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NASA NTRS1986

Modeling of coronal X-ray emission from active cool stars. I Hyades cluster

X-ray pulse height spectra of the most active cool stars in the Hyades cluster obtained with the Einstein IPC cannot be satisfactorily fitted using isothermal thin plasma emission models. Addition of a second isothermal component provides acceptable fits. However, a more physically meaningful set of coronal parameters is provided by models which consist of an ensemble of loops wih a single maximum temperature, but with the temperature distribution within the loop determined by loop physics. Such models have been successfully fitted to the IPC pulse height spectra. Constraints on loop parameters are discussed for the F-G dwarfs BD + 14 deg 693, BD + 14 deg 690, BD + 15 deg 640, and 71 Tau. Models with a large variation of loop cross section from base to top do not fit the data. A consistent physical description is an ensemble of small high-pressure loops of similar maximum temperature which dominate the coronal X-ray spectrum.

Stern, R. A.↗

NASA NTRS1987

Quasar energy distributions. I - Soft X-ray spectra of quasars

As the initial stage of a study of quasar energy distributions (QEDs), Einstein IPC spectra of 24 quasars are presented. These are combined with previously reported IPC spectra to form a sample of 33 quasars with well-determined soft X-ray slopes. A correlation analysis shows that radio loudness, rather than redshift or luminosity, is fundamentally related to the X-ray slope. This correlation is not followed by higher energy spectra of active galaxies. Two components are required to explain both sets of results. The best-fit column densities are systematically smaller than the Galactic values. The same effect is not present in a sample of BL Lac objects, implying that the effect is intrinsic to the quasars and is caused by a low-energy turnup in the quasar spectra.

Wilkes, Belinda J.↗

NASA NTRS1988

An X-ray perspective on quasar continua

Two of the major continuum features seen in normal quasars are discussed on the basis of new Einstein IPC spectral data. Particular attention is given to the possibility of a single IR-to-X-ray 'baseline' power law and to the connection between radio-loudness and the presence or absence of a turn-up in the X-ray spectrum toward higher energies. It is found that radio-loud quasars have systematically flatter X-ray spectra than do radio-quiet quasars in the present Einstein IPC survey which may be due to a mix of two components. These are a radio-associated component with a flat slope and an optical-associated component with a steeper slope.

Elvis, Martin↗

NASA NTRS1988

The scattered solar X-ray background in low earth orbit

In order to interpret X-ray observations of the sun-lit earth obtained with the IPC, a detailed model is developed that treats the radiative transfer of solar X-rays in single scattering approximation and employs the relevant scattering processes (elastic Thomson scattering and inelastic fluorescent scattering). The X-ray-bright earth, one of the strongest soft X-ray sources seen by the IPC, can then be understood in terms of solar X-rays scattered in the upper atmosphere. Using the CIRA 1972 Reference Atmosphere, it is possible to account for the observed 'bright earth' X-ray light curves under a variety of different viewing geometries. It is argued that the observed changes in hardness ratio of the scattered radiation can be interpreted as an indication of a change in the ratio of Thomson and fluorescently scattered photons as a function of zenith angle. The relevance of bright earth X-ray observations is further discussed in the context of operating X-ray telescopes in low-earth orbit, as well as in a broader astrophysical context.

Fink, H. H.↗

NASA NTRS1989

The complex X ray spectra of AGN

X-ray spectral surveys of large samples of Seyfert galaxies are discussed. The spectral shape in the 0.1 to 20 keV energy range is considered. Two new spectral survey are undertaken, one involving 105 Imaging Proportional Counter (IPC) observations of 75 Seyfert galaxies, the other using IPC and Monitor Proportional Counter (MPC) data from 28 observations of 23 Seyfert galaxies. The X-ray spectra of active galactic nuclei (AGN) are complex, with in most cases considerable steepening at the lowest energies. At higher energies (2 to 20 keV), the existence of a universal, canonical power law is confirmed, independent of X-ray luminosity over four orders of magnitude.

