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Lamb, F. K.

Publications and source records attributed to Lamb, F. K..

36 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Probing the interiors of neutron stars

This paper reviews the information on neutron stars that has been obtained from observations of pulsars and pulsing X-ray stars, with emphasis on results from timing studies. Included are summaries of the most recent theoretical work on neutron star structure and the current evidence concerning neutron star masses and radii. Also included are discussions of large frequency jumps and small-scale timing irregularities, the description of the latter phenomenon in terms of random noise processes, and the use of both as probes of the internal structure and dynamical properties of neutron stars. Finally, some of the most promising directions for future work are briefly noted.

Lamb, F. K.↗
NASA NTRS1983

Synchronization of magnetic stars in binary systems

Asynchronous rotation of magnetic stars in close binary systems drives substantial field-aligned electrical currents between the magnetic star and its companion. The resulting magnetohydrodynamic torque is able to account for the heretofore unexplained synchronous rotation of the strongly magnetic degenerate dwarf component in systems like AM Her, VV Pup, AN UMa, and EF Eri as well as the magnetic A type component in systems like HD 98088 and 41 Tauri. The electric fields produced by even a small asynchronism are large and may accelerate some electrons to high energies, producing radio emission. The total energy dissipation rate in systems with degenerate dwarf spin periods as short as 1 minute may reach 10 to the 33rd ergs/s. Total luminosities of this order may be a characteristic feature of such systems.

Lamb, F. K.↗
NASA NTRS1982

Observable properties of neutron stars

This paper reviews the information on neutron stars that has been obtained from observations of pulsars and pulsing X-ray stars, with emphasis on results from timing studies. Included are summaries of the most recent theoretical work on neutron star structure and the current evidence concerning neutron star masses and radii. Also included are discussions of large frequency jumps and small-scale timing irregularities, the description of the latter phenomenon in terms of random noise processes, and the use of both as probes of the internal structure and dynamical properties of neutron stars. Finally, some of the most promising directions for future work are briefly noted.

Lamb, F. K.↗
NASA NTRS1982

The low energy spectra of gamma-ray bursts

The implications of observed gamma-ray burst spectra for the physical conditions and geometries of the sources are examined. It is noted that an explanation of the continua in terms of optically thin thermal bremsstrahlung requires a relatively large area but a fairly shallow depth. On the other hand, a spectrum similar to that observed could be produced by rapid flickering of sources with less extreme geometries if each flicker emits a Comptonized thermal spectrum. Either field inhomogeneities or plasma motions are required to interpret the low energy features as cyclotron extinction. An alternative explanation is photoelectric absorption by heavy atoms; this requires a field strength high enough to make one-photon electron positron annihilation possible. Observational tests of these possibilities are proposed

Bussard, R. W.↗
NASA NTRS1981

Neutron star binaries, pulsars and burst sources

Unresolved issues involving neutron star binaries, pulsars, and burst sources are described. Attention is drawn to the types of observations most likely to resolve them. Many of these observations are likely to be carried out during the next decade by one or more missions that have been approved or proposed. Flux measurements with an imaging detector and broad-band spectroscopic studies in the energy range 30-150 keV are discussed. The need for soft X-ray and X-ray observations with an instrument which has arcminute angular resolution and an effective area substantially greater than of ROSAT or EXOSAT is also discussed.

Lamb, F. K.↗
NASA NTRS1981

Pointed soft X-ray observations of AM Herculis from HEAO 1

Extensive soft X-ray (0.1-1.5 keV) observations of AM Herculis are reported. The data, acquired during two pointed observations by HEAO 1 in 1978, span four binary cycles and reveal considerable detail in the 3.09 hour light curve. Rapid X-ray flickering is evident throughout the bright section of the light curve and is characterized by an average e-folding autocorrelation time of about 10 s. Analysis of the spectral data indicates a 30% variability; on occasion the X-ray spectrum becomes softer as the source intensity decreases. The current understanding of AM Her as an accreting magnetic white dwarf is discussed, and various models are related to the present HEAO-1 data.

Tuohy, I. R.↗
NASA NTRS1980

On the origin and persistence of long-period pulsating X-ray sources

Mounting observational evidence and theoretical results suggest that most pulsing X-ray sources, including those with massive companions, are disk fed when significant mass transfer is taking place. Regardless of whether or not substantial braking has occurred before the onset of disk accretion, once it begins the resulting strong braking torque on fast rotators is sufficient to spin-down a neutron star from an initial period approx. 1 sec to a period approx. greater than 100 sec in a time short compared to the main-sequence lifetime of a massive companion and even, in many cases, in a time short compared to the star's lifetime as a bright X-ray source. It is argued that sources with longer periods have larger magnetic moments, or lower mean luminosities, or both, whereas sources with shorter periods have smaller magnetic moments, or higher mean luminosities, or both.

