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NASA NTRS1979

HEAO A-2 extragalactic results

Results from the HEAO A-2 experiment concerning the extragalactic X-ray sky are discussed. All-sky maps of hard X-ray sources detected are presented reflecting the temporal variability of the sources that make up the extragalactic X-ray sky. Most of the identified extragalactic objects are not clusters of galaxies but isolated galaxies. Spectra for sources representing classes of active galz galaxies, BL Lacertae objects, and N galaxies are discussed in order of luminosity.

Boldt, E. A.↗
NASA NTRS1974

Interstellar and intergalactic matter and radiation

A brief summary of recent studies of the interstellar medium is given. Most attention is then devoted to a review of the evidence for the presence of intergalactic matter and radiation in the universe. It is concluded that the only important constituents which may make a sizable contribution to the total mass-energy are intergalactic gas and condensed objects with a very high mass-to-light ratio. If the QSOs are not at cosmological distances, cold atomic hydrogen may still be the most important constituent and may contribute much more mass than do the galaxies. The X-ray observations still do not unambiguously show that very hot gas is present, though it is very likely on general grounds that some hot gas is present in clusters of galaxies. The question of whether or not large amounts of matter, enough to close the universe, are present, remains unsettled. From the theoretical standpoint the answer depends almost completely on the approach taken to the problem of galaxy formation and to the cosmological model which is favored.

Burbidge, G. R.↗
NASA NTRS1974

Intergalactic matter and radiation and its bearing on galaxy formation and evolution

An up-dated review is given of the evidence for the presence of intergalactic matter and radiation in the Universe. It is concluded that the only important constituents which may make a sizable contribution to the total mass-energy are intergalactic gas and condensed objects with a very high mass-to-light ratio. If the QSOs are not at cosmological distances, cold atomic hydrogen may still be the most important constituent and may contribute much more mass than do the galaxies. The X-ray observations still do not unambiguously show that very hot gas is present, though it is very likely on general grounds that some hot gas is present in clusters of galaxies. The question of whether or not large amounts of matter, enough to close the Universe, are present, remains unsettled. From the theoretical standpoint the answer depends almost completely on the approach taken to the problem of galaxy formation and to the cosmological model which is favored.

Burbidge, G. R.↗
NASA NTRS1979

Faint extragalactic objects and the unidentified high latitude X-ray sources

Classes of extragalactic objects that have been identified with faint high-galactic-latitude X-ray sources in the Fourth Uhuru and Ariel 5 Catalogs are examined. Sources with declinations of at least -30 deg, galactic latitudes of at least 20 deg (absolute value), and error boxes of 1.0 sq deg are considered. It is shown that clusters of galaxies constitute the most numerous class of extragalactic sources and that Seyfert galaxies are the next largest class. Unidentified high-latitude X-ray sources are discussed. Preliminary results obtained with the A-1 instrument aboard HEAO 1 are reported which indicate that a substantial fraction of unidentified sources are highly variable and may be identified with active galaxies.

Shulman, S.↗
NASA NTRS1975

On the equilibrium distribution of gas in clusters of galaxies

The hydrostatic-equilibrium solution for a polytropic gas in a cluster of galaxies has been calculated. The gas has a wider distribution than the galaxies. If the central gas density is 0.0008 particles/cu cm for gamma = 5/3 or 0.002 per cu cm for gamma = 1.35, the total X-ray emission from a model Coma cluster is equal to the observed emission. The spatial extent and spectrum for the source are also well-represented by the model if gamma = 1.35.

Lea, S. M.↗
NASA NTRS1979

High-resolution X-ray spectroscopy of M87 with the Einstein observatory - The detection of an O VIII emission line

The paper deals with high-resolution X-ray spectroscopy performed to study the extended source surrounding the giant elliptical galaxy, M87, in the Virgo cluster. From observations carried out with a focal plane crystal spectrometer, L-alpha emission was detected from hydrogenic oxygen (O VIII). Upper limits could be set on lines from intermediate ionization states of iron. The presence of a quantity of cooler matter surrounding M87 was revealed, which has important implications for cluster models and favors a radiatively controlled accretion mechanism.

