The cosmic X-ray background: abundance and evolution of hidden black holes. (arXiv:1304.3665v1 [astro-ph.CO])

The cosmic X-ray background: abundance and evolution of hidden black holes. (arXiv:1304.3665v1 [astro-ph.CO]):
The growth of supermassive black holes across cosmic time leaves a radiative
imprint recorded in the X-ray background (XRB). The XRB spectral shape suggests
that a large population of distant, hidden nuclei must exist, which are now
being revealed at higher and higher redshifts by the deepest surveys performed
by Chandra and XMM.

Our current understanding of the XRB emission in terms of AGN population
synthesis models is here reviewed, and the evolutionary path of nuclear
accretion and obscuration, as emerging from the major X-ray surveys, is
investigated. The role of galaxy merging versus secular processes in triggering
nuclear activity is also discussed in the framework of recent galaxy/black hole
co-evolutionary scenarios. Finally, the limits of current instrumentation in
the detection of the most obscured and distant black holes are discussed and
some possible directions to overcome these limits are presented.

Discovery of a 168.8 s X-ray pulsar transiting in front of its Be companion star in the Large Magellanic Cloud. (arXiv:1302.4665v1 [astro-ph.HE])

Discovery of a 168.8 s X-ray pulsar transiting in front of its Be companion star in the Large Magellanic Cloud. (arXiv:1302.4665v1 [astro-ph.HE]):
Aims: We report the discovery of LXP169, a new high-mass X-ray binary in the
Large Magellanic Cloud. The optical counterpart is identified and exhibits an
eclipsing light curve. We performed follow-up observations to clarify the
eclipsing nature of the system. Methods: Energy spectra and time series were
extracted from two XMM-Newton observations to search for pulsations,
characterise the spectrum and measure spectral and timing changes. Long-term
X-ray variability was studied using archival ROSAT data. The XMM-Newton
positions were used to identify the optical counterpart. We obtained
ultraviolet to near-infrared photometry to characterise the companion, along
with its 4000 d long I-band light curve. We observed LXP169 with Swift at two
predicted eclipse times. Results: We found a spin period of 168.8 s which did
not change between two XMM-Newton observations. The X-ray spectrum, well
characterised by a power-law, was harder when the source was brighter. The
X-ray flux of LXP169 is found to be variable by a factor of at least 10. The
counterpart is highly variable on short and long timescales, and its photometry
is that of an early-type star with a near-infrared excess. This classifies the
source as a Be/X-ray binary pulsar. We observed a transit in the ultraviolet,
thereby confirming that the companion star itself is eclipsed. We give an
ephemeris for the transit of MJD56203.877(-0.197,+0.934)+N*(24.329+/-0.008). We
propose and discuss the scenario where the matter captured from the companion's
equatorial disc creates an extended region of high density around the neutron
star, which partially eclipses the companion as the neutron star transits in
front of it. Conclusions: LXP169 is the first confirmed eclipsing Be/X-ray
binary. For the first time we observe the compact object in an X-ray binary
eclipsing its companion star, and much can be learned by studying this
important system.

What X-ray source counts can tell about the large scale matter distribution. (arXiv:1212.5891v1 [astro-ph.CO])

What X-ray source counts can tell about the large scale matter distribution. (arXiv:1212.5891v1 [astro-ph.CO]):
Sources generating most of the X-ray background (XRB) are dispersed over a
wide range of redshifts. Thus, statistical characteristics of the source
distribution carry the information on the matter distribution on very large
scales. We test the possibility to detect the variation of the X-ray source
number counts over the celestial sphere. A large number of Chandra pointings
spread over both galactic hemispheres is investigated. A search for all the
point-like sources in the soft band of 0.5 - 2 keV is performed, and
statistical assessment of the population of sources below the detection
threshold is carried out. A homogeneous sample of the number counts at fluxes
above ~10^{-16} erg/s/cm^2 for more than 300 ACIS fields was constructed. The
counts correlations between overlapping fields were used to assess the accuracy
of the computational methods used in the analysis. It is shown that the source
number counts vary between fields at the level only slightly larger than the
fluctuation amplitude expected for the random (Poissonian) distribution.
Nevertheless, small asymmetry between galactic hemispheres is present. The
average number of sources in the northern hemisphere is larger than in the
southern at the 2.75 sigma level. Also the autocorrelation function of the
source density in both hemispheres are substantially different. Possible
explanations for the observed anisotropies are considered. If the effect is
unrelated to the observational selection, a large scale inhomogeneities in the
distribution of X-ray sources are required. Correlations of the source number
counts observed in the southern hemisphere could be generated by a coherent
structure extending over 1200 Mpc.

The Fractional Ionization of the Warm Neutral Interstellar Medium. (arXiv:1301.3144v1 [astro-ph.GA])

The Fractional Ionization of the Warm Neutral Interstellar Medium. (arXiv:1301.3144v1 [astro-ph.GA]):
When the neutral interstellar medium is exposed to EUV and soft X ray
radiation, the argon atoms in it are far more susceptible to being ionized than
the hydrogen atoms. We make use of this fact to determine the level of
ionization in the nearby, warm, neutral medium (WNM). By analyzing FUSE
observations of ultraviolet spectra of 44 hot subdwarf stars a few hundred pc
away from the Sun, we can compare column densities of Ar I to those of O I,
where the relative ionization of oxygen can be used as a proxy for that of
hydrogen. The measured deficiency [ Ar I/O I]= 0.427+/- 0.11dex below the
expectation for a fully neutral medium implies that the electron density n(e)
\sim 0.04cm^- 3 if n(H)=0.5cm^- 3. This amount of ionization is considerably
larger than what we expect from primary photoionizations resulting from cosmic
rays, the diffuse X-ray background, and X-ray emitting sources within the
medium, along with the additional ionizations caused by energetic secondary
photoelectrons, Auger electrons, and photons from helium recombinations. We
favor an explanation that bursts of radiation created by previous, nearby
supernova remnants that have faded by now may have elevated the ionization, and
the gas has not yet recombined to a quiescent level. A different alternative is
that the low energy portion of the soft X ray background is poorly shielded by
the H I because it is frothy and has internal pockets of very hot, X-ray
emitting gases.

