673 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
The polar ring galaxy AM1934-563 revisited
We report long-slit spectroscopic observations of the dust-lane polar-ring
galaxy AM1934-563 obtained with the Southern African Large Telescope (SALT)
during its performance-verification phase. The observations target the spectral
region of the Ha, [NII] and [SII] emission-lines, but show also deep NaI
stellar absorption lines that we interpret as produced by stars in the galaxy.
We derive rotation curves along the major axis of the galaxy that extend out to
about 8 kpc from the center for both the gaseous and the stellar components,
using the emission and absorption lines. We derive similar rotation curves
along the major axis of the polar ring and point out differences between these
and the ones of the main galaxy. We identify a small diffuse object visible
only in Ha emission and with a low velocity dispersion as a dwarf HII galaxy
and argue that it is probably metal-poor. Its velocity indicates that it is a
fourth member of the galaxy group in which AM1934-563 belongs. We discuss the
observations in the context of the proposal that the object is the result of a
major merger and point out some observational discrepancies from this
explanation. We argue that an alternative scenario that could better fit the
observations may be the slow accretion of cold intergalactic gas, focused by a
dense filament of galaxies in which this object is embedded (abridged).Comment: 19 pages, 13 figures, submitted to MNRAS. Some figures were bitmapped
to reduce the size. Full resolution version is available from
http://www.saao.ac.za/~akniazev/pub/AM1934_563.pd
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
Incidental finding of a microsporidian parasite from an AIDS patient
Light microscopic examination of feces from a human immunodeficiency virus-positive patient with chronic diarrhea, anorexia, and lethargy revealed the presence of numerous refractile bodies resembling microsporidian spores. They were subsequently identified as belonging to the genus Nosema on the basis of their ultrastructural characteristics. However, the microsporidia were enclosed within striated muscle cells, suggesting that they were probably ingested in food; thus, this represented an incidental finding rather than a true infection
Polyuronides in Avocado (Persea americana) and Tomato (Lycopersicon esculentum) Fruits Exhibit Markedly Different Patterns of Molecular Weight Downshifts during Ripening
Persistence in systems with conserved order parameter
We consider the low-temperature coarsening dynamics of a one-dimensional
Ising ferromagnet with conserved Kawasaki-like dynamics in the domain
representation. Domains diffuse with size-dependent diffusion constant, with . We generalize this model to arbitrary
, and derive an expression for the domain density, with , using a scaling argument. We also
investigate numerically the persistence exponent characterizing the
power-law decay of the number, , of persistent (unflipped) spins at
time , and find where depends on
. We show how the results for and are related to
similar calculations in diffusion-limited cluster-cluster aggregation (DLCA)
where clusters with size-dependent diffusion constant diffuse through an
immobile `empty' phase and aggregate irreversibly on impact. Simulations show
that, while is the same in both models, is different except for
. We also investigate models that interpolate between symmetric
domain diffusion and DLCA.Comment: 9 pages, minor revision
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
The rapidly pulsating sdO star, SDSS J160043.6+074802.9
A spectroscopic analysis of SDSS J160043.6+074802.9, a binary system
containing a pulsating subdwarf-O (sdO) star with a late-type companion, yields
Teff = 70 000 +/- 5000 K and log g = 5.25 +/- 0.30, together with a most likely
type of K3V for the secondary star. We compare our results with atmospheric
parameters derived by Fontaine et al. (2008) and in the context of existing
evolution models for sdO stars. New and more extensive photometry is also
presented which recovers most, but not all, frequencies found in an earlier
paper. It therefore seems probable that some pulsation modes have variable
amplitudes. A non-adiabatic pulsation analysis of uniform metallicity sdO
models show those having log g > 5.3 to be more likely to be unstable and
capable of driving pulsation in the observed frequency range.Comment: 14 pages, 12 figures, accepted for publication in MNRAS, 2009
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