148,115 research outputs found
A magnetically driven origin for the low luminosity GRB 170817A associated with GW170817
The gamma-ray burst GRB 170817A associated with GW170817 is subluminous and
subenergetic compared with other typical short GRBs. It may be due to a
relativistic jet viewed off-axis, or a structured jet, or cocoon emission.
Giant flares from magnetars may possibly be ruled out. However, the luminosity
and energetics of GRB 170817A is coincident with that of magnetar giant flares.
After the coalescence of the binary neutron star, a hypermassive neutron star
may be formed. The hypermassive neutron star may have magnetar-strength
magnetic field. During the collapse of the hypermassive neutron star, the
magnetic field energy will also be released. This giant-flare-like event may
explain the the luminosity and energetics of GRB 170817A. Bursts with similar
luminosity and energetics are expected in future neutron star-neutron star or
neutron star-black hole mergers.Comment: 6 pages, 1 figure, accepted in Research in Astronomy and Astrophysic
Study of a homotopy continuation method for early orbit determination with the Tracking and Data Relay Satellite System (TDRSS)
A recent mathematical technique for solving systems of equations is applied in a very general way to the orbit determination problem. The study of this technique, the homotopy continuation method, was motivated by the possible need to perform early orbit determination with the Tracking and Data Relay Satellite System (TDRSS), using range and Doppler tracking alone. Basically, a set of six tracking observations is continuously transformed from a set with known solution to the given set of observations with unknown solutions, and the corresponding orbit state vector is followed from the a priori estimate to the solutions. A numerical algorithm for following the state vector is developed and described in detail. Numerical examples using both real and simulated TDRSS tracking are given. A prototype early orbit determination algorithm for possible use in TDRSS orbit operations was extensively tested, and the results are described. Preliminary studies of two extensions of the method are discussed: generalization to a least-squares formulation and generalization to an exhaustive global method
Apollo gastrointestinal analysis
Fecal bile acid patterns for the Apollo 17 flight were studied to determine the cause of diarrhea on the mission. The fecal sterol analysis gave no indication of an infectious diarrhea, or specific, or nonspecific etiology occurring during the entire flight. It is assumed that the gastrointestinal problems encountered are the consequences of altered physiology, perhaps secondary to physical or emotional stress of flight
Control of tetrahedral coordination and superconductivity in FeSe0.5Te0.5 thin films
We demonstrate a close relationship between superconductivity and the
dimensions of the Fe-Se(Te) tetrahedron in FeSe0.5Te0.5. This is done by
exploiting thin film epitaxy, which provides controlled biaxial stress, both
compressive and tensile, to distort the tetrahedron. The Se/Te height within
the tetrahedron is found to be of crucial importance to superconductivity, in
agreement with the theoretical proposal that (pi,pi) spin fluctuations promote
superconductivity in Fe superconductors
Comparative study of Steel-FRP, FRP and steel reinforced coral concrete beams in their flexural performance
In this paper, a comparative study of Carbon Fiber Reinforced Polymer (CFRP) Bar and Steel-Carbon Fiber Composite Bar (SCFCB) reinforced coral concrete beams are made through a series experimental tests and theoretical analysis. The flexural capacity, crack development and failure modes of CFRP and SCFCB reinforced coral concrete were investigated in detail. They are also compared to ordinary steel reinforced coral concrete beams. The results show that under the same condition of reinforcement ratio, the SCFCB reinforced beam exhibits better performance than those of the CFRP reinforced beams, and its stiffness is slightly lower than that of the steel reinforced beam. Under the same load condition, the crack width of the SCFCB beam is between the steel reinforced beam and the CFRP bar reinforced beam. Before the steel core yields, the crack growth rate of SCFCB beam is similar to the steel reinforced beam. SCFCB has a higher strength utilization rate, about 70% -85% of its ultimate strength. The current design guidance was also examined based on the test results. It was found that the existing design specifications for FRP reinforced normal concrete is not suitable for SCFCB reinforced coral concrete structures
Analysis of the Movement of Chlamydomonas Flagella: The Function of the Radial-spoke System Is Revealed by Comparison of Wild-type and Mutant Flagella
The mutation uni-1 gives rise to uniflagellate Chlamydomonas cells which rotate around a fixed point in the microscope field, so that the flagellar bending pattern can be photographed easily . This has allowed us to make a detailed analysis of the wild-type flagellar bending pattern and the bending patterns of flagella on several mutant strains. Cells containing uni-1, and recombinants of uni-1 with the suppressor mutations, sup(_pf)-1 and sup(_pf)-3, show the typical asymmetric bending pattern associated with forward swimming in Chlamydomonas,
although sup(_pf)-1 flagella have about one-half the normal beat frequency, apparently as the result of defective function of the outer dynein arms. The pf-17 mutation has been shown to produce nonmotile flagella in which radial spoke heads and five characteristic axonemal
polypeptides are missing. Recombinants containing pf-17 and either sup(_pf)-1 or sup(_pf)-3 have
motile flagella, but still lack radial-spoke heads and the associated polypeptides . The flagellar
bending pattern of these recombinants lacking radial-spoke heads is a nearly symmetric, large
amplitude pattern which is quite unlike the wild-type pattern . However, the presence of an
intact radial-spoke system is not required to convert active sliding into bending and is not
required for bend initiation and bend propagation, since all of these processes are active in the
sup(_pf) pf-17 recombinants. The function of the radial-spoke system appears to be to convert the
symmetric bending pattern displayed by these recombinants into the asymmetric bending
pattern required for efficient swimming, by inhibiting the development of reverse bends during
the recovery phase of the bending cycle
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