Discrete X-ray Sources Research Articles

Detection of the Angular Correlation of Faint X-Ray Sources

We have analyzed a set of deep ROSAT observations with a total sky coverage of 40 square degrees to search for clustering of faint X-ray sources. Using the resulting catalog of discrete X-ray sources, we detect, for the first time in X-rays, a positive correlation on angular scales of 0.5'-10'. When corrected for a bias due to limited spatial resolution which amplifies the correlation, the observed angular correlation function agrees well with that expected from the spatial correlation of optically selected quasars, provided that they comprise an appreciable fraction (>= ~50%) of detected X-ray sources.

First Maps of the Soft X-Ray Diffuse Background from the ROSAT XRT/PSPC All-Sky Survey

view Abstract Citations (263) References (31) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS First Maps of the Soft X-Ray Diffuse Background from the ROSAT XRT/PSPC All-Sky Survey Snowden, S. L. ; Freyberg, M. J. ; Plucinsky, P. P. ; Schmitt, J. H. M. M. ; Truemper, J. ; Voges, W. ; Edgar, R. J. ; McCammon, D. ; Sanders, W. T. Abstract This paper presents an initial version of the diffuse background results from the ROSA T soft X-ray all-sky survey. These maps cover ∼98% of the sky in the ¼ keV, ¾ keV, and 1.5 keV bands, with ∼2° angular resolution and high sensitivity for low surface brightness extended features. The effects of non-X-ray contamination and X-rays of solar system origin have been eliminated to the greatest possible extent, but discrete X-ray sources have not been removed. The much improved angular resolution, statistical precision, and completeness of coverage of these maps reveal considerable structure over the entire 0.1-2.0 keV energy range that was not observed previously. The data agree well with previous all-sky surveys in terms of absolute normalization and zero point. Publication: The Astrophysical Journal Pub Date: December 1995 DOI: 10.1086/176517 Bibcode: 1995ApJ...454..643S Keywords: SURVEYS; X-RAYS: GALAXIES; X-RAYS: GENERAL; X-RAYS: ISM full text sources ADS | data products SIMBAD (11)

Detections of hard X-ray emissions from bright early-type galaxies with ASCA

Five bright elliptical galaxies in the Virgo Cluster, NGC 4365, NGC 4374 (M84), NGC 4406 (M86), NGC 4472 (M49), and NGC 4636, were observed with ASCA. In addition to the extended thermal X-ray emission of temperature kT approximately 1 keV, harder X-rays with color temperature kT greater than or equal to 2 keV were detected from all of them. The 2-10 keV luminosities of this hard component for the five galaxies, integrated within 5 min, are distributed within a relatively narrow range of (1-4) x 10(exp 40) ergs/s. The hard X-ray component is primarily attributed to the integrated emission from discrete X-ray sources. In NGC 4406 and NGC 4374 the data indicate that the hard component is contributed additionally by foreground/background emission from the hot intracluster medium (ICM) of the Virgo Cluster. The hard component of NGC 4472 seems also contributed by the Virgo ICM emission, but in this case there is evidence that the ICM brightness is locally enhanced within approximately 10 min of NGC 4472.

Active galactic nuclei and star-forming galaxies - contributing to the extragalactic X-ray flux

The nature of discrete sources contributing to the extragalactic X-ray background (XRB) in the energy range ∼0.1-3 keV has been investigated via optical identifications based on positional coincidence and spectroscopy. The results presented here address the counterparts to the discrete X-ray sources detected in the Einstein Observatory deep X-ray survey in Pavo. X-ray selected quasars, detected to a limiting flux of ∼10 -14 erg cm -2 s -1 and a limiting B J magnitude of 21.5, have a surface density of ∼32 per square degree, an average redshift of 0.94, and account directly for ∼15 per cent of the extragalactic X-ray background at an effective energy of 2 keV

The discrete X-ray source population in M31

Results of an analysis of the complete set of Einstein imaging observations of M31 are reported. One-hundred-eight individual X-ray sources are detected, with (0.2-4.0 keV) luminosities ranging from 5 x 10 exp 36 ergs/s to above 10 exp 38 ergs/s at the distance of M31. Fourteen of these sources are luminous enough to perform a spectral fit of the X-ray data. Fourteen sources are found to vary between Einstein observations, in addition to the two sources already reported by Collura, Reale, and Peres (1990). Contrary to earlier reports, no substantial difference is found between the luminosity distributions of X-ray sources in the bulge and in the disk. Contour maps of the X-ray emission in the central region is presented, and the radial distribution of the surface brightness is compared with the optical profiles. It is inferred that the disk is relatively underluminous in X-rays, relative to the stellar content. The spatial distribution of bulge sources is compared with that of optical novae and with emission-like images of the bulge. These comparisons constrain models of the formation of low-mass X-ray binaries.

