2,900 research outputs found
The XMM-Newton Slew Survey: Towards The Whole X-ray Sky and the Rarest X-ray Events
The data collected by XMM-Newton as it slews between pointings currently
cover almost half the entire sky, and many familiar features and new sources
are visible. The soft-band sensitivity limit of the Slew is close to that of
the RASS, and a large-area Slew-RASS comparison now provides the best
opportunity for discovering extremely rare high-variability objects.Comment: To appear in Proceedings of "X-ray Astronomy 2009: Present Status,
Multi-Wavelength Approach and Future Perspectives", Bologna, Italy, September
7-11, 2009, AIP, eds. A. Comastri, M. Cappi, and L. Angelin
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Assessment for Learning in International Contexts: exploring shared and divergent dimensions in teacher values and practices
The Assessment for Learning in International Contexts (ALIC) project sought to extend knowledge around teachersâ understandings of Assessment for Learning (AfL). Using a modified version of a survey item devised by James and Pedder (2006) for use with teachers in England, evidence was gathered about the assessment practices that were highly valued by teachers across international contexts. The extent of congruence between these values and teachersâ reported classroom practices was explored and dimensions of teachersâ assessment practices were derived through factor analysis. Whilst there was considerable congruence across the ALIC cohort of teachers and data sets derived from English teachers, particularly with respect to the items that have positive values-practice gaps, there were some interesting differences. Two components were derived from factor analysis, rather than the three derived by James and Pedder (2006). These components were âMaking learning explicit and promoting learner autonomyâ and âStudent control of assessment processesâ.This is an Accepted Manuscript of an article published by Taylor & Francis in The Curriculum Journal on the 6th of November 2014, available online: http://wwww.tandfonline.com/10.1080/09585176.2014.975732. This version will be under embargo until the 6th of May 2016
New insights into ultraluminous X-ray sources from deep XMM-Newton observations
The controversy over whether ultraluminous X-ray sources (ULXs) contain a new
intermediate-mass class of black holes (IMBHs) remains unresolved. We present
new analyses of the deepest XMM-Newton observations of ULXs that address their
underlying nature. We examine both empirical and physical modelling of the
X-ray spectra of a sample of thirteen of the highest quality ULX datasets, and
find that there are anomalies in modelling ULXs as accreting IMBHs with
properties simply scaled-up from Galactic black holes. Most notably, spectral
curvature above 2 keV in several sources implies the presence of an
optically-thick, cool corona. We also present a new analysis of a 100 ks
observation of Holmberg II X-1, in which a rigorous analysis of the temporal
data limits the mass of its black hole to no more than 100 solar masses. We
argue that a combination of these results points towards many (though not
necessarily all) ULXs containing black holes that are at most a few 10s of
solar mass in size.Comment: 5 pages, 2 figures, to appear in the proceedings of "The X-ray
Universe 2005", San Lorenzo de El Escorial (Spain), 26-30 September 200
The myth of the new: Mass digitization, distant reading, and the future of the book
This article presents the theoretical background to a wider project that is attempting to increase our understanding of the impact and uses of large-scale digitization, being undertaken by the first author at University College London with the working title âWhat is the impact of large-scale digitization upon researchers and the information sector?â It discusses the controversy surrounding the emergence of mass digitization: the creation and collection of huge resources containing millions of pages of textual cultural content. It demonstrates that the polarized nature of the literature about this technological development is far from unprecedented, and in fact can be traced through the theory of a number of varied fields: the debate surrounding mechanization and digital technologies, our understanding of the role of the sublime in modern representations of technology, the similarities between the sociology of city life and digital information overload, and the way in which innovations are diffused throughout society. It proposes that these theories explain why debates around technological innovation often become so hyperbolic, creating an almost mythological view of technological determination. It concludes that, as a result of the processes outlined in this theory, mass digitization has become stuck between two conflicting rhetorical movements, and that it is therefore necessary to begin working to increase our understanding of this technology and to move the debate onwards using evidence from the real world
Primary cancer of the liver in Kenyan children.
