10,136 research outputs found
A century of Zoology and Entomology at Rhodes University, 1905 to 2005
The history of the Department of Zoology & Entomology at Rhodes University is traced from its origin with a single staff member, through the era of permanent heads of department to the present day staff. Significant achievements and developments in teaching and research are highlighted, and some anecdotal material is included
Positive allosteric modulators of the a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor
L-glutamate is the major excitatory neurotransmitter in the mammalian central nervous system (CNS) and plays a fundamental role in the control of motor function, cognition and mood. The physiological effects of glutamate are mediated through two functionally distinct receptor families. While activation of metabotropic (G-protein coupled) glutamate receptors results in modulation of neuronal excitability and transmission, the ionotropic glutamate receptors (ligand-gated ion channels) are responsible for mediating the fast synaptic response to extracellular glutamate
Iris colour in passerine birds: why be bright-eyed?
An initial survey of iris coloration in passerine birds (Aves: Passeriformes) showed that a brightly pigmented iris is much more common in southern African and Australian birds than in those from Europe, temperate North America, and Venezuela. However, the only statistical correlation reflected the distribution of particular bird families in these regions. Ten family-level groups considered to represent monophyletic taxa were then selected for a more detailed analysis, comparing iris coloration with distribution, status, taxonomy, plumage patterns, and some biological and behavioural characters for 1143 species. No pattern associating iris colour with particular traits was common to all families, but within families there were statistically significant associations with both plumage and biology. Our expectation that social behaviour would be an important predictor of iris colour was not supported, but critical information is still lacking for many species. Future studies of avian behavioural ecology should examine critically the role of iris coloration in individual species.Rhodes Centenary issu
The complex accretion geometry of GX 339-4 as seen by NuSTAR and Swift
We present spectral analysis of five NuSTAR and Swift observations of GX
339-4 taken during a failed outburst in summer 2013. These observations cover
Eddington luminosity fractions in the range ~0.9-6%. Throughout this outburst,
GX 339-4 stayed in the hard state, and all five observations show similar X-ray
spectra with a hard power-law with a photon index near 1.6 and significant
contribution from reflection. Using simple reflection models we find
unrealistically high iron abundances. Allowing for different photon indices for
the continuum incident on the reflector relative to the underlying observed
continuum results in a statistically better fit and reduced iron abundances.
With a photon index around 1.3, the input power-law on the reflector is
significantly harder than that which is directly observed. We study the
influence of different emissivity profiles and geometries and consistently find
an improvement when using separate photon indices. The inferred inner accretion
disk radius is strongly model dependent, but we do not find evidence for a
truncation radius larger than 100 r_g in any model. The data do not allow
independent spin constraints but the results are consistent with the literature
(i.e., a>0). Our best-fit models indicate an inclination angle in the range
40-60 degrees, consistent with limits on the orbital inclination but higher
than reported in the literature using standard reflection models. The iron line
around 6.4 keV is clearly broadened, and we detect a superimposed narrow core
as well. This core originates from a fluorescence region outside the influence
of the strong gravity of the black hole and we discuss possible geometries.Comment: 11 pages, 6 figures, 6 tables, plus 9 tables in the appendix.
Submitted to Ap
Warm-Dense Molecular Gas in the ISM of Starbursts, LIRGs and ULIRGs
The role of star formation in luminous and ultraluminous infrared galaxies is
a hotly debated issue: while it is clear that starbursts play a large role in
powering the IR luminosity in these galaxies, the relative importance of
possible enshrouded AGNs is unknown. It is therefore important to better
understand the role of star forming gas in contributing to the infrared
luminosity in IR-bright galaxies. The J=3 level of 12CO lies 33K above ground
and has a critical density of ~1.5 X 10^4 cm^-3. The 12CO(J=3-2) line serves as
an effective tracer for warm-dense molecular gas heated by active star
formation. Here we report on 12CO (J=3-2) observations of 17 starburst spirals,
LIRGs and ULIRGs which we obtained with the Heinrich Hertz Submillimeter
Telescope on Mt. Graham, Arizona.
