Sub-pixel event repositioning algorithms for Chandra X-ray astronomical imaging

Chandra X-ray Observatory (CXO) with the Advanced CCD Imaging Spectrometer (ACIS) offers unprecedented spatial resolution for X-ray astronomical imaging. As focal plane arrays, ACIS CCDs were fabricated to satisfy CXO\u27s requirements in spatial and spectral specifications, and operate in photon-counting mode to record any photoelectric interactions into event lists with energy information and charge split morphology (grade). The charge cloud generated by a single X-ray photon has a relatively small size compared to ACIS pixel, and the shape is approximated by an axial-symmetric Gaussian with full width half maximum (FWHM) about 2 to 4 microns. This fact indicates that the impact positions of photons that generate split events are near the pixel boundaries, instead of the pixel centers. Considering ACIS CCDs have pixel size of 24 microns, subpixel event repositioning (SER) algorithms designed to refine the positions of split events should significantly improve the spatial resolution of Chandra/ACIS imaging. SER methods have been modified, from original corner-split events only model, to static SER (including 2-pixel and single pixel events), energy-dependent SER, and charge-split dependent SER, for both backside-illuminated (BI) and frontside-illuminated (FI) CCDs. Both simulated and CXO-observation data demonstrate the improvement for various SER methods. Chandra/ACIS data obtained for the Orion Nebula Cluster (ONC) was used to evaluate the SER algorithms, by reconstructing point-like sources in ONC and measuring their FWHM before and after applying SER methods. The improvement of FWHM for simulated and ONC sources was analyzed, so as to establish the degree of image improvement achieved by, as well as limitations on the success of, subpixel event repositioning algorithms. BI and FI CCDs exhibit different performance and, overall, BI applications benefit more from angular resolution improvement after applying SER techniques. The best performance after applying SER techniques can be as much as 62%, i.e., very close to theoretically available improvement, depending on applied SER method, source spectrum, off-axis angle, and employed CCD type

High Voltage Direct Current transmission

This thesis is focused on the application and development of HVDC transmission technology based on thyristor without turn-off capability. Compared with other macroelectronics in the power field, thyristor without turn-off capability has successful operation experience to ensure reliability and high power ratings to transfer bulk energy. This thesis covers converter station design and equipments, reactive power compensation and voltage stability, AC/DC filters design, control strategy and function, fault analysis, overvoltage and insulation co-ordination, overhead line and cable transmission, transmission line environmental effects, earth electrode design and development. With the development of new concepts and techniques, the cost of HVDC transmission will be reduced substantially, thereby extending the area of application

Refining Chandra/ACIS Subpixel Event Repositioning Using a Backside Illuminated CCD Model

Subpixel event repositioning (SER) techniques have been demonstrated to significantly improve the already unprecedented spatial resolution of Chandra X-ray imaging with the Advanced CCD Imaging Spectrometer (ACIS). Chandra CCD SER techniques are based on the premise that the impact position of events can be refined, based on the distribution of charge among affected CCD pixels. ACIS SER models proposed thus far are restricted to corner split (3- and 4-pixel) events, and assume that such events take place at the split pixel corners. To improve the event counting statistics, we modified the ACIS SER algorithms to include 2-pixel split events and single pixel events, using refined estimates for photon impact locations. Furthermore, simulations that make use of a high-fidelity backside illuminated (BI) CCD model demonstrate that mean photon impact positions for split events are energy dependent leading to further modification of subpixel event locations according to event type and energy, for BI ACIS devices. Testing on Chandra CCD X-ray observations of the Orion Nebula Cluster indicates that these modified SER algorithms further improve the spatial resolution of Chandra/ACIS, to the extent that the spreading in the spatial distribution of photons is dominated by the High Resolution Mirror Assembly, rather than by ACIS pixelization.Comment: 23 pages, 8 figures, 2nd version, submitted to Ap

On the Asymmetries of Extended X-ray Emission from Planetary Nebulae

Chandra X-ray Observatory (CXO) images have revealed that the X-ray emitting regions of the molecule-rich young planetary nebulae (PNs) BD+30 3639 (BD+30) and NGC 7027 are much more asymmetric than their optical nebulosities. To evaluate the potential origins of these X-ray asymmetries, we analyze X-ray images of BD+30, NGC 7027, and another planetary nebula resolved by CXO, NGC 6543, within specific energy bands. Image resolution has been optimized by sub-pixel repositioning of individual X-ray events. The resulting subarcsecond-resolution images reveal that the soft (E < 0.7 keV) X-ray emission from BD+30 is more uniform than the harder emission, which is largely confined to the eastern rim of the optical nebula. In contrast, soft X-rays from NGC 7027 are highly localized and this PN is more axially symmetric in harder emission. The broad-band X-ray morphologies of BD+30 and NGC 7027 are highly anticorrelated with their distributions of visual extinction, as determined from high-resolution, space- and ground-based optical and infrared imaging. Hence, it is likely that the observed X-ray asymmetries of these nebulae are due in large part to the effects of nonuniform intranebular extinction. However, the energy-dependent X-ray structures in both nebulae and in NGC 6543 -- which is by far the least dusty and molecule-rich of the three PNs, and displays very uniform intranebular extinction -- suggests that other mechanisms, such as the action of collimated outflows and heat conduction, are also important in determining the detailed X-ray morphologies of young planetary nebulae.Comment: 33 pages, 12 figures; to appear in the Astrophysical Journa

