652 research outputs found
Simultaneous X-ray/optical observations of GX 9+9 (4U 1728-16)
We report on the results of the first simultaneous X-ray (RXTE) and optical
(SAAO) observations of the luminous low mass X-ray binary (LMXB) GX 9+9 in 1999
August. The high-speed optical photometry revealed an orbital period of 4.1958
hr and confirmed previous observations, but with greater precision. No X-ray
modulation was found at the orbital period. On shorter timescales, a possible
1.4-hr variability was found in the optical light curves which might be related
to the mHz quasi-periodic oscillations seen in other LMXBs. We do not find any
significant X-ray/optical correlation in the light curves. In X-rays, the
colour-colour diagram and hardness-intensity diagram indicate that the source
shows characteristics of an atoll source in the upper banana state, with a
correlation between intensity and spectral hardness. Time-resolved X-ray
spectroscopy suggests that two-component spectral models give a reasonable fit
to the X-ray emission. Such models consist of a blackbody component which can
be interpreted as the emission from an optically thick accretion disc or an
optically thick boundary layer, and a hard Comptonized component for an
extended corona.Comment: 19 pages, 13 figures; accepted for publication in MNRA
Two new pulsating hot subdwarf stars from the Edinburgh-Cape survey
We report the discovery of very rapid pulsations in two hot subdwarf stars from the Edinburgh-
Cape blue object survey. The short periods, small amplitudes and multiperiodicity establish
these stars as members of the class of rapidly-pulsating sdB stars. The spectrograms of
both stars, however, show relatively strong He II 4686 and they are therefore more properly
classified as sdOB. The light curve of EC 01541?1409 is dominated by two strong
(?1 per cent) variations with frequencies near 7114 and 7870 ?Hz (periods near 140.6 and
127.1 s), though at least five frequencies are present with amplitudes above about 0.002 mag.
The light curve of EC 22221?3152 appears to be generated by at least 10 frequencies in the
range 5670-11850 ?Hz (about 175-85 s) with amplitudes between about 0.01 and 0.001 mag,
including the first overtone of the strongest variation. Somewhat surprisingly, this number of
frequencies is detectable in observing runs as short as 3 h, probably due to the fact that the
detected frequencies are well-separated.IS
Differentiation of human fetal mesenchymal stem cells into cells with an oligodendrocyte phenotype
This article is available open access through the publisherâs website at the link below. Copyright @ 2009 Landes Bioscience.The potential of mesenchymal stem cells (MSC) to differentiate into neural lineages has raised the possibility of autologous cell transplantation as a therapy for neurodegenerative diseases. We have identified a population of circulating human fetal mesenchymal stem cells (hfMSC) that are highly proliferative and can readily differentiate into mesodermal lineages such as bone, cartilage, fat and muscle. Here, we demonstrate for the first time that primary hfMSC can differentiate into cells with an oligodendrocyte phenotype both in vitro and in vivo. By exposing hfMSC to neuronal conditioned medium or by introducing the pro-oligodendrocyte gene, Olig-2, hfMSC adopted an oligodendrocyte-like morphology, expressed oligodendrocyte markers and appeared to mature appropriately in culture. Importantly we also demonstrate the differentiation of a clonal population of hfMSC into both mesodermal (bone) and ectodermal (oligodendrocyte) lineages. In the developing murine brain transplanted hfMSC integrated into the parenchyma but oligodendrocyte differentiation of these naĂŻve hfMSC was very low. However, the proportion of cells expressing oligodendrocyte markers increased significantly (from 0.2% to 4%) by pre-exposing the cells to differentiation medium in vitro prior to transplantation. Importantly, the process of in vivo differentiation occurred without cell fusion. These findings suggest that hfMSC may provide a potential source of oligodendrocytes for study and potential therapy
Aperiodic optical variability of intermediate polars - cataclysmic variables with truncated accretion disks
We study the power spectra of the variability of seven intermediate polars
containing magnetized asynchronous accreting white dwarfs, XSS J00564+4548,IGR
J00234+6141, DO Dra, V1223 Sgr, IGR J15094-6649, IGR J16500-3307 and IGR
J17195-4100, in the optical band and demonstrate that their variability can be
well described by a model based on fluctuations propagating in a truncated
accretion disk. The power spectra have breaks at Fourier frequencies, which we
associate with the Keplerian frequency of the disk at the boundary of the white
