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NASA NTRS1993

X-ray emission at the low-mass end - Results from an extensive Einstein Observatory survey

We have used available IPC data and a critical compilation of cataloged optical data to measure the 0.16-3.5 keV X-ray emission from 88 K and 169 M stars of luminosity classes IV, V, and VI within 25 pc from the Sun. The IPC detected 54 out of the 88 K stars, 70 out of the 138 M stars with M(v) less than 13.4, and 15 out of the 31 fainter M stars. We have identified a subsample of surveyed stars that is statistically representative of the population of K and M stars in the solar neighborhood. On the basis of this subsample (1) we have shown the occurrence of a drop in the level of X-ray emission for M stars later than approximately M5; (2) we have built unbiased maximum likelihood X-ray luminosity functions for the K, early M, and late M stars; (3) we have confirmed, both for K and M stars, the decrease of X-ray luminosity with increasing stellar age in the range of ages of disk population stars: and (4) we have shown that no obvious correlation is present between X-ray and bolometric luminosities in the entire representative samples of K and M stars, but only within flare stars which also seem to mark a saturation in X-ray luminosity level.

Barbera, M.↗

NASA NTRS1994

An X-ray and optical study of the supernova remnant W44

We report the results of a 8000 s observation of the supernova remnant W44 using the ROSAT Position Sensitive Proportional Counter (PSPC). The image shows the same centrally peaked morphology observed by the Einstein IPC and contrasts with the shell-like radio morphology. The eastern limb shows a lack of X-ray emission within the radio shell, probably due to the interaction between the Supernova Remnants (SNR) and a molecular cloud. No counterpart to the pulsar 1853 + 01 in W44 has been detected, with L(sub X) less than 1.3 x 10(exp 32) ergs/s in the 0.2 to 2.4 keV band. The spectral analysis of the central part of W44, combining EXOSAT ME and Einstein SSS data, shows that the shocked plasma has not reached ionization equilibrium. The best nonequilibrium fit to PSPC, ME, and SSS spectra gives Eta = 10(exp 51) ergs cm(exp -6), T(sub s) = 10(exp 7) K with T(sub e) = T(sub i), suggesting conditions are approaching ionization equilibrium. There is no evidence of enhanced abundances of Mg, Si, S, or Fe. The variation of temperature and column density was obtained region by region using the PSPC and Einstein IPC. The temperature is largely uniform over the remnant, but strong column density variations are found to be consistent with molecular clouds in the line of sight. An evaporation model with a two-phase interstellar medium structure of clumps and interclump gas (White & Long 1991) can explain the X-ray centrally peaked morphology of W44. The clumps remaining behind a SN shock provide a reservoir of material, and evaporat e to increase the density of X-ray emitting gas in the interior of a SNR. The uniform temperature distribution of W44 strongly supports the predictions of this model. In addition, mosaiced H alpha and (S II) images of W44, taken using the prime focus universal extragalactic instrument (PFUEI) camera on the Palomar 60 sec telescope, reveal the first discovery of optical filaments (both H alpha and (S II)) in the northwestern and southeastern portion of the remnant, within the X-ray emitting region. The optical filaments and the X-ray image showing locally brighter emission and clumps along the filaments suggest both are produced by the interaction between the supernova shock front and regions of enhanced ambient density.

Rho, Jeongee↗

NASA NTRS1994

The complex optical to soft x-ray spectrum of the low-redshift radio-quiet quasars

