On the state of low luminous accreting neutron stars

Observational appearance of a neutron star in the subsonic propeller state which is a companion of a wind-fed mass-exchange close binary system is discussed. During the subsonic propeller state the neutron star magnetosphere is surrounded by a spherical quasi-static plasma envelope, which is extended from the magnetospheric boundary up to the star accretion radius. The energy input to the envelope due to the propeller action by the neutron star exceeds the radiative losses and the plasma temperature in the envelope is of the order of the free-fall temperature. Under this condition the magnetospheric boundary is interchange stable. Nevertheless, I find that the rate of plasma penetration from the envelope into the magnetic field of the neutron star due to diffusion and magnetic field line reconnection processes is large enough for the accretion power to dominate the spindown power. I show that the accretion luminosity of the neutron star in the subsonic propeller state is 5*10**{30} - 10**{33} (dM/dt)_{15} erg/s, where dM/dt is the strength of the normal companion stellar wind which is parametrized in terms of the maximum possible mass accretion rate onto the neutron star magnetosphere. On this basis I suggest that neutron stars in the subsonic propeller state are expected to be observed as low luminous accretion-powered pulsars. The magnetospheric radius of the neutron star in this state is determined by the strength of the stellar wind, (dM/dt)_c, while the accretion luminosity is determined by the rate of plasma penetration into the star magnetosphere, (dM/dt)_a, which is (dM/dt)_a << (dM/dt)_c. That is why the classification of the neutron star state in these objects using the steady accretion model (i.e. setting (dM/dt)_a = (dM/dt)_c) can lead to a mistaken conclusion.Comment: 6 pages, accepted for publication in A&

On the duration of the subsonic propeller state of neutron stars in wind-fed mass-exchange close binary systems

The condition for the subsonic propeller - accretor state transition of neutron stars in wind-fed mass-exchange binary systems is discussed. I show that the value of the break period, at which the neutron star change its state to accretor, presented by Davies & Pringle (1981) is underestimated by a factor of 7.5. The correct value is P_{\rm br} = 450 \mu_{30}^{16/21} \dot{M}_{15}^{-5/7} (M/M_{\sun})^{-4/21} s. This result forced us to reconsider some basic conclusions on the efficiency of the propeller spindown mechanism.Comment: 3 pages, published in A&A 368, L

Large-scale circulation departures related to wet episodes in northeast Brazil

Large scale circulation features are presented as related to wet spells over northeast Brazil (Nordeste) during the rainy season (March and April) of 1979. The rainy season season is devided into dry and wet periods, the FGGE and geostationary satellite data was averaged and mean and departure fields of basic variables and cloudiness were studied. Analysis of seasonal mean circulation features show: lowest sea level easterlies beneath upper level westerlies; weak meridional winds; high relative humidity over the Amazon basin and relatively dry conditions over the South Atlantic Ocean. A fluctuation was found in the large scale circulation features on time scales of a few weeks or so over Nordeste and the South Atlantic sector. Even the subtropical High SLP's have large departures during wet episodes, implying a short period oscillation in the Southern Hemisphere Hadley circulation

Tungsten cladding of reactor fuels

Tungsten cladding of reactor fuel

Stabilized lanthanum sulphur compounds

Lanthanum sulfide is maintained in the stable cubic phase form over a temperature range of from 500 C to 1500 C by adding to it small amounts of calcium, barium, or strontium. This compound is an excellent thermoelectric material

Accretion by Isolated Neutron Stars

Accretion of interstellar material by an isolated neutron star is discussed. The point I address here is the interaction between the accretion flow and the stellar magnetosphere. I show that the interchange instabilities of the magnetospheric boundary under the conditions of interest are basically suppressed. The entry of the material into the magnetosphere is governed by diffusion. Due to this reason the persistent accretion luminosity of isolated neutron stars is limited to < 4E+26 erg/s. These objects can also appear as X-ray bursters with the burst durations of about 30 minutes and repetition time of about 1E+5 yr. This indicates that the number of the accreting isolated neutron stars which could be observed with recent and modern X-ray missions is a few orders of magnitude smaller than that previously estimated.Comment: 3 pages, to appear in Astrophysics and Space Science, in the proceedings of "Isolated Neutron Stars: from the Interior to the Surface", edited by D. Page, R. Turolla and S. Zan

