1,788 research outputs found
X-ray emission from star-forming galaxies - I. High-mass X-ray binaries
Based on a homogeneous set of X-ray, infrared and ultraviolet observations
from Chandra, Spitzer, GALEX and 2MASS archives, we study populations of
high-mass X-ray binaries (HMXBs) in a sample of 29 nearby star-forming galaxies
and their relation with the star formation rate (SFR). In agreement with
previous results, we find that HMXBs are a good tracer of the recent star
formation activity in the host galaxy and their collective luminosity and
number scale with the SFR, in particular, Lx~2.6 10^{39} SFR. However, the
scaling relations still bear a rather large dispersion of ~0.4 dex, which we
believe is of a physical origin. We present the catalog of 1057 X-ray sources
detected within the ellipse for galaxies of our sample and construct the
average X-ray luminosity function (XLF) of HMXBs with substantially improved
statistical accuracy and better control of systematic effects than achieved in
previous studies. The XLF follows a power law with slope of 1.6 in the
logLx~35-40 luminosity range with a moderately significant evidence for a break
or cut-off at Lx~10^{40} erg/s. As before, we did not find any features at the
Eddington limit for a neutron star or a stellar mass black hole. We discuss
implications of our results for the theory of binary evolution. In particular
we estimate the fraction of compact objects that once upon their lifetime
experienced an X-ray active phase powered by accretion from a high mass
companion and obtain a rather large number, fx~0.2 (0.1 Myr/tau_x) (tau_x is
the life time of the X-ray active phase). This is ~4 orders of magnitude more
frequent than in LMXBs. We also derive constrains on the mass distribution of
the secondary star in HMXBs.Comment: 23 pages, 14 figures, 5 tables, MNRAS - Accepted 2011 September 2
The Non-Linear Dependence of Flux on Black Hole Mass and Accretion Rate in Core Dominated Jets
We derive the non-linear relation between the core flux F_{nu} of accretion
powered jets at a given frequency and the mass M of the central compact object.
For scale invariant jet models, the mathematical structure of the equations
describing the synchrotron emission from jets enables us to cancel out the
model dependent complications of jet dynamics, retaining only a simple, model
independent algebraic relation between F_{nu} and M. This approach allows us to
derive the F_{nu}-M relation for any accretion disk scenario that provides a
set of input boundary conditions for the magnetic field and the relativistic
particle pressure in the jet, such as standard and advection dominated
accretion flow (ADAF) disk solutions. Surprisingly, the mass dependence of
F_{nu} is very similar in different accretion scenarios. For typical
flat-spectrum core dominated radio jets and standard accretion scenarios we
find F_{nu}~M^{17/12}. The 7-9 orders of magnitude difference in black hole
mass between microquasars and AGN jets imply that AGN jets must be about 3-4
orders of magnitude more radio loud than microquasars, i.e., the ratio of radio
to bolometric luminosity is much smaller in microquasars than in AGN jets.
Because of the generality of these results, measurements of this F_{nu}-M
dependence are a powerful probe of jet and accretion physics. We show how our
analysis can be extended to derive a similar scaling relation between the
accretion rate mdot and F_{nu} for different accretion disk models. For
radiatively inefficient accretion modes we find that the flat spectrum emission
follows F_{nu}~(mdot*M)^{17/12}.Comment: Added key words and acknowledgements, minor editorial corrections. 6
pages, to appear in MNRAS 343, L59-L6
Scale-dependent bias of galaxies and mu-type distortion of the cosmic microwave background spectrum from single-field inflation with a modified initial state
We investigate the phenomenological consequences of a modification of the
initial state of a single inflationary field. While single-field inflation with
the standard Bunch-Davies initial vacuum state does not generally produce a
measurable three-point function (bispectrum) in the squeezed configuration,
allowing for a non-standard initial state produces an exception. Here, we
calculate the signature of an initial state modification in single-field
slow-roll inflation in both the scale-dependent bias of the large-scale
structure (LSS) and mu-type distortion in the black-body spectrum of the cosmic
microwave background (CMB). We parametrize the initial state modifications and
identify certain choices of parameters as natural, though we also note some
fine-tuned choices that can yield a larger bispectrum. In both cases, we
observe a distinctive k^-3 signature in LSS (as opposed to k^-2 for the
local-form). As a non-zero bispectrum in the squeezed configuration correlates
a long-wavelength mode with two short-wavelength modes, it induces a
correlation between the CMB temperature anisotropy on large scales with the
temperature-anisotropy-squared on very small scales; this correlation persists
as the small-scale anisotropy-squared is processed into mu-type distortions.
