190 research outputs found
The Iron Line Background
We investigate the presence of iron line emission among faint X-ray sources
identified in the 1Ms Chandra Deep Field South and in the 2Ms Chandra Deep
Field North. Individual source spectra are stacked in seven redshift bins over
the range z=0.5-4. We find that iron line emission is an ubiquitous property of
X-ray sources up to z~3. The measured line strengths are in good agreement with
those expected by simple pre-Chandra estimates based on X-ray background
synthesis models. The average rest frame equivalent width of the iron line does
not show significant changes with redshift.Comment: 5 pages, 2 figures, ApJ Letters in press (include emulateapj.sty
Radio Loud AGNs are Mergers
We measure the merger fraction of Type 2 radio-loud and radio-quiet active
galactic nuclei at z>1 using new samples. The objects have HST images taken
with WFC3 in the IR channel. These samples are compared to the 3CR sample of
radio galaxies at z>1 and to a sample of non-active galaxies. We also consider
lower redshift radio galaxies with HST observations and previous generation
instruments (NICMOS and WFPC2). The full sample spans an unprecedented range in
both redshift and AGN luminosity. We perform statistical tests to determine
whether the different samples are differently associated with mergers. We find
that all (92%) radio-loud galaxies at z>1 are associated with recent or ongoing
merger events. Among the radio-loud population there is no evidence for any
dependence of the merger fraction on either redshift or AGN power. For the
matched radio-quiet samples, only 38% are merging systems. The merger fraction
for the sample of non-active galaxies at z>1 is indistinguishable from
radio-quiet objects. This is strong evidence that mergers are the triggering
mechanism for the radio-loud AGN phenomenon and the launching of relativistic
jets from supermassive black holes. We speculate that major BH-BH mergers play
a major role in spinning up the central supermassive black holes in these
objects.Comment: 16 pages, 6 figures, accepted for publication in the Ap
X-ray spectroscopy of the z=6.4 quasar J1148+5251
We present the 78-ks Chandra observations of the quasar SDSS
J1148+5251. The source is clearly detected in the energy range 0.3-7 keV with
42 counts (with a significance ). The X-ray spectrum is
best-fitted by a power-law with photon index absorbed by a gas
column density of .
We measure an intrinsic luminosity at 2-10 keV and 10-40 keV equal to , comparable with luminous local and
intermediate-redshift quasar properties. Moreover, the X-ray to optical
power-law slope value () of J1148 is consistent
with the one found in quasars with similar rest-frame 2500 \AA ~luminosity
(\AA). Then we use Chandra data
to test a physically motivated model that computes the intrinsic X-ray flux
emitted by a quasar starting from the properties of the powering black hole and
assuming that X-ray emission is attenuated by intervening, metal-rich () molecular clouds distributed on kpc scales in the host
galaxy. Our analysis favors a black hole mass and a molecular hydrogen mass , in good agreement with estimates obtained from previous studies. We
finally discuss strengths and limits of our analysis.Comment: 9 pages, 3 figures, 1 table, MNRAS in pres
X-ray variability with WFXT: AGNs, transients and more
The Wide Field X-ray Telescope (WFXT) is a proposed mission with a high
survey speed, due to the combination of large field of view (FOV) and effective
area, i.e. grasp, and sharp PSF across the whole FOV. These characteristics
make it suitable to detect a large number of variable and transient X-ray
sources during its operating lifetime. Here we present estimates of the WFXT
capabilities in the time domain, allowing to study the variability of thousand
of AGNs with significant detail, as well as to constrain the rates and
properties of hundreds of distant, faint and/or rare objects such as X-ray
Flashes/faint GRBs, Tidal Disruption Events, ULXs, Type-I bursts etc. The
planned WFXT extragalactic surveys will thus allow to trace variable and
transient X-ray populations over large cosmological volumes.Comment: Proceedings of "The Wide Field X-ray Telescope Workshop", held in
Bologna, Italy, Nov. 25-26 2009 (arXiv:1010.5889). To appear in Memorie della
Societ\`a Astronomica Italiana 2010 - Minor corrections to text
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