Subsonic propellers in a strong wind as anomalous X-ray pulsars

The appearance of subsonic propellers situated in a strong wind is discussed. We show that it is similar to the appearance of anomalous X-ray pulsars (AXPs) provided the mass and the magnetic moment of neutron stars are 1.4 solar masses, and 2E+30 G cm^3, respectively, and the strength of the wind is M_c = 3E+17 g/s. Under these conditions, the spin periods of subsonic propellers are limited within the range of 5-15 s, and the expected spin-down rates are close to 7E-11 s/s. The mass accretion rate onto the stellar surface is limited to the rate of plasma penetration into its magnetosphere at the boundary. As this process is governed by the reconnection of the field lines, the accretion rate onto the stellar surface constitutes 1-2% of M_c. In this case the X-ray luminosity of the objects under consideration can be evaluated as 4E+35 erg/s. The model predicts the existence of at least two spatially separated sources of the X-ray emission: hot spots at the stellar surface, and the hot atmosphere surrounding the magnetosphere of the star. The ages of the subsonic propellers under the conditions of interest are limited to 10^5 yr.Comment: 5 pages, 1 figure, accepted for publication in A&

Testing of Milliwatt Power Source Components

A milliwatt power source (MPS) has been developed to satisfy the requirements of several potential solar system exploration missions. The MPS is a small power source consisting of three major components: a space qualified heat source (RHU), a thermopile (thermoelectric converter or TEC) and a container to direct the RHU heat to the TEC. Thermopiles from Hi-Z Technology, Inc. of San Diego and the Institute of Thermoelectricity of Chernivtsi Ukraine suitable for the MPS were tested and shown to perform as expected, producing 40 mW of power with a temperature difference of about 170°C. Such thermopiles were successfully life tested for up to a year. A MPS container designed and built by Swales Aerospace was tested with both a TEC simulator and actual TEC. The Swales unit, tested under dynamic vacuum, provided less temperature difference than anticipated, such that the TEC produced 20 mW of power with heat input equivalent to a RHU

On the accretion flow geometry in A0535+26

The geometry of accretion flow in the Be/X-ray transient A0535+26 is explored. It is shown that neither moderate nor giant X-ray flaring events observed in the system can be interpreted within the spherically symmetrical accretion model and hence the formation of an accretion disk around the neutron star magnetosphere during the both types of flares is required. The accretion disk can be formed at the periastron if (i) the expansion velocity of the Be star envelope in the equatorial plane is V_wr < 150 km/s and (ii) the parameter accounting for the accretion flow inhomogeneities, xi, satisfies the following condition: xi > 0.16 (Mdot_17)^-1/7, where (Mdot_17)^-1/7 is the rate of mass capture by the neutron star expressed in units of 1017 g/s. We suggest that the `missing' outburst phenomenon can be associated with the spherically symmetrical accretion onto the interchange-stable magnetosphere of the neutron star. The average spin up rate of the neutron star during moderate flares < 3.5 x 10^-12 Hz/s is predicted.Comment: 6 pages, published in A&A 372, 227 (2001

Signs of magnetic accretion in young Be/X-ray pulsar SXP 1062

The spin behaviour of the neutron star in the newly discovered young Be/X-ray long-period pulsar SXP 1062 is discussed. The star is observed to rotate with the period of 1062s, and spin-down at the rate ~ - 2.6 \times 10^{-12} Hz s^{-1}. I show that all of the conventional accretion scenarios encounter major difficulties explaining the rapid spin-down of the pulsar. These difficulties can be, however, avoided within the magnetic accretion scenario in which the neutron star is assumed to accrete from a magnetized wind. The spin-down rate of the pulsar can be explained within this scenario provided the surface magnetic field of the neutron star is B_* ~ 4 \times 10^{13} G. I show that the age of the pulsar in this case lies in the rage (2-4) \times 10^4 yr, which is consistent with observations. The spin evolution of the pulsar is briefly discussed.Comment: accepted for publication in MNRA