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X-ray Cosmology: from
dark ages to the present Universe
With this mission we aim to
address three key themes of modern Cosmology and astrophysics:
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Discover
and study the first "X-ray light" from primordial gravitationally
bounded object in the Universe at z=10-30.
Our observational window on the Universe extends in distance up to
z=6.2, the reshift of the most distant object discovered so far (a
quasar, ref.), and then recovers at z=1000, the epoch of primordial
fluctuations measured by BOOMERANG and MAP. The formation of the
first objects, stars, and protogalaxies, should have taken place at
epochs corresponding to z=10-30, certainly beyond z=5. These first
gravitationally bound proto-systems are the result of the evolution
of the primordial fluctuations observed at z=1000, this evolution
depending on cosmological models and dark-matter properties. The big
observational gap in between these epochs is then particularly
serious
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Trace the cosmic dark
matter web at z<2 in X-rays. In the
local (z<1) Universe the evolution of large-scale structures
dominated by dark matter is challenging the observers. The sudden
decrease of the baryon density in the local Universe is one of the
unresolved issues of Cosmology. The most intriguing solution is that
most of the baryon are in a hot phase, that can be detected
primarily through X-ray measurements. Numerical simulations predict
that, at z<1, most of the baryons fall onto the cosmic web pattern
of the dark matter, and are heated at T~106 K by shock
mechanisms, forming filamentary and sheet-like structures. Such gas
is called Warm-Hot Intergalactic Medium (WHIM). One of the most
promising methods to study this component is by searching for the
narrow absorption features - the strongest of which is OVII (at
0.574 keV rest frame) - imprinted by the WHIM on the X-ray spectrum
of a bright background object.
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Study the history of metal
enrichment in the Universe from early epoch to the local Universe.
X-ray "light" emitted from distant sources will be selectively
absorbed at specific frequencies by metals at the source, thus
allowing to build up a "map" of metal abundances and hence star
formation rate as function of the redshift.
Gamma-Ray Bursts will be the beacons
leading the investigation of the evolving Universe, from early obscure
epochs, when the stars and primordial galaxies formed, to the local
Universe.
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They are the brightest and most distant sources
in the Universe. The radiation intensity of GRB's is so high that
they can be detectable out to much larger distances than those of
the most luminous quasars or galaxies observed so far. About 10 % of
GRB observed with BeppoSAX have an afterglow X-ray fluence (integrated
from 60 sec to 60.000 after the main pulse) greater than 4 10-6
erg cm-2, and 2% of the events a factor 5 times brighter.
In comparison, a primordial super-massive Black Hole (106
Msun) at z=10 accreting at 10% of the Eddington limit
would have a X-ray flux of about 2 10-18 erg cm-2
s-1, while a 1041 erg/s galaxy would have a
flux of 5 10-20 erg cm-2 s-1, at
the limit of the XEUS deep survey. Even for an integration time of
106 sec, the corresponding fluences would still be 106-8
times lower than compared to GRB afterglow.
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Gamma-ray burst (specifically the so called
long-burst) are now unquestionably associated with explosions of
massive stars taking place in star formation regions. This evidence
is independently supported by the presence of X-ray lines in their
spectra, the location of optical afterglow in the center of host
galaxies, the association with Supernova events.
Based on these properties, a mission able to localize GRB
and to perform fast (<60 sec) follow-up observations with X-ray
telescope and a focal-plane high resolution spectrometer will open a new
window to:
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Discover and study primordial star-forming
galaxies at z>5-10. These objects are obscure in the optical due to
the dusty environment (and Ly alpha forest absorption at z>5). On
the other hand, X-rays and gamma-ray photons produced by a GRB will
easily pierce through this environment, pin-pointing the location of
the host galaxy and allowing to measure its distance in X-rays by
measuring the redshift of X-ray features (X-ray redshift).
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Study the line-of-sight absorption features
imprinted on the bright X-ray spectrum of the GRB by the medium in
the line of sight between us and the GRB, in particular:
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Caption : a simulation of X-ray
edges produced by metals (Si, S, Ar, Fe) by a medium with
column density NH=5x1022 cm-2 and
solar-like abundances in the host galaxy of a bright GRB at
z=5, as observed ESTREMO with an observation starting 60 s
after the main pulse and lasting 60 ks |
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The narrow absorption lines
expected to be produced by the Warm Hot Intragalactic Medium at
z<2. Using bright GRB afterglows as background sources gives the
big advantage, with respect to AGN, to reach out much larger
distances, increasing the number of filaments through the line
of sight. For a burst at z>0.5 at least one system with an
equivalent width >0.4 eV is expected along a random line of
sight, while many more (8) are expected for just twice weaker
systems.
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Caption : simulations of WHIM
absorption features from OVII as expected from filaments (at
different z, with EW=0.2-0.5 eV) in the l.o.s. toward a GRB
with Fluence~10-5 erg cm-2 as observed
with ESTREMO (in 100 ks). We expect to detect about 10% of
GRB (10 events per year per 3 sr) with 4 million counts in
the TES focal plane detector. |
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