988 research outputs found
The large-scale bias of the hard X-ray background
Recent deep X-ray surveys combined with spectroscopic identification of the
sources have allowed the determination of the rest-frame 2-8 keV luminosity as
a function of redshift. In addition, an analysis of the HEAO1 A2 2-10 keV
full-sky map of the X-ray background (XRB) reveals clustering on the scale of
several degrees. Combining these two results in the context of the currently
favored Lambda-CDM cosmological model implies an average X-ray bias factor,
b_x, of b_x^2 = 1.12 +- 0.33, i.e., b_x = 1.06 +- 0.16. These error estimates
include only statistical error; the systematic error sources, while comparable,
appear to be sub-dominant. This result is in contrast to the large biases of
some previous estimates and is more in line with current estimates of the
optical bias of L* galaxies.Comment: 6 pages, 3 eps figures, accepted for ApJ, vol. 612, 10 September 200
Chromatic correlations at injection and related ejection problems in separated sector cyclotrons
International audienceInjection into a cyclotron, in order to preventemittance and phase spread dilution, requires propercouplings in the matching. One must first introduce aAP/r' associated with an r/AW coupling (through simplecticconditions) ; according to the angle of theaccelerating dees and the choice of harmonic numberthe(r,r') acceptance may also have to be tilted. Allthese effects are investigated in the case of theGANIL SSC's. At extraction corresponding correlationsexist. For a resonant system, extraction may be difficultwhen the energy spread is large because of thelarge coupling induced by resonance. A precessionalextraction which has been studied might in this casebe more efficient. Other ways for making extractioneasier are also considered. Moreover a new method ofphase compression at injection into the SSC is presentlyunder study at GANIL
The Large-Scale Structure of the X-ray Background and its Cosmological Implications
A careful analysis of the HEAO1 A2 2-10 keV full-sky map of the X-ray
background (XRB) reveals clustering on the scale of several degrees. After
removing the contribution due to beam smearing, the intrinsic clustering of the
background is found to be consistent with an auto-correlation function of the
form (3.6 +- 0.9) x 10^{-4} theta^{-1} where theta is measured in degrees. If
current AGN models of the hard XRB are reasonable and the cosmological
constant-cold dark matter cosmology is correct, this clustering implies an
X-ray bias factor of b_X ~ 2. Combined with the absence of a correlation
between the XRB and the cosmic microwave background, this clustering can be
used to limit the presence of an integrated Sachs-Wolfe (ISW) effect and
thereby to constrain the value of the cosmological constant, Omega_Lambda <
0.60 (95 % C.L.). This constraint is inconsistent with much of the parameter
space currently favored by other observations. Finally, we marginally detect
the dipole moment of the diffuse XRB and find it to be consistent with the
dipole due to our motion with respect to the mean rest frame of the XRB. The
limit on the amplitude of any intrinsic dipole is delta I / I < 5 x 10^{-3} at
the 95 % C.L. When compared to the local bulk velocity, this limit implies a
constraint on the matter density of the universe of Omega_m^{0.6}/b_X(0) >
0.24.Comment: 15 pages, 8 postscript figures, to appear in the Astrophysical
Journal. The postscript version appears not to print, so use the PDF versio
A strongly changing accretion morphology during the outburst decay of the neutron star X-ray binary 4U 1608â52
It is commonly assumed that the properties and geometry of the accretion flow in transient low-mass X-ray binaries (LMXBs) significantly change when the X-ray luminosity decays below âŒ10â»ÂČ of the Eddington limit (L_(Edd)). However, there are few observational cases where the evolution of the accretion flow is tracked in a single X-ray binary over a wide dynamic range. In this work, we use NuSTAR and NICER observations obtained during the 2018 accretion outburst of the neutron star LMXB 4U 1608â52, to study changes in the reflection spectrum. We find that the broad FeâKα line and Compton hump, clearly seen during the peak of the outburst when the X-ray luminosity is âŒ10Âłâ· erg sâ»Âč (âŒ0.05 L_(Edd)), disappear during the decay of the outburst when the source luminosity drops to âŒ4.5 Ă 10Âłâ” erg sâ»Âč (âŒ0.002 L_(Edd)). We show that this non-detection of the reflection features cannot be explained by the lower signal-to-noise ratio at lower flux, but is instead caused by physical changes in the accretion flow. Simulating synthetic NuSTAR observations on a grid of inner disc radius, disc ionization, and reflection fraction, we find that the disappearance of the reflection features can be explained by either increased disc ionization (logâΟ âł 4.1) or a much decreased reflection fraction. A changing disc truncation alone, however, cannot account for the lack of reprocessed FeâKα emission. The required increase in ionization parameter could occur if the inner accretion flow evaporates from a thin disc into a geometrically thicker flow, such as the commonly assumed formation of a radiatively inefficient accretion flow at lower mass accretion rates
