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NASA NTRS1979

X-ray observations of galaxies in the Virgo cluster

X-ray emission from individual galaxies (other than M87) in the Virgo cluster has been detected using observations from the Einstein X-ray Observatory. One of the galaxies, M86, exhibits extended emission which is interpreted as thermal bremsstrahlung from hot gas being stripped from the galaxy by the ram pressure of the intracluster medium. The observations are discussed in relation to models for the dynamical evolution of clusters of galaxies.

Forman, W.↗
NASA NTRS1980

The contribution of young galaxies to the X-ray background

The contribution of young galaxies to the diffuse X-ray background is estimated and two mechanisms by which young galaxies may emit hard X-rays are considered. The first mechanism is direct thermal bremsstrahlung emission from a hot galactic wind powered by supernovae. Estimating mass-loss rates from galaxies by arguments based on metallicity and on the X-ray spectroscopic observations of rich clusters, it is shown that galactic winds may contribute substantially to the background. The second mechanism relies on the decreased metallicity of young galaxies to increase the number of supergiant stars and hence the numbers of hard X-ray binaries. This effect quantitatively explains the relatively large X-ray luminosity of the Magellanic clouds and also indicates that binaries in young galaxies may contribute a significant fraction of the diffuse X-ray background. Both mechanisms require that the epoch of galaxy formation be recent (redshift less than 2-3) in order to account for the observed spectral temperature (45 keV) of the hard X-ray background.

Bookbinder, J.↗
NASA NTRS1980

The high energy X-ray spectrum of 4U 1700-37 observed from OSO 8

The most intense hard X-ray source in the confused region in Scorpius has been identified as 4U 1700-37 (=HD 153919). Observations extending over three binary periods in 1978 September were carried out with the high-energy X-ray spectrometer on OSO 8. The 3.4 day modulation is seen above 20 keV with the intensity during eclipse being consistent with zero flux. The photonumber spectrum from 20 to 150 keV is well represented by a single power law with a photonumber spectral index of -2.77 + or - 0.35 or by a thermal bremsstrahlung spectrum with kT = 27 (+15, -7)keV. The counting rate above 20 keV outside of eclipse shows no evidence for the 96.8 minute X-ray modulation previously reported at lower energies. Despite the difficulties that exist in reconciling both the lack of periodic modulation in the emitted X-radiation and the orbital dynamics of the system with our currently accepted theories of the evolution and physical properties of neutron stars, the observed properties of 4U 1700-37 are all consistent with the source being a spherically accreting neutron star rather than a black hole.

Dolan, J. F.↗
NASA NTRS1980

HEAO 1 identification of the X-ray emitting poor cluster of galaxies, AWM 7

The X-ray source 2A 0251 + 413 is identified with the AWM 7 group of galaxies. The source is extended, with a FWHM of roughly 9 arcmin if an isothermal sphere profile is assumed. The spectrum is best fitted by thermal bremsstrahlung at 4.0 + or - 0.5 keV, with a significant Fe line emission of equivalent width 0.63 keV centered at 6.6 keV. This temperature is consistent with a measured dispersion velocity of 600 km/s of the central galaxies.

Schwartz, D. A.↗
NASA NTRS1980

X-ray emission from compact groups of galaxies

The existence of very compact groups, or 'poor clusters,' of galaxies as a category of X-ray source is reported. By analogy with X-ray emission from clusters of galaxies, it is assumed that the extended X-ray emission is due to thermal bremsstrahlung from hot gas in the groups. The derived X-ray temperatures, luminosities, and sizes are similar to those for X-ray emitting, rich clusters of galaxies. It is inferred that these groups are in a late evolutionary stage because of the presence of a dominant galaxy, the absence of spiral galaxies, and the short cooling time inferred for 2A 0335 + 096.

Schwartz, D. A.↗
NASA NTRS1980

Optical properties of Morgan's poor clusters

The galaxy density, spiral content, and richness of the 23 poor clusters of galaxies cataloged by Morgan and co-workers are determined and compared with those of Abell clusters. It is found that Morgan's clusters generally represent a smooth continuation of the galaxy clustering spectrum from the rich Abell clusters. This continuity in optical properties implies a corresponding continuity in X-ray properties. The extended X-ray emission recently detected from several of Morgan's clusters is consistent with the amount of thermal bremsstrahlung emission expected on the basis of the present optical observations and previous correlations of X-ray-optical cluster properties. The fraction of cluster galaxies that are spirals is a sensitive function of the galaxy surface density, N(-)0.