Eight high signal-to-noise ROSAT Position Sensitive Proportional Counter (PSPC) observations of six low-redshift (o.048 less than z less than 0.155) radio-quiet quasars have been analyzed to study ant soft excess. All the spectra can, at least roughly, be described int eh 0.1-2.5 keV band by simple power laws reduced at low energies by Galactic absorption. The strong oxygen edges seen in the PSPC spectra of several Seyfert galaxies and quasars are not observed in this sample. The limits implied for the abount of absorbing gas intrinsic to the quasars are particularly tight: of the order of approximately 10(exp 20)/sq cm. THe range of energy indices is broad: 1.3 less than alpha(sub E) less than 2.3. The energy indices are systematically steeper than those found in the same sources at higher energies (by DELTA alpha(sub E) approximately 0.5-1 with respect to Ginga or EXOSAT (2-10 keV) measurements, and by DELTA alpha(sub E) approximately 0.5 with respect to IPC (0.2-3.5 keV) measurements). This suggests a break between the hard and soft components in the keV region and, therefore, that the PSPC spectra are strongly dominated by the soft compnents. In fact, a fit tot he composite, high signal-to-noise spectrum reveals a significant excess above approximately 1 keV withrespect to the simple power-law model. No evidence for strong emission lines is found in any of the quasars. This argues against emission from an ionized plasma as the main contributor to the soft X-ray compnentunless there is a distribution of te mperatures. If the soft X-ray spectrum of thee quasars is dominated by radiation reflected by the photoinonized surface of an accretion disk, the absence of strong emissionlines suggests high ionization parameters and therefore high accretion rates. We include in two Appendices a comarison of the two official PSPC resolution matrices, those released on1992 March and on 1993 January, a discussion of the amplitude of the residual systematic uncertainties in 1993 January matrix, and a compaison between the PSPC and IPC spectra of a sample of sources.

Fiore, Fabrizio↗

NASA NTRS1994

An X-ray image of the large magellanic cloud

We have used archival ROSAT Position Sensitive Proportional Counter (PSPC) pointed observations to construct maps of the Large Magellanic Cloud (LMC) in four energy bands between 0.5 and 2.0 keV. These represent the most complete, deepest, and most detailed X-ray images of the LMC to date. While confirming the general morphology of the diffuse LMC emission observed by Wang et al. with Einstein IPC data, these images reveal a wealth of detailed structure of high statistical significance on angular scales from a few arcminutes to a few degrees. In addition, at least twice as many discrete sources are detected as were found using the IPC.

Snowden, S. L.↗

NASA NTRS1995

A serach for 'failed clusters' of galaxies

We describe a search for a new type of object - large clouds of hot gas with no visible galaxies - which we call failed clusters of galaxies. We calculate the expected X-ray luminosity, temperature, and angular diameter of such objects as a function of total cloud mass and convert the results to expected X-ray fluxes from failed clusters at different redshifts. Using the Einstein Imaging Proportional Counter (IPC) database, we establish a strategy to search for candidate failed clusters. From this initial screening of 1435 IPC fields, 17 candidates are selected for more detailed analysis, which indicates that 10 of these are very probably extended X-ray sources. Optical follow-up on the 10 prime candidates finds eight clusters of galaxies (including six reproted for the first time in this paper), one stellar identification, and one without an obvious optical counterpart (the candidate with the weakest evidence for X-ray extent). Investigation of several candidates with less evidence for X-ray extent yields two additional new clusters of galaxies. A conservative comparison of our results with the Einstein Extended Medium Sensitivity Survey demonstrates that failed clusters are a relatively unimportant contributor to the mass density of the universe. Our inability to find failed clusters is consistent with the hierarchical clustering scenario for the formation of galaxies and clusters.

Tucker, W. H.↗

NASA NTRS1995

A deep PSPC obervation of the Cyg OB2 association

The Cyg OB2 Association contains six of the fifteen most luminous stars in the Galaxy as well as the most luminous and most heavily reddened Galactic star, Cyg OB2 #12. In addition, the association contains a number of Wolf-Rayet stars, at least one early-type binary (Cyg OB2 #5), and a trapezium-like system (Cyg OB2 #8). Though not physically associated with the association, the X-ray source Cyg X-3 lies within a half degree of the center of the association. The association is a known source of X-rays. The X-ray emission from this region includes a contribution from Cyg X-3, as well as coronal emission from OB stars and hot diffuse gas. The region was extensively sampled by the HEAO-2 IPC (136 ks, which includes a single pointing of 57 ks on the central region of the association). A 3.6 ks PSPC observation of this region was obtained in the AO1 pointing phase. In addition, the four brightest OB stars (Cyg OB2 #5, #8A, #9, and #12) have been extensively monitored with the VLA. These stars show evidence of strong radio variability, and nonthermal radio emission characteristics. Several of these radio observations were taken within about a month of an IPC observation, and the two PSPC observations. We obtained a 19.5 ks exposure of the Cyg OB2 Association using the ROSAT PSPC instrument. Scientific tasks included: (1) extracting the PSPC spectral distributions of the brightest OB stars; (2) comparing the 19.5 ks spectral data with the 3.6 ks spectral data; (3) investigating the long term X-ray variability (15 years); and (4) comparing the X-ray and radio variability.