The deepest X-ray look at the Universe
The origin of the X-ray background, in particular at hard (2-10 keV)
energies, has been a debated issue for more than 30 years. The Chandra deep
fields provide the deepest look at the X-ray sky and are the best dataset to
study the X-ray background. We searched the Chandra Deep Field South for X-ray
sources with the aid of a dedicated wavelet-based algorithm. We are able to
reconstruct the Log N-Log S source distribution in the soft (0.5-2 keV) and
hard (2-10 keV) bands down to limiting fluxes of 2x10^{-17} erg s^{-1} cm^{-2}
and 2x10^{-16} erg s^{-1} cm^{-2}, respectively. These are a factor ~5 deeper
than previous investigations. We find that the soft relation continues along
the extrapolation from higher fluxes, almost completely accounting for the soft
X-ray background. On the contrary, the hard distribution shows a flattening
below ~2x10^{-14} erg s^{-1} cm^{-2}. Nevertheless, we can account for >68% of
the hard X-ray background, with the main uncertainty being the sky flux itself.Comment: Accepted for publication on ApJL. Two figures, requires emulateapj5
(included
The 3-53 keV Spectrum of the Quasar 1508+5714: X-rays from z = 4.3
We present a high-quality X-ray spectrum in the 3--53 keV rest-frame band of
the radio-loud quasar 1508+5714, by far the brightest known X-ray source at z >
4. A simple power-law model with an absorption column density equal to the
Galactic value in the direction of the source provides an excellent and fully
adequate fit to the data; the measured power-law photon index Gamma = 1.42
(+0.13,-0.10). Upper limits to Fe K alpha line emission and Compton-reflection
components are derived. We offer evidence for both X-ray and radio variability
in this object and provide the first contemporaneous radio spectrum (alpha =
-0.25). The data are all consistent with a picture in which the emission from
this source is dominated by a relativistically beamed component in both the
X-ray and radio bands.Comment: 8 pages, TeX, 2 postscript figures; to appear in ApJ Letter
A Composite Seyfert 2 X-ray Spectrum: Implications for the Origin of the Cosmic X-ray Background
We present a composite 1-10 keV Seyfert 2 X-ray spectrum, derived from ASCA
observations of a distance-limited sample of nearby galaxies. All 29 observed
objects were detected. Above ~3 keV, the composite spectrum is inverted,
confirming that Seyfert 2 galaxies as a class have the spectral properties
necessary to explain the flat shape of the cosmic X-ray background spectrum.
Integrating the composite spectrum over redshift, we find that the total
emission from Seyfert 2 galaxies, combined with the expected contribution from
unabsorbed type 1 objects, provides an excellent match to the spectrum and
intensity of the hard X-ray background. The principal uncertainty in this
procedure is the cosmic evolution of the Seyfert 2 X-ray luminosity function.
Separate composite spectra for objects in our sample with and without polarized
broad optical emission lines are also presented.Comment: 11 pages (AASTeX), including 3 figures. Accepted for publication in
ApJ Letter
The resolved fraction of the Cosmic X-ray Background
We present the X-ray source number counts in two energy bands (0.5-2 and 2-10
keV) from a very large source sample: we combine data of six different surveys,
both shallow wide field and deep pencil beam, performed with three different
satellites (ROSAT, Chandra and XMM-Newton). The sample covers with good
statistics the largest possible flux range so far: [2.4*10^-17 - 10^-11] cgs in
the soft band and [2.1*10^-16 - 8*10^{-12}]cgs in the hard band. Integrating
the flux distributions over this range and taking into account the (small)
contribution of the brightest sources we derive the flux density generated by
discrete sources in both bands. After a critical review of the literature
values of the total Cosmic X--Ray Background (CXB) we conclude that, with the
present data, the 94.3%, and 88.8% of the soft and hard CXB can be ascribed to
discrete source emission. If we extrapolate the analytical form of the Log
N--Log S distribution beyond the flux limit of our catalog in the soft band we
find that the flux from discrete sources at ~3*10^-18 cgs is consistent with
the entire CXB, whereas in the hard band it accounts for only 93% of the total
CXB at most, hinting for a faint and obscured population to arise at even
fainter fluxes.Comment: Accepted for publication in Ap
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