Urry, C. M.↗

NASA NTRS1989

Spatial and spectral features of soft diffuse X ray background seen by the Einstein observatory

A survey of the diffuse soft X-ray background as seen directly by the Einstein Observatory Imaging Proportional Counter (IPC) is presented. A source free region of the detector 1 by 1 degree field is used. The background in the 0.16 to 3.5 keV spectral region is viewed. The data covers roughly 5 percent of the sky, with some bias in coverage towards the galactic plane. The moderate energy resolution of the IPC enables the characterization and the production of maps of the background as a function of energy within the Einstein passband. The results are compared with previous observations of the diffuse X-ray background. The implications for galactic structure and for the soft component of the extragalactic X-ray background are discussed.

Micela, G.↗

NASA NTRS1989

The soft X ray excess in Einstein quasar spectra

The soft X-ray excess component is studied for a signal to noise limited subsample of 14 quasars from the WE87 sample observed with the Einstein Imaging Proportional Counter (IPC). Detailed analysis of the IPC data, combined with Einstein Monitor Proportional Counter (MPC) data where possible, and use of accurate galactic N sub H values allows estimation of the strength of any excess and improvement of constraints on the spectral slope at higher X-ray energies. A significant excess in 9 of the 14 objects is found. It is confined in all but one case to below 0.6 keV and variable in the two cases where there are multiple observations. The relation of the soft excess to other continuum properties of the quasars is investigated.

Wilkes, Belinda J.↗

NASA NTRS1991

The CfA Einstein Observatory extended deep X-ray survey

All IPC exposures in the Einstein Extended Deep X-ray Survey program have been reanalyzed. The current survey covers about 2.3 sq deg with a typical limiting sensitivity of about 5 x 10 to the -14th ergs/sq cm/s in the energy range from 0.8-3.5 keV. A total of 25 IPC sources are detected above a threshold of 4.5 sigma. A total of 18 are detected independently in the HRI, leading to the identification of six with stars and 11 with extragalactic objects. The remaining sources are classified as extragalactic. The population of identified extragalactic objects is dominated by QSOs, with one or two possible clusters. The basic conclusions of the original survey remain unchanged.

Primini, F. A.↗

NASA NTRS1991

A complete database for the Einstein imaging proportional counter

A complete database for the Einstein Imaging Proportional Counter (IPC) was completed. The original data that makes up the archive is described as well as the structure of the database, the Op-Ed analysis system, the technical advances achieved relative to the analysis of (IPC) data, the data products produced, and some uses to which the database has been put by scientists outside Columbia University over the past year.

Helfand, David J.↗

NASA NTRS1992

Ultrasoft x ray bursts: A clue to the origin of gamma ray bursts?

A search was conducted for short timescale (less than or approx. 10 s) faint X ray bursts in the complete 201,000,000 photon Einstein Observatory imaging proportional counter (IPC) data base. Some 73,000,000 events were extracted which were distributed among 11,230 observing intervals with an average observing time of approx. 1400 seconds. To exclude all sources, only those spatial pixels were searched with a count rate less than 6 x 10(exp -4) counts/sec sq arcmin. The mean count rate was much less. All files with less than 400 secs of observing time were excluded. The data was then binned into overlapping cells 4'.3 by 4'.3 by 10 secs in volume, and all the space-time cells were searched to locate cells with more than 5 counts in a 10 sec interval; i.e., a minimum flux enhancement of 50 over the mean rate for that point in space. Some 52 such events were found. An annulus was inspected around the event to see if it coincided with a general rise in the counter's event rate. Ten events did coincide with counter phenomena; these were associated with sunrise, sunset, or the approach of the South Atlantic Anomaly and were excluded. The remaining 42 events were then examined in an attempt to associate them with instrumental or environmental artifacts. The event occurrence times are uniformly distributed over the course of the mission and show no correlation with local solar time; their positions are randomly distributed in geographic, Galactic, and celestial coordinates. Their summed radial distribution is consistent with the point response function of the IPC for a soft source, indicating that the events are focused by the Observatory's mirror system. The integrated spectrum is, indeed, quite soft and is very similar to that of the cataclysmic variable star U Gem in outburst. Most of the events have risetimes of less than or approx. 1 sec and decay times ranging from 1 to 20 sec. Their positions are not associated with any class of catalogued objects. Several possible origins are discussed for the events; if they are associated with gamma ray bursters, they provide interesting constraints on the space density and repetition rate of the underlying source population.