Bahcall, N. A.↗
NASA NTRS1980

Two component X-ray emission from RS CVn binaries

A summary of results from the solid state spectrometer on the Einstein Observatory for 7 RS CVn binaries is presented. The spectra of all require two emission components, evidenced by line emission characteristic of plasma at 4 to 8 x 10 to the 6th power and bremsstrahlung characteristic of 20 to 100 x 10 to the 6th power K. The data are interpreted in terms of magnetic coronal loops similar to those seen on the Sun, although with different characteristic parameters. The emission regions could be defined by separate magnetic structures. For pressure less than approximately 10 dynes/sq cm the low temperature plasma would be confined within the stellar radii, while the high temperature plasma would, for the synchronous close binaries, fill the binary orbits. However, for loop pressures exceeding 100 dynes/sq cm, the high temperature components would also be confined to within the stellar radii, in loops covering only small fractions of the stellar surfaces. While the radio properties and the occurrence of X-ray flares suggest the larger emission regions, the observations of time variations leave the ambiguity unresolved.

Swank, J. H.↗
NASA NTRS1981

Middle atmosphere response to measured relativistic electrons

During two independent rocket programs, Aurorozone 1 and 2 at Poker Flat Research Range, Alaska in September 1976 and March 1978 respectively, frequent penetration and absorption of relativistic electrons were observed between 40-70 km altitude. This relativistic electron source was compared with bremsstrahlung X-ray energy deposition in the same altitude domain. Correlated atmospheric electrical response was simultaneously observed. Ozone was also measured following each event to determine its departure from quiet nighttime values. Preliminary results for Aurorozone 1 show that the relativistic electrons observed on September 23, 1976 dominated X-ray bremmstrahlung as an energy source to an atmospheric depth of 55 km. Above this height, the deposition ratio rapidly grew to a value greater than 100, clearly demonstrating the dominant role of relativistic electrons as an important energy source for this lower mesospheric domain.

Barcus, J. R.↗
NASA NTRS1981

Auroral X-ray detection from rocket overflights

Remote sensing of auroral X-rays from the topside can provide both local and global information concerning X-ray and energetic particle deposition within the middle atmosphere. Usually, contamination of X-ray detectors by corpuscular radiation can severely affect the scope and accuracy of the measurement. Preliminary results obtained with a rocket-borne instrument designed to operate cleanly in a precipitating particle environment are reported. Two dimensional images of the atmospheric bremsstrahlung X-ray sources were also constructed from the detector scan produced by payload translation and coning.

Goldberg, A.↗
NASA NTRS1981

The Pinhole/Occulter Facility

A large occulting system in space can be used for high resolution X-ray observations and for large aperture coronagraphic observations in visible and UV light. The X-ray observations can combine high angular resolution in hand (10 keV) X-radiation with the high sensitivity of a multiple pinhole camera, and can permit sensitive observations of bremsstrahlung from nonthermal particles in the corona. The large aperture coronagraphs have two major advantages: high angular resolution and good photon collection. This will permit observations of small scale structures in the corona for the first time and will give sufficient counting rates above the coronal background rates for sensitive diagnostic analysis of intensities and line profiles for coronal structures in the solar wind acceleration region. The technical basis for performing observations with a large occulting system in these three wavelength ranges is described as well as a pinhole/occulter facility presently being considered for Spacelab. Some indications about future developments are included.

Hudson, H. S.↗
NASA NTRS1980

The Crimean Solar Maximum Year Workshop, selected reports

Problems associated with the transport of energy and acceleration of charged particles in solar flares are considered. Existing theories are compared with observation with a view to either discriminating between rival theories (such as whether hard X-rays are emitted by thermal or nonthermal bremsstrahlung), constraining existing theories (such as deduction of the number of nonthermal electrons present from spectroscopic diagnostics in the soft X-ray part of the spectrum), or suggesting theories (such as attempting to explain the observed spatial structure of microwave emission relative to alpha).

Emslie, A. G.↗
NASA NTRS1980

Observation with the VLA of a stationary loop structure on the sun at 6 centimeter wavelength

A looplike structure connecting two sunspots of opposite polarity in an active region has been observed at 6 cm with a resolution of 3.5 arcsec, using the Very Large Array. This loop structure is reminiscent of the X-ray loops, as observed, for example, from Skylab. The brightness temperature in the 'loop' is one-million K and five-million K near its foot points. Most of the bright peaks in the 'loop' are well aligned with a long neutral line. Several compact, highly circularly polarized emission peaks were observed over emerging flux regions near one of the spots. Some of these sources appear to be associated with arch filament systems. The low brightness emission in the 'loop' is attributed to optically thin thermal bremsstrahlung.

Kundu, M. R.↗
NASA NTRS1980

X-ray measurements during the Araks experiments

Measurements of the X-ray bremsstrahlung precipitation were made during the Araks experiments. The following conclusions were deduced from the data: with several exceptions the ambient X-ray flux observed was the normal quiet background expected for the location and existing conditions. The X-ray flux at energies below 20 keV was in excess of that predicted by theory. No obvious effects of the Araks experiments were seen in the data. However, a superposed epoch analysis of the X-ray data revealed a very small amount of precipitation associated with the electron gun.

