11,035 research outputs found
An exploratory study of the hard X-ray variability properties of PG quasars with RXTE
We have monitored with the RXTE PCA the variability pattern of the 2-20 keV
flux in four PG quasars (QSOs) from the Laor et al. (1994) sample. Six
observations of each target at regular intervals of 1 day were performed. The
sample comprises objects with extreme values of Balmer line width (and hence
soft X-ray steepness) and spans about one order of magnitude in luminosity. The
most robust result is that the variability amplitude decreases as energy
increases. Several options for a possible ultimate driver of the soft and hard
X-ray variability, such as the influx rate of Comptonizing relativistic
particles, instabilities in the accretion flow or the number of X-ray active
sites, are consistent with our results.Comment: Contributed talk presented at the Joint MPE,AIP,ESO workshop on
NLS1s, Bad Honnef, Dec. 1999, to appear in New Astronomy Reviews; also
available at http://wave.xray.mpe.mpg.de/conferences/nls1-worksho
Systematic literature review of Rift Valley fever virus seroprevalence in livestock, wildlife and humans in Africa from 1968 to 2016
Background: Rift Valley fever virus (RVFV) is a zoonotic arbovirus that causes severe disease in livestock and humans. The virus has caused recurrent outbreaks in Africa and the Arabian Peninsula since its discovery in 1931. This review sought to evaluate RVFV seroprevalence across the African continent in livestock, wildlife and humans in order to understand the spatio-temporal distribution of RVFV seroprevalence and to identify knowledge gaps and areas requiring further research. Risk factors associated with seropositivity were identified and study designs evaluated to understand the validity of their results. Methodology: The Preferred Reporting of Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines were used to produce a protocol to systematically search for RVFV seroprevalence studies in PubMed and Web of Science databases. The Strengthening the Reporting of Observational studies in Epidemiology (STROBE) statement guided the evaluation of study design and analyses. Principal findings: A total of 174 RVFV seroprevalence studies in 126 articles fulfilled the inclusion criteria. RVFV seroprevalence was recorded in 31 African countries from 1968 to 2016 and varied by time, species and country. RVFV seroprevalence articles including either livestock and humans or livestock and wildlife seroprevalence records were limited in number (8/126). No articles considered wildlife, livestock and human seroprevalence concurrently, nor wildlife and humans alone. Many studies did not account for study design bias or the sensitivity and specificity of diagnostic tests. Conclusions: Future research should focus on conducting seroprevalence studies at the wildlife, livestock and human interface to better understand the nature of cross-species transmission of RVFV. Reporting should be more transparent and biases accounted for in future seroprevalence research to understand the true burden of disease on the African continent
Partially Absorbed Comptonization Spectrum from the Nearly Edge-on Source X 1822-371
We report the results of a spectral analysis over the range 0.1-200 keV
performed on the dipping source X 1822-371 observed by BeppoSAX. We find the
best fit to the continuum using a partially covered Comptonization model, due
to scattering off soft seed photons by electrons at a temperature of ~4.8 keV,
without the presence of any soft blackbody emission. The equivalent hydrogen
column obtained for the absorbed component is ~4.5 10^{22} cm^{-2}, an order of
magnitude larger than the Galactic absorption for this source, and the covering
fraction is ~71%. Because the inclination angle of X 1822-371 to the line of
sight is ~85^\circ, this model gives a reasonable scenario for the source: the
Comptonized spectrum could come from an extended accretion disk corona (ADC),
probably the only region that can be directly observed due to the high
inclination. The excess of matter producing the partial covering could be close
to the equatorial plane of the system, above the outer disk, occulting the
emission from the inner disk and the inner part of the ADC. An iron emission
line is also present at ~6.5 keV with an equivalent width of ~150 eV. We argue
that this strong iron line cannot be explained as reflection of the Comptonized
spectrum by the accretion disk. It is probably produced in the ADC. An emission
line at ~1.9 keV (with an equivalent width of ~54 eV) and an absorption edge at
\~8.7 keV (with an optical depth of ~0.1) are also required to fit this
spectrum. These features are probably produced by highly ionized iron (Fe XXIV)
present in the outer part of the ADC, where the plasma density is
\~10^{11}-10^{12} cm^{-3} and ionized plasma is present.Comment: 15 pages, including 3 figures. Accepted by ApJ. Corrected typos and
Figure
Narrow Components within the Fe Kalpha Profile of NGC 3516: Evidence for the Importance of General Relativistic Effects?