Waldron, Wayne↗

NASA NTRS1994

X-Ray Emission from a Simulated Cluster of Galaxies

Using the 1993 cluster simulation of Katz & White, we analyze the intracluster medium and investigate the accuracy of the standard hydrostatic method for determining cluster masses. We show that the simulated cluster gas is in hydrostatic equilibrium with a subsonic flow toward the center. Inside a radius of (approx.) 100 kpc, this flow is in a steady state. The cooling time is shorter than a Hubble time within the central 50 kpc. The flow rate is regulated by the gas sink in the middle of the cluster and the PdV work done as the gas flows in, verifying the standard cooling flow scenario. We simulate observations of the cluster using the instrument parameters of the EXOSAT ME detector and the Einstein IPC detector. Even though the intracluster gas is not isothermal, isothermal models of the cluster, excluding regions within 100 kpc of galaxies, fit the EXOSAT X-ray spectra as well as they fit real clusters. The X- ray surface brightness distribution is similar to that of real clusters, again excluding the galaxies. We simulate the procedure used to determine the masses of real clusters. We use the equation of hydrostatic equilibrium together with the temperature derived from an isothermal fit to the simulated EXOSAT spectrum and the density profile derived from a fit to the simulated IPC surface brightness profile to determine the mass. A comparison of the derived mass profile to the actual mass profile shows that errors of a factor of 2 are possible. If the actual temperature profile is used, the cluster mass is found to an accuracy of better than 25% within the virial radius.

Tsai, John C.↗

NASA NTRS2011

Heterogeneity in Short Gamma-Ray Bursts

We analyze the Swift/BAT sample of short gamma-ray bursts, using an objective Bayesian Block procedure to extract temporal descriptors of the bursts' initial pulse complexes (IPCs). The sample comprises 12 and 41 bursts with and without extended emission (EE) components, respectively. IPCs of non-EE bursts are dominated by single pulse structures, while EE bursts tend to have two or more pulse structures. The medians of characteristic timescales - durations, pulse structure widths, and peak intervals - for EE bursts are factors of approx 2-3 longer than for non-EE bursts. A trend previously reported by Hakkila and colleagues unifying long and short bursts - the anti-correlation of pulse intensity and width - continues in the two short burst groups, with non-EE bursts extending to more intense, narrower pulses. In addition we find that preceding and succeeding pulse intensities are anti-correlated with pulse interval. We also examine the short burst X-ray afterglows as observed by the Swift/XRT. The median flux of the initial XRT detections for EE bursts (approx 6 X 10(exp -10) erg / sq cm/ s) is approx > 20 x brighter than for non-EE bursts, and the median X-ray afterglow duration for EE bursts (approx 60,000 s) is approx 30 x longer than for non-EE bursts. The tendency for EE bursts toward longer prompt-emission timescales and higher initial X-ray afterglow fluxes implies larger energy injections powering the afterglows. The longer-lasting X-ray afterglows of EE bursts may suggest that a significant fraction explode into more dense environments than non-EE bursts, or that the sometimes-dominant EE component efficiently p()wers the afterglow. Combined, these results favor different progenitors for EE and non-EE short bursts.

Norris, Jay P.↗

NASA NTRS1980

X-ray studies of clusters of galaxies with the Einstein Observatory

The paper reports on cluster observations made by the Columbia Astrophysics Laboratory with the Einstein Observatory. A plot of the 0.2-3.0 keV X-ray luminosity of two dozen randomly selected sources against the Bautz-Morgan class reveals no apparent correlation between X-ray brightness and cluster morphological type. Observing a field containing the Butcher and Oemler cluster CL0024+1654 (z = 0.39), the satellite detected a weak source coincident with the cluster center. An IPC (imaging proportional counter) image of the Coma cluster, the nearest and best studied of Abell's rich clusters, shows a smooth X-ray emission rising to a flat central plateau in the vicinity of the giant ellipticals, NGC4874 and NGC4889. Although individual galaxies emerge above the diffuse flux near the edge of the field at the level of 0.1% of the cluster emission, the central 40 minutes contain no fluctuations on the scale of approximately one minute to the 1%-level.

