Classification Supervisée de Questions : Rôle de l'Expansion Sémantique

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Evidence for a metalloprotein stucture of plasma membrane 5' -nucleotidase

AbstractTo point out the metalloprotein structure of bovine liver plasma membrane 5'-nucleotidase we studied the inhibition mechanism of the purified enzyme by EDTA: this apparently non-competitive inhibition seems to be dependent on EDTA concentration, pH, temperature and incubation time. When the restoration of activity was assayed by addition of divalent cations or by gel filtration, the inhibition became progressively irreversible with time. Incubation of the enzyme with [14C]EDTA allowed us to observe, after gel filtration as well as after sucrose gradient ultracentrifugation, that the chelating agent is bound to 5'-nucleotidase

Discovery of a New Transient Magnetar Candidate: XTE J1810-197

We report the discovery of a new X-ray pulsar, XTE J1810-197. The source was serendipitously discovered on 2003 July 15 by the Rossi X-ray Timing Explorer (RXTE) while observing the soft gamma repeater SGR 1806-20. The pulsar has a 5.54 s spin-period and a soft spectrum (photon index ~ 4). We detect the source in earlier RXTE observations back to 2003 January. These show that a transient outburst began between 2002 November 17 and 2003 January 23 and that the pulsar has been spinning down since then, with a high rate Pdot ~ 10^-11 s/s showing significant timing noise, but no evidence for Doppler shifts due to a binary companion. The rapid spin-down rate and slow spin-period imply a super-critical magnetic field B=3x10^14 G and a young characteristic age < 7600 yr. These properties are strikingly similar to those of anomalous X-ray pulsars and soft gamma repeaters, making the source a likely new magnetar. A follow-up Chandra observation provided a 2".5 radius error circle within which the 1.5 m Russian-Turkish Optical Telescope RTT150 found a limiting magnitude of R_c=21.5, in accord with other recently reported limits. The source is present in archival ASCA and ROSAT data as well, at a level 100 times fainter than the \~ 3 mCrab seen in 2003. This suggests that other X-ray sources that are currently in a state similar to the inactive phase of XTE J1810-197 may also be unidentified magnetars awaiting detection via a similar activity.Comment: Submitted to ApJL; 4 pages; 4 figure

Discovery of Cyclotron Resonance Features in the Soft Gamma Repeater SGR 1806-20

We report evidence of cyclotron resonance features from the Soft Gamma Repeater SGR 1806-20 in outburst, detected with the Rossi X-ray Timing Explorer in the spectrum of a long, complex precursor that preceded a strong burst. The features consist of a narrow 5.0 keV absorption line with modulation near its second and third harmonics (at 11.2 keV and 17.5 keV respectively). The line features are transient and are detected in the harder part of the precursor. The 5.0 keV feature is strong, with an equivalent width of ~ 500 eV and a narrow width of less than 0.4 keV. Interpreting the features as electron cyclotron lines in the context of accretion models leads to a large mass-radius ratio (M/R > 0.3 M_sun/km) that is inconsistent with neutron stars or that requires a low (5-7)x10^{11} G magnetic field that is unlikely for SGRs. The line widths are also narrow compared with those of electron cyclotron resonances observed so far in X-ray pulsars. In the magnetar picture, the features are plausibly explained as ion cyclotron resonances in an ultra-strong magnetic field that have recently been predicted from magnetar candidates. In this view, the 5.0 keV feature is consistent with a proton cyclotron fundamental whose energy and width are close to model predictions. The line energy would correspond to a surface magnetic field of 1.0x10^{15} G for SGR 1806-20, in good agreement with that inferred from the spin-down measure in the source.Comment: Published in the 2002 July 20 issue of the Astrophysical Journal Letters, 574, L5

Status of Scoping Nuclear Analyses for the Evolving Design of ITER TBM Port Cells

ITER is an international collaborative effort towards the realization of fusion energy via the magnetic confinement concept. Two of the equatorial ports in the facility are dedicated to the testing of tritium breeding concepts, which is essential for the tritium self-sufficiency of future fusion reactors. The concerned Test Blanket System (TBS) consists of a Test Blanket Module (TBM) residing inside the TBM–Port Plug (TBM-PP) and its associated ancillary systems in the Tokamak facility. In this paper, the results of a full suite of nuclear analyses concerning the shielding performance of the Pipe Forest (PF) and Bioshield Plug (BP), to reflect on the evolution of their designs, are discussed. On the BP side, the design of the peripheral part has been reviewed considering the ventilation openings and butterfly doors, to assure the design compliance with the Radiation Map (RadMap) requirements for the neutron flux in the Port Cell (PC), behind the BP. On the PF side, the pipes routing and maintenance corridor door have been redesigned, by taking into account results from previously concluded nuclear analyses. The neutronics model was developed from CAD and was used to perform transport simulations in two plasma modes: on and off. For plasma-on mode, the plasma neutron field in the Port Interspace (PI) as well as behind the BP was assessed and few shielding options were explored. The responses due to decay neutrons from 17N in activated cooling water were also considered. For the plasma-off mode, the focus was shifted to further refine the ShutDown Dose Rate (SDDR) maps, which is of importance for maintenance operations that are foreseen to take place at various stages of ITER operation, in particular following the FPO-1, FPO-2, and Short operation scenarios. In addition, detailed activation analyses were carried out to provide a provisional waste classification