The pyridoxal 5′-phosphate-dependent enzyme cystathionine β-lyase (CBL) catalyzes the penultimate step in the de novo biosynthesis of Met in microbes and plants. Absence of CBL in higher organisms ...makes it an important target for the development of antibiotics and herbicides. The three-dimensional structure of cystathionine β-lyase from Arabidopsis was determined by Patterson search techniques, using the structure of tobacco (Nicotiana tabacum) cystathionine γ-synthase as starting point. At a resolution of 2.3 Å, the model was refined to a final crystallographic R-factor of 24.9%. The overall structure is very similar to other pyridoxal 5′-phosphate-dependent enzymes of the γ-family. Exchange of a few critical residues within the active site causes the different substrate preferences between Escherichia coli and Arabidopsis CBL. Loss of interactions at the α-carboxyl site is the reason for the poorer substrate binding of Arabidopsis CBL. In addition, the binding pocket of Arabidopsis CBL is larger than that of E. coli CBL, explaining the similar binding of L-cystathionine and L-djenkolate in Arabidopsis CBL in contrast to E. coli CBL, where the substrate binding site is optimized for the natural substrate cystathionine.
This work presents numerical simulations of meteoroid streams released by comet 21P/Giacobini-Zinner over the period 1850–2030. The initial methodology, based on Vaubaillon et al. (2005), has been ...updated and modified to account for the evolution of the comet's dust production along its orbit. The peak time, intensity, and duration of the shower were assessed using simulated activity profiles that are calibrated to match observations of historic Draconid outbursts. The characteristics of all the main apparitions of the shower are reproduced, with a peak time accuracy of half an hour and an intensity estimate correct to within a factor of 2 (visual showers) or 3 (radio outbursts). Our model also revealed the existence of a previously unreported strong radio outburst on October 9, 1999, that has since been confirmed by archival radar measurements. The first results of the model, presented in Egal et al. (2018), provided one of the best predictions of the recent 2018 outburst. Three future radio outbursts are predicted in the next decade, in 2019, 2025 and 2029. The strongest activity is expected in 2025 when the Earth encounters the young 2012 trail. Because of the dynamical uncertainties associated with comet 21P's orbital evolution between the 1959 and 1965 apparitions, observations of the 2019 radio outburst would be particularly helpful to improve the confidence of subsequent forecasts.
•A new modeling of the Draconid meteoroid streams is presented.•Our simulations reproduce the time and intensity of the main historic Draconid outbursts.•Our simulations allowed identifying a previously unreported radio outburst in 1999.•The model led to one of the best predictions for the Draconid 2018.•Three potential outbursts are expected over the next decade.
We present a first catalog of sources detected by the
Mikhail Pavlinsky
ART-XC telescope on board the SRG observatory in the 4–12 keV energy band during its ongoing all-sky survey. The catalog ...comprises 867 sources detected on the combined map of the first two 6-month scans of the sky (December 2019–December 2020), ART-XC sky surveys 1 and 2, or ARTSS12. The achieved sensitivity to point sources varies between ~4 x 10
-12
erg s
-1
cm
-2
near the ecliptic plane and ~8 x 10
-13
erg s
-1
cm
-2
(4–12keV) near the ecliptic poles, and the typical localization accuracy is ~15”. Of the 750 sources of known or suspected origin in the catalog, 56% are extragalactic (mostly active galactic nuclei, AGN; and clusters of galaxies) and the rest are Galactic (mostly cataclysmic variables, CVs; and low- and high-mass X-ray binaries). For 114 sources, ART-XC has detected X-rays for the first time. Although the majority of these (~80) are expected to be spurious (given the adopted detection threshold), there can be a significant number of newly discovered astrophysical objects. We have started a program of optical follow-up observations of the new and previously unidentified X-ray sources, which has already led to the identification of several AGN and CVs. With the SRG all-sky survey planned to continue for a total of four years, we can expect the ART-XC survey in the 4–12 keV band to significantly surpass previous surveys that were carried out in similar (medium X-ray) energy bands in terms of the combination of angular resolution, sensitivity, and sky coverage.
