Engineering topics

Fabian, A. C.

Publications and source records attributed to Fabian, A. C..

At least 73 records · Page 4

NASA NTRS1990

Turbulent mixing layers in the interstellar and intracluster medium

Cold clouds embedded in hot turbulent gas are often observed in the interstellar medium of galaxies and in the intracluster medium. Mixing layers develop around the clouds by the action of the turbulent gas. It is shown that their column density is limited by cooling and their mean temperature by the momentum flux. Provided that mixing is efficient, the mean temperature is the geometric mean of the hot and cold gas temperatures. EUV radiation from mixing layers around filaments in cluster cooling flows photoionizes the colder gas and may produce the commonly observed optical and UV emission lines. Mixing layers around cold galactic clouds can produce absorption by O VI and similar species with a column density consistent with observations.

Begelman, Mitchell C.↗

NASA NTRS1990

Simultaneous multifrequency observations of the Seyfert 1 galaxy NGC 4051 - Constant optical-infrared emission observed during large-amplitude X-ray variability

Simultaneous infrared, optical, ultraviolet, and X-ray observations of the low-luminosity Seyfert 1 galaxy, NGC 4051 are reported. A new method of reduction was developed to correct the optical flux for atmospheric variations due to seeing and extinction. The X-ray flux varied by factors of up to 2 on time-scales of tens of minutes, while the optical flux remained steady to within 1 percent. The results rule out all models, including Compton scattering and synchrotron models, in which a single electron population is responsible for the formation of both the infrared-to-optical and the X-ray spectra. The optical emission region must be an order of magnitude larger than, or completely separate from, the X-ray source.

Done, C.↗

NASA NTRS1989

Radio-quiet QSO and the X-ray background

Upper limits on the extragalactic soft X-ray background flux (epsilon of about 0.1-0.3 keV) are used to constrain the contribution of X-ray sources with steep spectra to the X-ray background above 1 keV. The effective spectral index of the extragalactic X-ray background is less than 0.76 over the energy range 0.2 and 3.5 keV. Objects with power-law spectra of energy index 1.4, as recently found for a sample of radio-quiet QSO, cannot contribute more than 13 percent of the 2-10 keV X-ray background.

Fabian, A. C.↗

NASA NTRS1988

Do massive black holes reside in elliptical galaxies?

The accretion by a central black hole of the hot interstellar medium in an elliptical galaxy is investigated, and the minimum expected luminosity and manner of its emission is estimated. It is not obviously detected at any wavelength. The problem of 'starving the monster', if indeed there is a monster, is raised. The simplest conclusion from the evidence is that most bright elliptical galaxies do not contain massive black holes.

Fabian, A. C.↗

NASA NTRS1987

X-ray observations of the Ophiuchus, PKS 0745-191 and Cygnus-A clusters of galaxies

Observations are reported of the X-ray emission from three bright clusters of galaxies at low Galactic latitudes: the Ophiuchus, PKS 0745-191, and Cygnus-A clusters. Temperatures and iron abundances are determined from Exosat spectral data. Images of the Ophiuchus cluster show it to have a high central density and a cooling flow. Measurements of a high gas density around PKS 0745-191 and are confirmed, and it is shown that the X-ray emission is centered on the radio galaxy. The PKS 0745-191 cluster is the most distant object for which an abundance has been measured from X-ray spectra. The spectrum of Cygnus-A contains a highly absorbed power-law component that is identified with the nucleus of the radio source.

Arnaud, K. A.↗

NASA NTRS1987

The MIT spectroscopy investigations on AXAF and the study of supernova remnants

The MIT High Resolution X-Ray Spectroscopy experiment on the AXAF, which will study physical conditions in celestial sources by means of detailed measurements of emission and absorption features in their spectra, involves two complementary dispersive instruments: Bragg Crystal Spectrometer (BCS) and High Energy Transmission Grating (HETG). This paper discusses the principles of operation of BCS and HETG and the results that will be obtained by these instruments. Measurements of individual line strengths obtained by the AXAF spectrometers will allow the application of plasma diagnostic techniques to a study of the detailed physical conditions in celestial objects, particularly in the optically thin plasma of supernova remnants, which is particularly well suited to the application of plasma diagnostics.

