22,580 research outputs found
A 2 h periodic variation in the low-mass X-ray binary Ser X-1
No full textSpectroscopy of the low-mass X-ray binary Ser X-1 using the Gran Telescopio Canarias have revealed a ?2 h periodic variability that is present in the three strongest emission lines. We tentatively interpret this variability as due to orbital motion, making it the first indication of the orbital period of Ser X-1. Together with the fact that the emission lines are remarkably narrow, but still resolved, we show that a main-sequence K dwarf together with a canonical 1.4 M? neutron star gives a good description of the system. In this scenario, the most likely place for the emission lines to arise is the accretion disc, instead of a localized region in the binary (such as the irradiated surface or the stream-impact point), and their narrowness is due instead to the low inclination (?10°) of Ser X-1
[Letter from John P. Henry to Catherine Steele Ray, May 23, 1840]
No full textLetter from John P. Henry to his aunt Catherine Steele Ray. John writes briefly about money he owes and states he is sending $350 to Catherine
Improvements to MOS CCD technology for future X-ray astronomy missions
No full textThis thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.This thesis is concerned with the development of MOS charge-coupled device (CCD) technology for future applications in X-ray astronomy. Of particular interest is increased detection efficiency of high energy X-ray photons and increased pixel readout speed for large area sensors. Chapter 2 reviews the generation of X-rays, methods for extra-terrestrial X-ray observations, detectors and provides an overview of X-ray astronomy missions. Chapter 3 discusses the CCD and introduces some of the recent technological developments that improve their overall performance for optical and X-ray photon detection. Chapter 4 presents the basic laboratory equipment and methods used to carry out the experimental work of this thesis. Chapter 5 presents the characterisation of new high resistivity devices that were manufactured by e2v technologies during the work of this thesis. Chapter 6 describes a method for estimating the depletion depth of a CCD by analysing the X-ray event patterns that are generated in CCD image data. Chapter 7 presents the equipment developed and experimental measurements taken to evaluate the high energy X-ray quantum efficiency of a high resistivity CCD. Finally, Chapter 8 describes the ongoing development and characterisation of low noise ASICs that are intended for use in future X-ray astronomy missions
Atypical Interaction: An Introduction
No full textWilkinson, Rae and Rasmussen provide a thorough, state-of-the-art Introduction to the growing field of Atypical Interaction, that is, conversations and other types of social interaction where one of the participants has a communicative impairment or communication disorder. These can include, for example, autism, dementia, learning disability, stammering or hearing impairment. The authors discuss how within this field the method of conversation analysis is used to record, transcribe and analyse these types of social interaction. They describe similarities and differences in the way that the different forms of communicative impairment can impact on social interaction, and they summarise the contribution that work in this areas has made to both communication disorder research and conversation analysis research on the nature of talk and social interaction
New insights on accretion in supergiant fast X-ray transients from XMM-Newton and INTEGRAL observations of IGR J17544-2619
No full textXMM–Newton observations of the supergiant fast X-ray transient IGR J17544?2619 are reported and placed in the context of an analysis of archival INTEGRAL/IBIS data that provide a refined estimate of the orbital period at 4.9272?±?0.0004?d. A complete outburst history across the INTEGRAL mission is reported. Although the new XMM–Newton observations (each lasting ?15 ks) targeted the peak flux in the phase-folded hard X-ray light curve of IGR J17544?2619, no bright outbursts were observed, the source spending the majority of the exposure at intermediate luminosities of the order of several 1033?erg?s?1 (0.5–10 keV) and displaying only low level flickering activity. For the final portion of the exposure, the luminosity of IGR J17544?2619 dropped to ?4?×?1032?erg?s?1 (0.5–10 keV), comparable with the lowest luminosities ever detected from this source, despite the observations being taken near to periastron. We consider the possible orbital geometry of IGR J17544?2619 and the implications for the nature of the mass transfer and accretion mechanisms for both IGR J17544?2619 and the supergiant fast X-ray transients (SFXTs) population. We conclude that accretion under the ‘quasi-spherical accretion’ model provides a good description of the behaviour of IGR J17544?2619 and suggests an additional mechanism for generating outbursts based upon the mass accumulation rate in the hot shell (atmosphere) that forms around the neutron star under the quasi-spherical formulation. Hence, we hope to aid in explaining the varied outburst behaviours observed across the SFXT population with a consistent underlying physical model
Active X-ray optics for the next generation of X-ray space telescopes
No full textDescribed within is the design, manufacture, metrology and X-ray testing of an active X-ray
prototype intended for the next generation of X-ray telescopes. One of the challenges faced by
the X-ray telescope community is how to combine high resolution and high sensitivity into one
system, as weight limitations place constraints on the optics that can be launched. Therefore the
mandate of the active X-ray prototype is to provide high sensitivity through the ability of the optics
to be nested and to deliver high angular resolution through the active control of the optic’s form.
