E. A. Boldt

A complete X-ray sample of the high-latitude /absolute value of b greater than 20 deg/ sky from HEAO 1 A-2 - Log N-log S and luminosity functions

The HEAO 2 experiment A-2 has performed a complete x-ray survey of the 8.2 sr of the sky at Vertical BargVertical Bar by 20/sup 0/ down to a limiting sensitivity of approx.3.1 x 10/sup hyphen11/ ergs cm/sup -2/ s/sup -1/ in the 2-10 keV band. Of the 85 detected sources (excluding the LMC and SMC sources), have been identified with galactic objects, 61 have been identified with extragalactic objects, and 7 remain unidentified. The log N-log S relation for the nongalactic objects is well fitted by the Euclidean relationship. We have used the x-ray spectra of these objects to construct the log N-log S relation in physical units. The complete sample of identified sources have been used to construct x-ray luminosity functions, using the absolute maximum likelihood method, for clusters of galaxies and active galactic nuclei.

The diffuse X-ray background spectrum from 3 to 50 keV

The spectrum of the extragalactic diffuse X-ray background has been measured with the GSFC Cosmic X-Ray Experiment on HEAO 1 for regions of the sky away from known point sources and more than 20 /sup 0/ from the galactic plane. A total exposure of 80 m/sup 2/-s-sr is available at present. Free-free emission from an optically thin plasma of 40 +- 5 keV provides an excellent description of the observed spectrum from 3 to 50 keV. This spectral shape is confirmed by measurements from five separate layers of three independent detectors. With an estimated absolute precision of approx.10%, the intensity of the emission at 10 keV is 3.2 keV keV/sup -1/ cm/sup -2/ s/sup -1/ sr/sup -1/, a value consistent with the average of previously reported spectra. No other spectral features, such as iron line emission, are evident. This spectrum is not typical of known extra-galactic objects. A uniform hot intergalactic medium of approximately 36% of the closure density of the universe would produce such a flux, although nonuniform models indicating less total matter are probably more realistic.

The cosmic x-ray background

Abstract As the sky in the microwave band is dominated by a cosmic background, so too is the X-ray sky. In this report, the experimental approach used for measuring the X-ray background is explained and evaluated. The Compton-Getting interpretation of the dipole anisotropy in the microwave background is presented as a diagnostic of the weak asymmetry exhibited by the cosmic X-ray background. Spectral characteristics and spatial fluctuations of this X-ray background are described and then discussed within the context of what is known about individual extragalactic sources. It is concluded that the bulk of the cosmic X-ray background is yet to be understood. The critical apparatus of modern cosmology is reviewed and applied to this problem, providing constraints and indicating possible avenues for achieving a solution. The outlook for obtaining the new data called for is examined in terms of experiments now in preparation.

HEAO 1 measurements of the galactic ridge

The HEAO A2 experiment data was systematically searched for unresolved galactic disc emission. Although there were suggestions of non-uniformities in the emission, the data were consistent with a disc of half-thickness 241 + 22 pc and surface emissivity (2-10 keV) at galactic radius R(kpc) of 2.2 10 to the minus 7th power exp(-R/3.5) erg/sq cm to the (-2)power/s (R 7.8 kpc). giving a luminosity of approximately 4.4 10 to the 37th power erg S to the (-1) power. If the model is extrapolated to radii less than 7.8 kpc, the unresolved disc emission is approximately 1.4 10 to the 38th power erg S to the (-1) power (2-10 keV) i.e., a few percent of the luminosity of the galaxy in resolved sources. the disc emission has a spectrum which is significantly softer than that of the high galactic latitude diffuse X-ray background and it is most probably of discrete source origin.

