Electrostatic bremsstrahlung

Inverse Compton radiation due to scattering off electron plasma waves, called electrostatic bremsstrahlung by Colgate (1967), is discussed with the view of resolving a discrepancy which exists between various authors on the mean frequency radiated.     

The effect of wave generation on HXR bremsstrahlung spectra from flare thick-target beams

Fast electrons in the solar atmosphere are detected by their hard X-ray bremsstrahlung and by type III radio bursts caused by ‘bump-on-tail’ plasma wave generation. This paper investigates empirically the effect of wave generation on the HXR spectrum. Purely collisional propagation of an electron beam generates a bump in the distribution function, due to stopping of low-velocity electrons. The consequent positive gradient means there is a possibility of wave generation, production of type III radio bursts, and energy redistribution of the electron beam. We have represented this relaxation parametrically and calculated the global bremsstrahlung HXR emission spectrum. We show that for a range of relaxed forms, with different local electron spectral shapes, the bremsstrahlung spectrum integrated over the whole target is identical in shape to the purely collisionally evolved beam. Our results show that spatially integrated HXR spectral measurements would be unable to distinguish between the presence or absence of relaxation effects. Only spatially resolved hard X-ray spectra, such as anticipated from the HESSI mission, will be able to remove this ambiguity in HXR diagnostics of beam relaxation.     

Nonthermal electrons in the thick-target reverse-current model for hard X-ray bremsstrahlung

The behaviour of the accelerated electrons escaping from a high-temperature source of primary energy in a solar flare is investigated. The direct current of fast electrons is supposed to be balanced by the reverse current of thermal electrons in the ambient colder plasma inside flare loops. The self-consistent kinetic problem is formulated; and the reverse-current electric field and the fast electron distribution function are found from its solution. The X-ray bremsstrahlung polarization is then calculated from the distribution function. The difference of results from those in the case of thermal runaway electrons (Diakonov and Somov, 1988) is discussed. The solutions with and without account of the affect of a reverse-current electric field are also compared.     

Neutrino bremsstrahlung from a degenerate electron gas

The neutrino bremsstrahlung process is studied here according to the photon-neutrino weak coupling theory by considering the electrons as relativistic and degenerate and by adding some lattice effects at high density. The neutrino energy loss rate due to the process is then compared with that obtained according to the current-current coupling theory. It is concluded that the process is important only in cases of high dense stars when the temperature is below 10⁷K.     

A revised calculation of the bremsstrahlung cross-section in the high magnetic field of pulsars

It has been postulated that electron bremsstrahlung in a strong external magnetic field is the dominant radiation mechanism within the accretion plasma near the magnetic polar regions of binary X-ray sources. Earlier works on the cross section for such a process proved to be unsatisfactory.The present work uses the simple structure of the propagator obtained in a previous calculation for mildly relativistic electrons occupying no more than the first few Landau levels. Particular emphasis is put on the integration over the possible range of momentum transfer during a single collision with the ion. Typical behaviour for two linear polarization modes is illustrated for forward and backward electron scattering. It is found that the previously predicted behaviour at low frequencies for the two modes is only correct in the limit of weak field and large momentum transfer. At higher frequencies resonances are present irrespective of the polarization of the emitted radiation.     

Contribution of electron-electron bremsstrahlung to solar hard X-radiation during flares

The bremsstrahlung produced in collisions of energetic electrons with thermal electrons and protons of a hydrogen plasma is calculated. The importance of electron-electron bremsstrahlung to the X-radiation in solar flares is discussed.     

On the bremsstrahlung efficiency of nonthermal hard X-ray source models

It has often been stated, but never rigorously proven, that interpreting observed hard X-ray emission in terms of a thick-target source gives a lower limit to the flux of electrons which have to be injected into the source. The truth of this statement, for theburst-integrated emission, is rigorously established here. Also an explicit inversion for the injected electron flux in terms of the photon spectrum is given, for the case where all electrons traverse a single value of column density. This generalises the previous results for the thick- and thin-target limits.The use of the standard thick-target formalism for the interpretation of instantaneous (as opposed to burst-integrated) photon fluxes is also discussed. By considering the specific case of the thick-target trap model, it is shown that use of this formalism can either under-estimate or over-estimate the injected electron flux, at different times in the same event, but that integration of the inferred electron fluxes over the event nevertheless yields the true, burst-integrated electron flux.     

Thermal bremsstrahlung hard X-rays and primary energy release in flares

Various methods are explored for obtaining regularized solutions of the severely ill-posed Laplace inversion problem involved in deriving plasma temperature (T) structure (differential emission measure(T)) from bremsstrahlung spectra. Inversions of simulated data show that zero-order regularisation (Tikhonov regularisation inL ² space) is very unsatisfactory even with weighting, while first-order regularisation (Tikhonov regularisation in Sobolev space) yields reasonable results.The method is applied to a high-resolution hard X-ray flare spectrum observed by Lin and Schwartz (1987) and yields a positive solution for(T) showing that a purely thermal interpretation is possible for that event. The form of(T) found has two broad features: one peaking at around 10⁷ K and falling off steeply toward 2 × 10⁸ K; a second spread around a peak near 4.5 × 10⁸ K. The interpretation of such(T) in terms of plasma heating and conductive flux is discussed in terms of plasma heat fluxes and heating rates. For 1-D geometry, the distribution of the plasma heating rateH(T) per unit volume is inferred from(T) in the limits of classical diffusive conduction and of saturated heat flux, the former being relevant atT below around 5 × 10⁷ K and the latter at much higherT. We find there exists a maximum inH(T) around 2 × 10⁸ K, a fact which may be important for energy release theories.     

