Engineering topics

Ayasli, S.

Publications and source records attributed to Ayasli, S..

5 records

NASA NTRS1983

Discovery of 9.3 s X-ray pulsations from 2S 1553-542 and a determination of the orbit

X-ray pulsations with a period of 9.3 s from the transient galactic X-ray source 2S 1553-542 were detected. The data were obtained with SAS 3 during the only known outburst of the source in 1975 June. Pulse-timing analyses of the data, which span an interval of 20 days, show that there were large changes in the pulse period that are readily attributable to binary orbital motion. The inferred orbital period is 30.6 + or - 2.2 days and the X-ray mass function is 5.0 + or - 2.1 solar masses. These orbital parameters, the transient nature of the source, and the probable lack of an early-type giant companion star, lead to the tentative conclusion that 2S 1553-542 is a Be/X-ray binary system.

Kelley, R. L.↗

NASA NTRS1982

Thermonuclear processes on accreting neutron stars - A systematic study

A series of model calculations for the evolution of the surface layers of an accreting neutron star is carried out. The neutron star mass, radius, core temperature, and surface magnetic field strength are systematically varied, as are the accretion rate onto the neutron star surface and the metallicity of the accreting matter, in order to determine the effects of these parameters on the properties of thermonuclear flashes in the surface layers and the emitted X-ray bursts that result from such flashes. The core temperatures required for thermal equilibrium are found to be approximately a factor of 2 lower than estimated in earlier work. Owing to the effects of the gravitational redshift, the emitted X-ray bursts have lower peak luminosities and longer durations than those calculated in the Newtonian approximation. The entrainment of hydrogen into helium flashes can cause the flashes to exhibit a rather wide range of observable effects and can decrease by a factor of more than 2 the ratio of persistent accretion-driven luminosity to time-averaged burst luminosity emitted by the neutron star.

Ayasli, S.↗

NASA NTRS1981

The gamma-ray spectra of radio pulsars

A general expression for the gamma-ray spectra of pulsars is derived for models where the radiation is produced by particles accelerated along the open field lines of a neutron star. The accelerating potentials are assumed to be limited by pair production discharges and to decrease linearly with increasing number of electron-positron pairs created during these discharge processes. The corresponding energy distribution of particles is derived both for the case of strong radiation reaction and for that of no radiation reaction. The resultant formula for the gamma-ray spectrum fits well to the Crab and the Vela pulsar spectra assuming that radiation reaction is negligible in these two fast pulsars.

Ayasli, S.↗

NASA NTRS1977

Gamma ray pulsars

Data from the SAS-2 high-energy gamma-ray experiment reveal the existence of four pulsars emitting photons above 35 MeV. An attempt is made to explain the gamma-ray emission from these pulsars in terms of an electron-photon cascade that develops in the magnetosphere of the pulsar. Although there is very little material above the surface of the pulsar, the very intense magnetic fields (10 to the 12th power gauss) correspond to many radiation lengths which cause electrons to emit photons by magnetic bremsstrahlung and which cause these photons to pair-produce. The cascade develops until the mean photon energy drops below the pair-production threshold which is in the gamma-ray range; at this stage, the photons break out from the source.

Oegelman, H.↗

NASA NTRS1976

Gamma ray pulsars

Recent data from the high energy gamma ray experiment have revealed the existence of four pulsars emitting photons above 35 MeV. An attempt is made to explain the gamma ray emission from these pulsars in terms of an electron-photon cascade that develops in the magnetosphere of the pulsar. Although there is very little material above the surface of the pulsar, the very intense magnetic fields correspond to many radiation lengths which cause electrons to emit photons via magnetic bremsstrahlung and these photons to pair produce. The cascade develops until the mean photon energy drops below the pair production threshold which happens to be in the gamma ray range; at this stage the photons break out from the source.

Oegelman, H.↗