Helfand, D. J.↗

NASA NTRS1981

In orbit performance of the Einstein Observatory/HEAO-2 Imaging Proportional Counter

After two years of orbital operation, the IPC - a soft X-ray (0.2-4 keV) position sensitive proportional counter - continues to perform well after some initial gain changes, and is the most widely used focal plane instrument of the Einstein Observatory. X-ray images of several thousand cosmic X-ray sources have been obtained with a resolution of about 1.5 arcmin (FWHM) with a 1-deg field. Particle backgrounds are less than expected. The calibration of the instrument with respect to gain variations and spatial linearity is being refined by in-flight measurements.

Gorenstein, P.↗

NASA NTRS1981

Observations of the giant radio lobes region of Centaurus A with SAS 3

Observations of the X-ray flux from the extended radio lobes of the galaxy Centaurus A obtained with the SAS 3 observatory are reported. The 15 x 15 deg region that includes the lobes was mapped by superposing data from raster scans with the horizontal tubular collimator detectors, which have a 1.7 deg FWHM circular field of view. Data obtained is consistent with the presence of superposed emission from the point sources NGC 5128 and MX 1312-42, a previously unreported source detected by the Einstein Observatory IPC. Fluxes of 4.63 + or - 0.14 x 10 to the -10th and 4.6 + or - 0.6 x 10 to the -11th ergs/sq cm per sec were measured for NGC 5128 and MX 1312-42, respectively, with best fit power law spectra of indices 0.83 + or - 0.04 and 0.62 + or - 0.10 and hydrogen column densities of 1.23 + or - 0.10 x 10 to the 23rd and 9.7 + or - 15.0 x 10 to the 21st atoms/sq cm. Results imply an upper limit of 2.9 x 10 to the 11th for the X-ray emission from the extended radio lobes of Cen A between 2 and 10 keV, which indicates a lower limit to the magnetic field in the lobes of 1.6 x 10 to the -6th gauss.

Marshall, F. J.↗

NASA NTRS1981

Simultaneous X-ray, ultraviolet, optical, and radio observations of the flare star Proxima Centauri

Results of coordinated program of observations in the X-ray, UV, optical and radio regions of the dM5e flare star Proxima Centauri are presented. Simultaneous observations of the star were obtained on March 6 and March 7, 1979, by the Einstein Observatory IPC, the IUE SWP and LWR cameras at low dispersion, three ground-based optical telescopes in Australia and the Parkes 64-m radio telescope. A total of 10 radio bursts and six optical flares was detected during three nights of simultaneous radio and optical observations, which appear to be broadly correlated. A major X-ray flare event was detected with temperatures of 1.7 x 10 to the 7th and 1.2 x 10 to the 7th K during the rise and decay phases, respectively, respective X-ray fluxes of 3.0 x 10 to the -11th and 3.7 x 10 to the -11th ergs/sq cm per sec, and changes in spectral flux distribution. No radio, optical or UV flare emission corresponding to the X-ray flare was detected. The X-ray flare is interpreted in terms of an arch model with cooling predominantly by X-ray radiation, with an electron density of 1.0 x 10 to the 11th/cu cm during the decay phase and a total arch length comparable to the size of the star itself. The X-ray flare observed is thus more similar to a typical strong solar flare than heretofore seen on a flare star.

Haisch, B. M.↗

NASA NTRS1981

Observations of the X-ray sources in the nearby Sc galaxy M33

Two observations of the Local Group spiral M33 with the Imaging Proportional Counter (IPC) on the Einstein Observatory reveal the presence of 11 point sources which, if at the distance of M33, have X-ray luminosities ranging from 10 to the 37th ergs/sec to 10 to the 39th ergs/sec. The brightest source, coincident with the nucleus of the galaxy, comprises approximately 70% of the total X-ray luminosity of the galaxy. If this source is a single source, it must be qualitatively different from the compact binary sources which dominate the X-ray luminosity of the Galaxy.

