818 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
Evidence for a 2 hr Optical Modulation in GS1826-24
We report the discovery of a 2.1hr optical modulation in the transient source
GS1826-24, based on two independent high time-resolution photometric observing
runs. There is additional irregular variability on shorter timescales. The
source also exhibited an optical burst during each observation, with peak
fluxes consistent with those of the three X-ray bursts so far detected by
BeppoSAX. We compare the low-amplitude variation (0.06 mag) to that seen on the
orbital periods of the short period X-ray bursters, X1636-536 and X1735-444, as
well as the similarity in their non-periodic fluctuations. Other transient
neutron star LMXBs possess short periods in the range 3.8-7.1 hrs. However, if
confirmed as the orbital, a 2.1 hr modulation would make GS1826-24 unique and
therefore of great interest within the context of their formation and
evolution.Comment: 7 pages, 4 figures, to be published 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
An expanded phylogeny of the Entodiniomorphida (Ciliophora : Litostomatea)
The Entodiniomorphida are a diverse and morphologically complex group of ciliates which are symbiotic within the digestive tracts of herbivorous mammals. Previous phylogenies of the group have exclusively considered members of one family, the Ophryoscolecidae, which are symbiotic within ruminants. We sought to improve understanding of evolution within the entodiniomorphs by expanding the range of ciliates examined to include the Cycloposthiidae and Macropodimidae (symbionts of equids and macropodids respectively). The entire SSU-rRNA gene was sequenced for 3 species, Cycloposthium edentatum, Macropodinium ennuensis and M. yalanbense, and aligned against 14 litostome species and 2 postciliodesmatophoran outgroup species. Cycloposthium was consistently grouped as the sister-taxon to the Ophryoscolecidae although support for this relationship was low. This suggests that there is more evolutionary distance between the Cycloposthiidae and Ophryoscolecidae than previously inferred from studies of gross morphology, cell ontogeny or ultrastructure. In contrast, Macropodinium did not group with any of the entodiniomorphs, instead forming the sister group to the entire Trichostomatia (Entodiniomorphida + Vestibuliferida). This early diverging position for the macropodiniids is concordant with their morphology and ontogeny which failed to group the family with any of the entodiniomorph suborders. The currently accepted classification of the Trichostomatia is thus deficient and in need of review
Subdwarf B binaries in the Edinburgh-Cape Survey
We give an update of the results of a campaign to obtain orbital solutions of
subdwarf B stars from the Edinburgh-Cape survey (Stobie et al. 1997). To date
we have obtained blue spectra of 40 subdwarf B stars from the Edinburgh-Cape
catalogue using the grating spectrograph at the 1.9m Radcliffe telescope at the
South African Astronomical Observatory. We find that 17 out of these 40 are
certain binaries with a few other objects showing radial velocity variations of
small amplitude. The binary fraction found in our sample, after correcting for
our binary detection efficiency, is 48%. We have secured the orbital parameters
for 4 of the 17 systems and narrowed down the orbits of another 7 to a small
range of periods.Comment: To appear in Baltic Astronomy. Proceedings of the Second Meeting on
Hot Subdwarf Stars, La Palma, June 2005. Four pages, three figure
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