We present results from a simultaneous Chandra HETG and XMM-Newton
observation of NGC 3516. We find evidence for several narrow components of Fe
Kalpha along with a broad line. We consider the possibility that the lines
arise in an blob of material ejected from the nucleus with velocity ~0.25c. We
also consider an origin in a neutral accretion disk, suffering enhanced
illumination at 35 and 175 gravitational radii, perhaps due to magnetic
reconnection. The presence of these narrow features indicates there is no
Comptonizing region along the line-of-sight to the nucleus. This in turn is
compelling support for the hypothesis that broad Fe Kalpha components are, in
general, produced by strong gravity.Comment: 12 pages, 3 color figures. LaTeX with postscript figures. Resubmitted
June 7 2002, to Astrophysical Journal Letter
Resolving the Composite Fe K-alpha Emission Line in the Galactic Black Hole Cygnus X-1 with Chandra
We observed the Galactic black hole Cygnus X-1 with the Chandra High Energy
Transmission Grating Spectrometer for 30 kiloseconds on 4 January, 2001. The
source was in an intermediate state, with a flux that was approximately twice
that commonly observed in its persistent low/hard state. Our best-fit model for
the X-ray spectrum includes narrow Gaussian emission line (E = 6.415 +/- 0.007
keV, FWHM = 80 (+28, -19) eV, W = 16 (+3, -2) eV) and broad line (E = 5.82
(+0.06, -0.07) keV, FWHM = 1.9 (+0.5, -0.3) keV, W = 140 (+70, -40) eV)
components, and a smeared edge at 7.3 +/- 0.2 keV (tau ~ 1.0). The broad line
profile is not as strongly skewed as those observed in some Seyfert galaxies.
We interpret these features in terms of an accretion disk with irradiation of
the inner disk producing a broad Fe K-alpha emission line and edge, and
irradiation of the outer disk producing a narrow Fe K-alpha emission line. The
broad line is likely shaped predominantly by Doppler shifts and gravitational
effects, and to a lesser degree by Compton scattering due to reflection. We
discuss the underlying continuum X-ray spectrum and these line features in the
context of diagnosing the accretion flow geometry in Cygnus X-1 and other
Galactic black holes.Comment: Accepted for publication in Ap
Relativistic Iron Emission and Disk Reflection in Galactic Microquasar XTE J1748-288
We report evidence for an Fe K-alpha fluorescence line feature and disk
reflection in the Very High, High, and Low State X-ray spectra of the galactic
microquasar XTE J1748-288 during its June~1998 outburst. Spectral analyses are
made on data gathered throughout the outburst by the Rossi X-ray Timing
Explorer Proportional Counter Array. Gaussian line, relativistic disk emission
line, and ionized disk reflection models are fit to the data. In the Very High
State the line profile appears strongly redshifted, consistent with disk
emission from the innermost stable orbits around a maximally rotating Kerr
black hole. In the High State the line profile is less redshifted and
increasingly prominent. The Low State line profile is very strong (~0.5 keV
equivalent width) and centered at 6.7 +/- 0.10 keV; disk line emission model
fits indicate that the inner edge of the disk fluctuates between ~20 and ~100
gravitational radii in this state. The disk reflection fraction is traced
through the outburst; reflection from an ionized disk is preferred in the VHS
and HS, and reflection from a relatively neutral disk is preferred in the LS.