The Mechanical Energies of the Global Atmosphere in El Niño and La Niña Years

Two meteorological reanalysis datasets are analyzed to determine the mechanical energies of the global atmosphere in the El Niño and La Niña years. The general consistency of the mean energy components between the two datasets reveals ~1%–3% increase and ~2%–3% decrease in the mean energies in the El Niño years and La Niña years, respectively. These analyses further reveal that the tropospheric temperature responds to the sea surface temperature anomaly with a time lag of two months, which leads to the varying mean atmospheric energies in the El Niño and La Niña years

The Coronal X-ray Spectrum of the Multiple Weak-Lined T Tauri Star System HD 98800

We present high-resolution X-ray spectra of the multiple (hierarchical quadruple) weak-lined T Tauri star system HD 98800, obtained with the High Energy Transmission Gratings Spectrograph (HETGS) aboard the Chandra X-ray Observatory (CXO). In the zeroth-order CXO/HETGS X-ray image, both principle binary components of HD 98800 (A and B, separation 0.8'') are detected; component A was observed to flare during the observation. The infrared excess (dust disk) component, HD 98800B, is a factor ~4 fainter in X-rays than the apparently ``diskless'' HD 98800A, in quiescence. The line ratios of He-like species (e.g., Ne IX, O VII) in the HD 98800A spectrum indicate that the X-ray-emitting plasma around HD 98800 is in a typical coronal density regime (log n <~ 11). We conclude that the dominant X-ray-emitting component(s) of HD 98800 is (are) coronally active. The sharp spectral differences between HD 98800 and the classical T Tauri star TW Hya demonstrate the potential utility of high-resolution X-ray spectroscopy in providing diagnostics of pre-main sequence accretion processes.Comment: 11 pages, 5 figures; to appear in the Astrophysical Journal (Letters

UPPRESSO: Untraceable and Unlinkable Privacy-PREserving Single Sign-On Services

Single sign-on (SSO) allows a user to maintain only the credential at the identity provider (IdP), to login to numerous RPs. However, SSO introduces extra privacy threats, compared with traditional authentication mechanisms, as (a) the IdP could track all RPs which a user is visiting, and (b) collusive RPs could learn a user's online profile by linking his identities across these RPs. This paper proposes a privacypreserving SSO system, called UPPRESSO, to protect a user's login activities against both the curious IdP and collusive RPs. We analyze the identity dilemma between the security requirements and these privacy concerns, and convert the SSO privacy problems into an identity transformation challenge. In each login instance, an ephemeral pseudo-identity (denoted as PID_RP ) of the RP, is firstly negotiated between the user and the RP. PID_RP is sent to the IdP and designated in the identity token, so the IdP is not aware of the visited RP. Meanwhile, PID_RP is used by the IdP to transform the permanent user identity ID_U into an ephemeral user pseudo-identity (denoted as PID_U ) in the identity token. On receiving the identity token, the RP transforms PID_U into a permanent account (denoted as Acct) of the user, by an ephemeral trapdoor in the negotiation. Given a user, the account at each RP is unique and different from ID_U, so collusive RPs cannot link his identities across these RPs. We build the UPPRESSO prototype on top of MITREid Connect, an open-source implementation of OIDC. The extensive evaluation shows that UPPRESSO fulfills the requirements of both security and privacy and introduces reasonable overheads

Autotransplantation of Inferior Parathyroid glands during central neck dissection for papillary thyroid carcinoma: A retrospective cohort study

AbstractIntroduction: The management of inferior parathyroid glands during central neck dissection (CND) for papillary thyroid carcinoma (PTC) remains controversial. Most surgeons preserve inferior parathyroid glands in situ. Autotransplantation is not routinely performed unless devascularization or inadvertent parathyroidectomy occurs. This retrospective study aimed to compare the incidence of postoperative hypoparathyroidism and central neck lymph node (CNLN) recurrence in patients with PTC who underwent inferior parathyroid glands autotransplantation vs preservation in situ. Methods: This is a retrospective study which was conducted in a tertiary referral hospital. A total of 477 patients with PTC (pN1) who underwent total thyroidectomy (TT) and bilateral CND with/without lateral neck dissection were included. Patients' demographical characteristics, tumor stage, incidence of hypoparathyroidism, CNLN recurrence and the number of resected CNLN were analyzed. Results: Three hundred and twenty-one patients underwent inferior parathyroid glands autotransplantation (autotransplantation group). Inferior parathyroid glands were preserved in situ among 156 patients (preservation group). Permanent hypoparathyroidism rate was 0.9% (3/321) versus 3.8% (6/156) respectively (p = 0.028). Mean numbers of resected CNLN were 15 ± 3 (6–23) (autotransplantation group) versus 11 ± 3 (7–21) (preservation group) (p < 0.001). CNLN recurrence rate was 0.3% (1/321) versus 3.8% (6/156) respectively (p = 0.003). Conclusion: Inferior parathyroid glands autotransplantation during CND of PTC (pN1) might reduce permanent hypoparathyroidism and CNLN recurrence. Further study enrolling more patients with long-term follow-up is needed to support this conclusion

A Compact X-ray Source and Possible X-ray Jets within the Planetary Nebula Menzel 3

We report the discovery, by the Chandra X-ray Observatory, of X-ray emission from the bipolar planetary nebula Menzel 3. In Chandra CCD imaging, Mz 3 displays hot (3-6x10^6 K) gas within its twin, coaxial bubbles of optical nebulosity, as well as a compact X-ray source at the position of its central star(s). The brightest diffuse X-ray emission lies along the polar axis of the optical nebula, suggesting a jet-like configuration. The observed combination of an X-ray-emitting point source and possible X-ray jet(s) is consistent with models in which accretion disks and, potentially, magnetic fields shape bipolar planetary nebulae via the generation of fast, collimated outflows.Comment: 12 pages, 3 figures; to appear in Astrophysical Journal (Letters