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Higdon, J. C.

Publications and source records attributed to Higdon, J. C..

32 records · Page 2
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NASA NTRS1985

Galactic distribution of interstellar Al-26

A narrow cosmic gamma ray line at 1809 keV was discovered which was interpreted as resulting from the decay of approximately 3 M sub theta of Al-26 residing in the galactic disk. While its intrinsic width was unresolved by the HEAO 3 spectrometer, a (1 sigma) limit of 3 keV FWHM was obtained; this corresponds to bulk motions of v 250 km/s, which is consistent with material at rest in the ISM. Sites which have been suggested include type II supernovae and massive stars which are members of the extreme population I, as well as novae and red giants which are associated with an older disk population. The HEAO 3 data was used to distinguish between these two stellar populations.

Mahoney, W. A.↗
NASA NTRS1985

Diffuse galactic annihilation radiation from supernova nucleosynthesis

The propagation of MeV positrons in the outer ejecta of type I supernovae was investigated. It was found that the positrons created at times of approx 100 days propagated along magnetic field lines in the outer ejecta without any appreciable pitch-angle scattering or excitation of hydromagnetic waves. The lack of significant pitch-angle scattering is well consistent with models of wave excitation and scattering by resonant interactions. This occurs because time periods to scatter the particles or to excite waves are significantly longer than escape times. Thus it is expected that, when positrons are not coupled to the ejecta by Coulomb collisions, they escape from the relatively cold, dense ejecta and reside predominantly in the tenuous, hotter, shock-heated interstellar gas. In the tenuous shock-heated gas the positron lifetime against annihilation is much greater than lifetimes in the dense ejectra. Thus the production of steady-state diffuse annihilation radiation by some fraction of these escaped positrons seems probable.

Higdon, J. C.↗
NASA NTRS1985

Interstellar turbulence, random density variations, and scintillation measurements

The presence of random electron variations suggests that the ionized interstellar medium is turbulent. In the interstellar plasma the presence of power spectra of such variations extending to spatial scales much less than a Coulomb mean free path, Lambda sub c, is required by analyses of measurements of scintillation and angular broadening of pulsar radio signals. The existence of corresponding variations in magnetic field strength could efficiently scatter cosmic rays and thus constrain cosmic-ray propagation. Unfortunately both the origin of the electron density variations and mechanisms by which these variations couple to fluctuations in magnetic field strength are unknown. It is conjectured that the small-scale density variations are generated by the convective distortion of initially large-scale isobaric entropy structures in the turbulent interstellar plasma. An investigation of the spectra of turbulent entropy structures, velocity, and magnetic fields at small spatial scales is made. The modifier small is employed to characterize length scales much less than the dimension, L, containing the bulk of the turbulent energy.

Higdon, J. C.↗
NASA NTRS1984

Density fluctuations in the interstellar medium: Evidence for anisotropic magnetogasdynamic turbulence. I - Model and astrophysical sites

A self-consistent theoretical model of turbulent fluctuations in a compressible electrically conducting gas is developed in order to quantitatively study a turbulent fluid origin for the small spiral scale density fluctuations observed by Armstrong et al. (1981). A heuristic description of turbulent flows in compressible fluids is offered as an analogy for the small-scale turbulent velocity and magnetic field variations which derive density variations in the presence of a strong local magnetic field.

Higdon, J. C.↗
NASA NTRS1982

Derivation of the luminosity of a gamma ray burst

The intrinsic luminosity, erg/s, in a gamma ray burst is calculated using two assumptions: first, the power is independent of the burst duration, and, second, the spatial distribution of the bursters is similar to the spatial distribution of the pulsars. It is found that the experimentally determined distribution of burst durations is significantly biased by the observational selection effects. It is shown that a typical luminosity of 5 x 10 to the 35th erg/s can adequately reproduce the burst (log N, log S) curve, an isotropic latitude distribution, and the experimentally determined distribution of durations.

Higdon, J. C.↗
NASA NTRS1981

The Cygnus 'superbubble' - A supernova explosion in a tenuous intercloud medium

Cash et al. (1980) observed an incomplete X-ray shell in the Cygnus region with a temperature of 2 x 10 to the 6th K and a density of 0.02 per cu cm. This feature closely resembles Loop I, a nearby supernova remnant expanding in a tenuous (less than 0.01 per cu cm) medium. A similar origin - remnant expansion in a tenuous intercloud medium with the bulk of the interstellar gas concentrated in clouds - is suggested for the Cygnus X-ray feature. This model greatly reduces the required initial thermal energy. Remnant shock interaction with ambient interstellar clouds produces the observed H-alpha filaments.

Higdon, J. C.↗
NASA NTRS1980

Supernova radio remnants and the filling factor of the hot interstellar medium

If a hot (temperature of about one-million K), tenuous gas fills about 90% of interstellar space, then the observed number and surface brightness distribution of galactic radio remnants implies a galactic supernova rate of once about every 30 years. This is consistent with estimates based on observations of historical supernovae, pulsars, and supernovae in other spiral galaxies.

Higdon, J. C.↗
NASA NTRS1980

Galactic gamma rays produced by Compton scattering of cosmic ray electrons

Previous models of the Compton scattering of the galactic gamma rays have been based on starlight distributions determined from galactic mass models and a constant luminosity per unit mass. The fact that the spiral arms have a significantly higher photon density than does the base disk containing the bulk of the galactic mass has been neglected in calculating the Compton gamma-ray component. The inclusion of the spiral arm Compton component produces enhanced gamma-ray intensities along tangents to spiral arms. Irrespective of whether the cosmic-ray electron density is enhanced in the spiral arms, increased gamma-ray emission is produced along the spiral arm tangents due to increased photon density. Thus, cosmic ray sources are not necessarily located in the arms.

Higdon, J. C.↗
NASA NTRS1979

Acceleration and transport of galactic cosmic rays

A recently proposed cosmic-ray acceleration mechanism is combined with a model of transport and loss from the Galaxy to construct a model for galactic cosmic rays. Various aspects of the cosmic-ray spectrum, including the secondary-to-primary ratio as a function of energy, are found to fit naturally into the overall picture. A solution to the dynamical-halo model with energy-dependent diffusion coefficient (kappa) is presented. This model is consistent with the observed spectra if kappa is proportional to the square root of the kinetic energy. The observed flattening of the secondary-to-primary ratio below about 1-2 GeV per nucleon is shown to be a natural consequence of the model. Similarly, the primary electron spectrum is predicted to flatten below about 1-2 GeV, which is consistent with deconvolution of the local nonthermal radio spectrum, although the flattening is not quite as much as indicated by the observations.

Jokipii, J. R.↗
NASA NTRS1978

Nuclear gamma rays from compact objects

Accreting compact objects may be important gamma ray line sources and may explain recent observations of celestial gamma-ray line emission from a transient source in the direction of the galactic anti-center, from the galactic center, and possibly from the radio galaxy Centaurus A. The identification of the lines from the transient source requires a strong redshift. Such a redshift permits the identification of these lines with the most intense nuclear emission lines expected in nature, positron annihilation, and neutron capture on hydrogen and iron. Their production as a result of nuclear interactions in accreting gas around a neutron star is proposed. The gamma-ray line emission from the galactic center and possibly Centaurus A appears to have a surprisingly high luminosity, amounting to perhaps as much as 10% of the total luminosity of these sources. Such high gamma-ray line emission efficiencies could result from nuclear interactions in accreting gas around a massive black hole.

Lingenfelter, R. E.↗