

The disk model of Shakura and Sunyaev (1973) and Novikov and Thorne (1973) is used to calculate temperature distributions and integrated spectral fluxes for disks around a typical white dwarf and a typical neutron star, under the assumption that each element of the disk locally radiates as a blackbody. In addition, the disks' integrated UBV colors are calculated using the grid colors for real model atmospheres calculated by Buser and Kurucz (1978) and the observed colors given by Allen (1973). In all the calculations the effect of radiation from one part of the disk on all the other parts is included.

Pacharintanakul, P.↗
NASA NTRS1980

Implications of the high-state iron-line feature and soft X-ray emission of Hercules X-1

The interpretation that the soft X-ray emission of Her X-1 is due to reprocessing of the primary hard X-rays intercepted by a plasma layer in the Alfven surface imposes quite a stringent - but not an unreasonable - constraint on its parameters. By using the Alfven shell parameters constrained by this interpretation, the degree of ionization of the Alfven shell is calculated. Because the flux of hard X-ray from the neutron star is greatly attenuated inside the Alfven shell, except for a skin depth from its inner surface, the degree of ionization is determined by photoionization due to soft X-rays produced within the Alfven shell. On one hand, the large soft X-ray luminosity, which is about the same as the hard X-ray luminosity, indicates that a substantial fraction of primary hard X-rays is absorbed by the Alfven shell. On the other hand, the fact that the hard X-ray spectrum of Her X-1 observed during the high state shows neither a significant absorption feature down to about 3 keV nor an iron K-absorption dip tells us that the hard X-rays observed at the earth are mostly not reprocessed. This apparent conflict is resolved if the Alfven shell is clumpy (i.e., made of streams or blobs). The observed iron-line feature around 6.5 keV (E.W. about 300 keV) is explained by fluorescent iron K photons produced as a by-product of reprocessing of the hard X-rays by the Alfven shell; for this a large enhancement of the iron abundance with respect to the solar value is not required.

Bai, T.↗
NASA NTRS1980

White dwarf models for type 1 supernovae and quiet supernovae, and presupernova evolution

Supernova mechanisms in accreting white dwarfs are considered with emphasis on deflagration as a plausible mechanism for producing Type I supernovae and electron captures to form quiet supernovae leaving neutron stars. These outcomes depend on accretion rate of helium, initial mass and composition of the white dwarf. The various types of hydrogen shell burning in the presupernova stage are also discussed.

Nomoto, K.↗
NASA NTRS1980

X-ray bursters and the X-ray sources of the galactic bulge

Type 1 X-ray bursts, optical, infrared, and radio properties of the galactic bulge sources, are discussed. It was proven that these burst sources are neutron stars in low mass, close binary stellar systems. Several burst sources are found in globular clusters with high central densities. Optical type 1 X-ray bursts were observed from three sources. Type 2 X-ray bursts, observed from the Rapid Burster, are due to an accretion instability which converts gravitational potential energy into heat and radiation, which makes them of a fundamentally different nature from Type 1 bursts.

Lewin, W. H. G.↗
NASA NTRS1981

The 805s X-ray pulsar H2252-035

The X-ray flux from the 3.6 hr binary system H2252-035 is shown to be modulated at a period of 805s. The spectrum is consistent with either a 1.4 photon index power law or 20 keV thermal model. A 560t0r-350 eV equivalent width iron line is seen at approximately 6.7 keV. The possibility that this system contains a slowly rotating neutron star is discussed.

White, N. E.↗
NASA NTRS1981

Directionality effects in the transfer of X-rays from a magnetized atmosphere: Beam pulse shape

A formalism is presented for radiation transfer in two normal polarization modes in finite and semiinfinite plane parallel uniform atmospheres with a magnetic field perpendicular to the surface and arbitrary propagation angles. This method is based on the coupled integral equations of transfer, including emission, absorption, and scattering. Calculations are performed for atmosphere parameters typical of X-ray pulsars. The directionality of the escaping radiation is investigated for several cases, varying the input distributions. Theoretical pencil beam profiles and X-ray pulse shapes are obtained assuming the radiation is emitted from the polar caps of spinning neutron stars. Implications for realistic models of accreting magnetized X-ray sources are briefly discussed.

Meszaros, P.↗
NASA NTRS1980

X-rays from G74.9 + 1.2 - An elderly Crab Nebula

The supernova remnant G74.9 + 1.2, which resembles the Crab Nebula in many of its radio properties, has been detected in 0.15-3 keV X-rays with the Imaging Proportional Counter on the Einstein Observatory. The intrinsic luminosity of the object is about 100 times weaker than the Crab Nebula in this band. The X-ray morphology is centrally brightened (not a shell) with a total extent about 5 arcmin (17 pc). The nearby, intense, compact radio source 2013 + 370 is also probably detected as an extended X-ray source. It is argued that G74.9 + 1.2 represents an object similar to the Crab Nebula but of age 4000-7000 years. If this is the case, the period of the pulsar or neutron star (whose loss of rotational energy fuels the nebula) is predicted to be 0.1-0.2 s.

Wilson, A. S.↗