Canizares, C. R.↗
NASA NTRS1975

The SAS-3 X-ray observatory

The experiment section of the Small Astronomy Satellite-3 (SAS-3) launched in May 1975 is an X-ray observatory intended to determine the location of bright X-ray sources to an accuracy of 15 arc-seconds; to study a selected set of sources over a wide energy range, from 0.1 to 55 keV, while performing very specific measurements of the spectra and time variability of known X-ray sources; and to monitor the sky continuously for X-ray novae, flares, and unexpected phenomena. The improvements in SAS-3 spacecraft include a clock accurate to 1 part in 10 billion, rotatable solar panels, a programmable data format, and improved nutation damper, a delayed command system, improved magnetic trim and azimuth control systems. These improvements enable SAS-3 to perform three-axis stabilized observations of any point on the celestial sphere at any time of the year. The description of the experiment section and the SAS-3 operation is followed by a synopsis of scientific results obtained from the observations of X-ray sources, such as Vela X-1 (supposed to be an accreting neutron star), a transient source of hard X-ray (less than 36 min in duration) detected by SAS-3, the Crab Nebula pulsar, the Perseus cluster of galaxies, and the Vela supernova remnant.

Mayer, W. F.↗
NASA NTRS1976

An ultrasoft X-ray source in Coma Berenices

An intense soft X-ray source with an extraordinary spectrum has been observed in Coma Berenices, 4 deg northeast of and not associated with the Coma cluster of galaxies. Two spectra, obtained at different times in a sounding-rocket flight, indicate that the source temperature in thermal models is less than 1 million K; a power-law model requires photon power-law indices steeper than n = -3. The intensity in the 44-165 A band is of the order of 5 by 10 to the -10th power erg/sq cm/sec, but no flux is present at energies 0.3-2.1 keV to a limit of 1 by 10 to the -10th power erg/sq cm/sec. The lack of bright stars or a supernova remnant in the error box implies that this may be a new class of soft X-ray sources.

Margon, B.↗
NASA NTRS1973

Thermal-bremsstrahlung interpretation of cluster X-ray sources.

Analysis of the X-ray data for the extended sources in Coma, Perseus, and Virgo in terms of an isothermal gas-sphere model emitting thermal-bremsstrahlung radiation. It is found that the gas distribution is somewhat less centrally condensed than the galaxies, and that the gas mass is generally a small fraction of the binding mass of each cluster. In the case of the Coma cluster, approximately one-eighth of the binding mass (with an uncertainty of about 50%) may be in the form of hot gas at 100,000,000 K if H sub zero = 50 km/sec/Mpc.

Lea, S. M.↗
NASA NTRS1975

Optical studies of 10 high galactic latitude X-ray sources

Optical studies are reported for a representative sample of 10 unidentified high galactic latitude X-ray sources that are north of declination minus 25 deg and that have 3U error boxes less than or of the order of 0.5 square degrees. The source 3U 1410-03 is identified with a peculiar emission-line spiral galaxy, NGC 5506, and a distance group 4 rich cluster of galaxies, Abell 2142, is suggested as a possible identification for 3U 1555+27. It is shown that very few of the unidentified high galactic latitude sources are quasi-stellar sources brighter than 17.5 mag, although one possible X-ray quasar, NAB 0137-01, is reported.

Bahcall, J. N.↗
NASA NTRS1972

Upper limits to the soft X-ray emission of sources in Virgo.

Discussion of the soft X-rays from Virgo observed with a scanning focusing collector from an Aerobee 170 rocket. The region included the center of the Virgo cluster of galaxies, a source of higher energies which had been previously identified with M87, and QSO 3C 273. No discrete sources in Virgo were observed.

