577 research outputs found
Chandra Observation of the Persistent Emission from the Dipping Source XB 1916-053
We present the results of a 50 ks long Chandra observation of the dipping
source XB 1916-053. During the observation two X-ray bursts occurred and the
dips were not present at each orbital period. From the zero-order image we
estimate the precise X-ray coordinates of the source with a 90% uncertainty of
0.6''. In this work we focus on the spectral study of discrete absorption
features, during the persistent emission, using the High Energy Transmission
Grating Spectrometer on board the Chandra satellite. We detect, for the first
time in the 1st-order spectra of XB 1916-053, absorption lines associated to Ne
X, Mg XII, Si XIV, and S XVI, and confirm the presence of the Fe XXV and Fe
XXVI absorption lines with a larger accuracy with respect to the previous XMM
EPIC pn observation. Assuming that the line widths are due to a bulk motion or
a turbulence associated to the coronal activity, we estimate that the lines are
produced in a photoionized absorber distant from the neutron star 4 x 10^{10}
cm, near the disk edge.Comment: 20 pages, 10 figures, submitted to ApJ on 2005-09-22, accepted by ApJ
on 2006-05-0
Broad-band Spectral Evolution of Scorpius X-1 along its Color-Color Diagram
We analyze a large collection of RXTE archive data from April 1997 to August
2003 of the bright X-ray source Scorpius X-1 in order to study the broadband
spectral evolution of the source for different values of the inferred mass
accretion rate by studying energy spectra from selected regions in the Z-track
of its Color-Color Diagram. A two-component model, consisting of a soft thermal
component interpreted as thermal emission from an accretion disk and a thermal
Comptonization component, is unable to fit the whole 3--200 keV energy spectrum
at low accretion rates. Strong residuals in the highest energy band of the
spectrum require the addition of a third component that can be fitted with a
power-law component, that could represent a second thermal Comptonization from
a much hotter plasma, or a hybrid thermal/non-thermal Comptonization. We
discuss the physical implications derived from the results of our analysis,
with a particular emphasis on the hardest part of the X-ray emission and its
possible origins.Comment: 18 pages. Accepted for publication in Ap
The role of General Relativity in the evolution of Low Mass X-ray Binaries
We study the evolution of Low Mass X-ray Binaries (LMXBs) and of millisecond
binary radio pulsars (MSPs), with numerical simulations that keep into account
the evolution of the companion, of the binary system and of the neutron star.
According to general relativity, when energy is released, the system loses
gravitational mass. Moreover, the neutron star can collapse to a black hole if
its mass exceeds a critical limit, that depends on the equation of state. These
facts have some interesting consequences: 1) In a MSP the mass-energy is lost
with a specific angular momentum that is smaller than the one of the system,
resulting in a positive contribution to the orbital period derivative. If this
contribution is dominant and can be measured, we can extract information about
the moment of inertia of the neutron star, since the energy loss rate depends
on it. Such a measurement can therefore help to put constraints on the equation
of state of ultradense matter. 2) In LMXBs below the bifurcation period (\sim
18 h), the neutron star survives the period gap only if its mass is smaller
than the maximum non-rotating mass when the companion becomes fully convective
and accretion pauses. Therefore short period (P < 2h) millisecond X-ray pulsar
like SAX J1808.4-3658 can be formed only if either a large part of the
accreting matter has been ejected from the system, or the equation of state of
ultradense matter is very stiff. 3) In Low Mass X-ray binaries above the
bifurcation period, the mass-energy loss lowers the mass transfer rate. As side
effect, the inner core of the companion star becomes 1% bigger than in a system
with a non-collapsed primary. Due to this difference, the final orbital period
of the system becomes 20% larger than what is obtained if the mass-energy loss
effect is not taken into account.Comment: 7 pages, 3 figures, accepted by the MNRA
Evidence for a resonant cyclotron line in IGR J16493-4348 from the Swift-BAT hard X-ray survey
Resonant absorption cyclotron features are a key diagnostic tool to directly
measure the strength of the magnetic field of accreting neutron stars. However,