Piezoelectric unimorph actuators provide the active component: it is intended that they will correct
for figure errors within the optic and therefore increase the angular resolution capability.
The prototype’s design is based upon an ellipsoidal segment which provides point-to-point
focussing of an X-ray source. The prototype itself is composed of an electroformed nickel optic
where the non-reflective surface is populated with 30 piezoelectric actuators and it is the production
of the prototype that is the core of the presented research. Metrology of the actuators’ influence
functions is presented and highlight the prototype’s ability to deform its optic surface by microns.
In addition, the measured influence functions are compared against finite element models and a
distinct similarity between the functions is observed.
The prototype was tested at an X-ray beamline facility in November 2008 and the results
showed the prototype’s ability to correct the optic to achieve an improved angular resolution: from
0.786 arc-minutes to 0.686 arc-minutes in terms of full width half maximum. Finally, difficulties
in the manufacture of the prototype and X-ray testing shall be presented alongside future work in
conclusion to this thesis
A Novel Ray Analogy for Enrolment of Ear Biometrics
No full textThe ear is a maturing biometric with qualities that give it superiority over other biometrics in a number of situations; in particular the ear is relatively immune to variation due to ageing. Successful ear biometrics rely upon a well enrolled dataset, with ears normalised for position, scale and rotation. We present a novel ear enrolment technique using the image ray transform, based upon an analogy to light rays. The transform is capable of highlighting tubular structures such as the helix of the ear and spectacle frames and, by exploiting the elliptical shape of the helix, can be used as the basis of a method for enrolment for ear biometrics. The presented technique achieves 99.6% success at enrolment across 252 images of the XM2VTS database, displaying a resistance to confusion with hair and spectacles. These results show great potential for enhancing many other already existing enrolment methods through use of the image ray transform at a preprocessing stage
Towards improving the Statscan X-ray image quality through sliding mode control of the C-arm
No full textIncludes abstract.Includes bibliographical references.This dissertation investigates methods for improving the image quality of a digital radiography system. The Lodox Statscan™ X-ray system provides a full-body scanned image for initial diagnosis. The system is driven by a permanent magnet linear motor (PMLM) controlled by a cascaded proportional-proportional integral controller (P-PI). Transient errors in the trapezoidal motion profile of the scanning C-arm may cause mismatches between the detector and the collimated beams from the X-ray source. This results in a partial degradation of image quality. The Statscan™ X-ray system was investigated and the following constraints were identified: The scanning time is limited to 13 seconds and the maximum scan length is limited to 1.8 m. Since it was not possible to obtain the Lodox Statscan™ dynamics model, because of the similarity, a characteristic model was then developed using a DC motor in order to investigate the control dynamics. It is not advisable for a designer to manipulate the controllers on commercial machines except for changing the parameters for tuning. Therefore, a P-PI controller and a proportional-integral sliding mode controller (P-ISMC) as well as a Boundary Layer variant (P-ISMC+BD) were designed for fair comparison purposes. Root locus, Bode diagrams and integral sliding mode control methods, respectively, were used to design the P-PI and P-ISMC controller groups. Each controller consists of an inner loop and an outer position loop. Proportional integral (PI) and integral sliding mode controllers (ISMC) were used for the inner loop. The two inner loop controllers were tuned, and then tested, before cascading them with the outer position loop. The simulation and experiments were conducted to compare each controller’s performance on step set-point tracking, trapezoidal motion profile tracking, the time transient’s specifications and robustness against disturbances. In order to test image quality, 27 distance profiles were generated from P-PI, P-ISMC and P-ISMC+BD. In addition, four images captured by the Statscan™ were also selected. A time delay integrator (TDI) simulator was used on the distance profiles and the four images to generate 108 distorted image profiles
