Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The instrinsic UV/soft x-ray spectrum of quasars   The detection of a HeII absorption trough in the spectra of three
high-redshift quasars provides new constraints on the spectral shape of the UV
extragalactic background. The relative strengths of the observed flux
decrements at the rest-frame wavelengths of 1216 and 304 A require a relatively
soft radiation field at 4 ryd compared to 1 ryd. If the ionizing metagalactic
flux is dominated by the integrated light from QSOs, and the HeII reionization
of the universe was completed well before z~3, then the UV/soft X-ray spectrum
of individual quasars at these epochs must, once the cosmological ``filtering''
through material along the line of sight is taken into account, satisfy similar
constraints on the average. We model the propagation of AGN-like ionizing
radiation through the intergalactic medium using CUBA, a numerical code
developed in our prior work. We show that, in order to explain the reported
HeII absorption, any thermal component responsible for the ``soft X-ray
excess'' observed in the spectra of AGNs at z<0.3 must, at z~3, have a typical
temperature >80 eV, and a luminosity not exceeding 20% of that of the ``UV
bump''.

30
The categories of this paper are: q-alg
27
The categories of this paper are: comp-gas
11
The categories of this paper are: hep-lat
37
The categories of this paper are: supr-con
22
The categories of this paper are: chao-dyn
3
The categories of this paper are: physics
21
The categories of this paper are: adap-org
18
The categories of this paper are: econ
25
The categories of this paper are: patt-sol
1
The categories of this paper are: cs
31
The categories of this paper are: ao-sci
19
The categories of this paper are: eess
15
The categories of this paper are: nucl-ex
29
The categories of this paper are: funct-an
35
The categories of this paper are: mtrl-th
9
The categories of this paper are: nlin