Helfand, D. J.↗

NASA NTRS1992

Reanalysis of x ray emission from M87 II: Multiphase models

In a previous paper by the authors (hereafter TB), it was shown that the hot intracluster gas around M87 could not be adequately explained in terms of a spherically symmetric, single phase model. It was found that although data from the Einstein satellite High Resolution Imager (HRI), Imaging Proportional Counter (IPC), and Focal Point Crystal Spectrometer (FPCS) could indeed be simultaneously explained by a single phase model, data from the Solid State Spectrometer (SSS) and optically determined mass estimates could not be similarly explained. A qualitative discussion was then given to indicate how the adoption of a specific multiphase model could plausibly improve upon the single phase model, but a detailed discussion was delayed until now. The x ray data is reanalyzed for the gas around M87 in the same spirit as the work in TB. That is, by assuming several cooling flow models, the shortcomings are improved upon. The procedure of TB was adopted and the data reanalyzed in the following manner: a multiphase model is assumed which can compute the surface brightness as seen by HRI and IPC. Also, the line fluxes can be computed of the lines seen by the FPCS and the mass profile after assuming hydrostatic equilibrium. The parameters of the model are then adjusted to fit the data. A check is then made to see whether the resulting model is consistent with the SSS spectrum and the equivalent width of the 7 keV complex of Fe lines as seen by large field of view instruments.

Tsai, John C.↗

NASA NTRS1992

X-ray properties of bright far-infrared galaxies

The Einstein IPC was searched for observations of galaxies in the IRAS BCS. Of the 324 galaxies in this sample, 81 were observed with the IPC, and 51 were detected at a confidence level greater than 3-sigma. A strong correlation was found to exist between the FIR and X-ray luminosities among the normal and starburst (NSB) galaxies with LX varies as Lfir exp 0.90-1.01 (90-percent confidence limits). The X-ray luminosity of the 10 Seyfert galaxies in this sample is a factor of approximately 10 greater than the NSB galaxies with the same FIR luminosity. The efficiency of X-ray production through star formation is found to be nearly independent of the star formation rate with 10 exp 47.5 ergs of energy produced for each solar mass consumed into stars. It is shown that the X-ray emission from bright FIR galaxies can make a significant contribution to the observed X-ray background.

David, L. P.↗

NASA NTRS1992

The Einstein Slew Survey

A catalog of 819 sources detected in the Einstein IPC Slew Survey of the X-ray sky is presented; 313 of the sources were not previously known as X-ray sources. Typical count rates are 0.1 IPC count/s, roughly equivalent to a flux of 3 x 10 exp -12 ergs/sq cm s. The sources have positional uncertainties of 1.2 arcmin (90 percent confidence) radius, based on a subset of 452 sources identified with previously known pointlike X-ray sources (i.e., extent less than 3 arcmin). Identifications based on a number of existing catalogs of X-ray and optical objects are proposed for 637 of the sources, 78 percent of the survey (within a 3-arcmin error radius) including 133 identifications of new X-ray sources. A public identification data base for the Slew Survey sources will be maintained at CfA, and contributions to this data base are invited.