Thick target X-ray bremsstrahlung from partially ionised targets in solar flares

The effect of partial ionisation of a thick target bremsstrahlung source on the emitted X-ray intensity is analysed. It is shown that a totally ionised target produces an X-ray burst only about one third as intense as that from an unionised target.In the case of a solar flare plasma target, the ionisation decreases with increasing depth in the flare. Thus, in an X-ray flare model in which electrons are continuously accelerated down into the chromosphere, high energy photons are produced with increased efficiency in the deeper layers of the flare plasma with consequent hardening of the X-ray spectrum. As a result, the spectra of nonthermal electrons in flares, inferred from X-ray spectra, are steepened and their total energy correspondingly increased.     

Comparison of flare bremsstrahlung resulting from energetic thermal and nonthermal electrons

We have investigated the behavior of the X-ray bremsstrahlung spectra resulting from two distinct types of electron distribution functions impinging on a target atmosphere during flare activity. A power-law distribution function is compared with two double-peaked Maxwellians. The results of these calculations show that it would be difficult to rule out multithermal interpretations for the emitted high-energy X-rays.     

The derivation of parent electron spectra from bremsstrahlung hard X-ray spectra

We formulate a numerical method to derive the spectrum of the parent electrons from the hard X-ray spectrum produced in optically thin bremsstrahlung. The method can utilize any form for the bremsstrahlung cross sections, and it provides accurate estimates of uncertainties in the derived electron spectrum based on uncertainties in the photon measurements. This method is applied to test photon spectra, as well as to hard X-ray spectra of the 27 June, 1980 solar flare which was observed by high spectral resolution detectors. Future measurements with much more sensitive detectors will enable this method to be used to derive detailed, accurate flare electron spectra.     

The directivity and polarisation of thick target X-ray bremsstrahlung from solar flares

The directivity and polarisation of solar hard X-ray bursts is discussed in terms of two bremsstrahlung source models. These involve continuous and impulsive injection of electrons respectively, as described widely in the literature.A detailed analysis is made of the continuous injection model in which electrons are accelerated downward into the dense chromosphere where their velocity distribution is greatly modified by collisions. This thick target scattering is described by a simple analytical model. Directivity and polarisation of the bremsstrahlung emission are calculated in detail for a thick target model in which the guiding magnetic field is vertical.It is predicted for this model that X-ray sources should brighten from the centre to the limb of the solar disk, while the degree of polarisation should rise from zero to around 30 % near the limb. The plane of maximum intensity for any source is that containing the source and the disk centre. Both the directivity and polarisation should increase with increasing photon energy.Very recent observations agree with these predictions though they suggest that the field is significantly non-vertical. If scattering is not included in the model, or if an impulsive injection model is taken, agreement with observations is not obtained.Temporarily at the Lehrstuhl für Theoretische Astrophysik der Universität Tübingen, West Germany.     

The symbiotic system AG Draconis: Soft X-ray bremsstrahlung from the nebulae

The modeling of UV and optical spectra emitted from the symbiotic system AG Dra, adopting collision of the winds, predicts soft X-ray bremsstrahlung from nebulae downstream of the reverse shock with velocities > 150 km s^?1 and intensities comparable to those of the WD black body flux. At outbursts, the envelop of debris, which corresponds to the nebula downstream of the high velocity shocks (700–1000 km s^?1) accompanying the blast wave, absorbs the black body soft X-ray flux from the WD, explains the broad component of the H and He lines, and leads to low optical-UV–X-ray continuum fluxes. The high optical-UV flux observed at the outbursts is explained by bremsstrahlung downstream of the reverse shock between the stars. The depletion of C, N, O, and Mg relative to H indicates that they are trapped into dust grains and/or into diatomic molecules, suggesting that the collision of the wind from the WD with the dusty shells, ejected from the RG with a ～1 year periodicity, leads to the U-band fluctuations during the major bursts.     

Neutrino-pair bremsstrahlung in collisions between neutrons and nuclei in neutron star matter

In neutron star matter over the density range 4.3×10¹¹<-ϱ≤4×10¹⁴ g cm⁻³ there are both free neutrons and neutron-rich nuclei. If there is a weak neutral current interaction between neutrinos and neutrons, as suggested by recent experiments, then when neutrons scatter off nuclei they may emit pairs as bremsstrahlung radiation. We calculate the associated emissivity for degenerate (but not superfluid) neutrons and uncorrelated (not crystallized) nuclei. We find that, under these conditions. this emissivity can under some conditions compare with that calculated by Festa and Ruderman for bremsstrahlung in electron-nucleus collisions.     