We have developed thin film amorphous silicon alloy (a-Si:H) and nanocrystalline silicon (nc-Si:H) based multijunction solar cells on lightweight polymer substrate ~25 μm thick for space and ...near-space applications. The baseline cells use an a-Si:H/a-SiGe:H/a-SiGe:H structure deposited by conventional Radio Frequency (RF) plasma enhanced CVD using roll-to-roll deposition. The best initial performance for the baseline cells is aperture-area efficiency 9.84% and specific power ~1200 W/kg. The baseline cells are available to potential customers in large quantities. In order to increase the solar cell efficiency, we have pursued two new approaches. In the first, we use a Modified Very High Frequency (MVHF) technique to deposit the multijunction a-SiGe:H based cells. In the second, we have investigated nc-Si:H based multijunction cells. In this paper, we present the solar cell efficiency results on the three different device structures.
Video observations of lunar impact flashes have been made by a number of researchers since the late 1990's and the problem of determination of the impact energies has been approached in different ...ways Bellot Rubio et al. (2000a, b), Yanagisawa et al. (2008), Bouley et al. (2012), Suggs et al. (2014), Rembold and Ryan (2015), Ortiz et al. (2015), Madiedo et al. (2015). The wide spectral response of the unfiltered video cameras in use for all published measurements necessitates color correction for the standard filter magnitudes available for the comparison stars but this is not typically considered. In our approach, the published color of the comparison star and an estimate of the color of the impact flash is used to correct it to the chosen filter bandpass. Magnitudes corrected to standard filters are then used to determine the luminous energy in the filter bandpass according to the stellar atmosphere calibrations of Bessell et al. (1998). In this paper we compare the various photometric calibration techniques and calculation of luminous energy (radiometry) of impact flashes. This issue has significant implications for determination of luminous efficiency, predictions of impact crater sizes for observed flashes, and the determination of the flux of meteoroids in the 10s of grams to kilograms mass range.
•Radiometric calibration of video recordings of lunar impact flashes has been accomplished by a variety of techniques.•The use of unfiltered video cameras adds complexity to calibration and determination of luminous energy.•Color corrections for the comparison stars and the impact flashes are necessary.•Comparison of luminous energies from various researchers demonstrates differences due to calibration techniques.
We present an updated catalog of sources detected by the Mikhail Pavlinsky telescope aboard the Spektrum-Roentgen-Gamma (SRG) observatory during its all-sky survey. It is based on the data of the ...first four and the partially completed fifth scans of the sky ( The catalog comprises 1545 sources detected in the 4--12\,keV energy band. The achieved sensitivity ranges between $ near the ecliptic plane and $ near the ecliptic poles, which is a $ 30$--50<!PCT!> improvement over the previous version of the catalog based on the first two all-sky scans (ARTSS12). There are $ 130$ objects, excluding the expected contribution of spurious detections, that were not known as X-ray sources before the SRG/ all-sky survey. We provide information, partly based on our ongoing follow-up optical spectroscopy program, on the identification and classification of the majority of the sources (1463), of which 173 are tentative at the moment. The majority of the classified objects (964) are extragalactic, a small fraction (30) are located in the Local Group of galaxies, and 469 are Galactic. The dominant classes of objects in the catalog are active galactic nuclei (911) and cataclysmic variables (192).
ixpeobssim is a simulation and analysis framework, based on the Python programming language and the associated scientific ecosystem, specifically developed for the Imaging X-ray Polarimetry Explorer ...(IXPE). Given a source model and the response functions of the telescopes, it is designed to produce realistic simulated observations, in the form of event lists in FITS format, containing a strict super-set of the information provided by standard IXPE level-2 files. The core ixpeobssim simulation capabilities are complemented by a full suite of post-processing applications, allowing for the implementation of complex, polarization-aware analysis pipelines, and facilitating the inter-operation with the standard visualization and analysis tools traditionally in use by the X-ray community. We emphasize that, although a significant part of the framework is specific to IXPE, the modular nature of the underlying implementation makes it potentially straightforward to adapt it to different missions with polarization capabilities.
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