X-Ray emission from two-phase accretion disks

Summary We consider a ?two phase? accretion disk consisting of an optically thick layer with temperatureT≃10⁴ K embedded in a hot thin corona (T≃10⁹ K). The main energy input occurs through magnetic heating of the electrons in the corona, while cooling is due to Compton losses of the hot electrons on the soft photons provided by the thick layer. We write the balance equations for the two phases. We show that a possible mode of variability yields steeper spectra for increasing soft-photon luminosity as observed in Seyfert galaxies and compute composite model spectra in the X-ray range, via Monte Carlo simulations. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

A search for heavy leptons in cosmic radiation underground

An experiment to search massive long-lived, weakly interacting particles (leptons) in cosmic radiation has been conducted at Kolar Gold Fields at a depth of 7.6 hg cm⁻² (1 hg cm⁻²=100 g cm⁻²) below surface. The apparatus was senstive to sub-relativistic (velocity<0.75 c) charged leptons of mass greater than that of a proton and life times greater than a microsecond. The method consists of selecting charged particles using a scintillator counter telescope and vetoing relativistic particles (velocity >0.75 c) by using a water Čerenkov detector. The range of the particle is observed in arrays of neon flash tubes interspersed with iron absorbers. During 3000 hours of observation 28 events were recorded satisfying the trigger and event selection criteria. Bulk of these events were interpreted as due to recoil protons (low energy) from the inelastic scattering of high energy muons in the overhead absorber. The remaining events were interpreted as either atmospheric stopping protons or stopping muons that failed to generate a Čerenkov signal. The observed events are thus consistent with the background and no heavy leptons were seen. From our observations an upper limit of 2.12×10⁻⁷ (with 90% confidence level) is set on the ratio of the flux of heavy leptons to that of all muons at this depth.     

Terrestrial gamma-ray flashes and neutron pulses from direct simulations of gigantic upward atmospheric discharge

Gamma-ray pulses are calculated from 2D numerical simulations of upward atmospheric discharge in a self-consistent electric field using the multigroup approach to the kinetics of relativistic runaway electrons. Thecalculated numbers and spectra of γ-rays are consistent with the data on terrestrial γ-ray flashes observed aboard spacecraft. The calculated runaway electron flux is concentrated mainly within the domain of the blue jet fluorescence. This proves that exactly this domain is a source of the γ-ray flashes. One γ-ray flash generates ∼10¹⁴-10¹⁵ photonuclear neutrons. The text was submitted by the authors in English     

Atmospheric gamma-ray and neutron flashes

Gamma-ray pulses are calculated from 2D numerical simulations of an upward atmospheric discharge in a self-consistent electric field using the multigroup approach to the kinetics of relativistic runaway electrons (REs). Computed γ-ray numbers and spectra are consistent with those of terrestrial γ-ray flashes (TGFs) observed aboard spacecrafts. The RE flux is concentrated mainly within the domain of the Blue Jet fluorescence. This confirms that exactly the domain adjacent to a thundercloud is the source of the observed γ-ray flashes. The yield of photonuclear neutrons is calculated. One γ-ray pulse generates ∼10¹⁴–10¹⁵ neutrons. The possibility of the direct deposition of REs to the detector readings and the origin of the lightning-advanced TGFs are discussed. The text was submitted by the authors in English.     

Integral energy spectrum of primary cosmic rays at high energies

The integral energy spectrum of primary cosmic rays has been obtained. In the energy range (2.4×10³−1.1×10⁵ GeV), the spectrum of all nuclei is consistent with a power law of indexγ=1.55±0.06 and the flux of all nuclei is:N(⩾E ₀)⋍(5.1±1.8)×10⁻¹×E ₀ ^−1.55 particles/cm² sterad. sec., whereE ₀ is in GeV. The spectrum of primaryα-particles in the energy range (4.4×10³−4.8×10⁴) GeV is also consistent with a power law of indexγ=1.71±0.12 and the flux is:N(⩾E ₀)=(4.2±1.4)×10⁻¹×E ₀ ^−1.71 , particles per cm² sterad. sec, whereE ₀ is in GeV.     