Gorenstein, P.↗
NASA NTRS1978

Location of unidentified weak X-ray sources by the HEAO-1 scanning modulation collimator

Four weak sources for which precise positions have been obtained from the HEAO-1 Scanning Modulation Collimator (MC) experiment are discussed. One source (4U 0446+44) with the center of the cluster of galaxies containing 3C129 and 3C129.1 has been identified, and its angular size has been measured. One source with a galactic star is identified, although it is at galactic latitude b = 51 deg. The other two sources, 2A 0526-328 and 2A 0922-317, remain unidentified.

Schwartz, D. A.↗
NASA NTRS1978

Some problems with the interpretation of recent microwave background observations in the direction of galaxy clusters, or, beware of negative antenna temperatures

Recently reported observations of the 3-K microwave background in the direction of rich clusters of galaxies should be viewed as placing stringent limits on the mass of cooler ionized gas within the clusters, rather than as a verification of thermal bremsstrahlung models for cluster X-ray sources. At the high radio frequencies employed in the observations, there is a positive contribution to the observed source-brightness distributions from free-free emission by any cooler gas. This can overwhelm the anticipated inverse Compton diminution of the background radiation, even when the total mass in cooler gas is significantly less than the mass of hot plasma required to explain the X-ray source. Future experiments of this type should be conducted only when atmospheric stability is sufficient to permit lengthy drift scans across the clusters. Extreme care must be taken to remove the contributions from any discrete sources.

Tarter, j. C.↗
NASA NTRS1979

HEAO 1 scanning modulation collimator discovery of an extended X-ray source at Cygnus A

The HEAO 1 modulation-collimator data associate the X-ray emission from the Cygnus A region (4U 1957 + 40) with a small area including the radio galaxy. The source appears to be extended, with a size of about 2 arcmin; it may have a complex geometry. Any point-source component must have a flux no greater than 1.3 UFU. A lower limit of 1 microgauss is placed on the magnetic field in the radio lobes, based on the upper limit on the X-ray flux due to inverse Compton scattering of radio electrons on the 2.7-K microwave background radiation. The X-ray emission is more likely due to driven accretion of the intercluster gas onto the giant cD radio galaxy. Therefore this source appears to be similar to the Virgo and Perseus clusters. The density derived for the X-ray-emitting gas is too small for its thermal pressure to confine the radio source.

Fabbiano, G.↗
NASA NTRS1973

A study of galaxy groups and clusters - The case for a clumpy intergalactic medium.

A nonuniform model for a dense intergalactic medium is constructed under the assumption that all groups and clusters of galaxies are gravitationally bound by ionized gas. Catalogs by de Vaucouleurs and Abell are utilized to provide an almost complete sample of the spatial distribution of groups and clusters over a wide range of richness, and a distribution function is derived for galaxy groups and clusters as a function of velocity dispersion. A simple scaling law is applied to predict velocity dispersions for the very rich Abell clusters. Thermal bremsstrahlung emission from the intracluster gas accounts for the observed emission over 2 to 10 keV from several rich clusters, and also contributes up to 20% of the diffuse X-ray background over a considerable fraction of the observed range. The amount of X-ray emitting gas is restricted to a small fraction of the virial mass, with the remainder of the binding mass present as cooler ionized clouds. Available soft X-ray and ultraviolet diffuse background observations are used to define a narrow range of permissible temperatures and densities for these clouds.

Silk, J.↗
NASA NTRS1972

Extragalactic cosmic rays.

A detailed analysis is given of the possibility that a large part of the primary cosmic-ray flux is of extragalactic origin. The problems encountered in theories of galactic cosmic rays are first critically discussed. The objections that have previously been raised to extragalactic theories, particularly on energetic grounds, are then examined. The general problem of the propagation of cosmic rays which are ejected from galactic nuclei or in galactic winds to fill clusters and superclusters is dealt with next. Attention is also given to the limits which are set on the presence of extragalactic cosmic rays by observations of high-energy electrons and background X-rays and gamma rays, or the heating of a hypothetical intergalactic gas. Observable effects of cosmic rays between galaxies are studied, and the possible sources of the local cosmic-ray protons in the extragalactic model are considered.

Brecher, K.↗