While the local-form mu-distortion turns out to be too small to detect in the
near future, a modified initial vacuum state enhances the signal by a large
factor owing to an extra factor of k_1/k. For example, a proposed
absolutely-calibrated experiment, PIXIE, is expected to detect this correlation
with a signal-to-noise ratio greater than 10, for an occupation number of about
0.5 in the observable modes. Relatively calibrated experiments such as Planck
and LiteBIRD should also be able to measure this effect, provided that the
relative calibration between different frequencies meets the required
precision. (Abridged)Comment: 14 pages, 6 figures. Matches version in PRD. Improved explanation in
Sec. IV; added references and corrected typo
Hard X-ray emission of the Earth's atmosphere: Monte Carlo simulations
We perform Monte Carlo simulations of cosmic ray-induced hard X-ray radiation
from the Earth's atmosphere. We find that the shape of the spectrum emergent
from the atmosphere in the energy range 25-300 keV is mainly determined by
Compton scatterings and photoabsorption, and is almost insensitive to the
incident cosmic-ray spectrum. We provide a fitting formula for the hard X-ray
surface brightness of the atmosphere as would be measured by a satellite-born
instrument, as a function of energy, solar modulation level, geomagnetic cutoff
rigidity and zenith angle. A recent measurement by the INTEGRAL observatory of
the atmospheric hard X-ray flux during the occultation of the cosmic X-ray
background by the Earth agrees with our prediction within 10%. This suggests
that Earth observations could be used for in-orbit calibration of future hard
X-ray telescopes. We also demonstrate that the hard X-ray spectra generated by
cosmic rays in the crusts of the Moon, Mars and Mercury should be significantly
different from that emitted by the Earth's atmosphere.Comment: 12 pages, 16 figures, MNRAS accepte
Relativistic Corrections to the Sunyaev-Zel'dovich Effect for Clusters of Galaxies. IV. Analytic fitting formula for the Numerical Results
We present an accurate analytic fitting formula for the numerical results for
the relativistic corrections to the thermal Sunyaev-Zel'dovich effect for
clusters of galaxies. The numerical results for the relativistic corrections
have been obtained by numerical integration of the collision term of the
Boltzmann equation. The fitting is carried out for the ranges 0.02 < theta_{e}
< 0.05 and 0 < X < 20, where theta_{e} = k_{B}T_{e}/m_{e}c^{2}, X =
omega/k_{B}T_{0}, T_{e} is the electron temperature, omega is the angular
frequency of the photon, and T_{0} is the temperature of the cosmic microwave
background radiation. The accuracy of the fitting is generally better than
0.1%. The present analytic fitting formula will be useful for the analyses of
the thermal Sunyaev-Zel'dovich effect for high-temperature galaxy clusters.Comment: 11 pages + 1 table + 2 figures, LaTeX with AASMS macro. Accepted by
Astrophysical Journal for publicatio
Evidence of widespread degradation of gene control regions in hominid genomes
Although sequences containing regulatory elements located close to protein-coding genes are often only weakly conserved during evolution, comparisons of rodent genomes have implied that these sequences are subject to some selective constraints. Evolutionary conservation is particularly apparent upstream of coding sequences and in first introns, regions that are enriched for regulatory elements. By comparing the human and chimpanzee genomes, we show here that there is almost no evidence for conservation in these regions in hominids. Furthermore, we show that gene expression is diverging more rapidly in hominids than in murids per unit of neutral sequence divergence. By combining data on polymorphism levels in human noncoding DNA and the corresponding human¿chimpanzee divergence, we show that the proportion of adaptive substitutions in these regions in hominids is very low. It therefore seems likely that the lack of conservation and increased rate of gene expression divergence are caused by a reduction in the effectiveness of natural selection against deleterious mutations because of the low effective population sizes of hominids. This has resulted in the accumulation of a large number of deleterious mutations in sequences containing gene control elements and hence a widespread degradation of the genome during the evolution of humans and chimpanzees
