scholarly journals The Extragalactic X-Ray and γ-ray Backgrounds

1990 ◽  
Vol 139 ◽  
pp. 345-356
Author(s):  
G. Setti
Keyword(s):  
Big Bang ◽  
Prima Facie ◽  
Baryon Density ◽  
Primordial Nucleosynthesis ◽  
X Ray ◽  
Γ Ray ◽  
X Ray Background ◽  
Seyfert Type ◽  
Number Of Authors

The long-standing problem of the origin of the extragalactic X-ray background (XRB) is reviewed. Although the shape of the spectrum in the 3–100 keV interval is suggestive of an optically thin bremsstrahlung at ~ 40 keV, the interpretation in terms of a hot intergalactic gas (IGG) requires a rather extreme energy supply and a gas density conflicting with the baryon density upper limit derived from primordial nucleosynthesis calculations in the standard hot big-bang model. A summary discussion of the estimated contributions from the integrated X-ray emission of known classes of extragalactic discrete sources at a reference energy of 2 keV is given. Although these estimates are still uncertain, the subtraction of a “minimum” contribution drastically modifies the 40 keV thermal shape, which is the prima facie evidence of a hot IGG. AGNs are the main contributors. Low luminosity AGNs (Seyfert type 1 nuclei) at redshift z = 1 − 2 may in fact saturate the 2 keV XRB, but their observed hard X-ray spectra are on the average unlike (much too steep) that of the XRB. This has led a number of authors to postulate new classes of sources and some exotic models which are briefly summarized. However, if a recently proposed unified scheme of AGNs holds, then the bulk of the XRB intensity can be explained independently of the observed spectral differences and with a mild cosmological evolution. The origin of the extragalactic γ-ray background is briefly commented upon in the concluding remarks.

scholarly journals The Sources of the Hard X-Ray Background

1996 ◽  
Vol 168 ◽  
pp. 263-270
Author(s):  
Giancarlo Setti ◽  
Andrea Comastri
Keyword(s):  
Energy Spectrum ◽  
Energy Density ◽  
Energy Flux ◽  
Large Fraction ◽  
Power Laws ◽  
Prima Facie ◽  
Folding Energy ◽  
Hard Component ◽  
X Ray ◽  
X Ray Background

The hard component (3 keV – ~ MeV) of the X-ray background (XRB) comprises the largest portion, ~ 90%, of the overall XRB intensity. The observed isotropy (the entire Galaxy is transparent above 3 keV) provides aprima facieevidence of its prevailing extragalactic nature. A large fraction (~ 75%) of the energy flux falls in the 3 – 100 keV band, the corresponding energy density being ≃ 5×10−5eV cm−3, of which 50% is confined to the narrower 3 – 20 keV band. Although the energy flux carried by the XRB is relatively small compared to other extragalactic backgrounds, it was soon realized that it cannot be accounted for in terms of sources and processes confined to the present epoch. An analysis of the combined observed spectra (Gruber 1992) concludes that, while a thermal bremsstrahlung with an e-folding energy = 41.13 keV accurately fits the data up to 60 keV, above this energy the sum of two power laws is required with normalizations such that at 60 keV the spectral index is ~ 1.6, gradually flattening to ~ 0.7 at MeV energies. It should also be noted that below 10 keV the XRB energy spectrum is well represented by a power law of index α = 0.4 (I∝E−α).

scholarly journals High Energy Astrophysics and Cosmology

1980 ◽  
Vol 5 ◽  
pp. 753-761
Author(s):  
L. Woltjer
Keyword(s):  
High Energy ◽  
Selection Effects ◽  
High Energy Astrophysics ◽  
X Ray ◽  
Cosmological Tests ◽  
Γ Ray ◽  
X Ray Background ◽  
Galactic Sources

AbstractA brief review is given of cosmological tests based on quasars, of source evolution and of the X-ray background. The cosmological tests are still inconclusive because of the limited material available and the possibility of serious selection effects. Quasars and other extra-galactic sources account for most or all of the background in the 1-10 kev range and very possibly also at higher X- and γ-ray energies.