Fabrication and flight performance of a large area balloon borne hard X-ray telescope

We describe the fabrication and flight performance of a balloon-borne large area hard X-ray (20–100 keV) telescope for spectral studies of discrete cosmic X-ray sources. The telescope consists of two multi-wire Xenon filled proportional counters of effective area 1200 cm 2 each, mounted on an orientable platform. It can be pre-programmed to track any celestial source with a pointing accuracy of 0.5 degrees. For one hour of observation the telescope has a 5 σ detection sensitivity of 10 −5 ph cm −2 s −1. The laboratory test results and the performance in a series of balloon flights conducted in 1984–1986 period is discussed and the preliminary results obtained for some X-ray sources are presented.

Plasma Distribution Functions in Accretion onto Neutron Stars

In the usual model of discrete cosmic X-ray sources, it is assumed that a massive star loses matter to a companion neutron star, either through a stellar wind, or by means of Roche lobe overflow into an accretion disk around the neutron star (Pines 1980). The irifalling matter becomes ionized and is channelled along the magnetic field lines of the neutron star, so that it is accreted onto the star in a very small region at the magnetic polar caps. The physical conditions at these points are assumed to be: magnetic field B ~ 1012 G, plasma temperature ~107-108 K, and infall plasma velocity ~ 0.3 c-0.5 c.

X-Ray and Optical Observations of Quasars

The first Einstein observations established that quasars, as a class, are luminous X-ray emitters (Tananbaum et al. 1979). Since then, one of our major programs has been to carry out further X-ray and optical observations to improve our estimate of the contribution of quasars to the 2 keV extragalactic isotropic X-ray background and also to reconcile number counts of discrete X-ray sources with reported optical number counts of quasars (cf. Zamorani et al. 1981 and references therein). This paper is a preliminary report on our observations of the 1.7 square degree field studied originally by Braccesi and his colleagues (Formiggini et al. 1980 and Braccesi et al. 1980). A more detailed presentation of our results is in preparation (Marshall et al., 1981).

Cosmic X-ray background and intergalactic gas accretion onto intergalactic collapsed objects

The origin of the diffuse cosmic X-ray background (CXB) has become the subject of much speculation recently, with most theorists divided between the intergalactic hot gas bremstralung hypothesis and the QSO contribution theory. Stimulated by Sargent's recent argument about the existence of a cool, low density intergalactic gas, we investigate here the third possibility of its accretion onto intergalactic collapsed objects as the sources of the X-ray background. We conclude that if the CXB is hot thermal bremstralung (TB) from the accretion of Sargent type intergalactic gas onto intergalactic collapsed objects, a new class of faint (Lx≲1041 erg s−1) low mass (M∼107−1010M⊙) discrete X-ray sources must exist with no optical or IR images, some of which could be detected by the Einstein observatory.

X-ray emission from γ-ray sources in the galactic anticentre region

An X-ray survey of three high-energy γ-ray sources in the galactic anticentre region has been conducted using the large area sky survey (LASS) instrument on HEAO 1. The sensitivity of this survey ≲0.5 Uhuru flux units (UFU) is the highest yet reported. Although we do not confirm the recently published detection1 of an X-ray source associated with γ195 + 5 (CG195+4); we report the detection of two, and possibly three, other X-ray sources found in its vicinity. Other than the supernova remnant, IC443, no other discrete X-ray sources have been detected near CG189+1, but there is evidence for diffuse emission or emission from unresolved sources in its vicinity. Three new X-ray sources have been discovered within a few degrees of CG176−7, but none overlaps the γ-ray error box. These results demonstrate that γ-ray sources like CG195+4 and CG176−7 have emission characteristics significantly different from the Crab pulsar, but may be similar to the Vela pulsar. Alternatively, the lack of a detectable X-ray flux suggests that the γ-ray emission arises from cosmic-ray interactions in dense interstellar clouds. An interesting feature of the new X-ray sources detected in this survey is that four of them may be associated with early-type stars which exhibit emission lines.

The formation of molecules in interstellar clouds from singly and multiply ionized atoms

The suggestion is considered that multiply ionized atoms produced by K- and L-shell X-ray ionization and cosmic-ray ionization can undergo ion-molecule reactions and also initiate molecule production. The role of X-rays in molecule production in general is discussed, and the contribution to molecule production of the C(+) radiative association with hydrogen is examined. Such gas-phase reactions of singly and multiply ionized atoms are used to calculate molecular abundances of carbon-, nitrogen-, and oxygen-bearing species. The column densities of the molecules are evaluated on the basis of a modified version of previously developed isobaric cloud models. It is found that reactions of multiply ionized carbon with H2 can contribute a significant fraction of the observed CH in diffuse interstellar clouds in the presence of diffuse X-ray structures or discrete X-ray sources and that substantial amounts of CH(+) can be produced under certain conditions.

