3,579 research outputs found
Sic et Non: Are We So Sure of Matthean Dependence on Mark?
Let’s take another look at the view that Matthew depends on Mark, especially at the way in which G. M. Styler has recently defended it. The author then examines the pros and cons of the arguments and concludes that the case for Markan priority is not as open and shut as some would argue
Combined cerebellar and bilateral cervical posterior spinal artery stroke demonstrated on MRI
Combined cerebellar and spinal ischemic stroke is a rare, critical condition. We report a patient with combined cerebellar and bilateral posterolateral cervical spinal cord infarction due to bilateral stenosis of the vertebral arteries. MRI is the method of choice for imaging this condition; diffusion-weighted imaging of the spinal cord gives reliable results
Probing the BLR in AGNs using time variability of associated absorption line
It is know that most of the clouds producing associated absorption in the
spectra of AGNs and quasars do not completely cover the background source
(continuum + broad emission line region, BLR). We note that the covering factor
derived for the absorption is the fraction of photons occulted by the absorbing
clouds, and is not necessarily the same as the fractional area covered. We show
that the variability in absorption lines can be produced by the changes in the
covering factor caused by the variation in the continuum and the finite light
travel time across the BLR. We discuss how such a variability can be
distinguished from the variability caused by other effects and how one can use
the variability in the covering factor to probe the BLR.Comment: 12 pages, latex(aaspp4.sty), 2 figures, (To appear in ApJ
On X-ray pulsations in beta Cephei-type variables
Beta Cephei-type variables are early B-type stars that are characterized by
oscillations observable in their optical light curves. At least one Beta
Cep-variable also shows periodic variability in X-rays. Here we study the X-ray
light curves in a sample of beta Cep-variables to investigate how common X-ray
pulsations are for this type of stars. We searched the Chandra and XMM-Newton
X-ray archives and selected stars that were observed by these telescopes for at
least three optical pulsational periods. We retrieved and analyzed the X-ray
data for kappa Sco, beta Cru, and alpha Vir. The X-ray light curves of these
objects were studied to test for their variability and periodicity. While there
is a weak indication for X-ray variability in beta Cru, we find no
statistically significant evidence of X-ray pulsations in any of our sample
stars. This might be due either to the insufficient data quality or to the
physical lack of modulations. New, more sensitive observations should settle
this question.Comment: accepted in A&
The Metallicity of the Redshift 4.16 Quasar BR2248-1242
We estimate the metallicity in the broad emission-line region of the redshift
z=4.16 quasar, BR2248-1242, by comparing line ratios involving nitrogen to
theoretical predictions. BR2248-1242 has unusually narrow emission lines with
large equivalent widths, thus providing a rare opportunity to measure several
line-ratio abundance diagnostics. The combined diagnostics indicate a
metallicity of ~2 times solar. This result suggests that an episode of vigorous
star formation occurred near BR2248-1242 prior to the observed z=4.16 epoch.
The time available for this enrichment episode is only ~1.5 Gyr at z=4.16 (for
H_{0}=65 km s^-1 Mpc^-1, Omega_{m}=0.3 and Omega_Lambda ~< 1). This evidence
for high metallicities and rapid star formation is consistent with the expected
early-epoch evolution of dense galactic nuclei.Comment: 8 pages, 3 figures. Prepared in AAStex. Submitted to the
Astrophysical Journal Revised version: added 1 referenc
Increased plasma viscosity as a reason for inappropriate erythropoietin formation
The aim of this study was to examine whether altered plasma viscosity could contribute to the inappropriately low production rate of erythropoietin (EPO) observed in patients suffering from hypergammaglobulinemias associated with multiple myeloma or Waldenström's disease. We found that the EPO formation in response to anemia in these patients was inversely related to plasma viscosity. A similar inverse relationship between plasma viscosity and EPO production was seen in rats in which EPO formation had been stimulated by exchange transfusion and the plasma viscosity of which was thereby altered by using exchange solutions of different composition to alter plasma viscosity and thus whole blood viscosity independently from hematocrit. Raising the gammaglobulin concentration to approximately 40 mg/ml plasma in the rats almost totally blunted the rise in serum EPO levels despite a fall of the hematocrit to 20%. Determination of renal EPO mRNA levels by RNase protection revealed that the reductions in serum EPO levels at higher plasma viscosities were paralleled by reductions in renal EPO mRNA levels. Taken together, our findings suggest that plasma viscosity may be a significant inhibitory modulator of anemia-induced EPO formation. The increased plasma viscosity in patients with hypergammaglobulinemias may therefore contribute to the inappropriate EPO production, which is a major reason for the anemia developing in these patients