Long Term Variability of O VII Line Intensity toward the Lockman Hole Observed with Suzaku from 2006 to 2011. (arXiv:1301.5174v1 [astro-ph.HE])

Long Term Variability of O VII Line Intensity toward the Lockman Hole Observed with Suzaku from 2006 to 2011. (arXiv:1301.5174v1 [astro-ph.HE]):
Long-term time variabilities of the OVII (0.57 keV) emission in the soft
X-ray diffuse background were studied using six Suzaku annual observations of
blank sky towards the Lockman Hole made from 2006 to 2011. After time intervals
in which the emission was enhanced on time scales of a few tens of ks were
removed, the O VII intensity was found to be constant from 2006 to 2009 within
the 90% statistical errors. The intensity in 2010 and 2011 was higher by 2-3 LU
(photons/s/cm/sr) than the earlier values. The most plausible origin of the
fast variable component is Solar wind charge exchange (SWCX). The intensity
increase is not positively correlated with the proton flux at the L1 point.
Since all the observations were made in the same season of a year, the
variation cannot be explained by parallax of the SWCX induced X-ray emission
from the Heliosphere. We consider that it is related to the geometrical change
of slow and fast solar wind structures associated with the 11 year solar
activity. The observed variation was compared with that expected from the SWCX
induced X-ray emission model.

PRIMUS: An observationally motivated model to connect the evolution of the AGN and galaxy populations out to z~1. (arXiv:1301.1689v1 [astro-ph.CO])

PRIMUS: An observationally motivated model to connect the evolution of the AGN and galaxy populations out to z~1. (arXiv:1301.1689v1 [astro-ph.CO]):
We present an observationally motivated model to connect the AGN and galaxy
populations at 0.2<z<1.0 and predict the AGN X-ray luminosity function (XLF).
We start with measurements of the stellar mass function of galaxies (from the
Prism Multi-object Survey) and populate galaxies with AGNs using models for the
probability of a galaxy hosting an AGN as a function of specific accretion rate
(the rate of supermassive black hole growth scaled relative to the host stellar
mass). Our model is based on measurements indicating that the specific
accretion rate distribution is a universal function across a wide range of host
stellar mass with slope gamma_1=0.65 and an overall normalization that evolves
strongly with redshift. We test several simple assumptions to extend this model
to high specific accretion rates (beyond the measurements) and compare the
predictions for the XLF with the observed data. We find good agreement with a
model that allows for a break in the specific accretion rate distribution at a
point corresponding to the Eddington limit, with a steep power-law tail to
super-Eddington ratios with slope gamma_2=-2.1 +0.3 -0.5. We convert between
specific accretion rate and Eddington ratio by assuming a scaling between black
hole mass and host stellar mass with an intrinsic scatter of +/-0.38 dex. Our
results show that samples of low luminosity AGNs are dominated by moderately
massive galaxies (M* ~ 10^{10-11} M_sun) growing with a wide range of accretion
rates -- a consequence of the shape of the galaxy stellar mass function rather
than a preference for AGN activity at a particular stellar mass. The observed
population of the most luminous AGN may be severely skewed to the most extreme
sources with elevated black hole masses relative to their host galaxies and in
rare phases of very rapid accretion.

Black Holes in Ultra-Luminous X-ray sources: X-ray timing versus spectroscopy. (arXiv:1210.3965v1 [astro-ph.HE])

Black Holes in Ultra-Luminous X-ray sources: X-ray timing versus spectroscopy. (arXiv:1210.3965v1 [astro-ph.HE]):
Ultra-Luminous X-ray sources are accreting black holes that might represent
strong evidence of the Intermediate Mass Black Holes (IMBH), proposed to exist
by theoretical studies but with no firm detection (as a class) so far. We
analyze the best X-ray timing and spectral data from the ULX in NGC 5408
provided by XMM-Newton. The main goal is to study the broad-band noise
variability of the source. We found an anti-correlation of the fractional
root-mean square variability versus the intensity of the source, similar to
black-hole binaries during hard states.

X-ray photoionized bubble in the wind of Vela X-1 pulsar supergiant companion. (arXiv:1208.1827v1 [astro-ph.SR])

X-ray photoionized bubble in the wind of Vela X-1 pulsar supergiant companion. (arXiv:1208.1827v1 [astro-ph.SR]):
Vela X-1 is the archetype of high-mass X-ray binaries, composed of a neutron
star and a massive B supergiant. The supergiant is a source of a strong
radiatively-driven stellar wind. The neutron star sweeps up this wind, and
creates a huge amount of X-rays as a result of energy release during the
process of wind accretion. Here we provide detailed NLTE models of the Vela X-1
envelope. We study how the X-rays photoionize the wind and destroy the ions
responsible for the wind acceleration. The resulting decrease of the radiative
force explains the observed reduction of the wind terminal velocity in a
direction to the neutron star. The X-rays create a distinct photoionized region
around the neutron star filled with a stagnating flow. The existence of such
photoionized bubbles is a general property of high-mass X-ray binaries. We
unveiled a new principle governing these complex objects, according to which
there is an upper limit to the X-ray luminosity the compact star can have
without suspending the wind due to inefficient line driving