Search

Nanoscale hydroxyapatites (HAPs) with different morphologies and crystal phases including highly crystalline nanorods, single-crystal nanowires, and nanosheets with the c or a(b) lattice planes were selectively developed and a semiamorphous form was synthesized and structurally characterized. Powder X-ray diffraction, electron microscopy, and multinuclear magic angle spinning (MAS) solid-state nuclear magnetic resonance (ssNMR) were used for structure determination of the bulk and surface species. Proton MAS ssNMR provides considerable insights into surface hydroxyl defect sites that are significantly more abundant in c-lattice plane-dominated nanosheets compared to the other HAP forms. Through a series of hydration and amino acid adsorption experiments, it is proposed that these hydroxyl defects are likely to be OH groups that bind to surface calcium sites and may be important in biomineralization processes. For a(b) lattice plane-preferred HAP samples, the 1H MAS NMR reveals a low population of these environments with a spectrum dominated by structural hydroxyls and physisorbed water, while nanosheet morphologies with the a(b) lattice plane selectively developed reveal a considerably higher population of surface-associated defect hydroxyls. This work provides considerable new insights into the structure and surface chemistry of nanoscale HAP where surface hydroxyl environments show a strong dependence upon the morphologically exposed lattice planes.

Fast X-ray Transients (FXTs) have long been an enigma in x-ray astronomy. These non-recurrent and bright x-ray sources, lasting up to a day and without quiescent x-ray emission, have been studied with the Wide Field Cameras (WFCs) during the 6 years of operational life of BeppoSAX. They were among the prime motivations for building the instrument. The WFC observations allow for the determination of duration, lightcurves and spectra, while the positional information is sufficiently good to search for counterparts. The results show the heterogeneous nature of FXTs. If we exclude the x-ray counterparts of Gamma Ray Bursts, at least two types can be distinguished, both seen at high galactic latitude with a spatial distribution consistent with isotropy. A galactic population of FXTs has a typical duration of an hour and is associated with stellar sources exhibiting coronal activity. An extragalactic population of FXTs, called X-ray Flashes, has a duration of order minutes. They are explosive events whose origin has most likely a connection with the origin of Gamma Ray Bursts. Recently the first spectroscopic distance determination from the optical afterglow of an X-ray Flash has been found at z=0.25. These results, together with recent developments in GRBs, indicate a two component cosmic explosion in which the prompt ultra-relativistic γ-ray energy can be minor, whereas the total energy release of the explosion is similar in XRFs and GRBs. [Copyright &y& Elsevier]

We have observed the millisecond X-ray pulsar SAX J1808.4-3658 on three occasions during its 2000 outburst with the BeppoSAX satellite. The source was highly variable and erratic during this outburst, and by coincidence we obtained data only during times when the source had very low luminosities. During our observations, we detected four faint sources. The source closest to the position of SAX J1808.4-3658 is still ~1.'6 away. This source can be identified with SAX J1808.4-3658 only if we assume that the BeppoSAX positional reconstruction is not completely understood. We also reanalyzed a BeppoSAX observation taken in 1999 March by Stella et al. when the source was in quiescence and during which the source was thought to have been detected. Based on the similarities (position and luminosity) of this source with the above-mentioned source ~1.'6 away from SAX J1808.4-3658, it is possible that they are the same source. If this source is not the millisecond pulsar, then during all BeppoSAX observations of SAX J1808.4-3658 (the 2000 outburst ones and the 1999 quiescent one), the millisecond pulsar was not detected. A reanalysis of the ASCA quiescent data of SAX J1808.4-3658 recently performed by Dotani, Asai, & Wijnands confirms that during this observation the source was securely detected in quiescence. We discuss our results for SAX J1808.4-3658 in the context of the quiescent properties of low-mass X-ray binary transients.

Hot white dwarfs are objects that copiously emit in the Extreme Ultraviolet and soft X-ray range. They are the brightest sources seen in the Low Energy Telescope of EXOSAT, with countrates up to 25 cnts/s. in contrast to their optical and UV spectrum the total flux and spectral distribution at soft X-ray energies are highly sensitive to the effective temperature, structure and elemental composition of the dwarf's atmosphere. The imaging soft X-ray experiments onboard EXOSAT cover with large sensitivity the spectral region where the peak of emission of hot white dwarfs is expected to occur.

EXOSAT observations of the contact binary VW Cephei on 19th March 1984 are presented. The L1-telescope with CMA+thick Lexan filter was used. The observations cover one orbital revolution showing an asymmetrical X-ray light curve. This can be modelled by an active neck, connecting the two stars, and with enhanced coronal regions on the primary star. Nearly simultaneous IUE observations are also presented. The observations form a part of the program to observe contact binaries with EXOSAT.

It has been confirmed that absence of carbon dioxide may decrease the rate of oxygen production which accompanies the photochemical reduction of p-benzoquinone in algae and chloroplasts. This influence of carbon dioxide partial pressure does not apply to the overall oxygen yield. In the blue-green alga Anacystis nidulans the initially small carbon dioxide deficiency effect increases with time spent in the dark. The deterioration of reaction rates is counteracted by light. There seems to be no direct connection or interdependence between the photosynthetic reduction of carbon dioxide and the sensitivity of some part of the photochemical mechanism to loss of carbon dioxide. Not only does addition of quinone to living cells in these experiments destroy their capacity for photosynthesis, but mutant cells that never had this capacity still retain the sensitivity towards lack of carbon dioxide when tested for their ability to reduce quinone. Many different metabolic reactions have been seen to possess such dependency on traces of carbon dioxide, also in non-photosynthetic cells and tissues. The explanation for “catalytic” effects of carbon dioxide ought to be a general one–such as an influence on the efficiency of certain phosphorylations which occur everywhere in the living world.

We review the results obtained with the Galactic center campaigns of the BeppoSAX Wide Field X-ray Cameras (WFCs). This pertains to the study of luminous low-mass X-ray binaries (LMXBs). When pointed at the Galactic center, the WFC field of view contains more than half of the Galactic LMXB population. The results exemplify the excellent WFC capability to detect brief X-ray transients. Firstly, the WFCs expanded the known population of Galactic thermonuclear X-ray bursters by 50%. At least half of all LMXBs are now established to burst and, thus, to contain a neutron star as compact accretor rather than a black hole candidate. We provide a complete list of all 76 currently known bursters, including the new case 1RXS J170854.4-321857. Secondly, the WFCs have uncovered a population of weak transients with peak luminosities up to ∼ 1037 erg s-1 and durations from days to weeks. One is the first accretion-powered millisecond pulsar SAX J1808.4-3658. Thirdly, the WFCs contributed considerably towards establishing that nearly all (12 out of 13) luminous low-mass X-ray binaries in Galactic globular clusters contain neutron stars rather than black holes. Thus, the neutron star to black hole ratio in clusters differs from that in the Galactic disk at a marginal confidence level of 97%. [Copyright &y& Elsevier]