Roeder, J. L.↗
NASA NTRS1980

Upper limits on the power in solar oscillations at 1.2 mm, 9 mm, 3.7 cm, and 11.1 cm wavelengths

A search for solar oscillations has been made using the NRAO 36-ft telescope at 1.2 mm, the NRL 85-ft telescope at 9 mm, and the NRAO four-element interferometer at 3.7 and 11.1 cm wavelengths. After corrections for the small coherence length of the optically observed oscillations, for their known spectral bandwidth, and for the visibility function of the interferometer, upper limits have been placed on the fluctuating power at oscillation frequencies near 3 mHz. The interferometric observations at 3.7 cm and the single-dish observations at 1.2 and 9 mm imply that less than 0.3, 0.04, and 0.1%, respectively, of the bremsstrahlung photons emitted from the chromosphere show periodic fluctuations. This is 1 to 3 orders of magnitude less than the fraction (approximately 1 to 2%) reported at ultraviolet wavelengths. Simple arguments in terms of weak shock theory suggest that the observable oscillations at centimeter and millimeter wavelengths should be intrinsically weaker in the thermal bremmstrahlung radiation than the optical, but the 3.7 cm wavelength results are explained only by the absence of periodic fluctuations at the level of formation.

Kundu, M. R.↗
NASA NTRS1979

Hard X-ray properties of cataclysmic variables

A brief summary is presented of results for AM Her, EX Hya, SS Cyg, and U Gem obtained with the high-energy detectors of the HEAO A2 experiment from 2-60 keV and the Solid State Spectrometer Experiment with the Monitor Proportional Control on the Einstein Observatory. It is found that the magnetic variable AM Her and the dwarf novae are similar in having bremsstrahlung of similar luminosity with temperatures for the dwarf novae that can approach that for AM Her and in flickering on a time scale of 5-15 min. But in general these dwarf novae temperatures are about 2-10 times lower. Only in Am Her is there evidence in the spectrum and eclipses for a small spot very close to the white dwarf surface. Most of the hard X-rays in SS Cyg, EX Hya, and U Gem are not pulsed, and the hard X-ray fluxes observed have never been significantly absorbed or degraded.

Swank, J. H.↗
NASA NTRS1980

X-ray variability in active galaxy nuclei and quasars in less than one day

Data obtained from the Einstein Observatory demonstrating variations in X-ray emission from the nuclei of active galaxies and quasars on time scales of hours rather than previously observed days or years is presented. Light curves obtained from the Einstein imaging proportional counter for the Seyfert 1 galaxy NGC 6814 and from the High Resolution Imager for the quasars OX 169 and 3C 273 are illustrated, and variations by factors greater than two on time scales less than 20,000 sec for the first two objects and by a factor of 10% on a time scale over 50,000 sec for 3C 273 are pointed out. The measurements are also used to determine that thermal bremsstrahlung cannot be the cause of the intensity decay in OX 169, and that, in the absence of relativistic effects, the efficiency for energy release in the matter involved in the emission of 3C 273 is at least 0.1.

Elvis, M.↗
NASA NTRS1980

X-ray spectra of supernova remnants

The X-ray spectra of supernova remnants are discussed, and their implications for the emission mechanism are considered. A model-fitting procedure for analyzing spectral information is presented, and alternative data presentations are indicated. Model spectra expected from diffuse X-ray sources, in which X-ray transfer effects are minimized, are calculated for the cases of Compton interactions, synchrotron radiation and bremsstrahlung. The emission mechanisms of supernova remnants are then considered, and it is pointed out that whereas it is certain that the X radiation from the Crab pulsar arises from synchrotron emission, the kinetic mechanisms giving rise to X-ray emission in the nonpulsar SNRs are not understood. Results of Einstein Observatory solid state spectrometer measurements of the Cas A, Tycho and Puppis A are then presented which are fit by two-temperature thermal spectra, and the Crab SNR, fit by a power-law model. Spectral trends in the older and younger SNRs are pointed out, and the role of Einstein imaging data in future detailed modeling necessary to convert apparent abundances into true abundances is indicated.

Holt, S. S.↗
NASA NTRS1981

HEAO 1 observations of the Perseus cluster above 10 keV

Results are presented of HEAO 1 observations of the Perseus cluster from 10 to 150 keV in 1977 August and 1978 February and August. The spectrum exhibits a previously unknown hard (greater than 25 keV) component in addition to the previously known thermal bremsstrahlung emission. The data presented show no significant evidence of variability from 10.5 keV to 93.5 keV, and a comparison of our results with earlier results indicates no strong evidence for variability above 25 keV over a time scale of 4 yr. If the hard-component excess is due to NGC 1275, the data imply a 2-6 keV X-ray luminosity of 1 x 10 to the 44th ergs/s for the galaxy, or about 15% of the total cluster emission from 2 to 6 keV and a 25-40 keV luminosity of 8 x 10 to the 43rd ergs/s.

Primini, F. A.↗