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NASA NTRS1981

Two-phase models of quasar emission line regions

It is demonstrated that the emitting gas in clouds of quasar emission line regions must be confined by a hot intercloud medium, provided only that the heating mechanisms are strong enough to drive the low-density intercloud gas above a few tens of millions degrees K. The study of the thermal properties of the gas presented includes heating by photoionization, Compton scattering, suprathermal particles, absorption of radio frequency radiation, cloud friction, thermal conduction, and shocks. Cooling curves for photoionized gases are presented, and phase diagrams analogous to the pressure-temperature diagrams used in studying the interstellar medium are constructed for various conditions. It is shown that two-phase equilibria occur over a wide range of mean density, but over a much narrower range of pressure. The implications of these results for the emission line region are discussed, and it is shown that the emission clouds may be short-lived.

Krolik, J. H.↗
NASA NTRS1981

The high energy X-ray spectrum of 4U 0900-40 observed from OSO 8

The X-ray source 4U 0900-40 (= Vela XR-1) was observed with the high-energy X-ray spectrometer on OSO 8 for one week in 1976 and three weeks in 1978. Spectra of the source are presented above 16 keV. No systematic difference exists between the X-ray eclipse centers and the eclipse centers predicted from optical ephermerides. Short period intrinsic variability in the system's X-ray intensity may be related to changes in the Compton scattering optical depth in the system and does not require sporadic mass transfer via Roche lobe overflow. The 282 s modulation in the source's X-ray flux above 21 keV consists of two essentially similar pulses per period, most easily interpreted as arising from the two different magnetic poles of a rotating neutron star. The secondary appears to be a spherically accreting, magnetic neutron star.

Dolan, J. F.↗
NASA NTRS1981

The correlation of X-ray emission with strong millimeter activity in extragalactic sources

A sample of 25 strong extragalactic millimeter sources have been observed at about 1 keV with the Imaging Proportional Counter on the Einstein X-Ray Observatory and at 90 GHz with the NRAO 11 m telescope. A strong correlation between the millimeter and X-ray flux densities is found. This result is consistent with a synchronous self-Compton model in which the X-ray emission arises from the Compton scattering of millimeter radio photons off the relativistic electrons which created them.

Owen, F. N.↗
NASA NTRS1982

On intracluster Faraday rotation. II - Statistical analysis

The comparison of a reliable sample of radio source Faraday rotation measurements seen through rich clusters of galaxies, with sources seen through the outer parts of clusters and therefore having little intracluster Faraday rotation, indicates that the distribution of rotation in the former population is broadened, but only at the 80% level of statistical confidence. Employing a physical model for the intracluster medium in which the square root of magnetic field strength/turbulent cell per gas core radius number ratio equals approximately 0.07 microgauss, a Monte Carlo simulation is able to reproduce the observed broadening. An upper-limit analysis figure of less than 0.20 microgauss for the field strength/turbulent cell ratio, combined with lower limits on field strength imposed by limitations on the Compton-scattered flux, shows that intracluster magnetic fields must be tangled on scales greater than about 20 kpc.

Lawler, J. M.↗
NASA NTRS1982

Detection of X-rays during the outburst of supernova 1980k

X-ray emission from SN 1980k in NGC 6946 was detected about 35 days after maximum light using the imaging proportional counter on the Einstein Observatory. The absorption corrected X-ray flux of SN 1980k was about 0.03 micro-Jy at 0.24 x 10 to the 18th Hz (1 keV), which corresponds to a luminosity of about 2 x 10 to the 39th ergs/s (0.2-4 keV) at 10 Mpc. This detection of radio emission suggests the presence of relativistic electrons and a magnetic field strength of not greater than about 1 gauss. Inverse Compton scattering of the optical photons on the electrons can explain the X-ray emission without additional ad hoc assumptions, although other emission mechanisms are possible.

Canizares, C. R.↗
NASA NTRS1982

Processes in relativistic plasmas

The establishment and maintenance of a Boltzmann distribution in particle kinetic energies is investigated for a plasma with theta = KTe/mc-squared much greater than unity, where m is the electron mass. It is shown that thermalization of the electron gas by binary collisions is not sufficiently effective to maintain the equilibrium distribution when other processes that perturb the equilibrium are taken into account. Electron-positron pair production in electron-electron and electron-ion collisions, and perturbations of a Boltzmann distribution by nonthermal processes are evaluated. Thermalization by means of other mechanisms, such as interaction with plasma waves is discussed, and the opacity of a relativistic plasma is computed for Compton scattering, pair production in the fields of electrons and ions, inverse bremsstrahlung, and synchrotron self-absorption.

Gould, R. J.↗
NASA NTRS1981

The cosmic microwave background radiation

Because angular anisotropies and spectral distortions of the cosmic microwave background radiation are judged to be inevitable at some level, in a realistic cosmological model, the evidence for spectral distortions and its theoretical implications are described. The evidence for anisotropy is then discussed, and theoretical predictions of radiation anisotropy are summarized and compared with the data available. It is found that spectral distortions at the 3-sigma level near the peak of the blackbody spectrum, although inconsistent with the predicted distortions due to Compton scattering in the early universe, are elegantly interpreted in terms of radiation from an early, pregalactic generation of massive stars which had been thermalized by a modest amount of dust at high redshift. The quadrupole anisotropy at the 4-sigma level is most simply interpreted in terms of the large-scale structure of the universe.

