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At least 343 records · Page 19

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NASA NTRS1978

Extended X-ray observations of Scorpius X-1 with a Bragg crystal spectrometer

Observations covering multiple orbital periods of the X-ray source Sco X-1 were carried out in 1975 and 1976 with the graphite crystal spectrometer aboard OSO 8. Time-averaged continuum spectra are featureless and are reasonably well described in terms of a Comptonized thermal bremsstrahlung spectrum in which kT is about 1.5-2 keV and in which the number of electron scatterings that each photon is assumed to have undergone is of the order of 100. Line limits, typically with equivalent widths of a few electron volts, are established for important emission lines of Si XIII, Si XIV, S XV, S XVI, and Fe XXV as a function of binary phase.

Long, K. S.↗

NASA NTRS1980

Energy loss by resonance line photons in an absorbing medium

The mean path length of photons undergoing repeated scatterings in media of large optical thickness is calculated from accurate numerical solutions of the transfer equation including the effect of frequency redistribution characteristic of combined Doppler and natural broadening. Energy loss by continuous absorption processes, such as ionization or dust absorption, is discussed, and asymptotic scaling laws for the energy loss, the mean path length, and the mean number of scatterings are inferred from the numerical data.

Hummer, D. G.↗

NASA NTRS1984

Electron-positron pair equilibrium in strongly magnetized plasmas

Steady states of thermal electron-positron pair plasmas at mildly relativistic temperatures and in strong magnetic fields are investigated. The pair density in steady-state equilibrium, where pair production balances annihilation, is found as a function of temperature, magnetic field strength and source size, by a numerical calculation which includes pair production attenuation and Compton scattering of the photons. It is found that there is a maximum pair density for each value of temperature and field strength, and also a source size above which optically thin equilibrium states do not exist.

Harding, A. K.↗

NASA NTRS1984

Hydrogen on Venus - Exospheric distribution and escape

The spatial distribution of suprathermal H in the Venus atmosphere is calculated using an approximate numerical solution of the time-independent Boltzmann equation. Sources of suprathermal H are specified on the basis of Pioneer Venus measurements. Reactions involving H2 are neglected. The computed densities of suprathermal H are in satisfactory agreement with profiles derived from analysis of Lyman-alpha airglow by Mariners 5 and 10 and Veneras 11 and 12. The dayside emission at radial distances larger than 18,000 km is attributed to scattering of solar photons by fast H atoms produced primarily on the nightside near midnight. The nightside ionosphere, and consequently the source of suprathermal H, are both expected to vary in response to changes in the solar wind.

Rodriguez, J. M.↗

NASA NTRS1985

Power-law X-ray and gamma-ray emission from relativistic thermal plasmas

A common characteristic of cosmic sources is power-law X-ray emission. Extragalactic sources of this type include compact components of active galactic nuclei (AGN). The present study is concerned with a theoretical model of such sources, taking into account the assumption that the power-law spectra are produced by repeated Compton scatterings of soft photons by relativistic thermal electrons. This is one of several possible physical mechanisms leading to the formation of a power-law spectrum. Attention is given to the Comptonization of soft photon sources, the rates of pair processes, the solution of the pair equilibrium equation, and the constraints on a soft photon source and an energy source. It is concluded that the compactness parameters L/R of most of the cosmic sources observed to date lie below the maximum luminosity curves considered.

Zdziarski, A. A.↗

NASA NTRS1987

Prospects for observations of nucleosynthetic gamma-ray lines and continuum from SN 1987A

Expected flux levels for nucleosynthetic gamma-ray line and continuum emissions from SN 1987A are calculated for several models. The dominant line emission is from freshly synthesized Ni-56 and its decay daughters, and the continuum is from Compton scattering of line photons. For a 15 solar mass Type II model, the light curve for the 0.847 MeV gamma-ray line peaks in September 1988 at 3 x 10 to the -4th photons/sq cm/s. This is detectable only by new, ultrasensitive balloon-borne spectrometers. For models with substantial mass loss from their envelopes, the peak is in early 1988 at about 0.01 photons/sq cm/s, which is detectable at high significance levels by all current instruments. The continuum emission in the 0.05-0.5 MeV band peaks about 100 days before the 0.847 MeV line peak. This Compton precursor is observable at about the same significance level as the lines.

Gehrels, Neil↗

NASA NTRS1987

Critical fluid light scattering

Thermal fluctuations give rise to a host of thermodynamic anomalies near critical points. The background for these phenomena is presented, and an experiment design on STS flight experiment called 'Zeno' described. The objective is to measure the decay rates of critical density fluctuations in a simple fluid (xenon) very near its liquid-vapor critical point using laser light-scattering and photon-correlation spectroscopy. Such experiments have been limited on earth by the presence of gravity, which causes large density gradients in the sample. Fluctuation decay rates in xenon will be measured at least 100 times closer to the critical point than is possible on earth. This will require taking data as close as 145 microK to the critical temperature (Tc = 289.72 K), after locating it to + or - 10 microK. The minimum mission time of 100 hours will allow a complete range of temperature points to be covered, limited by the thermal response of the sample. An essential part of the apparatus is the thermostat. Its design principles and key features are given together with some observed performance characteristics of a prototype: + or - 20 microK stability for 18 hours, and one minute programmed step response for steps less than 10 mK.