typical values for cyclotron features lie in the high-energy part of the
spectrum between 20 keV and 50 keV, where detection is often damped by the low
statistics from single pointed observations. We show that long-term monitoring
campaign performed with Swift-BAT of persistently, but faint, accreting
high-mass X-ray binaries is able to reveal in their spectra the presence of
cyclotron features. We extracted the average Swift-BAT 15-150 keV spectrum from
the 54 months long Swift-BAT survey of the high-mass X-ray source IGR
J16493-4348. To constrain the broadband spectrum we used soft X-ray spectra
from Swift-XRT and Suzaku pointed observations. We model the spectra using a
set of phenomenological models usually adopted to describe the energy spectrum
of accreting high-mass X-ray binaries; irrespective of the models we used, we
found significant improvements in the spectral fits adding to the models a
broad (10 keV width) absorption feature, with best-fitting energy estimate
between 30 and 33 keV, that we interpret as evidence for a resonant cyclotron
absorption feature. We also discuss instrumental issues related to the use of
Swift-BAT for this kind of studies and the statistical method to weight the
confidence level of this detection. Correcting for the gravitational redshift
of a 1.4 M_{\sun} neutron star, the inferred surface magnetic field is Bsurf
3.7 x 10^{12} Gauss. The spectral parameters of IGR J16493-4348 fit well with
empirical correlations observed when the whole sample of high-mass binaries
with detected cyclotron features is considered.Comment: Published in Astronomy & Astrophysics, 2011, 532, A7
Resolving the Fe xxv Triplet with Chandra in Cen X-3
We present the results of a 45 ks Chandra observation of the high-mass X-ray
binary Cen X--3 at orbital phases between 0.13 and 0.40 (in the eclipse
post-egress phases). Here we concentrate on the study of discrete features in
the energy spectrum at energies between 6 and 7 keV, i.e. on the iron
K line region, using the High Energy Transmission Grating Spectrometer
on board the Chandra satellite. We clearly see a K neutral iron line
at keV and were able to distinguish the three lines of the
\ion{Fe}{25} triplet at 6.61 keV, 6.67 keV, and 6.72 keV, with an equivalent
width of 6 eV, 9 eV, and 5 eV, respectively. The equivalent width of the
K neutral iron line is 13 eV, an order of magnitude lower than
previous measures. We discuss the possibility that the small equivalent width
is due to a decrease of the solid angle subtended by the reflector.Comment: 11 pages, 2 figures, To appear in the Astrophysical Journal Letter
A complex environment around Cir X-1
We present the results of an archival 54 ks long Chandra observation of the
peculiar source Cir X--1 during the phase passage 0.223-0.261. A comparative
analysis of X-ray spectra, selected at different flux levels of the source,
allows us to distinguish between a very hard state, at a low countrate, and a
brighter, softer, highly absorbed spectrum during episodes of flaring activity,
when the unabsorbed source luminosity is about three times the value in the
hard state. The spectrum of the hard state clearly shows emission lines of
highly ionized elements, while, during the flaring state, the spectrum also
shows strong resonant absorption lines. The most intense and interesting
feature in this latter state is present in the Fe K alpha region: a very
broadened absorption line at energies ~ 6.5 keV that could result from a
smeared blending of resonant absorption lines of moderately ionized iron ions
(Fe XX - Fe XXIV). We also observe strong resonant absorption lines of Fe XXV
and Fe XXVI, together with a smeared absorption edge above 7 keV. We argue that
the emitting region during the quiescent/hard state is constituted of a purely
photo-ionized medium, possibly present above an accretion disk, or of a
photo-ionized plasma present in a beamed outflow. During the flaring states the
source undergoes enhanced turbulent accretion that modifies both the accretion
geometry and the optical depth of the gas surrounding the primary X-ray source.Comment: Accepted for publication in Ap
Spin up and phase fluctuations in the timing of the accreting millisecond pulsar XTE J1807-294
We performed a timing analysis of the 2003 outburst of the accreting X-ray
millisecond pulsar XTE J1807-294 observed by RXTE. Using recently refined
orbital parameters we report for the first time a precise estimate of the spin
frequency and of the spin frequency derivative. The phase delays of the pulse
