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At least 451 records · Page 25

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NASA NTRS1987

Numerical simulation of the generation of turbulence from cometary ion pick-up

Observations of magnetic field fluctuations near Comet Halley have revealed a rapid development of a Kolmogoroff-like turbulence spectrum extending from below 0.01 Hz to above 0.1 Hz. Spectra obtained far from the comet have a strong peak in power near the Doppler-shifted ion-cyclotron frequency of singly ionized water. Closer to the comet, the spectrum at higher frequencies is enhanced in power level over the background solar wind spectrum by approximately an order of magnitude. The equations of incompressible MHD are solved using a two-dimensional 256 x 256 mode spectral method code to simulate this spectral evolution as an inertial range turbulent cascade. The initial conditions contained a constant magnetic field and a single coherent wave mode at a low wave number. The solar wind turbulence was modeled by a background noise spectrum having a Kolmogoroff spectral index. The coherent mode decayed into an inertial range spectrum with Kolmogoroff slope within a few eddy-turnover times. Both the time scale and the increase in power level of the turbulence seen in the simulation are in accord with the Giotto observations.

Goldstein, M. L.↗

NASA NTRS1987

Energetic particle acceleration in spherically symmetric accretion flows and shocks

Steady state, spherically symmetric solutions of the cosmic-ray transport equation describing the acceleration of energetic particles in galactic accretion flows onto neutron stars, black holes, white dwarfs, and protostars are studied. The results indicate that astrophysical accretion flows can be partitioned into distinct classes depending upon whether the accretion rate lies above or below a critical value of a few times 10 to the -7th stellar masses/yr. When the particle transport is convection-dominated, both classes of accretion flows exhibit a spectral index appropriate for first-order Fermi acceleration at a plane shock in the absence of losses. As the particle transport becomes diffusion-dominated, both classes show a break and precipitous falloff in the particle spectrum due to the escape of these particles from the accretion flow. The precise nature of the spectrum depends on the relationship between the particle momentum and the spatial diffusion coefficient.

Webb, G. M.↗

NASA NTRS1987

Radio detection of the Be star Psi Persei

The results of a VLA survey for radio emission from a sample of Be stars having strong excess emission in the IRAS bands are presented. Radio emission was detected from one star, Psi Persei. The radio flux density, together with nearly simultaneous ultraviolet and near-infrared observations, argue strongly against a spherically symmetric model for the circumstellar envelope, thus favoring equatorial disk models. In general, it is found that the spectral index of the excess emission increases between the radio and far-infrared, suggesting either increased acceleration of the outflow at large distances from the star, partial recombination of the outer regions of the disk, or divergence of the equatorial disk at large radii.

Taylor, A. R.↗

NASA NTRS1987

Fermi acceleration at shocks with arbitrary velocity profiles

Diffusive particle acceleration in modified shock fronts is considered. A method by means of which to obtain the resulting particle spectrum for an arbitrary flow profile is outlined; this method is based on a reformulation of the transport equation as a nonlinear first-order equation. Besides the shape of the flow profile, the spectral index of particles depends on the Peclet number of the shock. Results are given for some sample profiles.

Schneider, Peter↗

NASA NTRS1988

Energy distributions of blazars

This paper presents observations of 162 blazars, over one-half of which were detected with data from the IRAS satellite using a coadded survey and pointed observations. The far-IR data were combined with published data at other wavelengths to complete 93 energy distributions from 10 to the 8th to 10 to the 18th Hz. The IRAS data were used to search for variability on time scales of 3-9 months in 24 objects. The detection rate of variability found for blazars was nearly 40 percent, as compared with a complete absence of detected variability in data sets for quasars and Seyfert galaxies. The range of variability in both spectral index and flux density increases with decreasing wavelength. Half of the blazars with total luminosities less than 10 to the 12th solar luminosity show evidence for thermal IR component. The global energy budget of blazars is dominated by IR emission from 1 to 100 microns.

Impey, C. D.↗

NASA NTRS1987

Uranian ring photometry - Results from Voyager 2

The rings of Uranus are characterized on the basis of an analysis of seven images obtained with a clear filter by Voyager 2 during its encounter with Uranus in January 1986. The data are presented graphically and compared with the predictions of theoretical models with different particle size distributions. Parameters determined include average particle Bond albedo 0.014 + or - 0.004, fractional area of the main rings in dust-size particles 0.002 or less, and average dust-particle size 1.0 + or - 0.3 microns for a Hansen-Hovenier (1974) distribution and a power-law distribution of spherical particles with spectral index 2.5 + or - 0.5.

