Diffuse X-ray Background Research Articles

Cold dense photoionized universes

This investigation employs a computer code, initially constructed for modeling the evolution of a hot dense intergalactic medium (IGM), to study some properties of a cold dense IGM photoionized by QSOs. Within the observational constraints set by flux measurements in the far-ultraviolet, night sky brightness, and hard X-ray region, and the optical depth limits on various QSO spectra, it determines some allowable scenarios for a range of QSO spectral indices (alpha) and cutoff energies. With H(0) = 50 km/s-Mpc, closure density of an IGM (composed of H and He in their cosmic ratio) is possible only if alpha is greater than or equal to 0.1, a circumstance not likely to prevail; the most probable value, alpha = -0.7, implies an upper limit density about 0.35 of closure, which is somewhat larger than previous estimates. These estimates are substantially independent of whether or not QSOs produce the observed diffuse extragalactic X-ray background; such QSO spectral details are critical, however, in determining the ionization of heavy elements that are expected to contaminate a pure primeval plasma.

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 deg from the galactic plane. A total exposure of 80 sq m-s-sr is available at present. Free-free emission from an optically thin plasma of 40 plus or minus 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 about 10%, the intensity of the emission at 10keV is 3.2 keV/keV/sq cm/s/sr, a value consistent with the average of previously reported spectra. 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.

Constraint on Quasar Number Counts from Their Contribution to The X-Ray Background

AbstractObservations with the Einstein X-ray telescope show that quasars, as a class, are luminous X-ray emitters. By coupling our X-ray observations with quasar optical number counts, we can show that the quasars contribute significantly to the diffuse X-ray background. In fact, the X-ray data strongly suggest that somewhat above 20m the slope of the optical counts must flatten.

A limit to the X-ray luminosity of nearby normal galaxies

The hypothesis that normal galaxies are on the average more luminous in the X-ray region than the Milky Way galaxy or M31 and therefore are possible candidates for the low-luminosity sources of the 2 to 60 keV extragalactic diffuse background is tested. Data from the A-2 detectors on the HEAO-1 spacecraft were examined for emission from positions coincident with 76 selected normal galaxies, and upper limits to the average galactic luminosity for various luminosity distributions resulting in the observed count rate distribution were determined. For uniform and exponential galactic luminosity distributions, limits of 2.7 x 10 to the 38th erg/sec and 3.4 x 10 to the 38th erg/sec, respectively, at the 90% confidence level were obtained. It is shown that the Hubble-constant-independent upper limit to galactic emissivity is less than 1% of the diffuse background emissivity, indicating that normal galaxies are not responsible for the diffuse X-ray background and have luminosities comparable to that of the Galaxy.

Cosmic far-ultraviolet background radiation - Probe of a dense hot intergalactic medium

Line and continuum radiation fluxes have been computed for a wide range of enriched intergalactic medium (IGM) models. Observations of the diffuse extragalactic light at optical and far-ultraviolet wavelengths are found to provide a potentially important probe of a dense hot intergalactic medium. If the diffuse X-ray background is produced by this gas, the models constrain the cosmological density parameter (Omega) to be less than 0.4. The associated Compton distortions of the cosmic blackbody background radiation and the optical depths to distant quasars at X-ray wavelengths are also evaluated.

Extragalactic X-ray sources and the diffuse X-ray background

Results from the Ariel 5 sky survey instrument relating to the properties and the spatial distribution of extragalactic X-ray sources are discussed. The lg N -lg S relation for sources in the 2A catalogue is consistent with a uniform distribution of sources in Euclidean space. In addition, measure­ments of fluctuations in the X-ray background suggest that the Euclidean form of the source counts can be extrapolated to flux levels at least an order of magnitude fainter than the 2A catalogue limit. Information is also available from the optical identification of 2A sources which, through redshift measurements, enables the X-ray luminosity functions of the two main classes of source, namely clusters of galaxies and active galaxies, to be determined. The luminosity functions can be used to calculate the contribution of clusters of galaxies and active galaxies to the diffuse X-ray background in the 2-10 keV range. It is found that cosmological evolution of one or both populations is required to account for the diffuse X-ray background entirely in terms of the integrated emission from these sources.