Role of equilibrium plasma flow on damping of slow MHD waves

In the solar corona waves and oscillatory activities are observed with modern imaging and spectral instruments. These oscillations are interpreted as slow magneto-acoustic waves excited impulsively in coronal loops. This study explores the effect of steady plasma flow on the dissipation of slow magneto-acoustic waves in the solar coronal loops permeated by uniform magnetic field. We have investigated the damping of slow waves in the coronal plasma taking into account viscosity and thermal conductivity as dissipative processes. On solving the dispersion relation it is found that the presence of plasma flow influences the characteristics of wave propagation and dissipation. We have shown that the time damping of slow waves exhibits varying behavior depending upon the physical parameters of the loop. The wave energy flux associated with slow magnetoacoustic waves turns out to be of the order of 10⁶ erg cm⁻² s⁻¹ which is high enough to replace the energy lost through optically thin coronal emission and the thermal conduction below to the transition region.     

The effect of the scale factor on the focusing properties of a vortex gas flow

An experimental study is carried out to investigate a bilateral direct-flow vortex tube of small (0.3–3.0)·10⁻² m) size and to compare its optical properties with those characteristics of vortex tubes of a larger ((3–16)\10⁻² m) scale. A model is suggested for forming transverse profiles of the intensity of laser beams with an intensity hole in the center and of annular beams. Translated from Zhural Prikladnoi Spektroskopii, Vol. 64, No. 3, pp. 386–389, May–June, 1997     

Bleaching effect in passive regions of powerful 1.02 μm InGaAs/AlGaAs DQW laser diodes with ridged waveguide structure

It has been shown that the cavity trapped amplified luminescence (CTAL) flux formed within the active region of a powerful 1.02 μm InGaAs/AlGaAs DQW laser diode (LD) with a ridged waveguide structure, can cause the bleaching of passive areas leading to abrupt hysteresis-type changes in the light–current and spectral characteristics. The post-threshold rise of the CTAL flux within the non-lasing parts of the active region is most likely to play significant role in the nonlinear optical phenomena observed in investigated LDs. The frequency-integrated CTAL flux density at which bleaching takes place is equal to 6.5×10⁸ W/m². The hysteresis-type loop can be removed through the LD “run-in” procedure or high-temperature annealing of the LD chip in an hydrogen atmosphere. PACS 42.55.Px; 42.70.Hj     

Experimental study of the pumping action of helical flow

A decrease in hydraulic friction coefficient Σ (Darcy number) is found in the range of smooth tubes. The maximum possible decrease in Σ is observed at Φ = 0.08 for numbers Re _D = V _x D / ν in the range Re _Dx = (5.8−6.6) × 10⁴.     

Efficiency of electron acceleration by shock waves in the solar corona according to observational data on the fine structure of type II radio bursts

Herringbone bursts (HB-bursts) are the type III-like fine structure in type II bursts of solar radio emission and are usually interpreted as plasma radiation arising from fast electrons accelerated by shock waves in the solar corona. In general outline, the radiation mechanism of HB-bursts is similar to that of type III bursts. However, HB-bursts have brightness temperatures that are about an order of magnitude higher than those of type III bursts. The frequency drift of BH-bursts is about two or three times lower than that for type III bursts. All this shows that the fast-electron beams responsible for HB-bursts and type III bursts differ markedly in their parameters. We calculated expected brightness temperatures of HB-bursts at the fundamental and second harmonic and compared our results with Culgoora radiometer and radioheliograph data and Tremsdorf spectrograph data in order to estimate parameters of fast-electron beams generating HB-bursts. We found that the observed brightness temperatures of HB-bursts give velocities of fast electrons accelerated by shock waves within the limits (0.02–0.17)c. These velocities are several times lower than those for type III bursts (0.15–0.5)c. The density of the fast electrons responsible for HB-bursts is in the interval 3·10⁻⁶ <n_b/n<6·10⁻⁵, which exceeds by 1–2 orders of magnitude the relative densities in type III sources. This gives a clue to understanding the markedly higher brightness temperatures of HB-bursts compared to those of type III bursts. We concluded that the parameters obtained for the agent exciting HB-bursts favor of turbulence mechanisms of electron acceleration by shock waves in the solar corona. Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod, Russia; Astrophysical Institute, Potsdam, Germany. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 2, pp. 164–176, February, 1998.     