Ockert, Maureen E.↗

NASA NTRS1988

Flat-spectrum, variable radio sources

The general behavior of flat-spectrum (compact) radio sources is examined in terms of adiabatic-jet models. Two puzzling properties - namely, (1) the broad, rather flat spectrum (over a large range of radio frequencies) and (2) the relatively slow decay of burst amplitude (with decreasing radio frequency) - are explained. Acceptable models are characterized by the following: (1) a nearly conical, adiabatic jet, with conserved magnetic flux transverse to the axis of the jet; (2) prolonged injection (for which the duration of an event exceeds the apparent transit time scale); and (3) a transparent spectral index which is not too steep. It is suggested that the acceleration mechanism in the core of compact jets may differ substantially from that far from the core, producing a flatter electron number index s = 1.4-2.6.

O'Dell, S. L.↗

NASA NTRS1988

Simultaneous X-ray and radio observations of GX 13 + 1

The results of simultaneous X-ray and radio observations of GX 13 + 1, intrinsically one of the brightest Galactic bulge X-ray sources, are presented and possible models are discussed. No evidence is found for correlated X-ray and radio flux density variations. The variation of the 6-20 cm wavelength spectral index from +0.5 to -1.0 indicates that the radio flux is probably produced by a nonthermal emission mechanism. Application of an equipartition synchrotron model to the data suggests that the radio emission originates in a region with a magnetic field of about 1 gauss. If the radio flux density variations are due to injection of relativistic electrons, the equivalent nonthermal particle energy is 10 to the 36th - 10 to the 41st ergs/s.

Garcia, M. R.↗

NASA NTRS1988

Studies of BL Lacertae objects with the Einstein Observatory - The soft X-ray spectra of OJ 287 and PKS 0735+178

Accurate, soft X-ray spectra of two BL Lac objects, OJ 287 and PKS 0735+178, are presented. The X-ray spectra are well described by a power-law model with a low-energy cutoff consistent with photoelectric absorption within the Galaxy. The best-fit values of the energy spectral index in the 0.2-4.0 keV band are 0.91 and 0.76 respectively. The X-ray flux from OJ 287 is variable by a ratio of three from low to high state; PKS 0735+178 shows no indication of X-ray variability. The X-ray emission in OJ 287 is interpreted to be due to the synchrotron process from a volume common with either a beamed radio component or a stationary optical component. In PKS 0735+178, where the X-ray emission is most likely due to the Compton process operating in one of the VLBI radio components. The synchrotron self-Compton process with modest kinematic Doppler factors predicts the measured X-ray flux from PKS 0735+178 and lower than the measured flux in OJ 287.

Madejski, Greg M.↗

NASA NTRS1988

A soft X-ray flare in the Seyfert I galaxy Markarian 335

Strong, erratic, and primarily soft X-ray flux variations observed in Mrk 335 with the Einstein high-resolution imager (HRI) and monitor proportional counter (MPC) are reported. The variability time scales lie from about 6000 s to the period of observation, 60,000 s. The variability consisted of a decrease followed by an increase at X-ray energies below 2-3 keV. The variability is most pronounced at the softest energies. The X-ray spectrum was harder before the flare than afterward, even after the flare had ended. Averaged over the time of the observations, the MPC data are well-fitted by a power-law spectrum with a spectral index of 1.25 + or - 0.19 with no evidence of absorption by foreground neutral hydrogen at energies above 1.2 keV. If the observed value of the Galactic H I column density is assumed, then the HRI observations require the existence of an additional soft and variable X-ray component.

Lee, M. G.↗

NASA NTRS1988

Interstellar scattering of the radio source 2013+370

Results are reported from a six-station 1663-MHz VLBI observation of the radio source 2013+370. The image of this source is dominated by interstellar scattering and is observed to be elliptical, with a major axis (FWHM) of 17 marcsec and an axial ratio of of 0.70. A fit of the observations to a model visibility function yields a value for alpha, the spectral index of the power-law density irregularity spectrum, of alpha=3.79 + or - 0.05. Corrections for the finite angular size of the source would probably produce a downward revision of alpha. The present observations are consistent with the interstellar density spectrum being close to Kolmogoroff for those scales which determine the angular broadening.

Spangler, Steven R.↗

NASA NTRS1988

VLBI structure of the QSO 3C286

Observations of the quasar 3C286 with the EVN at 609 and 1420 MHz are reported. Results are presented on source size, the steepening of the spectral index, the IC X-ray flux, and surface brightness temperature. It is concluded that 3C286 appears to be not as energetic as the other 3C sources of the class.

