824 research outputs found
Feasibility of targeted early detection for melanoma: a population-based screening study
The feasibility of targeted screening for cutaneous malignant melanoma in the UK using a postal questionnaire and invitation to screening by a consultant dermatologist was investigated in a population based cross-sectional survey. A total of 1600 people aged 25â69 years, stratified by the social deprivation score of wards within one general practice, were randomly selected from a population of 8000.1227 (77%) returned the questionnaire and 896 (56%) attended the screening clinic. Uptake was lower for men (P< 0.001), those aged under 50 (P< 0.001), people from deprived areas (P< 0.001) and skin types III and IV (men only, P< 0.001). Twenty per cent of women and 10% of men felt nervous about attending the clinic, but only 4% were worried by the questionnaire. The level of agreement between the self- and dermatologist's assessments of risk factors was best for hair colour (Kappa = 0.67, sensitivity 73% and specificity 98%). People tended to under-report their level of risk. Over 95% knew about at least one major sign, but 54% reported incorrect signs of melanoma. Targeted screening for melanoma in the UK will be hampered by difficulties in accurately identifying the target population. Strategies to improve skin self-awareness rather than screening should be developed and evaluated. © 2000 Cancer Research Campaig
The Role of Magnetic Field Dissipation in the Black Hole Candidate Sgr A*
The compact, nonthermal radio source Sgr A* at the Galactic Center appears to
be coincident with a 2.6 million solar mass point-like object. Its energy
source may be the release of gravitational energy as gas from the interstellar
medium descends into its potential well. Simple attempts at calculating the
spectrum and flux based on this picture have come close to the observations,
yet have had difficulty in accounting for the low efficiency in this source.
There now appear to be two reasons for this low conversion rate: (1) the plasma
separates into two temperatures, with the protons attaining a significantly
higher temperature than that of the radiating electrons, and (2) the magnetic
field, B, is sub-equipartition, which reduces the magnetic bremsstrahlung
emissivity, and therefore the overall power of Sgr A*. We investigate the
latter with improvement over what has been attempted before: rather than
calculating B based on a presumed model, we instead infer its distribution with
radius empirically with the requirement that the resulting spectrum matches the
observations. Our ansatz for B(r) is motivated in part by earlier calculations
of the expected magnetic dissipation rate due to reconnection in a compressed
flow. We find reasonable agreement with the observed spectrum of Sgr A* as long
as its distribution consists of 3 primary components: an outer equipartition
field, a roughly constant field at intermediate radii (~1000 Schwarzschild
radii), and an inner dynamo (more or less within the last stable orbit for a
non-rotating black hole) which increases B to about 100 Gauss. The latter
component accounts for the observed sub-millimiter hump in this source.Comment: 33 pages including 2 figures; submitted to Ap
Radio Synchrotron Emission from Secondary Leptons in the Vicinity of Sgr A*
A point-like source of ~TeV gamma-rays has recently been seen towards the
Galactic center by HESS and other air Cerenkov telescopes. In recent work
(Ballantyne et al. 2007), we demonstrated that these gamma-rays can be
attributed to high-energy protons that (i) are accelerated close to the event
horizon of the central black hole, Sgr A*, (ii) diffuse out to ~pc scales, and
(iii) finally interact to produce gamma-rays. The same hadronic collision
processes will necessarily lead to the creation of electrons and positrons.
Here we calculate the synchrotron emissivity of these secondary leptons in the
same magnetic field configuration through which the initiating protons have
been propagated in our model. We compare this emission with the observed ~GHz
radio spectrum of the inner few pc region which we have assembled from archival
data and new measurements we have made with the Australia Telescope Compact
Array. We find that our model predicts secondary synchrotron emission with a
steep slope consistent with the observations but with an overall normalization
that is too large by a factor of ~ 2. If we further constrain our theoretical
gamma-ray curve to obey the implicit EGRET upper limit on emission from this
region we predict radio emission that is consistent with observations, i.e.,
the hadronic model of gamma ray emission can, simultaneously and without
fine-tuning, also explain essentially all the diffuse radio emission detected
from the inner few pc of the Galaxy.Comment: 11 pages, 2 figures. Two references missing from published version
added and acknowledgements extende
Spin-Induced Disk Precession in the Supermassive Black Hole at the Galactic Center
Sgr A* is a compact radio source at the Galactic Center, thought to be the
radiative manifestation of a supermassive black hole.