Elsner, R. F.↗
NASA NTRS1979

Accretion by rotating magnetic neutron stars. III - Accretion torques and period changes in pulsating X-ray sources

The solutions of the two-dimensional hydromagnetic equations are used to calculate the torque on a magnetic neutron star accreting from a Keplerian disk. It is found that the magnetic coupling between the star and the plasma in the outer transition zone is appreciable; that as a result, the spin-up torque on fast rotators is substantially less than that on slow rotators, and that for sufficiently high stellar angular velocities or sufficiently low mass accretion rates, the rotation of the star can be braked while accretion continues. These results are applied to pulsating X-ray sources, revealing that at high luminosities a star of given spin period rotating in the same direction as the disk can experience either spin-up or spin-down, depending on its luminosity. Also considered are the general problem of interpreting period changes in pulsating X-ray sources, and the dipole magnetic moments of nine pulsating X-ray sources are estimated by fitting the theoretical spin-up equation to estimates of the average luminosity and spin-up rate of each source.

Ghosh, P.↗
NASA NTRS1979

The HEAO-1 neutron star timing experiment

The HEAO-1 neutron star timing experiment is presented. The scientific motivation for the experiment, the reasons for the choice of Her X-1 and Vela X-1 as promising sources to study, the methodology, and the current status of the experiment are summarized.

Lamb, F. K.↗
NASA NTRS1979

Accretion by rotating magnetic neutron stars. II - Radial and vertical structure of the transition zone in disk accretion

The radial and vertical structure of the transition zone at the magnetospheric boundary of an aligned rotating neutron star accreting matter from a Keplerian disk are calculated. The results obtained indicate that: (1) the inner edge of the disk is located where the integrated magnetic stress acting on the disk plasma becomes comparable to the integrated material stress associated with its inward radial drift and orbital motion; (2) the stellar magnetic field threads the disk near its inner edge via the Kelvin-Helmholtz instability, turbulent diffusion, and reconnection, producing a broad transition zone between the unperturbed disk flow and corotating magnetosphere; (3) the transition zone consists of two qualitatively different regions, viz., a broad outer transition zone where the motion is Keplerian and a narrow inner zone, or boundary layer, where the departure from Keplerian motion is substantial; (4) the stellar magnetic field is largely but not entirely screened by currents flowing in the boundary layer; and (5) there are no steady-flow solutions for sufficiently fast stellar rotation.

Ghosh, P.↗
NASA NTRS1979

HEAO-1 observations of AM Herculis between 0.1 and 3 keV

Results are reported for HEAO A2 scanning observations of AM Her in the energy range from 0.1 to 3 keV. A light curve of AM Her is plotted, a typical flux of 8 x 10 to the -10th erg/sq cm-sec in the 0.15-0.5-keV range at X-ray maximum is estimated, and no pronounced spectral variation with phase is detected. It is found that both a 0.025-keV blackbody spectrum and an exponential spectrum with an energy-dependent Gaunt factor provide an acceptable fit to the data. It is concluded that the spectrum of the radiation from the plasma at the surface of the strongly magnetic and relatively massive white dwarf in AM Her consists of three components: (1) a high-temperature (about 100 million to 1 billion K) component produced by electron-ion bremsstrahlung in the hot postshock plasma; (2) a blackbody component of about 100,000 to 1 million K produced by the heated surface of the white dwarf; and (3) a still softer component produced by optically thick electron cyclotron emission in the postshock region.

Tuohy, I. R.↗
NASA NTRS1978

New evidence on the nature of the soft X-ray source in AM Herculis from HEAO 1

Results are reported for scanning observations of AM Herculis from HEAO 1 during the period September 22-October 12, 1977. The soft X-ray light curve constructed from the observations shows considerable scatter in intensity, together with clear evidence for a residual flux during X-ray minimum. At X-ray maximum the flux in the energy range 0.15-0.5 keV is typically 8 x 10 to the -10th power erg/sq cm per sec. The spectrum of AM Her is characterized by a sharp turnover at 0.2 keV and is well described by a blackbody model with absorption. There is no appreciable change in the spectrum with binary phase. The present observations, when combined with theoretical models, imply that the soft X-rays are produced by the heated surface of the degenerate dwarf component, and that the surface has a temperature of approximately 400,000 K and a luminosity about 30 times that observed at optical or hard X-ray wavelengths. AM Her is predicted to be a spectacular UV source, with an intrinsic UV luminosity comparable to that in soft X-rays.