Canizares, C. R.↗

NASA NTRS1986

The detection of distant cooling flows and the formation of dark matter

Cooling flows involving substantial mass inflow rates appear to be common in many nearby rich and poor clusters and in isolated galaxies. The extensive optical and ultraviolet filaments produced by the thermal instability of large flows are detectable out to redshifts greater than 1. It is proposed that this may explain the extended optical line emission reported in, and around, many distant radio galaxies, narrow-line quasars, and even nearby normal and active galaxies. An important diagnostic to distinguish cooling flows from other possible origins of emission line filaments is the presence of extensive regions at high thermal pressure. Other evidence for distant cooling flows and the resultant star formation is further discussed, together with the implications of cooling flow initial-mass functions for galaxy formation and the nature of 'dark' matter.

Fabian, A. C.↗

NASA NTRS1985

An X-ray, optical and radio study of PKS 0745-191 - A massive cooling flow

The X-ray source 3A 0745-191 is identified with intracluster gas surrounding the giant galaxy containing the radio source, PKS 0745-191, which lies at z = 0.1. The gas is cooling at the unprecedented rate of 1000 solar mass/yr. Optical line-emitting gas extends more than 10 kpc from the centre of the galaxy and corroborates the high mass-flow rate. The optical spectrum is similar to that of distant 3CR radio galaxies reported by others. It is concluded that formation of a giant galaxy was observed.

Fabian, A. C.↗

NASA NTRS1985

O VIII resonant absorption in PKS 2155 - 304 - A hot wind

The broad X-ray absorption feature in PKS 2155 - 304 could be due to O VIII Ly-alpha resonant absorption in a wind with outflow speeds up to about 30,000 km/s. The properties of such a wind are calculated. A homogeneous, spherically symmetric wind would involve a mass flux of about a 1000 solar masses/year, but this estimate could be greatly reduced if the outflow were beamed, and/or the absorbing gas were clumped into small clouds. In the latter case, the O VIII absorption offers clues to the physical state of thermal material closer to the central energy source than the clouds which produce optical emission lines in quasars.

Krolik, J. H.↗

NASA NTRS1984

The prevalence of cooling flows in clusters of galaxies

The central density and temperature of the intracluster gas in 36 clusters of galaxies have been determined by deprojection of the X-ray surface brightness distribution. The cooling time of the gas in half of these clusters is less than a Hubble time (50 km/s per Mpc). For these clusters mass inflow rates between 2 and 400 solar mass per year are derived. These cooling flows may provide a significant fraction of the mass of the central cD galaxies.

Stewart, G. C.↗

NASA NTRS1984

Cooling flows in clusters of galaxies

Cooling gas in the center of many clusters of galaxies is compressed and pushed inward by the thermal pressure of the hot, uncooled outer gas. Gravity acts as a focusing agent until the gas has cooled to galactic temperatures. The cooling process is thermally unstable, giving rise to optical filamentation. Once the gas cools below 10.000 K it presumably forms stars which add to the mass of a central galaxy. Accretion rates of up to 400 solar masses/yr are inferred from X-ray measurements so that the mass of such central galaxies may be due to cooling flows. Their optical and UV appearance emphasize that the gas predominantly turns into low-mass stars with an initial mass function unlike that of the solar neighborhood. Optical and X-ray observations of cooling flows can be used to investigate the continuing formation of the largest known galaxies.

Fabian, A. C.↗

NASA NTRS1984

Models of the hard X-ray spectrum of AM Herculis and implications for the accretion rate

Phenomenological fits to the hard X-ray spectrum of AM Hercules have failed to explain the high equivalent width (0.8 + or - 0.1 keV) of Fe K-alpha emission. The scattering and fluorescent line production in a cold or partially ionized accretion column of hard X-rays emitted at the base are investigated using the Monte Carlo method. It is shown that the Fe emission equivalent width observed in the spectrum of AM Her can be explained without resorting to an overabundance of Fe in the accreting material. For sufficient optical depth across the accretion column, equivalent widths of K-alpha emission even larger than the observed values can be obtained. The accretion rate and gravitational power can be deduced from the optical depth across the column, if the column size is known, and, together with the observed hard X-ray and polarized light luminosities, imply a lower limit for the luminosity in the UV to soft X-ray range, for which the observations give model-dependent values.