At least a portion of its spectrum--notably the mm/sub-mm ``bump''--appears to
be produced within the inner portion () of a hot, magnetized
Keplerian flow, whose characteristics are also consistent with the
linear polarization detected from this source at mm wavelengths. (The
Schwarzschild radius, , for an object of this mass is cm, or roughly 1/20 A.U.) The recent detection of a 106-day
cycle in Sgr A*'s radio variability adds significant intrigue to this picture,
since it may signal a precession of the disk induced by the spin of the
black hole. The dynamical time scale near the marginally stable orbit around an
object with this mass is mins. Thus, since the physical conditions
associated with the disk around Sgr A* imply rigid-body rotation, a precession
period of 106 days may be indicative of a small black hole spin if the
circularized flow is confined to a region , for which . The precession of a larger structure would require
a bigger black hole spin. We note that a small value of () would
be favored if the non-thermal ( cm) portion of Sgr A*'s spectrum is
powered with energy extracted via a Blandford-Znajek type of process, for which
the observed luminosity would correspond to an outer disk radius . Such a small disk size is also suggested by earlier hydrodynamical
simulations, and is implied by Sgr A*'s spectral and polarimetric
characteristics.Comment: 12 pages, 1 figure. Accepted for publicaton in ApJ Letter
Viewing the Shadow of the Black Hole at the Galactic Center
In recent years, the evidence for the existence of an ultra-compact
concentration of dark mass associated with the radio source Sgr A* in the
Galactic Center has become very strong. However, an unambiguous proof that this
object is indeed a black hole is still lacking. A defining characteristic of a
black hole is the event horizon. To a distant observer, the event horizon casts
a relatively large ``shadow'' with an apparent diameter of ~10 gravitational
radii due to bending of light by the black hole, nearly independent of the
black hole spin or orientation. The predicted size (~30 micro-arcseconds) of
this shadow for Sgr A* approaches the resolution of current
radio-interferometers. If the black hole is maximally spinning and viewed
edge-on, then the shadow will be offset by ~8 micro-arcseconds from the center
of mass, and will be slightly flattened on one side. Taking into account
scatter-broadening of the image in the interstellar medium and the finite
achievable telescope resolution, we show that the shadow of Sgr A* may be
observable with very long-baseline interferometry at sub-millimeter
wavelengths, assuming that the accretion flow is optically thin in this region
of the spectrum. Hence, there exists a realistic expectation of imaging the
event horizon of a black hole within the next few years.Comment: 5 pages, 1 figure (color), (AAS)Tex, to appear in The Astrophysical
Journal Letters, Vol. 528, L13 (Jan 1, 2000 issue); also available at
http://www.mpifr-bonn.mpg.de/staff/hfalcke/publications.html#bhimag
Urological referral of asymptomatic men in general practice in England
The Prostate Cancer Risk Management Programme (PCRMP) launched in November 2002 provides guidelines for general practitioners (GPs) on age-specific prostate-specific antigen (PSA) cutoff levels in asymptomatic men. The impact of the PCRMP on GP referrals is unknown. This study investigates whether there was a change in the proportion of asymptomatic men with raised PSA levels (â©Ÿ3ângâmlâ1) who were referred to urologists since the launch of the guidelines. Sixty-nine general practices in four areas of England and the main pathology laboratory in each area, which had participated in our previous research, were asked to provide data. Forty-eight practices (70%) provided retrospective data on urological referrals in men who had a PSA test taken in the periods 1 December 2001 to 31 May 2002 (pre-launch) and 1 December 2003 to 31 May 2004 (post-launch). Data on referrals were completed for 709 (79%) out of 898 and 1040 (90%) out of 1157 raised records pre- and post-launch, respectively. The percentage of men with raised PSA levels who were asymptomatic was similar in both time periods (19â20%) and the proportion referred to urologists according to the PCRMP guidelines did not increase significantly over time (24% pre-launch and 29% post-launch, P=0.42). The referral rate was lower than expected if the guidelines had been followed. The influence of the guidelines seems to have been low. At the time of data collection, 56% (112 out of 200) of GP partners reported that they were aware of receiving the PCRMP pack. To ensure future, effective implementation of guidelines requires evaluation
Ubiquitous equatorial accretion disc winds in black hole soft states
High resolution spectra of Galactic Black Holes (GBH) reveal the presence of
highly ionised absorbers. In one GBH, accreting close to the Eddington limit
for more than a decade, a powerful accretion disc wind is observed to be
present in softer X-ray states and it has been suggested that it can carry away
enough mass and energy to quench the radio jet. Here we report that these
winds, which may have mass outflow rates of the order of the inner accretion
rate or higher, are an ubiquitous component of the jet-free soft states of all
GBH. We furthermore demonstrate that these winds have an equatorial geometry
with opening angles of few tens of degrees, and so are only observed in sources
in which the disc is inclined at a large angle to the line of sight. The
decrease in Fe XXV / Fe XXVI line ratio with Compton temperature, observed in
the soft state, suggests a link between higher wind ionisation and harder
spectral shapes. Although the physical interaction between the wind, accretion
flow and jet is still not fully understood, the mass flux and power of these
winds, and their presence ubiquitously during the soft X-ray states suggests
they are fundamental components of the accretion phenomenon.Comment: Accepted for publication in MNRAS Letter
Towards transforming community eye care: an observational study and time-series analysis of optometrists' prescribing for eye disorders
Objectives:
This study aimed to provide evidence on the therapeutic prescribing activity by community optometrists in Scotland and to determine its impact on workload in general practice and ophthalmology clinics.
Study design:
Scottish administrative healthcare data for a 53-month period (November 2013âApril 2018) were used to analyse non-medical prescribing practice by optometrists.
Methods:
Using interrupted time-series regression (Autoregressive Integrated Moving Average), we assessed the impact of optometrist prescribing on ophthalmology outpatient attendances and general practice prescribing for eye disorders.
Results:
A total of 54,246 items were prescribed by 205 optometrists over the study period. Since the commencement of data recording, optometrist prescribing activity increased steadily from a baseline of zero to 1.2% of all ophthalmic items prescribed. Neither the monthly number of items prescribed nor the size of optometric workforce were associated with a reduction in ophthalmology outpatient appointments over time.
Conclusions:
Optometrists increasingly contribute to community ophthalmic prescribing in Scotland, releasing capacity and lessening general practice, but not secondary care workload. There appears to be an underutilisation of optometrists related to the management of dry eye, which represents an opportunity to release further capacity
Safety, the Preface Paradox and Possible Worlds Semantics
This paper contains an argument to the effect that possible worlds semantics renders
semantic knowledge impossible, no matter what ontological interpretation is given
to possible worlds. The essential contention made is that possible worlds semantic
knowledge is unsafe and this is shown by a parallel with the preface paradox
- âŠ