1,166 research outputs found
Spectral Statistics and Local Luminosity Function of a Hard X-ray Complete Sample of Brightest AGNs
We have measured the X-ray spectral properties of a complete flux-limited
sample of bright AGNs from HEAO-1 all-sky catalogs to investigate their
statistics and provide greater constraints on the bright-end of the hard X-ray
luminosity function (HXLF) of AGNs and the AGN population synthesis model of
the X-ray background. Spectral studies using data from ASCA, XMM-Newton and/or
Beppo-SAX observations have been made for almost all AGNs in this sample.
The spectral measurements enable us to construct the neutral absorbing column
density (Log nH) distribution and separate HXLFs for absorbed (Log nH[cm-2]>
21.5) and unabsorbed AGNs in the local universe. Our results show evidence for
a difference in the shapes of HXLFs of absorbed and unabsorbed AGNs in that
absorbed AGN HXLF drops more rapidly at higher luminosities than that of
unabsorbed AGNs, which is similar to that previously reported. In the Lx - nH
plot, we found no AGN in the high-luminosity high-intrinsic absorption regime
(Log Lx[erg/s]> 44.5, Log nH[cm-2]> 21.5) in our sample, where we expect about
5 AGNs if we assume that absorbed and unabsorbed having identical AGN HXLF
shapes. We also find that the observed flux with ASCA or XMM-Newton is smaller
than that with HEAO-1 by a factor of 0.29 on average, which is expected for
re-observation of sources with a factor 2.5 variability amplitude scale.Comment: 43 pages(one column), 10 figures(5 electronic only figures have been
included in the preprint source (tar.gz file)), accepted by The Astronomical
Journal, 9. Mar 200
A population of intermediate-mass black holes in dwarf starburst galaxies up to redshift=1.5
We study a sample of 50,000 dwarf starburst and late-type galaxies
drawn from the COSMOS survey with the aim of investigating the presence of
nuclear accreting black holes (BHs) as those seed BHs from which supermassive
BHs could grow in the early Universe. We divide the sample into five complete
redshift bins up to and perform an X-ray stacking analysis using the
\textit{Chandra} COSMOS-Legacy survey data. After removing the contribution
from X-ray binaries and hot gas to the stacked X-ray emission, we still find an
X-ray excess in the five redshift bins that can be explained by nuclear
accreting BHs. This X-ray excess is more significant for . At higher
redshifts, these active galactic nuclei could suffer mild obscuration, as
indicated by the analysis of their hardness ratios. The average nuclear X-ray
luminosities in the soft band are in the range 10 erg s.
Assuming that the sources accrete at 1\% the Eddington rate, their BH
masses would be 10 M, thus in the intermediate-mass BH
regime, but their mass would be smaller than the one predicted by the
BH-stellar mass relation. If instead the sources follow the correlation between
BH mass and stellar mass, they would have sub-Eddington accreting rates of
10 and BH masses 1-9 10 M. We thus
conclude that a population of intermediate-mass BHs exists in dwarf starburst
galaxies, at least up to =1.5, though their detection beyond the local
Universe is challenging due to their low luminosity and mild obscuration unless
deep surveys are employed.Comment: 10 pages, 7 figures, ApJ in pres
Heating of the IGM
Using the cosmic virial theorem, Press-Schechter analysis and numerical
simulations, we compute the expected X-ray background (XRB) from the diffuse
IGM with the clumping factor expected from gravitational shock heating. The
predicted fluxes and temperatures are excluded from the observed XRB. The
predicted clumping can be reduced by entropy injection. The required energy is
computed from the two-point correlation function, as well as from
Press-Schechter formalisms. The minimal energy injection of 1 keV/nucleon
excludes radiative or gravitational heating as a primary energy source. We
argue that the intergalactic medium (IGM) must have been heated through violent
processes such as massive supernova bursts. If the heating proceeded through
supernova explosions, it likely proceeded in bursts which may be observable in
high redshift supernova searches. Within our model we reproduce the observed
cluster luminosity-temperature relation with energy injection of 1 keV/nucleon
if this injection is assumed to be uncorrelated with the local density. These
parameters predict that the diffuse IGM soft XRB has a temperature of ~1 keV
with a flux near 10 keV/cm^2 s str keV, which may be detectable in the near
future.Comment: to appear in ApJ Lett., 11 pages incl 1 figur
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