Electron density of the magnesium XII doublet in the solar corona by use of a collision excitation model

For accurate spectroscopic diagnostics in the solar corona, the principle of spectral diagnostics of electron density is discussed by collision excitation model. Variation in electron density in the solar corona is calculated by this method using the observed signal ratio of spectral lines produced by the magnesium XII ion in the solar corona. Results show that with increasing signal ratio, the electron density will decrease; furthermore, the electron density is on the order of 10¹⁰?cm^?3, which is a reasonable value in the solar corona. Finally, variation in temperature with different signal ratio is discussed. This investigation will be significant for solar plasma diagnostics and study on the solar coronal.     

Spectral properties of backward stimulated scattering in liquid carbon disulfide

The spectral structure of backward stimulated scattering from a 10 cm-long CS₂-liquid cell is investigated by using Q-switched 10-ns and 532-nm laser pulses with different spectral linewidths. Under a narrow spectral line (∼0.1 cm⁻¹) pump condition, very strong sharp lines near the pump wavelength (λ ₀) position and the first-order stimulated Raman scattering (λ _s1) position can be observed. However, under a wide line (≈1 cm⁻¹) pump condition, only a strong and superbroadening spectral band can be observed mainly in the red-shift side of the pump wavelength. The different spectral features under these two conditions can be explained by a competition between stimulated Brillouin, Raman, and Rayleigh-Kerr scattering. Under both pump conditions, the broadening spectral distributions are not consistent with the predictions given by stimulated Rayleigh-wing scattering theories, but can be interpreted well utilizing the theoretical model of stimulated Rayleigh-Kerr scattering. Zh. éksp. Teor. Fiz. 112, 1563–1573 (November 1997) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Experimental study of the variation of the plasma temperature with the laser flux

Experiments performed with a 50 MW — 60 nsec ruby laser to estimate the temperature of the plasma produced on the planar targets of carbon as well as polyethylene are reported. Temperatures were estimated by two foil ratio technique. The temperatures of carbon and polyethylene plasma show aφ ^2/9 dependance on flux in the flux regime of 10¹⁰ W/cm² to 5 × 10¹¹ W/cm². The comparatively slower dependance is explained on the basis of purely collisional absorption, the effect being enhanced due to relatively long duration of the laser pulses. Scaling laws of plasma temperature against laser flux obtained by different workers in different flux regimes have been analysed on the basis of collisional and non-collisional absorption.     

EID study on the system platinum-hydrogen

Electron induced desorption of hydrogen on polycrystalline platinum has been studied in continuation of former work. The results remain consistent with a supposed adsorption model implying that electron induced H⁺ emission is restricted to hydrogen adsorbed only at particular sites occupying not more than 1 per cent coverage. These sites are ascribed to characteristic and stable surface impurities which are not influenced by the improved target treatment applied. Depletion of sites by sputter cleaning is cancelled by recovery during annealing. The highest observed H⁺ yield (all available sites filled with hydrogen) is 2 · 10⁻⁵ ions per electron. The total cross section for desorption by electron impact is 2 · 10⁻¹⁷ cm². Complex interaction phenomena between the electrons and the hydrogen covered platinum surface have been observed.     

Production of transuranium elements in a binary model under conditions of pulse nucleosynthesis

A model for the production of transuranium isotopes under the conditions of pulse nucleosynthesis in a powerful neutron flux (∼10²⁴−10²⁵ neutron cm⁻²) is considered. The explosive nature of the process allows us to separate it into two parts: the process of multiple neutron captures (t < 10⁻⁶ s) and the process of the subsequent β-decays for neutron-rich nuclei. The model of multiple captures neutron includes a variation of the cross section of the (n, γ) reaction as a result of the adiabatic expansion of the explosive nucleosynthesis area. A binary mixture of ²³⁸U and ²³⁹Pu is used as the initial isotope composition.     

