Engineering topics

Skibo, J. G.

Publications and source records attributed to Skibo, J. G..

5 records

NASA NTRS1997

The source and distribution of Galactic positrons

The oriented scintillation spectrometer experiment (OSSE) observations of the Galactic plane and the Galactic center region were combined with observations acquired with other instruments in order to produce a map of the Galactic 511 keV annihilation radiation. Two mapping techniques were applied to the data: the maximum entropy method, and the basis pursuit inversion method. The resulting maps are qualitatively similar and show evidence for a central bulge and a weak galactic disk component. The weak disk is consistent with that expected from positrons produced by the decay of radioactive Al-26 in the interstellar medium. Both maps suggest an enhanced region of emission near l = -4 deg, b = 7 deg, with a flux of approximately 50 percent of that of the bulge. The existence of this emission appears significant, although the location is not well determined. The source of this enhanced emission is presently unknown.

Purcell, W. R.↗

NASA NTRS1994

Compton scattering in jets: A mechanism for approximately 0.4 and less than or approximately 0.2 MeV line production

We show that gamma-ray line emission at approximately 0.4 and less than or approximately 0.2 MeV can be produced by Compton scattering of beamed radiation in the jets of Galactic black hole candidates. This mechanism has the novel feature of not invoking the presence of e(exp +)-e(exp -) pairs. To produce the two lines, we employ a symmetric double-sided jet with bulk flow velocity of about 0.5c and incident beam radiation with a hard energy spectrum. We show that the two lines can be seen at viewing-angle cosines relative to the jet ranging from 0.2 to 0.6. This comprises 40% of the total solid angle. In addition, the line radiation is approximately 10% polarized. Depending on the bulk flow and viewing angle, the model can produce lines at other energies as well. In particular, a broad feature near 1 MeV can be seen by viewing the jet close to its axis. Our model can also accommodate single-line spectra if the beamed gamma-ray emission or the jets themselves are asymmetric.

Skibo, J. G.↗

NASA NTRS1994

Galactic 0.511 MeV line emission

Observations over two decades with various balloon-borne instruments and instruments on HEAO 3, SMM, and Compton Gamma Ray Observatory (CGRO) have revealed the existence of strong 0.511 MeV line emission from the direction of the Galactic center. We have attempted to fit the spatial distribution of this emisssion with distributions of various Galactic constituents, but found that none of these distributions alone could fit the data. Of the assumed distributions, those of Galactic novae and hot (10(exp 8) K) plasma are the most strongly peaked at the Galactic center. But we found that the 0.511 MeV line emission is even more strongly peaked. This new result is the direct consequence of the analysis of recent data obtained with the OSSE instrument on CGRO. We have thus considered two-component models consisting of an enhanced central Galactic component superposed on a Galactic plane component given by any one of our previously assumend distributions. For the central Galactic component we take either a single point source at the location of the source of annihilation radiation 1E 1740.7-2942 or an extended spheroidal distribution of several hundred parsecs in size. We find acceptable fits in both cases. We suggest that this additional component of the emission from the Galactic center region is due to positrons produced near one or more black holes. Such a component was also suggested by the variability of the 0.511 MeV line flux observed with balloon-borne high-resolution detectors. To produce the very narrow line observed by these high-resolution detectors at almost precisely 0.511 MeV the positrons must escape from the vicinity of the holes and annihilate in the surrounding interstellar medium. We have reexamined the issure of the time variability of the 0.511 MeV line emission. Although the significance of the variability implied by all of the earlier observations is diminished by a large number of recent OSSE observations which show no variability, the latter measurements merely indicate that the flux remained relatively constant only over a period of about 1 year (1991-1992). Positron producation in the Galactic center region by a small number of variable sources would lead to variable 0.511 MeV line emission. The timescale of the variation depends on the density of the gas in which the positrons annihilate. It could be as short as several months if the annihilation is in dense moleduar clouds, but would be much longer and essentially undetectable if the positrons annihilate in lower density interstellar gas.

Ramaty, R.↗

NASA NTRS1993

Diffuse Galactic low energy gamma ray continuum emission

We investigate the origin of diffuse low-energy Galactic gamma-ray continuum down to about 30 keV. We calculate gamma-ray emission via bremsstrahlung and inverse Compton scattering by propagating an unbroken electron power law injection spectrum and employing a Galactic emmissivity model derived from COSB observations. To maintain the low energy electron population capable of producing the observed continuum via bremsstrahlung, a total power input of 4 x 10 exp 41 erg/s is required. This exceeds the total power supplied to the nuclear cosmic rays by about an order of magnitude.

Skibo, J. G.↗

NASA NTRS1992

Diffuse 0.511 MeV line emission and the distribution of positron annihilation in the galaxy

We model the distribution in the Galaxy of 0.511 MeV line emission and the implied positron annihilation using plausible distributions based on observations at other photon energies. We use 0.511 MeV line observations from the Galactic center and from directions away from the Galactic center to normalize the two-dimensional 0.511 MeV sky maps. We find that in order to understand all of the available data it is necessary to invoke the presence of a time-variable component of 0.511 MeV line emission in addition to an underlying diffuse component. The large 0.511 MeV fluxes observed with broad field of view detectors, such as the SMM gamma-ray spectrometer, can be reconciled with the small fluxes seen with narrow field-of-view instruments (GRIS and OSSE) from directions away from the Galactic center if the diffuse emission follows the distribution of Galactic novae for which the recently detected 0.511 MeV line emission from the Galactic center with OSSE is predominantly of diffuse origin. For future GRIS and HEXAGONE observations from the direction of the Galactic center we predict a minimum 0.511 MeV line flux of (6.0 +/- 0.9) x 10 exp -4 photons/sq cm/s.

Skibo, J. G.↗