Spectral Models of Convection-Dominated Accretion Flows

For small values of the dimensionless viscosity parameter, namely $\alpha\lesssim 0.1$, the dynamics of non-radiating accretion flows is dominated by convection; convection strongly suppresses the accretion of matter onto the central object and transports a luminosity $\sim 10^{-3}-10^{-2} \dot M c^2$ from small to large radii in the flow. A fraction of this convective luminosity is likely to be radiated at large radii via thermal bremsstrahlung emission. We show that this leads to a correlation between the frequency of maximal bremsstrahlung emission and the luminosity of the source, $\nu_{\rm peak} \propto L^{2/3}$. Accreting black holes with X-ray luminosities $10^{-4} L_{Edd}\gtrsim L_X(0.5-10{\rm keV}) \gtrsim 10^{-7}L_{Edd}$ are expected to have hard X-ray spectra, with photon indices $\Gamma\sim2$, and sources with $L_X\lesssim 10^{-9}L_{Edd}$ are expected to have soft spectra, with $\Gamma\sim3.5$. This is testable with {\it Chandra} and {\it XMM}.Comment: final version accepted by ApJ; significant modifications from previous versio

Properties of Intercalated 2H-NbSe2, 4Hb-TaS2 and 1T-TaS2

The layered compounds 2H-NbSe, 24Hb-TaS, 2and 1T-TaS2 have been intercalated with organic molecules; and the resulting crystal structure, heat capacity, conductivity, and superconductivity have been studied. The coordination in the disulfide layers was found to be unchanged in the product phase. Resistance minima appear and the superconducting transition temperature is reduced in the NbSe2 complex. Conversely, superconductivity is induced in the 4Hb-TaS2 complex. Corresponding evidence of a large change of the density of states, negative for 2H-NbSe2 and positive for 4Hb-TaS2, was also observed upon intercalation. The transport properties of all the intercalation complexes show a pronounced dependence upon the coordination of the transition metal

A Comparison and Strategy of Semantic Segmentation on Remote Sensing Images

In recent years, with the development of aerospace technology, we use more and more images captured by satellites to obtain information. But a large number of useless raw images, limited data storage resource and poor transmission capability on satellites hinder our use of valuable images. Therefore, it is necessary to deploy an on-orbit semantic segmentation model to filter out useless images before data transmission. In this paper, we present a detailed comparison on the recent deep learning models. Considering the computing environment of satellites, we compare methods from accuracy, parameters and resource consumption on the same public dataset. And we also analyze the relation between them. Based on experimental results, we further propose a viable on-orbit semantic segmentation strategy. It will be deployed on the TianZhi-2 satellite which supports deep learning methods and will be lunched soon.Comment: 8 pages, 3 figures, ICNC-FSKD 201

Low heat conduction in white dwarf boundary layers?

X-ray spectra of dwarf novae in quiescence observed by Chandra and XMM-Newton provide new information on the boundary layers of their accreting white dwarfs. Comparison of observations and models allows us to extract estimates for the thermal conductivity in the accretion layer and reach conclusions on the relevant physical processes. We calculate the structure of the dense thermal boundary layer that forms under gravity and cooling at the white dwarf surface on accretion of gas from a hot tenuous ADAF-type coronal inflow. The distribution of density and temperature obtained allows us to calculate the strength and spectrum of the emitted X-ray radiation. They depend strongly on the values of thermal conductivity and mass accretion rate. We apply our model to the dwarf nova system VW Hyi and compare the spectra predicted for different values of the thermal conductivity with the observed spectrum. We find a significant deviation for all values of thermal conductivity that are a sizable fraction of the Spitzer conductivity. A good fit arises however for a conductivity of about 1% of the Spitzer value. This also seems to hold for other dwarf nova systems in quiescence. We compare this result with thermal conduction in other astrophysical situations. The highly reduced thermal conductivity in the boundary layer requires magnetic fields perpendicular to the temperature gradient. Locating their origin in the accretion of magnetic fields from the hot ADAF-type coronal flow we find that dynamical effects of these fields will lead to a spatially intermittent, localized accretion geometry at the white dwarf surface.Comment: 8 pages, 5 figs, to appear in Astronomy & Astrophysic

Radioactivity and Electron Acceleration in Supernova Remnants

We argue that the decays of radioactive nuclei related to $^{44}$Ti and $^{56}$Ni ejected during supernova explosions can provide a vast pool of mildly relativistic positrons and electrons which are further accelerated to ultrarelativistic energies by reverse and forward shocks. This interesting link between two independent processes - the radioactivity and the particle acceleration - can be a clue for solution of the well known theoretical problem of electron injection in supernova remnants. In the case of the brightest radio source Cas A, we demonstrate that the radioactivity can supply adequate number of energetic electrons and positrons for interpretation of observational data provided that they are stochastically pre-accelerated in the upstream regions of the forward and reverse shocks.Comment: 6 pages, 1 figure, revised version accepted to Phys.Rev.