Diagnosing and mapping pulmonary emphysema on X-ray projection images
No full textTo assess whether grating-based X-ray dark-field imaging can increase the sensitivity of X-ray projection images in the diagnosis of pulmonary emphysema and allow for a more accurate assessment of emphysema distribution. Lungs from three mice with pulmonary emphysema and three healthy mice were imaged ex vivo using a laser-driven compact synchrotron X-ray source. Median signal intensities of transmission (T), dark-field (V) and a combined parameter (normalized scatter) were compared between emphysema and control group. To determine the diagnostic value of each parameter in differentiating between healthy and emphysematous lung tissue, a receiver-operating-characteristic (ROC) curve analysis was performed both on a per-pixel and a per-individual basis. Parametric maps of emphysema distribution were generated using transmission, dark-field and normalized scatter signal and correlated with histopathology. Transmission values relative to water were higher for emphysematous lungs than for control lungs (1.11 vs. 1.06, p<0.001). There was no difference in median dark-field signal intensities between both groups (0.66 vs. 0.66). Median normalized scatter was significantly lower in the emphysematous lungs compared to controls (4.9 vs. 10.8, p<0.001), and was the best parameter for differentiation of healthy vs. emphysematous lung tissue. In a per-pixel analysis, the area under the ROC curve (AUC) for the normalized scatter value was significantly higher than for transmission (0.86 vs. 0.78, p<0.001) and dark-field value (0.86 vs. 0.52, p<0.001) alone. Normalized scatter showed very high sensitivity for a wide range of specificity values (94% sensitivity at 75% specificity). Using the normalized scatter signal to display the regional distribution of emphysema provides color-coded parametric maps, which show the best correlation with histopathology. In a murine model, the complementary information provided by X-ray transmission and dark-field images adds incremental diagnostic value in detecting pulmonary emphysema and visualizing its regional distribution as compared to conventional X-ray projections
X-ray polarization in relativistic jets
No full textWe investigate the polarization properties of Comptonized X-rays from relativistic jets in active galactic nuclei (AGN) using Monte Carlo simulations. We consider three scenarios commonly proposed for the observed X-ray emission in AGN: Compton scattering of blackbody photons emitted from an accretion disc; scattering of cosmic microwave background (CMB) photons and self-Comptonization of intrinsically polarized synchrotron photons emitted by jet electrons. Our simulations show that for Comptonization of disc and CMB photons, the degree of polarization of the scattered photons increases with the viewing inclination angle with respect to the jet axis. In both cases, the maximum linear polarization is ≈20 per cent. In the case of synchrotron self-Comptonization (SSC), we find that the resulting X-ray polarization depends strongly on the seed synchrotron photon injection site, with typical fractional polarizations P≈ 10–20 per cent when synchrotron emission is localized near the jet base, while P≈ 20–70 per cent for the case of uniform emission throughout the jet. These results indicate that X-ray polarimetry may be capable of providing unique clues to identify the location of particle acceleration sites in relativistic jets. In particular, if synchrotron photons are emitted quasi-uniformly throughout a jet, then the observed degree of X-ray polarization may be sufficiently different for each of the competing X-ray emission mechanisms (synchrotron, SSC or external Comptonization) to determine which is the dominant process. However, X-ray polarimetry alone is unlikely to be able to distinguish between disc and CMB Comptonization
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