Magnetic fields,bremsstrahlung and synchrotron emission in the flare of 24 October 1969

An impulsive flare October 24, 1969 produced two bursts with virtually identical time profiles of 8800 MHz emission and X-rays above 48 keV. The two spikes of hard X-rays correspond in time to the times of sharp brightening and expansion in the H flare. The first burst was not observed at frequencies below 3000 MHz. This cut off is ascribed to plasma cutoff above the low-lying flare.A model of the flare based on H observations at Big Bear shows that the density of electrons with energy above 10 keV is 5 × 10⁷ if the field density is 10¹¹. The observed radio flux would be produced by this electron distribution with the observed field of 200 G. The H emission accompanying the hard electron acceleration is presumed due to excitation of the field atoms by the hard electrons.     

Hard X-ray bremsstrahlung produced by electrons escaping a high-temperature thermal source in a solar flare

The problem of production of flare hard X-rays by bremsstrahlung from hot thermal escaping electrons (Skrynnikov and Somov, 1982) in a chromospheric plasma is studied.The Landau kinetic equation is solved near the thermal source of energized electrons in a homogeneous magnetic tube to compute the anisotropic inhomogeneous distribution of the thermal escaping electrons.The intensity and polarization of hard X-rays is also computed and a comparison of theoretical results with observational data is made.On leave from: Istituto di Astronomia, Largo E, Fermi 5, I-50125 Firenze, Italy.     

Complex absorption and reflection of a multitemperature cyclotron–bremsstrahlung X-ray cooling shock in BY Cam

We re-analyse the ASCA Ginga X-ray data from BY Cam, a slightly asynchronous magnetic accreting white dwarf. The spectra are strongly affected by complex absorption, which we model as a continuous (power-law) distribution of covering fraction and column of neutral material. This absorption causes a smooth hardening of the spectrum below ∼ 3 keV, and is probably produced by material in the pre-shock column which overlies the X-ray emission region. The ASCA data show that the intrinsic emission from the shock is not consistent with a single-temperature plasma. Significant iron L emission coexisting with iron K shell lines from H- and He-like iron clearly shows that there is a wide range of temperatures present, as expected from a cooling shock structure. The Ginga data provide the best constraints on the maximum temperature emission in the shocked plasma, with kT _max = 21⁺¹⁸₋₄ keV. Cyclotron cooling should also be important; it suppresses the highest temperature bremsstrahlung components, so the X-ray data provide only a lower limit on the mass of the white dwarf of M  ≥ 0.5 M⊙. Reflection of the multitemperature bremsstrahlung emission from the white dwarf surface is also significantly detected.   We stress the importance of modelling all these effects in order to gain a physically self-consistent picture of the X-ray spectra from polars in general and BY Cam in particular.     

Thermal electrons runaway from a hot plasma during a flare in the reverse-current model and their X-ray bremsstrahlung

The behaviour of the thermal electrons escaping from a hot plasma to a cold one during a solar flare is investigated. We suppose that the direct current of fast electrons is compensated by the reverse current of the thermal electrons in ambient plasma. It is shown that the direct current strength is determined only by the regular energy losses due to Coulomb collisions. The reverse-current electric field and the distribution function of fast electrons are found in the form of an approximate analytical solution to the self-consistent kinetic problem of the dynamics of a beam of escaping thermal electrons and its associated reverse current.The reverse-current electric field in solar flares leads to a significant reduction of the convective heat flux carried by fast electrons escaping from the high-temperature plasma to the cold one. The spectrum and polarization of hard X-ray bremsstrahlung, and its spatial distribution along flare loops are calculated and can be used for diagnostics of flare plasmas and escaping electrons.Send offprint requests to B. V. Somov.     

A simplified model for a nonlinear tidal effect on accretion disks in CVs

This paper investigates the tidal effect on accretion disk in CVs and sets up a simplified model in which the secondary's gravitation is substituted by a mean tidal torque. We find that a linear tidal torque will not be able to maintain an equilibrium disk. By using the result of the radius of the equilibrium disk approximately equals to the tidal radius, which was obtained by using the two dimensional numerical simulation invoking nonlinear tidal effect, we give the modified tidal dissipation function for our simplified model which could be used to interpret the outburst of the dwarf nova with tidal effect. The paper also shows that the radius of an equilibrium disk with a torus is slightly small than the Lubow-Shu radius, and the tidal effect may also cause the cycle of quiescence-superoutburst in addition to the cycle of quiescence-outbursts-superoutburst.     

A polarization-color effect in the K-corona

An observation of the corona during the 7 March, 1970 eclipse through a Wollaston prism-red and blue filter combination was carried out for the purpose of confirming the prediction that the component of the K-corona with electric vector radial to the Sun is redder than the tangential component. An analysis of a portion of the resulting data seems to confirm this prediction, and also indicates that the method is useful for determining the three-dimensional configuration of the very inner corona. A three-dimensional configuration is suggested for a portion of the NE quadrant of the Sun, including the large NE streamer and an adjacent low-lying loop.