1,760 research outputs found
What's wrong with AGN models for the X-ray background ?
The origin of the hard X-ray background (XRB) as a superposition of
unabsorbed and absorbed Active Galactic Nuclei is now widely accepted as the
standard model. The identification of faint X-ray sources in ROSAT, ASCA, and
BeppoSAX medium-deep surveys and their average spectral properties are in broad
agreement with the model predictions. However, AGN models, at least in their
simplified version, seem to be at odds with some of the most recent findings
calling for substantial revisions. I will review the recent XRB "best fit"
models and discuss how the foreseen XMM and Chandra surveys will be able to
constrain the allowed parameter space.Comment: 10 pages, 5 figures. Invited talk to appear in the Proceedings of the
Conference X-ray Astronomy '999: Stellar Endpoints, AGNs and the Diffuse
X-ray Background. (September 6-10 - 1999
The Missing X-ray Background
The fraction of the hard X-ray background (XRB) resolved into individual
sources by the deep Chandra and XMM-Newton surveys strongly depends on the
adopted energy range and decreases with increasing energy. As a consequence,
the nature of the sources of the even harder (>10 keV) XRB remains
observationally poorly constrained. I will briefly discuss the need for X-ray
observations above 10 keV.Comment: to appear in the proceedings of 'Multiwavelength AGN surveys'
(Cozumel, December 8-12 2003), ed. R. Maiolino and R. Mujic
ASCA spectroscopy of the luminous infrared galaxy NGC6240: X-ray emission from a starburst and a buried active nucleus
We present an X-ray spectral study of the prototype far-infrared galaxy
NGC6240 from ASCA. The soft X-ray spectrum (below 2 keV) shows clear signatures
of thermal emission well described with a multi-temperature optically-thin
plasma, which probably originates in a powerful starburst. Strong hard X-ray
emission is also detected with ASCA and its spectrum above 3 keV is extremely
flat with a prominent iron K line complex, very similar to that seen in the
Seyfert 2 galaxy NGC1068 but about an order of magnitude more luminous
[L(3-10keV)=1.4E42 erg/s]. The hard X-ray spectrum indicates that only
reflected X-rays of an active galactic nucleus (AGN) buried in a heavy
obscuration [N(H)>2E24 cm-2] are visible. This is evidence for an AGN in
NGC6240 emitting possibly at a quasar luminosity and suggests its significant
contribution to the far-infrared luminosity.Comment: 9 pages, 6 Postscript figures, to appear in MNRA
On the low detection efficiency of disk water megamasers in Seyfert 2 AGN
Disk megamasers are a unique tool to study active galactic nuclei (AGN)
sub-pc environment, and precisely measure some of their fundamental parameters.
While the majority of disk megamasers are hosted in heavily obscured (i.e.,
Seyfert 2, Sy2) AGN, the converse is not true, and disk megamasers are very
rarely found even in obscured AGN. The very low detection rate of such systems
in Sy2 AGN could be due to the geometry of the maser beaming, which requires a
strict edge-on condition. We explore some other fundamental factors which could
play a role in a volume-limited survey of disk megamasers in Sy2 galaxies, most
importantly the radio luminosity.Comment: 2 pages, 2 figures. To appear in the Proceedings IAU Symposium No.
336, 2017 "Astrophysical Masers: Unlocking the Mysteries of the Universe
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
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