dwarfs' magnetospheres. We propose that the properties of the optical power
spectra can be used to deduce the geometry of the inner parts of the accretion
disk, in particular: 1) truncation radii of the magnetically disrupted
accretion disks in intermediate polars, 2) the truncation radii of the
accretion disk in quiescent states of dwarf novaeComment: Accepted for publication in A&
Hot subdwarf stars in close-up view IV. Helium abundances and the 3He isotopic anomaly of subdwarf B stars
Atmospheric parameters and helium abundances of 44 bright subdwarf B stars have been determined. More than half of our sample
consists of newly discovered stars from the Edinburgh Cape survey. We showed that effective temperatures and surface gravities can
be derived from high resolution echelle spectra with sufficient accuracy. Systematic uncertainties have been determined by comparing
the parameters derived from the high resolution data with the ones derived from medium resolution spectra. Helium abundances have
been measured with high accuracy. Besides the known correlation of helium abundance with temperature, two distinct sequences in
helium abundance have been confirmed. Significant isotopic shifts of helium lines due to an enrichment in 3He have been found in
the spectra of 8 subluminous B stars (sdBs). Most of these stars cluster in a small temperature range between 27 000K and 31 000K
very similar to the known 3He-rich main sequence B stars, which also cluster in such a small strip, but at different temperatures. Both
the helium sequences and the isotopic anomaly are discussed.Web of Scienc
Dwarf Nova Oscillations and Quasi-Periodic Oscillations in Cataclysmic Variables - VIII. VW Hyi in outburst observed with the Southern African Large Telescope
We analyse four light curves obtained at high time resolution (~ 0.1 s) with
the 11-m Southern African Large Telescope, at the ends of two normal outbursts
and one superoutburst of the dwarf nova VW Hyi. All of these contain at least
some Dwarf Nova Oscillations (DNOs), which, when at their highest amplitudes,
are seen in unprecedented detail. In addition to the expected DNOs with periods
> 20 s we find a previously unknown modulation at 13.39 s, but none at shorter
periods. The various DNOs and their interaction with the longer period
Quasi-periodic Oscillations are interpreted in terms of the model of
magnetically controlled flow from an accretion disc proposed earlier in this
series of papers. Our observations include rare DNOs very late in outburst; we
find that the fundamental period does not increase beyond ~ 90 s, which is the
same value that the independent ``longer period DNOs'' converge on.Comment: Accepted for publication in MNRAS. 7 pages, 10 figures, 2 table
Whole Earth Telescope observations of the hot helium atmosphere pulsating white dwarf EC 20058-5234
We present the analysis of a total of 177h of high-quality optical
time-series photometry of the helium atmosphere pulsating white dwarf (DBV) EC
20058-5234. The bulk of the observations (135h) were obtained during a WET
campaign (XCOV15) in July 1997 that featured coordinated observing from 4
southern observatory sites over an 8-day period. The remaining data (42h) were
obtained in June 2004 at Mt John Observatory in NZ over a one-week observing
period. This work significantly extends the discovery observations of this
low-amplitude (few percent) pulsator by increasing the number of detected
frequencies from 8 to 18, and employs a simulation procedure to confirm the
reality of these frequencies to a high level of significance (1 in 1000). The
nature of the observed pulsation spectrum precludes identification of unique
pulsation mode properties using any clearly discernable trends. However, we
have used a global modelling procedure employing genetic algorithm techniques
to identify the n, l values of 8 pulsation modes, and thereby obtain
asteroseismic measurements of several model parameters, including the stellar
mass (0.55 M_sun) and T_eff (~28200 K). These values are consistent with those
derived from published spectral fitting: T_eff ~ 28400 K and log g ~ 7.86. We
also present persuasive evidence from apparent rotational mode splitting for
two of the modes that indicates this compact object is a relatively rapid
rotator with a period of 2h. In direct analogy with the corresponding
properties of the hydrogen (DAV) atmosphere pulsators, the stable low-amplitude
pulsation behaviour of EC 20058 is entirely consistent with its inferred
effective temperature, which indicates it is close to the blue edge of the DBV
instability strip. (abridged)Comment: 19 pages, 8 figures, 5 tables, MNRAS accepte
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