profile show a strong erratic behavior superposed to what appears as a global
spin-up trend. The erratic behavior of the pulse phases is strongly related to
rapid variations of the light curve, making it very difficult to fit these
phase delays with a simple law. As in previous cases, we have therefore
analyzed separately the phase delays of the first harmonic and of the second
harmonic of the spin frequency, finding that the phases of the second harmonic
are far less affected by the erratic behavior. In the hypothesis that the
second harmonic pulse phase delays are a good tracer of the spin frequency
evolution we give for the first time a estimation of the spin frequency
derivative in this source. The source shows a clear spin-up of Hz sec (1 confidence level). The
largest source of uncertainty in the value of the spin-up rate is given by the
uncertainties on the source position in the sky. We discuss this systematics on
the spin frequency and its derivative.Comment: 17 pages, 4 figures, Accepted by Ap
A broad iron line in the Chandra/HETG spectrum of 4U 1705-44
We present the results of a Chandra 30 ks observation of the low mass X-ray
binary and atoll source 4U 1705-44. Here we concentrate on the study of
discrete features in the energy spectrum at energies below 3 keV, as well as on
the iron Kalpha line, using the HETG spectrometer on board of the Chandra
satellite. Below 3 keV, three narrow emission lines are found at 1.47, 2.0, and
2.6 keV. The 1.47 and 2.6 keV are probably identified with Ly-alpha emission
from Mg XII and S XVI, respectively. The identification of the feature at 2.0
keV is uncertain due to the presence of an instrumental feature at the same
energy. The iron Kalpha line at ~6.5 keV is found to be intrinsically broad
(FWHM ~ 1.2 keV); its width can be explained by reflection from a cold
accretion disk extending down to 15 km from the neutron star center or by
Compton broadening in the external parts of a hot (~2 keV) Comptonizing corona.
We finally report here precise X-ray coordinates of the source.Comment: 8 pages including 2 figures. ApJ Letters, in pres
Chandra Observation of the Dipping Source XB 1254-690
We present the results of a 53 ks long Chandra observation of the dipping
source XB 1254--690. During the observation neither bursts or dips were
observed. From the zero-order image we estimated the precise X-ray coordinates
of the source with a 90% uncertainty of 0.6\arcsec. Since the lightcurve did
not show any significant variability, we extracted the spectrum corresponding
to the whole observation. We confirmed the presence of the \ion{Fe}{xxvi}
K absorption lines with a larger accuracy with respect to the previous
XMM EPIC pn observation. Assuming that the line width were due to a bulk motion
or a turbulence associated to the coronal activity, we estimate that the lines
were produced in a photoionized absorber between the coronal radius and the
outer edge of the accretion disk.Comment: 8 pages, 10 figs, accepted by A&A on 6 December 200
Post-neurosurgical multidrug-resistant Acinetobacter baumannii meningitis successfully treated with intrathecal colistin. A new case and a systematic review of the literature
Introduction: Post-neurosurgical nosocomial meningitis has become an important subgroup of bacterial
meningitis in the hospital setting. The increase in meningitis caused by multidrug-resistant (MDR)
Acinetobacter baumannii has resulted in a significant reduction in available treatment options.
Case report and literature review: We report the case of a 36-year-old man with a complex craniofacial
trauma, who developed a nosocomial meningitis due to MDR A. baumannii that was cured by intrathecal
colistin. The case is contextualized among all the published cases of Acinetobacter meningitis treated
with topical colistin found through a MEDLINE search of the literature. To date, including the present
case, eight reported cases of Acinetobacter meningitis have been treated with colistin administered by an
intrathecal route and 24 by an intraventricular route. The daily dose of colistin used ranged from 1.6 mg
every 24 h to 20 mg every 24 h in adult patients. Themedian time necessary to obtain cerebrospinal fluid
sterilization was 4.1 days, and treatment was always successful even if in two cases Acinetobacter
meningitis relapsed. Toxicity probably or possibly related to the topical administration of colistin was
noted in five out of the 32 patients.
Conclusions: Topical colistin can be an effective and safe treatment for MDR Acinetobacter meningitis
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