In 9 years in Kenya, 34 examples of primary liver cancer wer diagnosed in patients in the first two decades of life. This represents 4.7% of all liver cancers during this period. The larger proportion (29) were hepatocellular carcinoma. In the second decade, there was a notable association with macronodular cirrhosis. Analogy with experimental work suggests that cells in mitotic cycle may be more vulnerable to the effect of environmental carcinogens. Five examples of hepatoblastoma were identified at ages from 2 months to 14 years; none showed the features of "mixed" tumours. The ratio of hepatoblastoma to hepatocellular carcinoma was the reverse of that found in other large series of juvenile hepatic tumours. The histopathological features of these tumours are described and problems of their classification are discussed
X-ray Spectroscopy of Candidate Ultracompact X-ray Binaries
We present high-resolution spectroscopy of the neutron star/low-mass X-ray
binaries (LMXBs) 4U 1850-087 and 4U 0513-40 as part of our continuing study of
known and candidate ultracompact binaries. The LMXB 4U 1850-087 is one of four
systems in which we had previously inferred an unusual Ne/O ratio in the
absorption along the line of sight, most likely from material local to the
binaries. However, our recent Chandra X-ray Observatory LETGS spectrum of 4U
1850-087 finds a Ne/O ratio by number of 0.22+/-0.05, smaller than previously
measured and consistent with the expected interstellar value. We propose that
variations in the Ne/O ratio due to source variability, as previously observed
in these sources, can explain the difference between the low- and
high-resolution spectral results for 4U 1850-087. Our XMM-Newton RGS
observation of 4U 0513-40 also shows no unusual abundance ratios in the
absorption along the line of sight. We also present spectral results from a
third candidate ultracompact binary, 4U 1822-000, whose spectrum is well fit by
an absorbed power-law + blackbody model with absorption consistent with the
expected interstellar value. Finally, we present the non-detection of a fourth
candidate ultracompact binary, 4U 1905+000, with an upper limit on the source
luminosity of < 1 x 10^{32} erg s^{-1}. Using archival data, we show that the
source has entered an extended quiescent state.Comment: 8 pages, 3 figures, accepted for publication to the Astrophysical
Journa
Minimum requirements for feedback enhanced force sensing
The problem of estimating an unknown force driving a linear oscillator is
revisited. When using linear measurement, feedback is often cited as a
mechanism to enhance bandwidth or sensitivity. We show that as long as the
oscillator dynamics are known, there exists a real-time estimation strategy
that reproduces the same measurement record as any arbitrary feedback protocol.
Consequently some form of nonlinearity is required to gain any advantage beyond
estimation alone. This result holds true in both quantum and classical systems,
with non-stationary forces and feedback, and in the general case of
non-Gaussian and correlated noise. Recently, feedback enhanced incoherent force
sensing has been demonstrated [Nat. Nano. \textbf{7}, 509 (2012)], with the
enhancement attributed to a feedback induced modification of the mechanical
susceptibility. As a proof-of-principle we experimentally reproduce this result
through straightforward filtering.Comment: 5 pages + 2 pages of Supplementary Informatio
Evanescent field optical readout of graphene mechanical motion at room temperature
Graphene mechanical resonators have recently attracted considerable attention
for use in precision force and mass sensing applications. To date, readout of
their oscillatory motion has typically required cryogenic conditions to achieve
high sensitivity, restricting their range of applications. Here we report the
first demonstration of evanescent optical readout of graphene motion, using a
scheme which does not require cryogenic conditions and exhibits enhanced
sensitivity and bandwidth at room temperature. We utilise a high
microsphere to enable evanescent readout of a 70 m diameter graphene drum
resonator with a signal-to-noise ratio of greater than 25 dB, corresponding to
a transduction sensitivity of 2.6 m
. The sensitivity of force measurements using this
resonator is limited by the thermal noise driving the resonator, corresponding
to a force sensitivity of N
with a bandwidth of 35 kHz at room temperature (T = 300
K). Measurements on a 30 m graphene drum had sufficient sensitivity to
resolve the lowest three thermally driven mechanical resonances.Comment: Fixed formatting errors in bibliograph
The impact of reservoir conditions on the residual trapping of carbon dioxide in Berea sandstone
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