Spectral evolution of a long X-ray burst

Results are reported for spectroscopic observations of an X-ray burstlike event that had a duration of 100 sec and a peak count range 1.5 times that of the Crab Nebula. The best position determined is 356.4 deg galactic longitude, 2.3 deg galactic latitude. It is shown that the spectrum of the source changed as the event evolved, being very soft during the 20-sec peak of the burst, quickly hardening during the decay, and again softening as the decay proceeded. The spectra are fit best by a blackbody model with kT of 0.87 to 2.33 keV, and it is noted that a weak flux was detected some time after the burst and was well fitted by a 10-keV thermal bremsstrahlung spectrum with an indication of iron line emission. A source size is obtained which suggests a neutron star or a black hole of stellar mass.

X-radiation from clusters of galaxies - Spectral evidence for a hot evolved gas

OSO-8 observations of the X-ray flux in the range between 2 and 60 keV from the Virgo, Perseus, and Coma clusters provide strong evidence for the thermal origin of the radiation, including iron-line emission. The data are adequately described by emission from an isothermal plasma with an iron abundance in near agreement with cosmic levels. A power-law description is generally less acceptable and is ruled out in the case of Perseus. Implications of the origin of the cluster gas are discussed.

Evidence for a 16.6 day period from Circinus X-1

Analysis of All-Sky Monitor observations of Cir X-1 (3U1516-56) over the period October 1975 -- April 1976 revealed a well-defined modulation of the 3--6 keV flux at a period of 16.585 +- 0.01. The light-curve is characterized by an abrupt drop in emission occurring on a timescale of 0.07, with epoch JD 2,442,877.181 +- 0.07. No clear, correspondingly sharp increase in emission is observed during the cycle, so that a noneclipse origin for this effect cannot be ruled out. (auth)

X-ray spectral constraints on the broad-line cloud geometry of NGC 4151

X-ray spectral data from NGC 4151 taken with the Einstein Solid-State Spectrometer (SSS) and the HEAO 1 A-2 experiment cannot be simply reconciled with absorption from a uniform column of cold gas. The SSS data can, however, be explained in terms of a clumped absorber with approximately 10% uncovered fraction and factor-of-two overabundances in Z equal to or greater than 14 elements relative to solar oxygen. It is shown that these and previously reported spectral and variability data can be quantitatively reconciled with absorption arising in the cold clouds responsible for the broad optical line emission if the cloud dimensions are small compared to the central source size. It is suggested that the lack of significant X-ray absorption observed from much higher luminosity Seyferts and quasars is a natural consequence of the proposed picture for NGC 4151.

HEAO-1 spectra of X-ray emitting Seyfert 1 galaxies

The paper presents the 2-50-keV X-ray spectra and time variability information on seven Seyfert 1 galaxies NGC 3783, NGC 4151, NGC 5548, NGC 6814, MK 509, MCG 8-11-11, and ESO 141-G55, obtained with the A2(2) experiment on HEAO 1. It is concluded that the spectra of Seyfert 1 galaxies can be well represented by flat power laws of high-temperature thermal breamsstrahlung models with relatively low column densities.

A new X-ray spectral observation of NGC 1068

A new X-ray observation of NGC 1068, in which improved spectral resolution (R is approximately equal to 40) and broad energy range provide important new constraints on models for this galaxy, is reported. The observed X-ray continuum of NGC 1068 from 0.3 to 10 keV is well fitted as the sum of two power-law spectra with no evidence for absorption intrinsic to the source. Strong Fe K emission lines with a total equivalent width of 2700 eV were detected due to iron less ionized than Fe XX and to iron more ionized than Fe XXIII. No evidence was seen for lines due to the recombination of highly ionized oxygen with an upper limit for the O Ly-alpha emission line of 40 eV. The discovery of multiple Fe K and Fe L emission lines indicates a broad range of ionization states for this gas. The X-ray emission from the two components is modeled for various geometries using a photoionization code that calculates the temperature and ionization state of the gas. Typical model parameters are a total Compton depth of a few percent, an inner boundary of the hot component of about 1 pc, and an inner boundary of the warm component of about 20 pc.