Gammon, Robert W.↗

NASA NTRS1988

The anomalous C 4 intensity ratio in symbiotic stars

The C IV lambda lambda 1548.2,1550.8 resonance doublet in a symbiotic stars was shown to exhibit anomalous line intensity ratios in which I (lambda 1548.2)/I(lambda 1550.8) less than 1, or less than the optically-thick limit of unity. The R Aquarii-central HII region and RX Puppis exhibit this phenomena. The I(lambda 1548.2)/I(lambda 1550.8) ratio in RX Puppis is found to vary inversely with the total C IV line intensity, and with the FES-visual light, as the object declined over a 5 yr period following a brightening in UV and optical emission which peaked in 1982. This doublet intensity behavior could be explained by a wind which has a narrow velocity range of 600 approx. less than sup v wind approx. less than 1000 km/sec, or by the pumping of the Fe II (mul. 45.01) transition a sup 4 F sub 9/2 - y sup 4 H(o) sub 11/2 by C IV lambda 1548.2, which effectively scatters C IV photons into the Fe II spectrum in these objects.

Michalitsianos, A. G.↗

NASA NTRS1989

Spectral line profiles of nickel and argon in supernova 1987A - Expansion velocity and electron scattering effects

Spectra of SN 1987A showing the Ni II 6.634-micron and Ar II 6.983-micron fine-structure lines were obtained from the Kuiper Airborne Observatory in April 1988. The signal-to-noise ratio of 100 near the peaks and resolving power of 200 are sufficient to show the average velocity of expansion from the core of about 1400 km/s and to indicate the range of velocities. An asymmetry in the profiles of both lines and a redshift of the line centroids of about 440 km/s above the 280-km/s recessional velocity of the LMC can be explained in terms of scattering of the photons by electrons in the expanding hydrogen envelope of the supernova. A mass of 0.0030 solar masses of Ni II can be deduced from the line strength of the Ni II line and a mass of 0.0009 solar masses of Ar II from the Ar II line strength.

Witteborn, F. C.↗

NASA NTRS1991

A nonthermal radio halo surrounding M82

Radio maps at several frequencies are presented which reveal an extended halo of nonthermal emission surrounding the starburst galaxy M82. The origin of this halo is probably synchrotron radiation from relativistic electrons originally produced in SNR, swept out of the disk by the extensive wind associated with the galaxy. The halo appears to be asymmetrical, with an enhancement occurring about 1 arcmin to the south, and on a larger scale there is excess emission to the north. Possible causes of this effect are addressed. The spectral index measured using data at 6, 20, 49, and 90 cm is about -0.4 + or - 0.1 in the nucleus, steepening to about -1.0 in the halo. A simple model is invoked which involves outward convection of relativistic particles with electron energy losses by inverse Compton scattering against IR photons and adiabatic expansion.

Seaquist, E. R.↗

NASA NTRS1991

Do clusters of galaxies affect the spectrum of the microwave background?

The distortion, averaged over the sky, due to the Compton scattering of background photons with electrons in the hot gas in clusters of galaxies is calculated. Using an existing sample of X-ray clusters, various values of the density parameter Omega, and plausible models for cluster evolution, Monte Carlo realizations of the microwave sky are generated. The spatial structure of these simulations shows a network of discrete sources whose properties can be a strong function of both evolution and Omega. The amount of spectral distortion in the models is greatest for models characterized by self-similar cluster evolution in an open universe and is within an order of magnitude of the current upper limits. Thus, improved observational sensitivity must inevitably detect some deviation from a blackbody spectrum.

Markevitch, M.↗

NASA NTRS1990

Scientific applications of frequency-stabilized laser technology in space

A synoptic investigation of the uses of frequency-stabilized lasers for scientific applications in space is presented. It begins by summarizing properties of lasers, characterizing their frequency stability, and describing limitations and techniques to achieve certain levels of frequency stability. Limits to precision set by laser frequency stability for various kinds of measurements are investigated and compared with other sources of error. These other sources include photon-counting statistics, scattered laser light, fluctuations in laser power, and intensity distribution across the beam, propagation effects, mechanical and thermal noise, and radiation pressure. Methods are explored to improve the sensitivity of laser-based interferometric and range-rate measurements. Several specific types of science experiments that rely on highly precise measurements made with lasers are analyzed, and anticipated errors and overall performance are discussed. Qualitative descriptions are given of a number of other possible science applications involving frequency-stabilized lasers and related laser technology in space. These applications will warrant more careful analysis as technology develops.