On the lack of absorption of 1 keV diffuse X-rays by the Small Magellanic Cloud

view Abstract Citations (9) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the lack of absorption of 1 keV diffuse X-rays by the Small Magellanic Cloud. Bunner, A. N. ; Sanders, W. T. ; Nousek, J. A. Abstract The contribution to the diffuse X-ray background near 1 keV cosmic (extragalactic) origin is poorly known. Data from a slow scan across the Small Magellanic Cloud by the Wisconsin soft X-ray experiment on OSO 8 are analyzed to search for absorption of a possible cosmic component to the diffuse flux. No indication of an X-ray shadow near 1 keV is found, setting an upper limit to the intensity of the flux originating beyond the SMC that is inconsistent with a cosmic spectrum of 11 E to the -1.4 photons per (sq cm s sr keV), even allowing for depletion of the absorbing elements in the SMC. Unless emission by the SMC or an upward fluctuation in the galactic or cosmic diffuse flux at the position of the SMC has canceled out the expected absorption, the contribution to the diffuse flux at 1 keV from beyond the SMC must be less than 50% of that expected for an extrapolated E to the -1.4 source spectrum. Publication: The Astrophysical Journal Pub Date: February 1979 DOI: 10.1086/182896 Bibcode: 1979ApJ...228L..29B Keywords: Cosmic X Rays; Magellanic Clouds; Radiant Flux Density; X Ray Absorption; Abundance; Cosmic Gases; Hydrogen Atoms; Interstellar Gas; Luminous Intensity; Thermal Emission; Space Radiation; Magellanic Clouds:X Rays; X-Ray Background full text sources ADS | data products SIMBAD (3)

On the contribution of normal galaxies to the diffuse X-ray background

A calculation has been made of the contribution of normal galaxies to the diffuse X-ray background, based on a model of the X-ray luminosity evolution of galaxies in which two populations of galactic X-ray sources are distinguished. With plausible estimates of the behavior of the galactic X-ray source populations with time, normal galaxies provide between 1.5% and 9% of the X-ray background between 2 and 10 keV. Unless massive halos exist around galaxies and their formation is accompanied by the production of early-type Population I X-ray binaries, a firm upper limit of about 25% can be set on the contribution of normal galaxies to the X-ray background between 2 and 10 keV.

A balloon observation of the diffuse cosmic X-radiation above 20 keV

view Abstract Citations (37) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A balloon observation of the diffuse cosmic X-radiation above 20 keV. Kinzer, R. L. ; Johnson, W. N. ; Kurfess, J. D. Abstract Balloon-borne equipment has been used for measurements of the diffuse cosmic X-ray background radiation between 20 and 165 keV. Observations were made by two directional phoswich scintillation detectors having active and passive shutters. The detectors had apertures of about 20 degrees FWHM and were sensitive to radiation between 15 and 180 keV. Data were collected on the vertically incident radiation between 2.6 and 20 g/sq cm residual atmosphere. The X-ray spectra were unfolded from the measured energy-loss spectra by decoder-response functions found with a Monte Carlo simulation of the detector. The diffuse cosmic X-ray spectrum was found by extrapolating the measured growth curves to the top of the atmosphere. Publication: The Astrophysical Journal Pub Date: May 1978 DOI: 10.1086/156150 Bibcode: 1978ApJ...222..370K Keywords: Balloon Sounding; Balloon-Borne Instruments; Cosmic X Rays; Diffuse Radiation; X Ray Spectra; Data Processing; Energy Dissipation; Energy Spectra; Monte Carlo Method; Photoelectric Effect; Photopeak; Scintillation Counters; Space Radiation; X-Ray Background:Cosmic full text sources ADS |