Silk, J.↗
NASA NTRS1982

High-resolution X-ray observations of M87 - Nucleus, jet and radio halo

X-ray observations of M87 made using the Einstein Observatory HRI show the presence of four components. A broad, diffuse component associated with hot gas in the galaxy follows an approximate inverse power law; the pressure of the gas could be sufficient to confine the inner and outer radio lobes. A diffuse, asymmetrical X-ray distribution to the east and southwest appears similar to the radio morphology and is consistent with an inverse Compton model. An X-ray source centered on the nucleus of the galaxy is extended by 2-3 arcsec and may be due to thermal bremsstrahlung; a possible compact source of less than 10 to the 41st ergs/sec is weak compared with other active galaxies, and its implications for the presence of a massive black hole are discussed. A structure correlated with the radio-optical jet is also seen; the X-ray flux is consistent with an extrapolation of the optical synchrotron spectrum. The four components suggest that all three canonical astrophysical X-ray emission mechanisms are present in M87: thermal bremsstrahlung, inverse Compton scattering, and synchrotron radiation.

Schreier, E. J.↗
NASA NTRS1982

On the interpretation of gamma-ray burst continua and possible cyclotron absorption lines

It is pointed out that most gamma-ray bursts have spectra that are fit by an optically thin thermal bremsstrahlung shape. The small emitting volume implied by the observation of spectral features identified as cyclotron absorption lines from a 10 to the 12th Gauss field is shown to prevent thermal bremsstrahlung from providing sufficient luminosities to account for the observed fluxes unless the optical depth to Compton scattering is large. It is also demonstrated that complicated detector response functions can introduce artifacts that mimic absorption features below 60 keV unless the absolute gain of the detector is established and correctly applied in analysis. Comparing the continuum as observed by the Berkeley/Los Alamos ISEE-3 gamma-ray detector with the continuum observed by the Konus (Mazets et al. 1981) experiments, a lack of agreement is found, suggesting that the gain of one of these experiments may be incorrect.

Fenimore, E. E.↗
NASA NTRS1983

Gamma ray detection with long NaI/Tl/ scintillator bars

Test measurements with a prototype NaI(Tl) scintillator for energy, position, and timing measurements in gamma ray astronomy are reported. The scintillator bar is 100 x 5 x 5 cu cm in size, and allows detection of the arrival times and pulse heights of signals from two photomultiplier tubes, one at each end of the bar. Data is gathered on the energy loss, linear position, and time-of-flight of gamma ray interactions within the bar over an energy range of 0.5-20 MeV. A mean attenuation coefficient of 0.015/cm has been determined, as have a FWHM resolution of 5 cm, 9.4%, and 10 nsec at an energy of 0.662 MeV. At 1.25 MeV the timing resolution was 6 nsec, and at 6.13 MeV the spatial resolution was 2.2 cm. The instrument is a prototype of a Compton scatter telescope being constructed for two balloon flights, one each in the Northern and Southern Hemispheres, in 1984.

Zych, A. D.↗
NASA NTRS1982

The galactic center; Proceedings of the Workshop, California Institute of Technology, Pasadena, CA, January 7, 8, 1982

Observations and theoretical models of the galactic center are discussed, together with future observational capabilities. Radio observations of the molecular gas clouds and continuum around Sgr A are reported, as are IR continuum observations of the central 30 arcmin of the galaxy. IR data at 150, 200, and 300 microns are also cited, together with balloon-borne near-IR observations of the galactic central bulge. The galactic center has been examined with IR spectroscopy to characterize the ionized gas, the B-alpha and Ne II lines, the O I and O III lines in Sgr A, and the 10 micron emissions. Gamma ray observations have indicated the presence of positron annihilation, and further studies have been made in the 0.3-1.0 MeV range. Models for the compact source at the galactic center have considered accreting black holes, an accretion disk featuring Compton scattering to emit the positron radiation, and a protostar in the center of Sgr A. The gas motions and the potential triaxiality of the galactic center are discussed, and future NASA, ESA, Franco-Soviet, and Japanese spacecraft for X ray and gamma ray detection and mapping are described.

Riegler, G. R.↗
NASA NTRS1983

Radio and X-ray observations of the radio halo source in A1367

Radio and X-ray observations of the 'radio halo' in A1367 are presented. The radio spectral index distribution is derived combining data at 610 and 1415 MHz taken with the Westerbork Synthesis Radio Telescope (WSRT). A high-resolution Very Large Array (VLA) map allows us to discriminate between the extended cluster emission and a triple radio source associated with a background stellar object (most probably a QSO) which appears projected onto it. X-ray observations taken with the high-resolution imager on board the Einstein satellite reveal an X-ray source associated with this background object. A second weak X-ray enhancement is found projected onto the extended radio component. Inverse Compton scattering of the synchrotron-emitting particles provides a possible interpretation of such a source. The presence of large-scale magnetic fields and particle acceleration due to the turbulent motion of galaxies through the intergalactic medium is discussed in relationship to the observed peculiarities of the galaxies found in this region.

