The soft X-ray diffuse background observed with the HEAO 1 low-energy detectors

Garmire, G. P.; Nousek, J. A.; Apparao, K. M. V.; Burrows, D. N.; Fink, R.; Kraft, Ralph

doi:10.1086/171962

Cited by 62 publications

(45 citation statements)

References 0 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Between 3 and 60 keV, the spectrum of thermal bremsstrahlung from a thin intergalactic plasma with kT = 40 keV provides an excellent fit to the data, but this physical origin has been ruled out (Barcons, Fabian, & Rees 1991) by COBE measurements (Mather et al 1990). In the energy range of 3 -10 keV, the spectrum is well fitted by a power law, ~10E -0.4 keV cm -2 s -1 sr -1 keV -1 , with some 20% uncertainty on the normalization factor derived by different experiments Fabian & Barcons 1992;Garmire et al 1992;Gendreu et al 1995). At even lower energies, the extragalactic X-ray background is still poorly constrained due to confusion with the bright Galactic soft X-ray emission.…”

Section: Introductionmentioning

confidence: 90%

Limits to the 14 keV Extragalactic X-Ray Background

Cui

Sanders

McCammon

et al. 1996

ApJ

View full text Add to dashboard Cite

We observed several nearby face-on spiral galaxies with the ROSAT PSPC. The apparent deficiency in soft X-ray surface brightness observed at the outer portion of their disks is consistent with the absorption of the extragalactic soft X-ray background by material associated with these galaxies, and allows us to place a lower limit on the intensity of this cosmologically important background. From the depth of the soft X-ray shadow observed in NGC 3184, a 95% confidence lower limit was derived to be 32 keV cm -2 s -1 sr -1 keV -1 at 1/4 keV. This was obtained by assuming that there is no unresolved 1/4 keV X-ray emission from the outer region of the galaxy which may otherwise partially fill in the shadow: any such emission, or any unresolved structure in the absorbing gas, would imply a larger value. In the deepest exposure to date in this energy range, Hasinger et al. (1993) resolved about 30 keV cm -2 s -1 sr -1 keV -1 at 1/4 keV into discrete sources; our current limit is therefore consistent with an extragalactic origin for all of these sources. Our result can also be directly compared with the corresponding upper limit derived from the ROSAT PSPC detection of soft X-ray shadows cast by high latitude clouds in Ursa Major, ~65 keV cm -2 s -1 sr -1 keV -1 at 1/4 keV. The lower and upper limits are only a factor of 2 apart, and begin to provide a reasonable measurement of the intensity of the 1/4 keV extragalactic X-ray background. INTRODUCTIONDespite much attention paid to the extragalactic X-ray background over the past few decades, its origin still remains a mystery. Recently, debate has been focused on whether it originates solely from the 1 Present address: Center for Space Research, Room 37-571, MIT, Cambridge, MA 02139 2 integrated emission of known types of active galactic nuclei (AGN), or whether there are also significant contributions from unknown populations of discrete X-ray sources. The X-ray spectrum of this background has been well measured over a large energy range by several experiments (see review

show abstract

Section: Introductionmentioning

confidence: 90%

Limits to the 14 keV Extragalactic X-Ray Background

Cui

Sanders

McCammon

et al. 1996

ApJ

View full text Add to dashboard Cite

show abstract

“…The 1Ms dataset of the CDFS is the result of the coaddition of 11 individual Chandra ACIS-I (Garmire et al 1992;Bautz et al 1998) exposures with aimpoints only a few arcsec from each other. The nominal aim point of the CDFS is α = 3:32:28.0, δ = −27:48:30 (J2000).…”

Section: The Datamentioning

confidence: 99%

X-ray spectral properties of active galactic nuclei in the Chandra Deep Field South