Measurement of Partial-Wave Contributions in pp --> pp pi^0

We report a measurement of the spin-dependent total cross section ratios delta_sigma_T/sigma_tot and delta_sigma_L/sigma_tot of the pp --> pp pi^0 reaction between 325 MeV and 400 MeV. The experiment was carried out with a polarized internal target in a storage ring. Non-vertical beam polarization was obtained by the use of solenoidal spin rotators. Near threshold, the knowledge of both spin-dependent total cross sections is sufficient to deduce the strength of certain participating partial waves, free of any model.Comment: 6 pages, 4 figure

The Formation of the First Stars II. Radiative Feedback Processes and Implications for the Initial Mass Function

We consider the radiative feedback processes that operate during the formation of the first stars, including the photodissociation of H_2, Ly-alpha radiation pressure, formation and expansion of an HII region, and disk photoevaporation. These processes may inhibit continued accretion once the stellar mass has reached a critical value, and we evaluate this mass separately for each process. Photodissociation of H_2 in the local dark matter minihalo occurs relatively early in the growth of the protostar, but we argue this does not affect subsequent accretion since by this time the depth of the potential is large enough for accretion to be mediated by atomic cooling. However, neighboring starless minihalos can be affected. Ionization creates an HII region in the infalling envelope above and below the accretion disk. Ly-alpha radiation pressure acting at the boundary of the HII region is effective at reversing infall from narrow polar directions when the star reaches ~20-30Msun, but cannot prevent infall from other directions. Expansion of the HII region beyond the gravitational escape radius for ionized gas occurs at masses ~50-100Msun, depending on the accretion rate and angular momentum of the inflow. However, again, accretion from the equatorial regions can continue since the neutral accretion disk has a finite thickness and shields a substantial fraction of the accretion envelope from direct ionizing flux. At higher stellar masses, ~140Msun in the fiducial case, the combination of declining accretion rates and increasing photoevaporation-driven mass loss from the disk act to effectively halt the increase in the protostellar mass. We identify this process as the mechanism that terminates the growth of Population III stars... (abridged)Comment: 31 pages, including 10 figures, accepted to Ap

Electronic correlations in vanadium chalcogenides: BaVSe3 versus BaVS3

Albeit structurally and electronically very similar, at low temperature the quasi-one-dimensional vanadium sulfide BaVS3 shows a metal-to-insulator transition via the appearance of a charge-density-wave state, while BaVSe3 apparently remains metallic down to zero temperature. This different behavior upon cooling is studied by means of density functional theory and its combination with the dynamical mean-field theory and the rotationally-invariant slave-boson method. We reveal several subtle differences between these chalcogenides that provide indications for the deviant behavior of BaVSe3 at low temperature. In this regard, a smaller Hubbard U in line with an increased relevance of the Hund's exchange J plays a vital role.Comment: 16 pages, 11 figures, published versio

High efficiency of soft X-ray radiation reprocessing in supersoft X-ray sources due to multiple scattering

Detailed analysis of the lightcurve of CAL 87 clearly has shown that the high optical luminosity comes from the accretion disc rim and can only be explained by a severe thickening of the disc rim near the location where the accretion stream impinges. This area is irradiated by the X-rays where it faces the white dwarf. Only if the reprocessing rate of X-rays to optical light is high a luminosity as high as observed can be understood. But a recent detailed study of the soft X-ray radiation reprocessing in supersoft X-ray sources has shown that the efficiency is not high enough. We here propose a solution for this problem. As already discussed in the earlier lightcurve analysis the impact of the accretion stream at the outer disc rim produces a ``spray'', consisting of a large number of individual gas blobs imbedded in a surrounding corona. For the high mass flow rate this constitutes an optically thick vertically extended screen at the rim of the accretion disc. We analyse the optical properties of this irradiated spray and find that the multiple scattering between these gas blobs leads to an effective reprocessing of soft X-rays to optical light as required by the observations.Comment: 7 pages, 6 figures, accepted for publication in A&