Tuohy, I. R.↗
NASA NTRS1978

Period variations in pulsating X-ray sources. II - Torque variations and stellar response

A statistical description of variations in the torque acting on a rotating neutron star is developed in terms of stationary random processes and is used to calculate the torque power spectrum, stellar response power spectrum, and the total mean square variation in the crustal angular velocity of the star. The response of a star with a finite frequency internal mode is calculated with the aid of phenomenological equations which correspond to a generalized two-component model of the star. The form of the stellar response functions is examined in a number of limiting cases of physical interest, and the dependence of the response power on both the period of observation and the time over which fluctuating torques have been acting is discussed.

Lamb, F. K.↗
NASA NTRS1978

Period variations in pulsating X-ray sources. I - Accretion flow parameters and neutron star structure from timing observations

Torque fluctuations which can lead to variations in the periods of pulsating X-ray sources are examined. A description of torque variations in terms of noise processes is developed, and the resulting noise models are applied to observations of several pulsating X-ray sources. It is shown that fluctuations in accretion torque could account for the observed period variations and spindown episodes in Her X-1 and Cen X-3. The values of the torque noise strengths inferred from either a nonresonant response or, in the case of Her X-1, a Tkachenko-mode interpretation of the data are found to be consistent with those expected from processes at the magnetospheric boundary of an accreting neutron star. Ways to distinguish among the various interpretations of the period variations are considered. It is noted that fluctuating mass-flow rates may be responsible for other phenomena observed in compact X-ray sources, such as wobble with zero initial amplitude and binary period variations in close binary systems experiencing mass transfer.

Lamb, F. K.↗
NASA NTRS1978

Disk accretion by magnetic neutron stars

A model for disk accretion by a rotating magnetic neutron star is proposed which includes a detailed description of matter flow in the transition region between the disk and the magnetosphere. It is shown that the disk plasma cannot be completely screened from the stellar magnetic field and that the resulting magnetic coupling between the star and the disk exerts a significant torque on the star. On the assumption that the distortion of the residual stellar field lines threading the disk is limited by reconnection, the total accretion torque on the star is calculated. The calculated torque gives period changes in agreement with those observed in the pulsating X-ray sources and provides a natural explanation of why a fast rotator like Her X-1 has a spin-up rate much below the conventional estimate for slow rotators. It is shown that for such fast rotators, fluctuations in the mass-accretion rate can produce fluctuations in the accretion torque about 100 times larger. For sufficiently fast rotators or, equivalently, for sufficiently low accretion rates, the star experiences a braking torque even while accretion continues and without any mass ejection from its vicinity.

Ghosh, P.↗
NASA NTRS1976

An opaque shell around Hercules X-1

It is suggested that the observed soft X-rays from Her X-1 are the result of an opaque gas shell which surrounds the neutron star at a radius where centrifugal force and the magnetic field impede the gravitational infall of the gas and which absorbs a substantial fraction of the hard X-ray flux, reradiating it as soft X-rays. Two highly idealized models for the shell are constructed in which the radius and temperature are 7000 km and 550,000 K or 1300 km and 1.5 million K, respectively. These models are intended to show that a gas shell with interesting spectral characteristics is likely to occur at a radius of 2000 to 7000 km from the neutron star if the magnetic field impedes the gas infall at this radius and that such a shell is indicated by the soft X-ray observations. A possible geometry is considered wherein the shell is a wide opaque ring at the magnetic equator, becomes transparent at high latitudes, and becomes opaque again at the magnetic poles.

Mccray, R.↗
NASA NTRS1976

Information about accretion flows from X-ray timing of pulsating sources

The response was studied of a rotating neutron star to fluctuating torques and it was found that the observed variations in the pulsation periods of the compact X-ray sources Cen X-3 and Her X-1 could be caused by short time scale fluctuations in the accretion torques acting on the neutron stars. The sizes and rates of the required fluctuations are consistent with current accretion models. Such fluctuations can cause period variations either (a) directly, by causing a random walk of the star's angular velocity or (b) indirectly, by exciting a long-period mode of the neutron star, such as the Tkachenko mode of the rotating neutron superfluid. Phenomena in compact X-ray sources and cataclysmic variables which may be caused by fluctuating mass flow rates are also discussed.

Lamb, F. K.↗