Long, K. S.↗

NASA NTRS1982

X-ray and UV spectra of two quasars: PKS0637-75 and PKS1004+13 (4C13.41)

Two quasars, 4C13.41 (1004+13, V approx. 15.2, z = 0.140) and PKS0637-75 (V = 15.75, z = 0.651) were observed in December 1980 with IUE. Both were detected in the short wavelength region. 4C13.41 has the lowest X-ray luminosity/optical luminosity measured in quasars. PKS0637-75 was observed quasi-simultaneously with the Einstein Observatory and a spectrum obtained with the IPC. The UV and soft X-ray spectra have been combined and are discussed together with the optical and infrared data for both objects.

Elvis, M.↗

NASA NTRS1981

The smothered coronae of T Tauri stars

Einstein IPC observations of 10 fields containing pre-main-sequence stars are presented. The detection of significant soft X-ray flux from eight cataloged T Tauri stars plus the optical identification of two new T Tauri-like stars associated with X-ray sources are reported. It is demonstrated than an inverse correlation between the H-alpha equivalent width and the detection of X-ray flux exists. This is interpreted in terms of a model wherein the X-ray emission arises in a corona similar to that of active late-type stars, which is small compared to the extent of the circumstellar envelope. It is postulated that the lack of correlation of X-ray flux (or H-alpha) with A(v) is due to a lack of grains in the envelopes of the more active T Tauri stars.

Walter, F. M.↗

NASA NTRS1981

X-ray and optical observations of the November 19, 1978 gamma-ray burst source region

The Nov. 19, 1978 gamma-ray burst (GRB) has a very well determined error box, 10 square arcmin (Cline et al., 1981). An 8000-sec IPC exposure with the Einstein Observatory detected, at a 3.4-sigma level, one low intensity (less than 3 x 10 to the -13th erg/sq cm per sec) X-ray source inside the error box. The probability of a serendipitous detection was 0.01. Inside the X-ray source error box there are two weak radio sources, one of them highly polarized (Hjellming and Ewald, 1981), and two 20-magnitude objects, not coincident with the radio sources visible in the ESO/SRC J and R plates. With the exception of N49, this is the first possible detection of X-ray emission inside a GRB box. Its low intensity justifies, in fact, the lack of detection for other events.

Pizzichini, G.↗

NASA NTRS1981

X-ray and optical observations of HD 155638 - A remarkably active cool star

X-ray and optical observations of the 8.5-mag star HD 155638 are reported which reveal a remarkably active chromosphere and corona for the star. The object was first observed in soft X-rays with the HEAO 1 A2 low-energy detectors, and later reobserved with the HEAO 2 IPC. X-ray fluxes of about 2 x 10 to the -11th and 3 x 10 to the -12th ergs/sq cm per sec were obtained on the two occasions, indicative of source variability over the 2.5-year period between observations. SIT spectrographs taken in July-August 1980 show evidence of Ca II H and K core emission reversals and filled-in H alpha, while spectra taken in March and July 1981 show evidence of asymmetry and variability in the Ca II reversals. The spectral data are consistent with a G8 IV-V classification, from which is inferred a distance of 35-120 pc, and an X-ray luminosity from 4 x 10 to the 29th to 4 x 10 to the 31st ergs/sec. An absence of emission lines characteristic of mass transfer in binary systems indicates the X-rays to be coronal in origin. The properties of the star are noted to be similar to those of the RS CVn binaries.

Stern, R. A.↗

NASA NTRS1981

X-ray sources in globular clusters

The Einstein X-ray Observatory globular cluster observation program has completed a survey of 30 clusters, using the HRI detector. Preliminary results are reported for a more sensitive survey being conducted with the imaging proportional counter (IPC) of the satellite, which has found four possible new sources in the first 32 clusters observed. Also described are detailed studies of several luminous, globular cluster sources, including the X-ray burst from Terzan 2, and an effort is made to locate the X-ray sources with respect to cluster centers. Sources of systematic error, required optical studies of globular cluster structure, and the implications of the plausible binary systems for cluster core evolution are discussed.

Grindlay, J. E.↗