Our main results are the following: 1. We find a nearly linear relation
between the infrared luminosity and warm-dense molecular gas such that the
infrared luminosity increases as the warm-dense molecular gas to the power
0.92; We interpret this to be roughly consistent with the recent results of Gao
& Solomon (2004a,b). 2. We find L_IR/M_H2 ratios ranging from ~10 to ~128
L_sun/M_sun using a standard CO-H2 conversion factor of 3 X 10^20 cm^-2 (K km
s^-1)^-1. If this conversion factor is ~an order of magnitude less, as
suggested in a recent statistical survey (Yao et al. 2003), then 2-3 of our
objects may have significant contributions to the L_IR by dust-enshrouded AGNs.Comment: 15 Pages, 2 figures, Accepted for Publication in Ap
NuSTAR Observations of the Black Hole GS 1354-645: Evidence of Rapid Black Hole Spin
We present the results of a NuSTAR study of the dynamically confirmed
stellar-mass black hole GS 1354-645. The source was observed during its 2015
"hard" state outburst; we concentrate on spectra from two relatively bright
phases. In the higher-flux observation, the broadband NuSTAR spectra reveal a
clear, strong disk reflection spectrum, blurred by a degree that requires a
black hole spin of a = cJ/GM^2 > 0.98 (1 sigma statistical limits only). The
fits also require a high inclination: theta = 75(2) degrees. Strong "dips" are
sometimes observed in the X-ray light curves of sources viewed at such an
angle; these are absent, perhaps indicating that dips correspond to flared disk
structures that only manifest at higher accretion rates. In the lower-flux
observation, there is evidence of radial truncation of the thin accretion disk.
We discuss these results in the context of spin in stellar-mass black holes,
and inner accretion flow geometries at moderate accretion rates.Comment: Accepted for publication in ApJ Letter
A Soft X-Ray Spectral Episode for the Clocked Burster, GS 1826-24 as Measured by Swift and NuSTAR
We report on NuSTAR and Swift observations of a soft state of the neutron
star low-mass X-ray binary GS 1826-24, commonly known as the "clocked" burster.
The transition to the soft state was recorded in 2014 June through an increase
of the 2-20 keV source intensity measured by MAXI, simultaneous with a decrease
of the 15-50 keV intensity measured by Swift/BAT. The episode lasted
approximately two months, after which the source returned to its usual hard
state. We analyze the broad-band spectrum measured by Swift/XRT and NuSTAR, and
estimate the accretion rate during the soft episode to be about 13% of
Eddington, within the range of previous observations. However, the best fit
spectral model, adopting the double Comptonization used previously, exhibits
significantly softer components. We detect seven type-I X-ray bursts, all
significantly weaker (and with shorter rise and decay times) than observed
previously. The burst profiles and recurrence times vary significantly, ruling
out the regular bursts that are typical for this source. One burst exhibited
photospheric radius expansion, and we estimate the source distance at about
(5.7 / xi_b^1/2) kpc, where xi_b parameterizes the possible anisotropy of the
burst emission. Interpreting the soft state as a transition from an optically
thin inner flow to an optically thick flow passing through a boundary layer, as
is commonly observed in similar systems, is contradicted by the lower optical
depth measured for the double Comptonization model we find for this soft state.
The effect of a change in disk geometry on the burst behavior remains unclear.Comment: 40 pages (single-column, doubled spaced format), 9 figures, 3 tables;
submitted to Ap
A Hard X-Ray Study of Ultraluminous X-ray Source NGC 5204 X-1 with NuSTAR and XMM-Newton
We present the results from coordinated X-ray observations of the
ultraluminous X-ray source NGC 5204 X-1 performed by NuSTAR and XMM-Newton in
early 2013. These observations provide the first detection of NGC 5204 X-1
above 10 keV, extending the broadband coverage to 0.3-20 keV. The observations
were carried out in two epochs separated by approximately 10 days, and showed
little spectral variation, with an observed luminosity of Lx = (4.95+/-0.11)e39
erg/s. The broadband spectrum confirms the presence of a clear spectral
downturn above 10 keV seen in some previous observations. This cutoff is
inconsistent with the standard low/hard state seen in Galactic black hole
binaries, as would be expected from an intermediate mass black hole accreting
at significantly sub-Eddington rates given the observed luminosity. The
continuum is apparently dominated by two optically thick thermal-like
components, potentially accompanied by a faint high energy tail. The broadband
spectrum is likely associated with an accretion disk that differs from a
standard Shakura & Sunyaev thin disk.Comment: 7 pages, 5 figures. Accepted for publication in Ap
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