Swank, J. H.↗

NASA NTRS1984

X-ray spectral signatures of accreting black holes

The X-ray properties of the binary black hole candidates include distinctive bimodal spectral behavior where the X-ray spectra switch between being very soft (alpha greater than 3) and hard (alpha about 0.7). It is pointed out that this spectral transition occurs at about 1 percent L(edd) (for accretion onto a 10 solar masses black hole), and that this corresponds to the luminosity where an optically thick electron-positron pair plasma will form. Active galactic nuclei (AGN) are often considered to be accreting black holes. An X-ray spectral similarity between the above binary sources and both the Seyfert I and BL Lac classes of AGN is demonstrated. As many of these objects radiate most of their luminosity as X-rays, such a similarity may be expected if the underlying mechanisms are similar.

White, N. E.↗

NASA NTRS1984

The mass profile and gas content of M87

X-ray images and spectroscopic observations, in particular of the Fe L lines, are used to constrain the structure of the hot gas and gravitating matter around M87 with respect to allowable temperature gradient, and therefore gravitational potential, over the 1-10 arcmin radius range. Models in which the gravitating mass has a core radius of about 25 kpc are favored, suggesting that the material is associated with M87 rather than with the Virgo cluster as a whole. The gas temperature is noted to vary slowly beyond 5 arcmin, while the variation in the inner regions is not consistent with thermal conduction models for powering the central X-ray emission.

Stewart, G. C.↗

NASA NTRS1983

Models of the hard X-ray spectrum of AM Herculis and implications for the accretion rate

Phenomenological fits to the hard X-ray spectrum of AM Herculis left unexplained the high equivalent width (0.8 + or - 0.1 keV) of Fe K alpha emission. A purely thermal origin implies a much steeper spectrum than was observed. With Monte Carlo calculations, scattering and fluorescent line production in a cold or partially ionized accretion column of hard X-rays emitted at the base were investigated. The strength of the iron emission and the flat spectral continuum can be explained by the effects of fluorescence and absorption within the accretion column and the surface of the white dwarf on a thermal X-ray spectrum. Thomson optical depths across the column in the range 0.2 to 0.7 are acceptable. The accretion rate and gravitational power can be deduced from the optical depth across the column, if the column size is known, and, together with the observed hard X-ray and polarized light luminosities, imply a lower limit for the luminosity in the UV to soft X-ray range, for which the observations give model-dependent values. Estimates of the column size differ by a factor of 40. Small spot sizes and low luminosities would be consistent with the soft component being the expected reprocessed bremsstrahlung and cyclotron radiation, although the constraint of matching the spectrum confines one to solutions with fluxes exceeding 20% the Eddington limits.

Swank, J. H.↗

NASA NTRS1983

Radiative accretion of intracluster gas onto dominant galaxies in poor clusters

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.

Stewart, G. C.↗

NASA NTRS1983

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

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.

Ku, W. H.-M.↗

NASA NTRS1983

The (an)isotropy of the X-ray sky

An assessment is made of the extent to which the study of the isotropy of the X-ray sky has contributed to the present understanding of the structure of the universe at moderate redshifts. It is, of course, the anisotropic character of the sky flux that is valuable in this context. Although it is not currently possible to undertake measurements with the precision and small solid angles that are typically achieved in the microwave range, the comparatively crude limits from the X-ray fluctuations place limits on the largest scale structure of the universe. After indicating the nature of measurements made, with the HEAO 1 A-2 experiment, of the X-ray sky and its anisotropies, it is shown how these place limits on the origin of the X-ray sky and on any large scale structure of the universe.

Shafer, R. A.↗