Elvis, Martin↗

NASA NTRS1992

A deep image of the diffuse soft X-ray background at the Galactic plane

A deep Einstein IPC image of the Galactic plane provides important information about the Galactic diffuse X-ray background. Free from contamination by sources with fluxes greater than about 4 x 10 exp -14 ergs/sq cm s and by X-rays from beyond the Galactic plane, the intensity and spectrum of the background at the Galactic plane are measured for the first time in the 0.16-3.5 keV band. The spectrum can be characterized by a thermal hot gas model with a temperature of about 4,000,000 K, and the M-band Galactic flux is about 3 x 10 exp -8 ergs/sq cm s sr. This background is enhanced in the M band at the Galactic plane, but it is keV regime. If discrete sources are to be responsible for the background, their surface density must be much greater than one source per resolution element of the IPC (i.e., about 0.4 sources/sq arcmin) because of the lack of a correlation among observed X-ray on scales comparable to the instrument resolution. This density constraint, together with a model study of the Galactic emission, leads to the conclusion that no known stellar population could be responsible for the bulk of the emission.

Wang, Qingde↗

NASA NTRS1992

An Einstein Observatory SAO-based catalog of B-type stars

About 4000 X-ray images obtained with the Einstein Observatory are used to measure the 0.16-4.0 keV emission from 1545 B-type SAO stars falling in the about 10 percent of the sky surveyed with the IPC. Seventy-four detected X-ray sources with B-type stars are identified, and it is estimated that no more than 15 can be misidentified. Upper limits to the X-ray emission of the remaining stars are presented. In addition to summarizing the X-ray measurements and giving other relevant optical data, the present extensive catalog discusses the reduction process and analyzes selection effects associated with both SAO catalog completeness and IPC target selection procedures. It is concluded that X-ray emission, at the level of Lx not less than 10 exp 30 ergs/s, is quite common in B stars of early spectral types (B0-B3), regardless of luminosity class, but that emission, at the same level, becomes less common, or nonexistent, in later B-type stars.

Grillo, F.↗

NASA NTRS1992

The x ray properties of a large, uniform QSO sample: Einstein observations of the LBQS

Although there are large numbers of Quasi Stellar Objects (QSO's) now observed in X rays, extensive X-ray observations of uniformly selected, 'complete' QSO samples are more rare. The Large Bright QSO Survey (LBQS) consists of about 1000 objects with well understood properties, most brighter than B = 18.8 and thus amenable to X-ray detections in relatively brief exposures. The sample is thought to be highly complete in the range 0.2 less than z less than 3.3, a significantly broader interval than many other surveys. The Einstein IPC observed 150 of these objects, mostly serendipitously, during its lifetime. We report the results of an analysis of these IPC data, considering not only the 20 percent of the objects we find to have positive X-ray detections, but also the ensemble X-ray properties derived by 'image stacking'.

Margon, B.↗

NASA NTRS1992

Where have all the sources gone?

In the continuing quest for an understanding of the cosmic x-ray background, the most difficult technical barrier remains the mismatch between the wavelength band in which the discrete source populations are best characterized (0.1 to 3.5 keV), and the band in which the spectrum of the background is well determined and the bulk of the energy density resides (3.5 to 50 keV). A progress report in an attempt to resolve this conundrum using the Einstein Imaging Proportional Counter (IPC) database and analysis system is presented. The IPC data base is described and how it can be used to search both for missing clusters of galaxies and highly obscured active galactic nuclei is addressed. All 4083 imaging proportional counter observations were examined after flat fielding and editing to optimize sensitivity for diffuse emission on various size scales ranging up to 10 feet. A '2 sigma source' catalog which, when filtered by matching with source catalogs from other wavelength regimes, is shown to reveal new source classes which may be significant contributors to the cosmic x-ray background is being constructed. Optimization techniques are described and the source catalogs which result are presented. Limits on the contributions both classes of sources make to resolving the discrepancy with the higher energy observations are set.

Helfand, D. J.↗

NASA NTRS1992

An imaging study of the Galactic diffuse soft X-ray background

A study is performed of the Galactic diffuse soft X-ray background with the data collected with the imaging proportional counter (IPC) on board the Einstein X-ray Observatory. It is demonstrated that the IPC imaging data can provide important information on the nature of the background and significant constraints on the physical properties of the hot ISM component. The estimated pressure and density in the coronal gas are consistent with both theoretical arguments and UV observations made with IUE, HST, and rocket tests.

Wang, Qingde↗