We discuss the implications of our findings for the binary system dynamics and
accretion flow geometry in XTE J1748-288.Comment: 32 pages, 8 figures, 5 tables. ApJ accepted 8/16/0
BeppoSAX view of NGC 526A: a Seyfert 1.9 galaxy with a flat spectrum
In the present work we report the BeppoSAX observation of the Seyfert 1.9
galaxy NGC 526A in the band 0.1-150 keV. The high energy instrument onboard,
PDS, has succeeded in measuring for the first time the spectrum of this source
in the 13-150 keV range. The combined analysis of all Narrow Field Instruments
provides a power law spectral index of ~ 1.6 and confirms the flat spectral
nature of this source. Although NGC 526A varies strongly in the 2-10 keV over
period of months/years, its spectral shape remains constant over these
timescales. An Fe K-alpha line, characterized by a complex structure, has been
detected in the 6-7 keV range. The line, which has an equivalent width of 120
eV, is not compatible with being produced in an absorbing torus with N_H ~
10^22 cm^-2, but most likely originates by reflection in an accretion disk
viewed at an intermediate inclination angle of ~ 42 deg. The reflection
component is however small (R < 0.7) and so it is not sufficient to steepen the
spectrum to photon index values more typical of AGNs. Instead, we find that the
data are more consistent with a flat power law spectrum cut-off at around 100
keV plus a small reflection component which could explain the observed iron
line. Thus NGC 526A is the only bona-fide Seyfert 2 galaxy which maintains a
"flat spectrum" even when broad band data are considered: in this sense its
properties, with respect to the general class of Seyfert 2's, are analogous to
those of NGC 4151 with respect to the vast majority of Seyfert 1's.Comment: 8 pages, 6 PostScript figures, Latex manuscript, new A&A file style
included, accepted for publication on Astronomy and Astrophysic
A Medium-Resolution Near-Infrared Spectral Library of Late Type Stars: I
We present an empirical infrared spectral library of medium resolution
(R~2000-3000) H (1.6 micron) and K (2.2 micron) band spectra of 218 red stars,
spanning a range of [Fe/H] from ~-2.2 to ~+0.3. The sample includes Galactic
disk stars, bulge stars from Baade's window, and red giants from Galactic
globular clusters. We report the values of 19 indices covering 12 spectral
features measured from the spectra in the library. Finally, we derive
calibrations to estimate the effective temperature, and diagnostic
relationships to determine the luminosity classes of individual stars from
near-infrared spectra.
This paper is part of a larger effort aimed at building a near-IR spectral
library to be incorporated in population synthesis models, as well as, at
testing synthetic stellar spectra.Comment: 34 pages, 12 figures; accepted for publication at ApJS; the spectra
are available from the authors upon reques
BeppoSAX average spectra of Seyfert galaxies
We have studied the average 3-200 keV spectra of Seyfert galaxies of type 1
and 2, using data obtained with BeppoSAX. The average Seyfert 1 spectrum is
well-fitted by a power law continuum with photon spectral index Gamma~1.9, a
Compton reflection component R~0.6-1 (depending on the inclination angle
between the line of sight and the reflecting material) and a high-energy cutoff
at around 200 keV; there is also an iron line at 6.4 keV characterized by an
equivalent width of 120 eV. Seyfert 2's on the other hand show stronger neutral
absorption (NH=3-4 x 10^{22} atoms cm-2) as expected but are also characterized
by an X-ray power law which is substantially harder (Gamma~1.75) and with a
cut-off at lower energies (E_c~130 keV); the iron line parameters are instead
substantially similar to those measured in type 1 objects. There are only two
possible solutions to this problem: to assume more reflection in Seyfert 2
galaxies than observed in Seyfert 1 or more complex absorption than estimated
in the first instance. The first possibility is ruled out by the Seyfert 2 to
Seyfert 1 ratio while the second provides an average Seyfert 2 intrinsic
spectrum very similar to that of the Seyfert 1. The extra absorber is likely an
artifact due to summing spectra with different amounts of absorption, althought
we cannot exclude its presence in at least some individual sources. Our result
argues strongly for a very similar central engine in both type of galaxies as
expected under the unified theory.Comment: 12 pages, 4 figures, to be published in ApJ Letter
Current-Driven Conformational Changes, Charging and Negative Differential Resistance in Molecular Wires
We introduce a theoretical approach based on scattering theory and total
energy methods that treats transport non-linearities, conformational changes
and charging effects in molecular wires in a unified way. We apply this
approach to molecular wires consisting of chain molecules with different
electronic and structural properties bonded to metal contacts. We show that
non-linear transport in all of these systems can be understood in terms of a
single physical mechanism and predict that negative differential resistance at
high bias should be a generic property of such molecular wires.Comment: 9 pages, 4 figure
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