On a new method of separating the diffuse cosmic X-ray component in balloon-borne measurements

The diffuse X-ray background from 20 to 200 keV has been measured many times by rocket-, satellite-, and balloon-borne detectors. As has been noted by a number of authors, the balloon measurements do not agree with those outside the atmosphere. This suggests some systematic difference in the derivation of the diffuse flux. In the present article we propose a method for the analysis of balloon data in which the source function (atmospherically produced and scattered primary) is derived from the experimental data and is then used to calculate the residual counting rate due only to the transmitted primary photons.

The diffuse soft X-ray sky

The current status of the investigation of the soft X-ray diffuse background in the energy range 0.1–2.0 keV is reviewed. A consistent model, based on the soft X-ray brightness distribution and the energy spectrum over the sky, is derived. The observed diffuse background is predominantly of galactic origin and considered as thermal emission for the most part from a local hot region of temperature ≈106 K which includes the solar system. Several pronounced features of enhanced emission are interpreted in terms of hot regions with temperatures up to 3×106K, some of which are probably old supernova remnants. The properties of the soft X-ray emitting regions are discussed in relation to the observational results on O vi absorption.

Soft diffuse X-rays in the southern galactic hemisphere

A map is presented of the soft X-ray diffuse background flux in the C band (approximately 0.13-0.28 keV) covering almost all of the southern galactic hemisphere. A comparison at constant galactic latitude of both C band and B band (about 0.1-0.18 keV) soft X-ray data with neutral-hydrogen maps shows that the intensity does decrease with increasing neutral-hydrogen column density but in a manner that is inconsistent with photoelectric absorption. It is suggested that the inverse correlation is a displacement effect. X-ray emission regions appear to be where the cool gas is not. Further, the evidence against photoelectric absorption implies that the bulk of the cool gas is beyond the X-ray emitting regions. Fewer than 10 to the 20th power H atoms per sq cm can be between the sun and the X-ray emitting regions. The sun appears to be surrounded by a soft X-ray emission region consisting of gas at a temperature of about 1 million K.

Extragalactic X-Ray Sources

Those extragalactic objects that are likely to be intrinsic x-ray sources are reviewed. Topics covered include active galaxies, x-ray emitting clusters of galaxies, the diffuse x-ray background, and prospects for future observations. (GHT)

Coordinated X-ray, optical, and radio observations of YZ Canis Minoris

We report coordinated X-ray, optical, and radio observations of the flare star YZ CMi, including the first occasion on which such a star has been monitored in all three spectral regions simultaneously. Thirty-one minor optical flares and 11 radio events were recorded. No major optical flares greater than 3 magnitudes were observed during the program. Although no flare- related X-ray emission was observed, the measured upper limits in this band enable meaningful comparisons with published flare-star models. Three of the five models predicting the relative X-ray to optical or radio flare luminosities are in serious disagreement with the observations. For the largest optical flare with coincident X-ray co1verage, the 3 a upper limit on X-ray emission in the 0.15-0.8 keV band is about 9 x 1028 ergs 5 - , corresponding to a ratio of X-ray to B-band luminosity of <0.3. Based on the present results, the fraction of the galactic component of the diffuse soft X-ray background contributed by UV Ceti-type flare stars is <9 x H, where H is the mean density of interstellar hydrogen within a few hundred parsecs of the Sun. Subject headings: radio sources: variable - stars: flare - stars: individual - X-rays: bursts

The 20keV Break in the Diffuse X-ray Background Spectrum

In this paper we report an independent determination of the Location of the break (change in spectral index) in the spectrum of the diffuse X-ray background by applying a simple analysis technique to data already in the literature.