Optical Follow-Up Observations of the ASCA Large Sky Survey

1998 ◽  
Vol 188 ◽  
pp. 465-466
Author(s):  
M. Akiyama ◽  
K. Ohta ◽  
T. Yamada ◽  
Y. Ueda ◽  
T. Takahashi ◽  
...  
Keyword(s):  
Clusters Of Galaxies ◽  
X Ray ◽  
Sigma Level ◽  
Sky Survey ◽  
X Ray Background ◽  
Band Survey

To reveal the origin of the cosmic X-ray background (CXB) in the hard band, we are now conducting a wide (~ 7 deg2) and deep (~ 1 × 10−13 erg sec−1 cm−2 in the 2-10 keV band) survey with the ASCA (the ASCA Large Sky Survey, hereafter LSS). We have detected 83 sources above 4 sigma level in the 0.7-10 keV band with the GIS and resolved ~30% of the CXB in the 2-10 keV band into discrete sources (Ueda 1996). AGNs (type 1 and type 2) and clusters of galaxies are expected to be major contributers to these X-ray sources.

PRIMORDIAL NUCLEOSYNTHESIS CONSTRAINTS ON NEUTRINO DEGENERACY

1993 ◽  
Vol 02 (04) ◽  
pp. 381-400
Author(s):  
HO-SHIK KANG
Keyword(s):  
Critical Density ◽  
Big Bang ◽  
Baryon Density ◽  
Electron Neutrino ◽  
Light Elements ◽  
Big Bang Nucleosynthesis ◽  
Primordial Nucleosynthesis ◽  
Extended Range ◽  
Free Parameters ◽  
Η Value

Based on the work by Kang and Steigman, I review the effects of neutrino degeneracy on big bang nucleosynthesis (BBN). Since the electron-neutrino degeneracy and the non-electron-neutrino degeneracy play a different role in the synthesis of the light elements ( D , 3 He , 4 He , 7 Li ), besides the baryon asymmetry (the nucleon-to-photon ratio; η ≡ nB/nγ) there are two additional free parameters in our scenario of degenerate BBN. An extended range of these parameters has been explored. It is shown that at a given η value, the agreement of the predicted primordial abundances of the light elements with those observationally inferred abundances restricts the permitted range of neutrino degeneracies, particularly the electron-neutrino degeneracy. Furthermore, we find that a large baryon density, even baryon-dominated, critical density (ΩB=1) Universe successfully provides the consistency between the predicted and observed abundances of all the light elements if neutrinos are degenerate enough. For an ΩB=1 Universe, for example, η10=80 is permitted if the electron-neutrino degeneracy and the expansion rate due to the non-electron-neutrino degeneracies fall in the ranges 1.2 ≲ ξνe ≲ 1.5, 17 ≲ S (ξνμ,τ) ≲ 33, respectively.

scholarly journals Big Bang Nucleosynthesis

2002 ◽  
Vol 187 ◽  
pp. 1-15
Author(s):  
D.N. Schramm
Keyword(s):  
Systematic Error ◽  
Big Bang ◽  
Baryon Density ◽  
Galactic Evolution ◽  
Big Bang Nucleosynthesis ◽  
X Ray ◽  
Baryonic Density ◽  
The Big Bang ◽  
The Universe

Big Bang Nucleosynthesis (BBN) is on the verge of undergoing a transformation now that extragalactic deuterium is being measured. Previously, the emphasis was on demonstrating the concordance of the Big Bang Nucleosynthesis model with the abundances of the light isotopes extrapolated back to their primordial values using stellar and Galactic evolution theories. Once the primordial deuterium abundance is converged upon, the nature of the field will shift to using the much more precise primordial D/H to constrain the more flexible stellar and Galactic evolution models (although the question of potential systematic error in 4He abundance determinations remains open). The remarkable success of the theory to date in establishing the concordance has led to the very robust conclusion of BBN regarding the baryon density. The BBN constraints on the cosmological baryon density are reviewed and demonstrate that the bulk of the baryons are dark and also that the bulk of the matter in the universe is non-baryonic. Comparison of baryonic density arguments from Lyman-α clouds, x-ray gas in clusters, the Sunyaev-Zeldovich effect, and the microwave anisotropy are made and shown to be consistent with the BBN value.