Effects of the stellar wind on X-ray spectra of Cygnus X-3
We study X-ray spectra of Cyg X-3 from BeppoSAX, taking into account
absorption and emission in the strong stellar wind of its companion. We find
the intrinsic X-ray spectra are well modelled by disc blackbody emission, its
upscattering by hot electrons with a hybrid distribution, and by Compton
reflection. These spectra are strongly modified by absorption and reprocessing
in the stellar wind, which we model using the photoionization code cloudy. The
form of the observed spectra implies the wind is composed of two phases. A hot
tenuous plasma containing most of the wind mass is required to account for the
observed features of very strongly ionized Fe. Small dense cool clumps filling
<0.01 of the volume are required to absorb the soft X-ray excess, which is
emitted by the hot phase but not present in the data. The total mass-loss rate
is found to be (0.6--1.6) x 10^-5 solar masses per year. We also discuss the
feasibility of the continuum model dominated by Compton reflection, which we
find to best describe our data. The intrinsic luminosities of our models
suggest that the compact object is a black hole.Comment: MNRAS, in pres
On the binary nature of massive blue hypergiants: high-resolution X-ray spectroscopy suggests that Cyg OB2 12 is a colliding wind binary
The blue hypergiant Cyg OB2-12 (B3Ia+) is a representative member of the
class of very massive stars in a poorly understood evolutionary stage. We
obtained its high-resolution X-ray spectrum using Chandra observatory. PoWR
model atmospheres were calculated to provide realistic wind opacities and to
establish the wind density structure. We find that collisional de-excitation is
the dominant mechanism de-populating the metastable upper levels of the
forbidden lines of the He-like ions SiXIV and MgXII. Comparison between the
model and observations reveals that X-ray emission is produced in a dense
plasma, which could reside only at the photosphere or in a colliding wind zone
between binary components. The observed X-ray spectra are well fitted by
thermal plasma models, with average temperatures in excess of 10 MK. The wind
speed in Cyg OB2-12 is not high enough to power such high temperatures, but the
collision of two winds in a binary system can be sufficient. We used archival
data to investigate the X-ray properties of other blue hypergiants. In general,
stars of this class are not detected as X-rays sources. We suggest that our new
Chandra observations of Cyg OB2-12 can be best explained if Cyg OB2-12 is a
colliding wind binary possessing a late O-type companion. This makes Cyg OB2-12
only the second binary system among the 16 known Galactic hypergiants. This low
binary fraction indicates that the blue hypergiants are likely products of
massive binary evolution during which they either accreted a significant amount
of mass or already merged with their companion.Comment: accepted to Ap
A combined HST and XMM-Newton campaign for the magnetic O9.7 V star HD 54879: towards constraining the weak-wind problem of massive stars
Context: HD 54879 (O9.7 V) is one of a dozen O-stars for which an organized
atmospheric magnetic field has been detected. To gain insights into the
interplay between atmospheres, winds, and magnetic fields of massive stars, we
acquired UV and X-ray data of HD 54879 using the Hubble Space Telescope and the
XMM-Newton satellite. In addition, 35 optical amateur spectra were secured to
study the variability of HD 54879. A multiwavelength (X-ray to optical)
spectral analysis is performed using the Potsdam Wolf-Rayet (PoWR) model
atmosphere code and the xspec software.
Results: The photospheric parameters are typical for an O9.7 V star. The
microturbulent, macroturbulent, and projected rotational velocities are lower
than previously suggested (<4 km/s). An initial mass of 16 and an
age of 5 Myr are inferred from evolutionary tracks. We derive a mean X-ray
emitting temperature of [K] and an X-ray luminosity of
[erg/s]. Short- and long-scale variability is seen in
the H-alpha line, but only a very long period of yr could be
estimated. Assessing the circumstellar density of HD 54879 using UV spectra, we
can roughly estimate the mass-loss rate HD 54879 would have in the absence of a
magnetic field as . The
magnetic field traps the stellar wind up to the Alfv\'en radius >
, implying that its true mass-loss rate is . Hence, density enhancements around magnetic stars
can be exploited to estimate mass-loss rates of non-magnetic stars of similar
spectral types, essential for resolving the weak wind problem.
Conclusions: Our study confirms that strongly magnetized stars lose little or
no mass, and supplies important constraints on the weak-wind problem of massive
main sequence stars.Comment: Accepted for publication in A&A on Aug. 9, 2017, 12 + 1 pages, 15
figures. Paper replaced due to typos and missing acknowledgment
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