Showing papers by "A. C. Fabian published in 1983"

Spectral and variability constraints on compact sources

Abstract: Les spectres des sources compactes qui rayonnent au-dessus de 511 keV peuvent etre modifies par les paires electron-positron crees par les collisions photon-photon. L'opacite Thomson due aux paires peut facilement depasser l'unite dans des sources comme les noyaux galactiques actifs. La coupure spectrale aux hautes energies est inversement reliee a la luminosite de la source. On discute des contraintes generales sur la variabilite impliquant l'efficacite de la conversion masse au repos-energie, la geometrie et les effets des paires. Les corrections relativistes peuvent etre grandes, meme dans une source isotrope

Do galactic bulge X-ray sources evolve into millisecond pulsars?

Abstract: The recently discovered1 pulsar PSR1937 + 21 in 4C21.53 is remarkable for the shortness of its period (P = 1.5 ms) and for the length2 of its characteristic age, τP = P/2Ṗ ≳ 105 yr. The simple explanation that this is an ordinary supernova-formed pulsar but with an unusually weak magnetic field and hence weak magnetic dipole radiation (ref. 3), that is, B≊2×1010(τP/105 yr)−1/2(P/1.5 ms) G fits in with the position of the pulsar close to the galactic plane. We suggest here an alternative origin for the pulsar as being the remnant of one of the ‘galactic bulge’ X-ray binary stars.

Radiative accretion of intracluster gas onto dominant galaxies in poor clusters

Abstract: An analysis of X-ray images of MKW 4, MKW 3, AMW 4, and AMW 7 indicates that the hot gas in poor clusters is cooling and accreting onto the central, dominant galaxies. The deprojection technique of Fabian et al (1980) is applied to deduce the detailed density and temperature profiles for a wide range of assumed parameters. For each cluster, the mass accretion rates computed vary by less than a factor of 2 over the full range of the assumed parameters. Examinations of the gas and light density profiles, and of the effects of contraction of the gravitational potential, lead to the conclusion that the high gas densities giving rise to cooling flows are not primarily the result of dynamical evolution of the cluster, especially the merger of galaxies to form the dominant galaxy. Instead, the general initial condition of high material density favors both mergers and cooling flows.

X-ray observations of the southern cluster CA 0340 – 538 and the Horologium supercluster

Abstract: X-ray observations of the Horologium supercluster are examined. Results show that the optically Coma-like cluster CA 0340-538 has a smooth X-ray structure which also resembles that of Coma. The density structure of its X-ray emitting gas, determined in a model-independent way, is found to be remarkably featureless, being consistent with a nearly isothermal polytrope out to approximately 2 Mpc from the cluster center. The total mass of the gas is determined to exceed 10% of the cluster virial mass and may contribute a substantial fraction of the cluster binding mass. Although CA 0329-527 is found to be an X-ray double cluster, its structure appears to be confused. CA 0325-539 is found to show no substantial X-ray emission. An upper limit of 1.3 x 10 to the 44th erg/s is determined for any diffuse 2-10 keV emission from the Horologium supercluster.