Radio-frequency absorption study of the parameters of high-T c materials irradiated with fast reactor neutrons

The magnetic properties of YBa₂Cu₃O_7−x ceramics of various densities and Bi₂Sr₂CaCu₂O₈ quasicrystals irradiated with neutrons at fluences of 10¹⁶–10¹⁹ cm⁻² are investigated by a radio-frequency method (in the frequency range f=100–150 MHz). The electromagnetic absorption is used to estimate the parameters of a Josephson medium: the lower critical field, the critical current density, the grain size and intergranular distances, and the penetration depth of the magnetic field in the sample. The hysteresis of electromagnetic absorption in the indicated superconductors is investigated; the hysteresis effect can be used to obtain data on flux pinning processes and flux creep dynamics. It is shown that an increase in the critical current density is observed as a result of the injection of radiation-induced pinning centers. Fiz. Tverd. Tela (St. Petersburg) 39, 28–34 (January 1997)     

Origin of fine structures in solar radio bursts

The radiation, resulting from the nonlinear interaction of whistler solitons, which act as localized antennae, with the upperhybrid waves in the coronal loop, is shown to give rise to fine structures in solar radio bursts. All the observed features of microwave spikes in radio flares,e.g. their frequency, polarization and short duration can be explained by the presence of about 10⁶ solitons occupying a volume of ∼ 10⁸ m³, provided this interaction takes place at low altitudes. However, if this interaction takes place close to the top of the coronal loop, it gives rise to the isolated tadpole ‘eyes’ features in the dynamic spectra. About 10⁹ solitons are needed to account for the observed flux of these ‘eyes’.     

Measurement of the proton structure function F 2 at low x and low Q 2 at HERA

We report on a measurement of the proton structure functionF ₂ in the range 3.5×10⁻⁵≤x≤4×10⁻³ and 1.5 GeV²≤Q ²≤15GeV² at theep collider HERA operating at a centre-of-mass energy of √s=300GeV. The rise ofF ₂ with decreasingx observed in the previous HERA measurements persists in this lowerx andQ ² range. TheQ ² evolution ofF ₂, even at the lowestQ ² andx measured, is consistent with perturbative QCD. supported by EU HCM contract ERB-CHRX-CT93-0376     

Conductivity tensor of the coronal plasma

We study the plasma conductivity within the framework of the magnetohydrodynamic approximation under conditions typical of the coronas of the Sun and stars of the late spectral classes. The problem of finding the elements of the plasma conductivity tensor is reduced to derivation of approximate formulas and numerical values. It is shown that two regimes of dissipation of the current running across the magnetic field can be realized in the corona, namely, one due to the friction of the ion and neutral plasma components (Cowling conductivity) and the other, due to the friction of two different ion components. The first regime is realized in the larger part of the lower corona, whereas the condition for realization of the second regime, i.e., B/n _e < 10⁻⁹ G·cm³, where B is the magnetic field and ne is the electron number density, can be satisfied in the base of the solar corona or in coronal streamers. In the second regime, the rate of the transverse-current dissipation increases tenfold compared with the hydrogen plasma. The longitudinal and Hall components of the conductivity tensor differ only slightly from their values for the electron–proton plasma. The paper draws attention to the fact that along with the momentum exchange due to the charge exchange, the momentum exchange due to ionization and recombination during electron collisions plays a significant part in the friction of neutral and ionized hydrogen atoms.     

GZK photons as ultra-high-energy cosmic rays

We calculate the flux of “GZK photons,” namely, the flux of ultra-high-energy cosmic rays (UHECRs) consisting of photons produced by extragalactic nucleons through the resonant photoproduction of pions, the so-called GZK effect. We show that for primary nucleons, the GZK-photon fraction of the total UHECR flux is between 10⁻⁴ and 10^–2 above 10¹⁹ eV and up to the order of 0.1 above 10²⁰ eV. The GZK-photon flux depends on the assumed UHECR spectrum, the slope of the nucleon flux at the source, and the distribution of sources and intervening backgrounds. Detection of this photon flux would open the way for UHECR gamma-ray astronomy. Detection of a larger photon flux would imply the emission of photons at the source or new physics. We compare the photon fractions expected for GZK photons and the minimal fractions predicted by top-down models. We find that the photon fraction above 10¹⁹ eV is a crucial test for top-down models. The text was submitted by the authors in English.