Kus, A. J.↗

NASA NTRS1989

Soft X-ray diagnostics of electron-heated solar flare atmospheres

The dependence of the impulsive-phase Ca XIX w line profile on the form of the flare energy input (assumed to be due to the collisional degradation of a beam of high-energy electrons) is considered. The injected flux spectrum has the form of a power law with a low-energy 'knee', and the effects of varying the total energy flux, spectral index, and knee energy on the w line profile during the impulsive phase have been evaluated. Early in the burst, blueshifts of order 400 km/s are noted, and the peak intensity of the blueshifted component together with spatially unresolved hard X-ray burst spectra can be used to determine the beam filling factor.

Li, Peng↗

NASA NTRS1989

Combined first and second order Fermi acceleration at comets

Two-dimensional, time-dependent model calculations indicate that an interplay between velocity and spatial diffusion may be responsible for the acceleration of implanted heavy ions in the cometary preshock region. Velocity diffusion (second order Fermi acceleration) accelerates the pickup ions to moderate energies thus creating a seed population for the more efficient diffusive-compressive shock acceleration. Solar wind convection limits the time available for diffusive-compressive acceleration, therefore the resulting energy spectrum above 100 keV is a power law with a spectral index of 5 - 6, a value which is in reasonably good agreement with observations.

Gombosi, T. I.↗

NASA NTRS1989

Radio-quiet QSO and the X-ray background

Upper limits on the extragalactic soft X-ray background flux (epsilon of about 0.1-0.3 keV) are used to constrain the contribution of X-ray sources with steep spectra to the X-ray background above 1 keV. The effective spectral index of the extragalactic X-ray background is less than 0.76 over the energy range 0.2 and 3.5 keV. Objects with power-law spectra of energy index 1.4, as recently found for a sample of radio-quiet QSO, cannot contribute more than 13 percent of the 2-10 keV X-ray background.

Fabian, A. C.↗

NASA NTRS1989

High-resolution microwave spectra of solar bursts

A phenomenological and statistical study of flares observed in total power with the frequency-agile interferometer at the Owens Valley Radio Observatory during several months of high solar activity in 1981 is reported. Roughly 80 percent of the events have a complex spectrum consisting of more than one spectral component, implying that the microwave radiation of a burst usually does not come from a single homogeneous source. The presence of more than one component can lead to significant errors when data with low spectral resolution are used to determine the low-side spectral index. The low-frequency slope of a single spectra component is often steeper than expected, and the peak frequency stays nearly constant throughout a microwave event.

Stahli, M.↗

NASA NTRS1989

The Hard X-rays and Gamma-rays from Solar Flares

Radiation of energies from 10 KeV to greater than 10 MeV has been observed during solar flares, and is interpreted to be due to bremsstrahlung by relativistic electrons. A complete treatment of this problem requires solution of the kinetic equation for relativistic electrons and inclusion of synchrotron energy losses. Using the electron distributions obtained from numerical solutions of this equation the bremsstrahlung spectra in the impulsive x ray and gamma-ray regimes are calculated, and the variation of these spectral indices and directivities with energy and observation angle are described. The dependences of these characteristics of the radiation of changes in the solar atmospheric model, including the convergence of the magnetic field, the injected electron spectral index, and most importantly, in the anisotropy of the injected electrons and of the convergence of the magnetic field are also described. The model results are compared with stereoscopic observations of individual flares and the constraints that this data sets on the models are discussed.

Mctiernan, James M.↗

NASA NTRS1989

The dependence of the soft X ray spectral slope with radio property, luminosity, and redshift, for a large sample of AGN from the Einstein IPC data base

The dependence of the soft X-ray spectral slope on radio, optical and X-ray properties, and on redshift are reported for a large sample of Active Galactic Nuclei (AGN). The sample includes 317 optically and radio-selected AGN from a preliminary version of the Einstein Imaging Proportional Counter (IPC) quasar and AGN data base. The main results are: the difference in X-ray slope between radio-loud and radio-quiet AGN were confirmed for an independent and much larger sample of sources; a difference in X-ray slope between flat and steep radio spectrum AGN is observed only in high luminosity sub-sample; in flat radio spectrum AGNs there is an indication for a dependence of the X-ray spectral index on X-ray luminosity redshift and alpha sub 0x.

Brunner, H.↗