Primeval clusters of galaxies and the X-ray background

The X-ray telescope for HEAO-B, due for launch next year, will extend our observing power in X-ray astronomy by a very large factor. For example, its sensitivity to point sources will be 103 greater than the limit of existing X-ray sky surveys. Even more intriguing will be its capability to image the all-sky X-ray background radiation on an arc minute or sub arc minute scale. the origin of this background is still a mystery. the simplest hypothesis, that it is the integral of radiation from the more distant members of the classes of discrete X-ray sources such as clusters of galaxies, Seyferts, QSO's and other active galaxies, can only explain part of the background.

CygX-1/A candidate of the black hole

Among discrete galactic X-ray sources, Cyg X-1 has been noted for its peculiar features in several respects. It is one of the few sources with a hard power law spectrum extending beyond several hundred keV. Cyg X-1 also distinguishes itself by its profound time variability over a wide range of time scales. The most remarkable incident was that its optical identification with a spectroscopic binary HDE226868 has led to a presumption that it is a black hole. This possibility has induced continuous interests in the physical character of this source in conjunction with the nature of the black hole. The purpose of this paper is to summarize presently available pieces of knowledge on this source to help the design of future experimental and theoretical works, while the complexity of the source characteristics still has prevented us to construct a clear, coherent picture of this source in spite of the fact that numerous observational facts have been accumulated.

Neutron stars as X-ray burst sources

X-ray bursts are interpreted in the light of a model in which accreted matter accumulates at the magnetopause of a rotating neutron star until the surface becomes unstable for Kruskal-Schwarzschild instabilities. An instability releases matter from the magnetopause and produces a burst. A qualitative explanation is presented for the observed bursting patterns as well as the shapes and spectral properties of the bursts. A classification for the different types of discrete X-ray sources is suggested.

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)

Diffuse X and Gamma Radiation

The most reliable measurements in the energy range from 0.1 MeV to 100 MeV are presented in graphs. Studies in the region from 1 keV to 100 keV are discussed, giving attention to nonthermal mechanisms, thermal emission from a uniform intergalactic medium, the contribution of discrete extragalactic sources to the isotropic X-ray background, discrete diffuse X-ray sources in galaxy clusters, the contribution of galaxy clusters to the isotropic X-ray background, and fluctuations in the X-ray background. Diffuse X rays below 1 keV are also considered together with diffuse galactic X and gamma rays above 1 keV and the significance of diffuse X and gamma radiation for astronomy.

Hard X-ray Emission from Uhuru Sources

Giacconi et al.1 have published a catalogue of 125 discrete X-ray sources detected by Uhuru in the 2–6 keV band. Spectral information extending up to the hard X-ray region is available for only 8–10 of these sources, which have been studied earlier by balloon and rocket experiments. In two balloon flights carried out from Hyderabad by the TIFR group, one on April 16, 1969 and the other on May 5, 1970, a large part of the sky defined by RA = 190°−330° and δ = −20° to + 50° was scanned for hard X-ray emission in the 16–150 keV band. The sky map of hard X-ray intensity, the spectral results for Sco X-1 and Cyg X-1 and the evidence for two new hard X-ray sources obtained from these nights have already been published2–4. In the flight of April 16, 1969, the region of the sky corresponding to 13 of the Uhuru sources was also scanned with reasonably good efficiency. We have re-analysed the data of this flight to elicit some information on the spectral characteristics of these sources (Table 1, part a). The exposures were calculated using the positional coordinates given in the Uhuru catalogue. illustration Open image in new window

Optical Candidate for SMC X-1

THE one known discrete X-ray source in the Small Magellanic Cloud, SMC X-1, has been shown to be a binary system with eclipses of the X-ray component occurring every 3.8927 day1. We wish to point out that there are two relatively bright stars within the X-ray 90% region. One of these, which we shall call star A, seems to be a foreground F star; the other, Sanduleak2 number 160 (Sk 160), is an early B supergiant which, by analogy with HD 226868 (refs. 3,4), may be emitting the X-rays.

Radio Identifications of Weak Extragalactic X-ray Sources

OBSERVATIONS from the Uhuru satellite1 have been used to compile a catalogue of 116 discrete cosmic X-ray sources in the 2-6 keV energy range. Many of the weaker Uhuru sources are at high galactic latitudes and may represent a hitherto undetected population of extragalactic X-ray emitters2. Some are near the positions of rich clusters of galaxies1; of these, three (the Coma, Virgo and Perseus clusters) contain bright radio galaxies thought to be active sources of high-energy cosmic-ray electrons. We propose similar “radio identifications” for two further Uhuru sources and suggest that they are produced by inverse-Compton losses of electrons which have escaped from radio galaxies.