Gavazzi, G.↗
NASA NTRS1983

A search for the Sunyaev-Zel'dovich effect at millimeter wavelengths

It is believed that X-ray emission from clusters of galaxies represents thermal bremsstrahlung from a hot plasma. According to Sunyaev and Zel'dovich (1972), the plasma column density and temperature derived from this model imply a measurable distortion of the cosmic background radiation (CBR) in the cluster direction. This distortion results from the Compton scattering of the CBR photons by the electrons in the plasma, resulting in an average increase of each photon. This process, known as the Sunyaev-Zel'dovich effect, is photon conserving and 'shifts' the CBR spectrum to higher frequencies. The result is a decrease of flux at frequencies below 7.5 per cm (the Rayleigh-Jeans region), and an increase above. The investigation is concerned with measurements of the Sunyaev-Zel'dovich effect at frequencies in the range from 3 to 10 per cm. Attention is given to the employed observing and analysis technique, and an initial null result for the cluster Abell 1795.

Meyer, S. S.↗
NASA NTRS1984

Photon-photon absorption and the uniqueness of the spectra of active galactic nuclei

The effects of the feedback of e(+)-e(-) pair reinjection in a plasma due to photon-photon absorption of its own radiation was examined. Under the assumption of continuous electron injection with a power law spectrum E to the minus gamma power and Compton losses only, it is shown that for gamma 2 the steady state electron distribution function has a unique form independent of the primary injection spectrum. This electron distribution function can, by synchrotron emission, reproduce the general characteristics of the observed radio to optical active galactic nuclei spectra. Inverse Compton scattering of the synchrotron photons by the same electron distribution can account for their X-ray spectra, and also implies gamma ray emission from these objects. This result is invoked to account for the similarity of these spectra, and it is consistent with observations of the diffuse gamma ray background.

Kazanas, D.↗
NASA NTRS1984

Observations of NGC 4151 at 2 keV to 2 MeV from HEAO 1

Broad-range X-ray and gamma-ray spectra of the Seyfert galaxy NGC 4151 obtained by the UCSD/MIT and GSFC/CIT detectors of HEAO 1 are presented. For two out of the three observational epochs the best-fit model is a power law with photon index 1.6 + or - 0.1 in the 2 keV-2 MeV spectral range. The third observation, in June, 1978, when the intensity was a factor of 2 higher, requires a change above 50 keV from a similar power law to a steeper power law or an exponential. Through comparisons with previous observations, a weak correlation is found between the high-energy spectral index and the flux level. Comparison of these data and other X-ray and gamma-ray observations with the spectra expected from several possible emission mechanisms supports a synchrotron-self-Compton model using infrared target photons or a Compton-scattering process involving X-rays of less than 10 keV energies.

Baity, W. A.↗
NASA NTRS1984

X-ray spectra and time variability of active galactic nuclei

X ray measurements taken with the HEAO-1 and -2 satellites are examined for clues to the source of emissions from active galactic nuclei (AGN). The data covered the energy range 2-100 keV and 0.5-4.5 keV and encompassed quasars, BL Lac objects, Seyfert I galaxies and broadline radio galaxies. The broadline spectra were best fit with a power law with a slope of about 0.68. The continua displayed roughly equivalent energy at all wavelengths except for radio bands. Spectral variability was detected in the BL Lac objects, which may emit optical, UV and soft X ray radiation from optically thin synchrotron emission with a hard X ray tail from inverse Compton scattering. No strong variability was evidenced with other AGN, suggesting that the emitting regions are below the Eddington luminosity and thus exhibit stable flow characteristics.

Mushotzky, R. F.↗
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 NTRS1985

Gamma rays of 0.3 to 30 MeV from PSR 0531+21

Pulsed gamma rays from the Crab Pulsar PSR 0531+21 are reported for energies of 0.3 to 30 MeV. The observations were carried out with the UCR gamma ray double Compton scatter telescope launched on a balloon from Palestine, Texas at 4.5 GV, at 2200 LT, September 29, 1978. Two 8 hr observations of the pulsar were made, the first starting at 0700 UT (0200 LT) September 30 just after reaching float altitude of 4.5 g/sq cm. Analysis of the total gamma ray flux from the Crab Nebula plus pulsar using telescope vertical cell pairs was published previously. The results presented supersede the preliminary ones. The double scatter mode of the UCR telescope measures the energy of each incident gamma ray from 1 to 30 MeV and its incident angle to a ring on the sky. The time of arrival is measured to 0.05 ms. The direction of the source is obtained from overlapping rings on the sky. The count rate of the first scatter above a threshold of 0.3 MeV is recorded every 5.12 ms. The Crab Pulsar parameters were determined from six topocentric arrival times of optical pulses.

White, R. S.↗