Classification of radiating compact stars

A classification of compact stars, depending on the electron distribution in velocity space and the density profiles characterizing their magnetospheric plasma, is proposed. Fast pulsars, such as NP 0532, X-ray sources such as Sco-X1, and slow pulsars are suggested as possible evolutionary stages of similar objects. The heating mechanism of Sco-X1 is discussed in some detail

Volcanic Rocks from the Ross Island, Marguerite Bay, andMt.Weaver Areas, Antarctica

During the last several years volcanic rocks from the Ross Island, Marguerite Bay, and Mt. Weaver areas have been studied. Field data and some laboratory work indicate that the portions of the Ross Island area already studied consist of an older olivine basalt-trachyte sequence and a younger olivine basalt-basalt sequence. A K/Ar date determined from anorthoclase indicates an age of 0.68 (±0.14) m. y. for the Antarctic kenyte of the Cape Royds area. Chemical, isotopic, and petrographic analyses of the rocks from the Ross Island area continue. Future investigations will be concerned with the geology of the high peaks and the character of the volcanic activity and products of Mt. Erebus. The Terra Firma volcanic rocks occur in the Marguerite Bay area and in other parts of the Antarctic Peninsula. They constitute a unit that may be a few thousand or more feet thick (NICHOLS, 1955) and consist, primarily, of andesite, dacite and rhyolite. Tuffs, breccias, and agglomerates are also common. The rocks may be Mesozoic or Cenozoic in age. In the Mt. Weaver area volcanic rocks occur on the flanks of Mt. Saltonstall and constitute Mt. Early, which is apparently a partially dissected cone (DOUMANI and MINSHEW, 1965). Mt. Saltonstall consists of olivine basalt, basalt, tuff, and volcanic breccia

Representational capacity of a set of independent neurons

The capacity with which a system of independent neuron-like units represents a given set of stimuli is studied by calculating the mutual information between the stimuli and the neural responses. Both discrete noiseless and continuous noisy neurons are analyzed. In both cases, the information grows monotonically with the number of neurons considered. Under the assumption that neurons are independent, the mutual information rises linearly from zero, and approaches exponentially its maximum value. We find the dependence of the initial slope on the number of stimuli and on the sparseness of the representation.Comment: 19 pages, 6 figures, Phys. Rev. E, vol 63, 11910 - 11924 (2000

An associative network with spatially organized connectivity

We investigate the properties of an autoassociative network of threshold-linear units whose synaptic connectivity is spatially structured and asymmetric. Since the methods of equilibrium statistical mechanics cannot be applied to such a network due to the lack of a Hamiltonian, we approach the problem through a signal-to-noise analysis, that we adapt to spatially organized networks. The conditions are analyzed for the appearance of stable, spatially non-uniform profiles of activity with large overlaps with one of the stored patterns. It is also shown, with simulations and analytic results, that the storage capacity does not decrease much when the connectivity of the network becomes short range. In addition, the method used here enables us to calculate exactly the storage capacity of a randomly connected network with arbitrary degree of dilution.Comment: 27 pages, 6 figures; Accepted for publication in JSTA

The Neutron Stars Census

The paucity of old isolated accreting neutron stars in ROSAT observations is used to derive a lower limit on the mean velocity of neutron stars at birth. The secular evolution of the population is simulated following the paths of a statistical sample of stars for different values of the initial kick velocity, drawn from an isotropic Gaussian distribution with mean velocity $0\leq < V>\leq 550$ ${\rm km s^{-1}}$. The spin--down, induced by dipole losses and the interaction with the ambient medium, is tracked together with the dynamical evolution in the Galactic potential, allowing for the determination of the fraction of stars which are, at present, in each of the four possible stages: Ejector, Propeller, Accretor, and Georotator. Taking from the ROSAT All Sky Survey an upper limit of $\sim 10$ accreting neutron stars within $\sim 140$ pc from the Sun, we infer a lower bound for the mean kick velocity, $\gtrsim 200-300$ ${\rm km s^{-1}},$ corresponding to a velocity dispersion $\sigma_V\gtrsim 125-190$ km s$^{-1}$. The same conclusion is reached for both a constant magnetic field ($B\sim 10^{12}$ G) and a magnetic field decaying exponentially with a timescale $\sim 10^9$ yr. Such high velocities are consistent with those derived from radio pulsar observations. Present results, moreover, constrain the fraction of low velocity stars, which could have escaped pulsar statistics, to less than 1%.Comment: 13 pages, 6 PostScript figures, accepted to Ap