Schumaker, Bonny L.↗

NASA NTRS1992

Resonant Compton cooling and annihilation line production in gamma-ray bursts

Attention is given to a synchrotron self-Compton emission model for gamma-ray bursts which produces narrow annihilation features for a variety of field strengths, primary electron injection energies, and injection rates. In this model, primary electrons are injected and cooled by synchrotron emission in a strong, homogeneous magnetic field, resulting in a pair cascade. Multiple resonant scattering with cyclotron photons efficiently traps and cools pairs in the ground state to an average energy where the Compton energy loss rate is zero, which is in agreement with previous estimates of a Compton temperature. The particle distributions in the ground state are determined by numerically solving the Fokker-Planck equation in the steady state. In the case of isotropic injection of primary electrons, a significant narrow-line feature appears in the overall emission. In the case of beamed injection, the annihilation line is broadened to the extent that it would not be observable.

Preece, R. D.↗

NASA NTRS1991

Bremsstrahlung emission of the SN1987A interior

The down-scattering of gamma photons stores a significant fraction of the total energy in recoil electrons. This paper presents the bremsstrahlung emission of those recoil electrons in SN1987A. If the bolometric luminosity derives from radioactive decay, the bremsstrahlung emissivity must be present, but has not previously been treated. Initially it was our hope to account for the Ginga 16-28 keV measurement of SN1987A or to find a detectable X-ray luminosity near 1 keV for ROSAT. Although this bremsstrahlung emission fails to explain Ginga fluxes, it dominates the X-ray fluxes below 20 keV and can have detectable consequences for the X-ray lines from circumstellar matter. Results for day 175, 500, 1000, and 1500 are shown.

The, Lih-Sin↗

NASA NTRS1992

The expected high-energy to ultra-high-energy gamma-ray spectrum of the Crab Nebula

The inverse Compton scattering model for the unpulsed TeV emission from the Crab Nebula is reexamined using the magnetic field distribution derived from MHD flow models of the nebula. It is shown that the observed flux can be explained if the average nebular field is indeed about 0.0003, as is predicted by the spectral break between radio and optical. The brightness distribution of the TeV gamma-ray signal is expected to extend out to about 1.5 arcmin from the pulsar. The present estimates predict a steady flux of unpulsed ultrahigh-energy gamma-rays due to the inverse Compton scattering of soft photons by shock-accelerated electrons and/or positrons in the vicinity of the shock.

De Jager, O. C.↗

NASA NTRS1992

Angle-dependent Green's functions for relativistic Compton reflection

Using analytic methods, we derived the first-order Green's functions, in terms of the observing angle, for the reflected spectra after a single Compton scattering of relativistic photons by a plane-parallel semiinfinite medium of cold electrons. We also calculated the total Green's functions for incident photons of 511 keV, using Monte Carlo methods. We show that both the first-order and total Green's functions have strong angular dependences and that the first-order Green's functions play a dominant role in determining the properties of the total reflected spectra.

Hua, Xin-Min↗

NASA NTRS1993

Relativistic electron-positron beams in gamma-ray bursters

Beams of relativistic electrons and/or positrons leaving the surface of a strongly magnetized neutron star may give rise to gamma-ray bursts. The beams could be accelerated by strong, magnetically aligned electric fields that are produced by oscillations of the stellar surface. Here we investigate the particle acceleration in these electric fields, the resulting electron-positron pair cascade, and the gamma-ray emission. We find that beams of electrons and positrons moving parallel to the magnetic field are generated, with a reported differential energy distribution. These beams produce the bulk of the gamma-ray burst radiation below about 1 MeV by the resonant Compton scattering of thermal photons emitted from the stellar surface. The escaping synchrotron radiation from the cascade dominates the radiation spectrum above about 1 MeV.

Smith, I. A.↗

NASA NTRS1994

Compton scattering of the microwave background by quasar-blown bubbles

At least 10% of quasars drive rapid outflows from the central regions of their host galaxies. The mass and energy flow rates in these winds are difficult to measure, but their kinetic luminosities probably exceed 10(exp 45) ergs/s. This kind of outflow easily sunders the interstellar medium of the host and blows a bubble in the intergalactic medium. After the quasar shuts off, the hot bubble continues to shock intergalactic gas until its leading edge merges with the Hubble flow. The interior hot gas Compton scatters microwave background photons, potentially providing a way to detect these bubbles. Assuming that quasar kinetic luminosities scale with their blue luminosities, we integrate over the quasar luminosity function to find the total distortion (y) of the microwave background produced by the entire population of quasar wind bubbles. This calculation of y distortion is remarkably insensitive to the properties of the intergalactic medium (IGM), quasar lifetimes, and cosmological parameters. Current Cosmic Background Explorer (COBE) limits on y constrain the kinetic luminosities of quasars to be less than several times their bolometric radiative luminosities. Within this constraint, quasars can still expel enough kinetic luminosity to shock the entire IGM by z = 0, but cannot heat and ionize the IGM by z = 4 unless omega(sub IGM) much less than 10(exp -2).

Voit, G. Mark↗