608 research outputs found
Identification of four RXTE Slew Survey sources with nearby luminous active galactic nuclei
Based on RXTE scans and observations with the SWIFT/XRT telescope and
INTEGRAL observatory, we report the identification of four X-ray sources
discovered during the RXTE Slew Survey of the |b|>10deg sky with nearby (z ~
0.017-0.098) luminous (log L_2-10keV ~ 42.7-44 erg/s) active galactic nuclei.
Two of the objects exhibit heavily intrinsically absorbed X-ray spectra
(NHL~10^23 cm^-2).Comment: 4 pages, 2 figures. Sibmitted to Astronomy and Astrophysics Letter
Scattering of emission lines in galaxy cluster cores: measuring electron temperature
The central galaxies of some clusters can be strong emitters in the
Ly and H lines. This emission may arise either from the
cool/warm gas located in the cool core of the cluster or from the bright AGN
within the central galaxy. The luminosities of such lines can be as high as
erg/s. This emission originating from the core of the
cluster will get Thomson scattered by hot electrons of the intra-cluster medium
(ICM) with an optical depth 0.01 giving rise to very broad ( 15%) features in the scattered spectrum. We discuss the
possibility of measuring the electron density and temperature using information
on the flux and width of the highly broadened line features.Comment: 9 pages, 5 figures, accepted in MNRA
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
Hybrid LQG-Neural Controller for Inverted Pendulum System
The paper presents a hybrid system controller, incorporating a neural and an
LQG controller. The neural controller has been optimized by genetic algorithms
directly on the inverted pendulum system. The failure free optimization process
stipulated a relatively small region of the asymptotic stability of the neural
controller, which is concentrated around the regulation point. The presented
hybrid controller combines benefits of a genetically optimized neural
controller and an LQG controller in a single system controller. High quality of
the regulation process is achieved through utilization of the neural
controller, while stability of the system during transient processes and a wide
range of operation are assured through application of the LQG controller. The
hybrid controller has been validated by applying it to a simulation model of an
inherently unstable system of inverted pendulum
An upper limit on nickel overabundance in the supercritical accretion disk wind of SS 433 from X-ray spectroscopy
We take advantage of a long (with a total exposure time of 120 ks) X-ray
observation of the unique Galactic microquasar SS 433, carried out with the
XMM-Newton space observatory, to search for a fluorescent line of neutral (or
weakly ionized) nickel at the energy 7.5 keV. We consider two models of the
formation of fluorescent lines in the spectrum of SS 433: 1) due to reflection
of hard X-ray radiation from a putative central source on the optically thick
walls of the accretion disk "funnel"; and 2) due to scattering of the radiation
coming from the hottest parts of the jets in the optically thin wind of the
system. It is shown, that for these cases, the photon flux of Ni I K
fluorescent line is expected to be 0.45 of the flux of Fe I K
fluorescent line at 6.4 keV, for the relative nickel overabundance , as observed in the jets of SS 433. For the continuum model without the
absorption edge of neutral iron, we set a 90 per cent upper limit on the flux
of the narrow Ni I K line at the level of ph
s cm. For the continuum model with the absorption edge, the
corresponding upper limit is ph s cm. At the
same time, for the Fe I K line, we measure the flux of
ph s cm. Taken at the face
value, the results imply that the relative overabundance of nickel in the wind
of the accretion disc should be at least 1.5 times less than the corresponding
excess of nickel observed in the jets of SS 433.Comment: 17 pages, 12 figures, 4 tables, Astronomy Letters, in press, 2018,
Volume 44, Issue
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&
Origin of the Galactic ridge X-ray emission
We analyze a map of the Galactic ridge X-ray emission (GRXE) constructed in
the 3-20 keV energy band from RXTE/PCA scan and slew observations. We show that
the GRXE intensity closely follows the Galactic near-infrared surface
brightness and thus traces the Galactic stellar mass distribution. The GRXE
consists of two spatial components which can be identified with the bulge/bar
and the disk of the Galaxy. The parameters of these components determined from
X-ray data are compatible with those derived from near-infrared data. The
inferred ratio of X-ray to near-infrared surface brightness I(3-20 keV) (1e-11
erg/s/cm2/deg2)/I_(3.5micron)(MJy/sr)=0.26+/-0.05, and the ratio of X-ray to
near-infrared luminosity L_(3-20 keV)/L_(3-4 micron)=(4.1+/-0.3)e-5. The
corresponding ratio of the 3-20 keV luminosity to the stellar mass is
L_x/M_Sun=
(3.5\pm0.5) 10^{27} erg/s, which agrees within the uncertainties with the
cumulative emissivity per unit stellar mass of point X-ray sources in the Solar
neighborhood, determined in an accompanying paper (Sazonov et al.). This
suggests that the bulk of the GRXE is composed of weak X-ray sources, mostly
cataclysmic variables and coronally active binaries. The fractional
contributions of these classes of sources to the total X-ray emissivity
determined from the Solar neighborhood data can also explain the GRXE energy
spectrum. Based on the luminosity function of local X-ray sources we predict
that in order to resolve 90% of the GRXE into discrete sources a sensitivity
limit of ~10^{-16} erg/s/cm2 (2--10 keV) will need to be reached in future
observations.Comment: 12 pages, 9 figures. Submitted to A&
Identification of 8 INTEGRAL hard X-ray sources with Chandra
We report the results of identification of 8 hard X-ray sources discovered by
the INTEGRAL observatory during the ongoing all-sky survey. These sources have
been observed by Chandra. In 6 cases a bright X-ray source was found within the
INTEGRAL localization region, which permitted to unambigously identify 5 of the
objects with nearby galaxies, implying that they have an active galactic
nucleus (AGN), whereas one source is likely an X-ray binary in LMC. 4 of the 5
newly discovered AGNs have measured redshifts in the range 0.025-0.055. The
X-ray spectra reveal the presence of significant amounts of absorbing gas (NH
in the range 10^22-10^24 cm^-2) in all 5 AGNs, demonstrating that INTEGRAL is
starting to fill in the sample of nearby obscured AGNs.Comment: 4 pages, 2 figures, corrected Fig.
A hard X-ray survey of the Crux Galactic spiral arm tangent. A catalog of sources
This work is part of a large solid angle hard X-ray survey. We analized a
number of observations by the IBIS telescope aboard the INTEGRAL observatory
covering the Crux Galactic spiral arm tangent. We have detected 46 hard X-ray
sources, with 15 of them being new. Among the identified sources there are 12
AGNs, 11 HMXBs, 6 LMXBs and 2 active stars. 13 sources remain unidendified.Comment: Accepted for publication in Astronomy Letter
Constraints on the late X-ray emission from the low-energy GRB 031203: INTEGRAL data
Comparison of the INTEGRAL upper limits on the hard X-ray flux before and
after the low-energy GRB 031203 with the XMM measurements of the dust-scattered
radiation at lower energies suggests that a significant fraction of the total
burst energy could be released in the form of soft X-rays at an early afterglow
stage with a characteristic duration of ~100-1000s. The overall time evolution
of the GRB 031203 afterglow may have not differed qualitatively from the
behavior of standard (i.e., more intense) bursts studied by the SWIFT
observatory. The available data also admit the possibility that the
dust-scattered radiation was associated with an additional soft component in
the spectrum of the gamma-ray burst itself.Comment: 9 pages, 3 figure
- …