Tozzi

Gilli

Mainieri

et al. 2006

A&A

389

649

View full text Add to dashboard Cite

We present a detailed X-ray spectral analysis of the sources in the 1Ms catalog of the Chandra Deep Field South (CDFS) taking advantage of optical spectroscopy and photometric redshifts for 321 extragalactic sources out of the total sample of 347 sources. As a default spectral model, we adopt a power law with slope Γ with an intrinsic redshifted absorption N H , a fixed Galactic absorption and an unresolved Fe emission line. For 82 X-ray bright sources, we are able to perform the X-ray spectral analysis leaving both Γ and N H free. The weighted mean value for the slope of the power law is Γ 1.75 ± 0.02, and the distribution of best fit values shows an intrinsic dispersion of σ int 0.30. We do not find hints of a correlation between the spectral index Γ and the intrinsic absorption column density N H . We then investigate the absorption distribution for the whole sample, deriving the N H values in faint sources by fixing Γ = 1.8. We also allow for the presence of a scattered component at soft energies with the same slope of the main power law, and for a pure reflection spectrum typical of Compton-thick AGN. We detect the presence of a scattered soft component in 8 sources; we also identify 14 sources showing a reflection-dominated spectrum. The latter are referred to as Compton-thick AGN candidates. By correcting for both incompleteness and sampling-volume effects, we recover the intrinsic N H distribution representative of the whole AGN population, f (N H )dN H , from the observed one. f (N H ) shows a lognormal shape, peaking around log(N H ) 23.1 and with σ 1.1. Interestingly, such a distribution shows continuity between the population of Compton-thin and that of Compton-thick AGN. We find that the fraction of absorbed sources (with N H > 10 22 cm −2 ) in the sample is constant (at the level of about 75%) or moderately increasing with redshift. Finally, we compare the optical classification to the X-ray spectral properties, confirming that the correspondence of unabsorbed (absorbed) X-ray sources to optical type I (type II) AGN is accurate for at least 80% of the sources with spectral identification (1/3 of the total X-ray sample).

show abstract

“…The earlier authors took the locally-determined soft X-ray flux from Garmire et al (1992) and decomposed it into three components, each of which they represented by a (physically-motivated) plasma emissivity which was then separately attenuated (or not) by an assumed column of warm gas; the three contributions were then summed to provide the assumed incident X-ray spectrum at a typical location. Their final spectrum is characterized by a quantity they called Nw, the overall column of warm gas attenuating part of the incident spectrum 2 whose standard value they took to be Nw = 10 19 H atoms cm −2 .…”

Section: Particulars and Parameter Sensitivitiesmentioning

confidence: 99%

Two-phase equilibrium and molecular hydrogen formation in damped Lyman-alpha systems

Liszt

2002

A&A

View full text Add to dashboard Cite

Abstract. Molecular hydrogen is quite underabundant in damped Lyman-α systems at high redshift, when compared to the interstellar medium near the Sun. This has been interpreted as implying that the gas in damped Lyman-α systems is warm like the nearby neutral intercloud medium, rather than cool, as in the clouds which give rise to most H I absorption in the Milky Way. Other lines of evidence suggest that the gas in damped Lyman-α systems -in whole or part -is actually cool; spectroscopy of neutral and ionized carbon, discussed here, shows that the damped Lyman-α systems observed at lower redshift z < 2.3 are largely cool, while those seen at z > 2.8 are warm (though not devoid of H2). To interpret the observations of carbon and hydrogen we constructed detailed numerical models of H2 formation under the conditions of two-phase thermal equilibrium, like those which account for conditions near the Sun, but with varying metallicity, dust-gas ratio, etc. We find that the low metallicity of damped Lyman-α systems is enough to suppress H2 formation by many orders of magnitude even in cool diffuse clouds, as long as the ambient optical/uv radiation field is not too small. For very low metallicity and under the most diffuse conditions, H2 formation will be dominated by slow gas-phase processes not involving grains, and a minimum molecular fraction in the range 10 −8 −10 −7 is expected.

show abstract

The soft X-ray diffuse background observed with the HEAO 1 low-energy detectors

Cited by 62 publications

References 0 publications

Limits to the 14 keV Extragalactic X-Ray Background

Limits to the 14 keV Extragalactic X-Ray Background

X-ray spectral properties of active galactic nuclei in the Chandra Deep Field South

Two-phase equilibrium and molecular hydrogen formation in damped Lyman-alpha systems

Contact Info

Product

Resources

About