scholarly journals Constraints on Dark Matter from Primordial Nucleosynthesis

1987 ◽  
Vol 117 ◽  
pp. 499-523
Author(s):  
Jean Audouze
Keyword(s):  
Dark Matter ◽  
Chemical Evolution ◽  
Big Bang ◽  
Baryon Density ◽  
Cosmological Parameter ◽  
Big Bang Nucleosynthesis ◽  
Primordial Nucleosynthesis ◽  
The Galaxy ◽  
Predicted Values ◽  
The Universe

Primordial nucleosynthesis which is responsible for the formation of the lightest elements (D, 3He, 4He and 7Li) provides a unique way to determine the present baryon density pB in the Universe and therefore the corresponding cosmological parameter ΩB. After a brief summary of the relevant abundance determinations and of the consequences of the Standard Big Bang nucleosynthesis, it is argued that one needs to call for specific models of chemical evolution of the Galaxy in order to reconcile the observations with the predictions of this model. In this context the predicted values for ΩB should range from 4 10−3 to 6 10−2. These values are significantly lower than those deduced from current M/L determinations.

scholarly journals NON-CRITICAL STRING COSMOLOGIES

2007 ◽  
Vol 22 (31) ◽  
pp. 5622-5637 ◽  
Author(s):  
A. B. LAHANAS
Keyword(s):  
Dark Matter ◽  
Big Bang ◽  
Baryon Density ◽  
Primordial Nucleosynthesis ◽  
Cosmological Data ◽  
Late Transition ◽  
Relic Abundance ◽  
Supersymmetric Theories

Non-critical String Cosmologies are offered as an alternative to Standard Big Bang Cosmology. The new features encompassed within the dilaton dependent non-critical terms affect the dynamics of the Universeś evolution in an unconventional manner being in agreement with the cosmological data. Non-criticality is responsible for a late transition to acceleration at redshifts z =0.2. The role of the uncoupled rolling dilaton to relic abundance calculations is discussed. The uncoupled rolling dilaton dilutes the neutralino relic densities in supersymmetric theories by factors of ten, relaxing considerably the severe WMAP Dark Matter constraints, while at the same time leaves almost unaffected the baryon density in agreement with primordial Nucleosynthesis.

Extragalactic X-ray observations - The 2A catalogue and supplementary results from the Ariel 5 sky survey

1979 ◽  
Vol 366 (1726) ◽  
pp. 375-390 ◽  
Keyword(s):  
Radio Galaxies ◽  
Status Report ◽  
Galactic Latitude ◽  
Clusters Of Galaxies ◽  
X Ray ◽  
Major Class ◽  
Sky Survey ◽  
Emission Line Galaxies ◽  
Seyfert Type

A status report is given on the Ariel 5 sky survey for X -ray sources at galactic latitude | b | ≽ 10°. The operation of the sky survey instrument (s. s. i.) is briefly reviewed and the production of the recently published 2A catalogue of X -ray sources is described. A remarkably high percentage ( ca . 75%) of the 105 sources in the 2A catalogue have now been identified with extragalactic, or, in a few cases, galactic, counterparts, and this is attributed to the improved location of sources (cf. the earlier 3U catalogue), and the strong correlation with optically bright or outstanding objects. The 2A catalogue and subsequent studies have established active galaxies, especially of the Seyfert type 1, to be a major class of powerful X -ray emitter. Present members of this class, which also includes a number of emission line galaxies, radio galaxies and quasars, are listed and their basic X -ray properties reviewed. The second major class of extragalactic X -ray source, clusters of galaxies, first established with the Uhuru satellite, has been substantially extended by Ariel 5. This work also is briefly reviewed. Finally, additional X -ray data from the s. s. i. (still operative) is being used to revise and add to the 2A catalogue. Several examples of this more recent work are given, together with an estimate of the expected content of the final s. s. i. catalogue when the mission is terminated (now planned for October 1979).

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