Engineering topics

Melia, F.

Publications and source records attributed to Melia, F..

5 records

NASA NTRS1989

The probability of detecting absorption features in gamma-ray burst spectra

It has recently been suggested that several mechanisms for the primary release of energy in gamma-ray bursts may result in the excitation of magnetospheric plasma oscillations (MPO) above the polar cap of a neutron star. The basic MPO idea is refined by determining the soft photon source (which damps the particle motion via inverse Compton scattering interactions) self-consistently by means of a Monte Carlo simulation for the deposition of back-scattered gamma-ray energy below the stellar photosphere. The observed fraction of bursts displaying low-energy absorption features may be understood in the context of a model wherein the overall spectrum changes with inclination angle due to the superposition of several components with different angular distributions. GRB spectra should display these cyclotron lines about 18 percent of the time, which is consistent with the KONUS and Ginga sets of data.

Melia, F.↗

NASA NTRS1988

Evolution of magnetic cataclysmic binaries

The evolution of magnetic cataclysmic binaries is reviewed, with emphasis on the synchronization process by which DQ Herculis stars become AM Herculis stars. The various mechanisms that are thought to drive the evolution of cataclysmic binaries are discussed, and the criterion for stream versus disk accretion, the physics of the accretion and synchronization torques, and the conditions required for synchronization are described. The different physical regimes to which magnetic cataclysmic binaries belong are summarized, and how synchronization may be achieved, and how it may be broken, are considered.

Lamb, Don Q.↗

NASA NTRS1987

Evolution of magnetic cataclysmic binaries

The physics of magnetic cataclysmic binary evolution is considered, and it is suggested that DQ Her stars will not evolve into AM Her stars (assuming that mu1 remains constant throughout their evolution), in contradiction to previous assertions. Four regimes that depend on mu1 values are identified, and it is suggested that the mu1 values of the known DQ Her stars are significantly less than those of the known AM Her stars (unless all known stars of both groups have M1 greater than 1 solar mass). A new way of producing ultrashort period binaries is proposed in which synchronization of the magnetic degenerate dwarfs injects angular momentum into the binary, driving the system apart and producing a synchronization-induced period gap. The secondaries in these binaries are shown to be hydrogen-rich rather than hydrogen depleted or pure helium.

Lamb, D. Q.↗

NASA NTRS1986

Optical reprocessing of gamma-ray bursts

One model for the optical flashes associated with three cosmic gamma-ray burst sources invokes the reprocessing of some of the gamma-radiation emitted by a hypothesized collapsed object in the surface layers of a nearby companion star. This model was investigated by carrying out detail, fully hydrodynamical calculations of such reprocessing in the surface layers of very low mass stars. It is found that, at most, 7 percent of the gamma-ray fluence incident on the companion star is reprocessed into the blue band; the time scale for this reprocessing is typically 100 s, which is long compared to the duration of the gamma-ray burst itself. Using this result, it is shown that there is marginal agreement between the observed and calculated ratios of gamma-ray fluence to optical fluence at earth.

Melia, F.↗

NASA NTRS1985

The wind-disk interaction in X-ray burst sources

The interaction between a radiatively driven wind from the surface of a neutron star and an accretion disk surrounding the star is considered. A bow shock forms if the wind is supersonic when it reaches the inner edge of the disk. The resultant heating of the wind material can enhance the X-ray luminosity of the source, but this effect is important only if the wind is very strong. If the terminal velocity of the wind is at least about 0.99c and the integrated kinetic energy flux of the wind is at least about 30 times the Eddington limit, then super-Eddington X-ray luminosities will be generated. The conditions that are required for the formation of such a wind are briefly discussed. Also developed are estimates for the spectral shape, color temperature, and X-ray luminosity resulting from the wind-disk interaction, and it is shown that these results are consistent with the observed properties of type I cosmic X-ray bursts.

Melia, F.↗