Cumulative hard X-ray spectrum of local AGN: a link to the cosmic X-ray background
We determine the cumulative spectral energy distribution (SED) of local AGN
in the 3-300 keV band and compare it with the spectrum of the cosmic X-ray
background (CXB) in order to test the widely accepted paradigm that the CXB is
a superposition of AGN and to place constraints on AGN evolution. We performed
a stacking analysis of the hard X-ray spectra of AGN detected in two recent
all-sky surveys, performed by the IBIS/ISGRI instrument aboard INTEGRAL and by
the PCA instrument aboard RXTE, taking into account the space densities of AGN
with different luminosities and absorption column densities. We derived the
collective SED of local AGN in the 3-300 keV energy band. Those AGN with
luminosities below 10^43.5 erg/s (17-60 keV) provide the main contribution to
the local volume hard X-ray emissivity, at least 5 times more than more
luminous objects. The cumulative spectrum exhibits (although with marginal
significance) a cutoff at energies above 100-200 keV and is consistent with the
CXB spectrum if AGN evolve over cosmic time in such a way that the SED of their
collective high-energy emission has a constant shape and the relative fraction
of obscured AGN remains nearly constant, while the AGN luminosity density
undergoes strong evolution between z~1 and z=0, a scenario broadly consistent
with results from recent deep X-ray surveys. The first direct comparison
between the collective hard X-ray SED of local AGN and the CXB spectrum
demonstrates that the popular concept of the CXB being a superposition of AGN
is generally correct. By repeating this test using improved AGN statistics from
current and future hard X-ray surveys, it should be possible to tighten the
constraints on the cosmic history of black hole growth.Comment: 12 pages, 9 figures. Revised version accepted for publication in A&
Turbulence in Clusters of Galaxies and X-Ray Line Profiles
Large-scale bulk motions and hydrodynamic turbulence in the intergalactic gas
inside clusters of galaxies significantly broaden X-ray emission lines. For
lines of heavy ions (primarily helium-like and hydrogen-like iron ions), the
hydrodynamic broadening is significantly larger than the thermal broadening.
Since cluster of galaxies have a negligible optical depth for resonant
scattering in forbidden and intercombination lines of these ions, these lines
are not additionally broadened. At the same time, they are very intense, which
allows deviations of the spectrum from the Gaussian spectrum in the line wings
to be investigated. The line shape becomes an important indicator of bulk
hydrodynamic processes because the cryogenic detectors of new generation of
X-ray observatories will have a high energy resolution (from 5 eV for ASTRO-E2
to 1-2 eV for Constellation-X and XEUS). We use the spectral representation of
a Kolmogorov cascade in the inertial range to calculate the characteristic
shapes of X-ray lines. Significant deviations in the line profiles from the
Gaussian profile (shape asymmetry, additional peaks, sharp breaks in the
exponential tails) are expected for large-scale turbulence. The kinematic SZ
effect and the X-ray line profile carry different information about the
hydrodynamic velocity distribution in clusters of galaxies and complement each
other, allowing the redshift, the peculiar velocity of the cluster, and the
bulk velocity dispersion to be measured and separatedComment: 29 pages, 12 figures, Astronomy Letters 2003, v.29, p.79
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