466 research outputs found
Intergalactic UV Background Radiation Field
We have performed proximity effect analysis of low and high resolution data,
considering detailed frequency and redshift dependence of the AGN spectra
processed through galactic and intergalactic material. We show that such a
background flux, calculated using the observed distribution of AGNs, falls
short of the value required by the proximity effect analysis by a factor of
2.7. We have studied the uncertainty in the value of the required flux
due to its dependence on the resolution, description of column density
distribution, systemic redshifts of QSOs etc. We conclude that in view of these
uncertainties the proximity effect is consistent with the background
contributed by the observed AGNs and that the hypothesized presence of an
additional, dust extinct, population of AGNs may not be necessary.Comment: To be published in the Journal of Astronomy and Astrophysics aasms, 2
figures, 2 tables. Paper replaced to include the figure
Chandra Detection of X-ray Absorption Associated with a Damped Lyman Alpha System
We have observed three quasars, PKS 1127-145, Q 1331+171 and Q0054+144, with
the ACIS-S aboard the Chandra X-ray Observatory, in order to measure soft X-ray
absorption associated with intervening 21-cm and damped Ly absorbers.
For PKS 1127-145, we detect absorption which, if associated with an intervening
z_{abs}=0.312 absorber, implies a metallicity of 23% solar. If the absorption
is not at z_{abs}=0.312, then the metallicity is still constrained to be less
than 23% solar. The advantage of the X-ray measurement is that the derived
metallicity is insensitive to ionization, inclusion of an atom in a molecule,
or depletion onto grains. The X-ray absorption is mostly due to oxygen, and is
consistent with the oxygen abundance of 30% solar derived from optical nebular
emission lines in a foreground galaxy at the redshift of the absorber.
For Q1331+171 and Q 0054+144, only upper limits were obtained, although the
exposure times were intentionally short, since for these two objects we were
interested primarily in measuring flux levels to plan for future observations.
The imaging results are presented in a companion paper.Comment: 23 pages, 6 figures, accepted for publication in the Astrophysical
Journa
The X-ray absorber of PKS2126-158
BeppoSAX observed the z=3.27 quasar PKS2126-158 on 1999 May 24-28 when its
2-10keV and 0.1-2.5keV fluxes were 1.1E-11 and 4.4E-12 cgs respectively, a
factor of 2 higher than in all previous ROSAT and ASCA observations and 40%
higher than in two more recent Chandra and XMM-Newton observations. The
shortest detected rest frame variability timescale is of a few months,
comparable to the causal timescale associated to an emission region of ~10
Schwarzschild radii around a few 1E10 solar masses black hole. The source is
detected with a signal to noise ratio S/N>=3 up to ~50 keV, 215 keV rest frame.
The BeppoSAX observations confirm the presence of low energy absorption along
the line of sight, independent on the continuum model adopted, at high
confidence level. Despite the limited spectral resolution of the BeppoSAX LECS
and MECS it is possible to put constraints on different absorption and
continuum models, but not to unambiguously determine the redshift of the
absorber. If the absorber is not significantly ionized the BeppoSAX data do
prefer an absorber at z<=2.7. Strong and complex metal line systems along the
line of sight to PKS2126-158 have been found at z=0.6631 and at 2.64<z<2.82.
They could well be associated to the X-ray absorption. Conversely, an ionized
(``warm'') absorber at the quasar redshift provides a good fit only if the iron
abundance is smaller than ~0.3 solar, while that of the other elements is fixed
to the solar value.Comment: A&A in pres
X-ray Properties of the GigaHertz-Peaked and Compact Steep Spectrum Sources
We present {\it Chandra} X-ray Observatory observations of Giga-Hertz Peaked
Spectrum (GPS) and Compact Steep Spectrum (CSS) radio sources. The {\it
Chandra} sample contains 13 quasars and 3 galaxies with measured 2-10 keV X-ray
luminosity within erg s. We detect all of the
sources, five of which are observed in X-ray for the first time. We study the
X-ray spectral properties of the sample. The measured absorption columns in the
quasars are different than those in the galaxies in the sense that the quasars
show no absorption (with limits ) while the galaxies
have large absorption columns () consistent with
previous findings. The median photon index of the sources with high S/N is
and it is larger than the typical index of radio loud
quasars. The arcsec resolution of {\it Chandra} telescope allows us to
investigate X-ray extended emission, and look for diffuse components and X-ray
jets. We found X-ray jets in two quasars (PKS 1127-145, B2 0738+32), an X-ray
cluster surrounding a CSS quasar (z=1.1, 3C 186), detected a possible binary
structure in 0941-080 galaxy and an extended diffuse emission in galaxy PKS B2
1345+12. We discuss our results in the context of X-ray emission processes and
radio source evolution. We conclude that the X-ray emission in these sources is
most likely unrelated to a relativistic jet, while the sources' radio-loudness
may suggest a high radiative efficiency of the jet power in these sources.Comment: 15 pages, to be published in Ap
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