Three-band observations of the soft X-ray background and some implications of thermal emission models

The galactic component of the very soft diffuse X-ray background in three bands, covering the energy ranges of approximately 100 to 188 eV, 150 to 284 eV, and 500 to 850 eV, is mapped primarily at middle and low galactic latitudes. Intense regions on the maps are identified, and the dissimilarity of the three maps is shown to indicate spectral as well as intensity variations over the sky. Three emission-mechanism models involving hot gas at a single temperature are examined, a multitemperature model is found to be required, and several such models based on hot gas in a steady state or in equilibrium are considered. The problem of overpressure raised by Shapiro and Field (1976) is briefly discussed.

OSO-7 hard X-ray observations of 3U 0352+30 = X Persei

view Abstract Citations (23) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS OSO-7 hard X-ray observations of 3U 0352+30 = X Persei. Mushotzky, R. F. ; Roberts, D. H. ; Baity, W. A. ; Peterson, L. E. Abstract Results are reported for OSO 7 observations of 3U 0352+30 (X Per) over the energy range from 7 to 110 keV. It is found that this source has either a hard power-law spectrum or a thermal bremsstrahlung spectrum over the observed energy range, indicating that X Per may be the prototype of a new class of weak galactic hard X-ray sources. The data are shown to be consistent with spectra calculated for accretion onto a degenerate dwarf of at least 1 solar mass. Geometrical and dynamical constraints are considered for a model in which an accreting degenerate dwarf is in a close orbit about the primary of X Per, and it is concluded that such a model is not ruled out at present. It is suggested that X-ray sources associated with Be stars may make a significant contribution to the diffuse hard X-ray background in the galactic plane. Publication: The Astrophysical Journal Pub Date: February 1977 DOI: 10.1086/182356 Bibcode: 1977ApJ...211L.129M Keywords: B Stars; Galactic Radiation; Oso-7; X Ray Sources; Bremsstrahlung; Spaceborne Astronomy; Stellar Models; Astrophysics full text sources ADS | data products SIMBAD (4)

Ariel results on extragalactic X-ray sources

Two of the instruments on Ariel 5 are designed to study the weak high galactic latitude X-ray sources, a number of which have been identified with extragalactic objects. The survey experiment of the University of Leicester has detected sources with strengths down to 2-3 Uhuru counts. Several high latitude sources catalogued by Uhuru have apparently dropped in intensity below this level and improved positions have been obtained for other high latitude sources which support the tentative identification with optical counterparts. The pointed instrument from M. S. S. L. has a programme of spectral measurements of identified extragalactic and other weak sources at high galactic latitudes. The field of view of the M. S. S. L. experiment makes the instrument well suited to the study of the diffuse X-ray background. Results are presented from both experiments on a number of the extragalactic sources and, in particular, discussed in relation to their potential contribution to the diffuse X-ray background. Preliminary measurements of the diffuse X-ray background by the M. S. S. L. instruments are presented.

Flares of red dwarf stars and the galactic X-ray background

The mean density of the UV Cet-type flare stars in the solar neighbourhood is estimated. If differences of activity levels on different flare stars are taken into account, their summary flare activity is equivalent to 0.03 YZ CMi's flare activity per cubic parsec or to 4×1026 erg s−1 pc−3 in U-passband. From the X-ray flare observation on YZ CMi of 19.10.74 we estimate the luminosity of stellar flares in soft and intermediate X-ray. The ratio of X-ray to optical radiation for stellar flares is close to the respective ratio for strong solar chromospheric flares. It is shown the set of red-dwarf flare stars has all essential features of an ensemble of discrete X-ray sources to represent the galactic diffuse X-ray background.

Soft X-Ray Sources

Objects that have already been detected as discrete soft x-ray sources are reviewed. They include supernova remnants, a recurrent nova, flare stars, the coronas of nearby stars, SS Cygni, a white dwarf, and a soft component of emission from binary x-ray stars. A brief mention is made of those aspects of the diffuse cosmic soft x-ray background that may be related to the discrete sources. 10 figs., 3 tables, 177 refs. (GHT)