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NASA NTRS1994

Angular power spectrum of the microwave background anisotropy seen by the COBE differential microwave radiometer

The angular power spectrum estimator developed by Peebles (1973) and Hauser & Peebles (1973) has been modified and applied to the 2 yr maps produced by the Cosmic Background Explorer Satellite Differential Microwave Radiometer (COBE DMR)). The power spectrum of the real sky has been compared to the power spectra of a large number of simulated random skies produced with noise equal to the observed noise and primordial density fluctuation power spectra of power-law form, with P(k) proportional to k(exp n). Within the limited range of spatial scales covered by the COBE DMR, corresponding to spherical harmonic indices 3 less than or = l is less than or approximately = 30, the best-fitting value of the spectral index is n = 1.25(sup +0.39 sub -0.44) with the Harrisson-Zel'dovich value n = 1 approximately 0.5 sigma below the best fit. For 3 less than or = l less than or approximately = 19, the best fit is n = 1.46(sup +0.39 sub -0.44). Comparing the COBE DMR delta-T/T at small l to the delta-T/T at l approximately = 50 from degree scale anisotropy experiments gives a smaller range of acceptable spectral indices which includes n = 1.

Wright, E. L.↗
NASA NTRS1994

Concurrent X-ray and optical observations of two dwarf novae during eruption

The dwarf novae SU UMa and RU Peg were each observed with the ROSAT Observatory during outburst. SU UMa was also observed twice between outbursts. The spectrum of SU UMa during quiescence is well fitted by a 2.1 keV thermal bremsstrahlung model with a Gaussian line at 0.97 keV absorbed by a column density of 1.4 x 10(exp 20)/sq cm. During the outburst the X-ray count rate dropped by a factor of 3. The outburst spectrum is well fitted to a similar spectrum with a significantly larger absorption. In outburst the mass transfer rate, dot-M, approaches the critical value for the optically thin/thick transition predicted by theoretical models of the boundary layer while the X-ray emission remains optically thin. The best-fit model to the outburst spectrum of RU Peg is a 4.9 keV thermal bremsstrahlung plus a line at 0.89 keV absorbed by a column density of 2.8 x 10(exp 20)/sq cm. The column density is consistent with the upper limit determined for interstellar absorption. The boundary layer at outburst is expected to be optically thick based on the value of dot-M determined from the IUE spectra. There are no ROSAT observations of RU Peg between outbursts, but the flux detected by ROSAT is a factor of 10 lower than that seen with the Einstein Observatory, probably during quiescence. For both CVs the X-ray flux from the boundary layer significantly decreases at the same time that the optical-UV flux greatly increases. We suggest that during outburst most of the boundary layer energy is carried away as kinetic energy of the wind.

Silber, A.↗
NASA NTRS1994

Far-ultraviolet emission from the Taurus molecular cloud

Far-ultraviolet spectra of diffuse emission from a region in the Taurus molecular cloud are presented. These data were obtained with the Berkeley spectrometer on NASA's Ultraviolet Experiment (UVX) payload. The intensity of the continuum emission drops as the optical depth increases toward the denser part of the cloud, an effect which is the opposite of that observed for the diffuse flux at high galactic latitudes. At the center of the aperture, the optical depth is very large, and the continuum emission must be produced by backscattering. At the edges of the cloud, the continuum intensity rises by a factor of 2. The data are interpreted with a new radiative transfer model, which is significantly more sophisticated than its predecessors. If the grain scattering properties are constant across the cloud, a unique best-fit combination can be determined from the data. The best-fit albedo is 0.6 +/- 0.1 at 1600 A. The best-fit value for the phase factor g is 0.5 +/- 0.15. The intensity of the flourescence of H2, combined with an upper limit to the column density of atomic hydrogen associated with the cloud, sets a lower limit of about 130/cm to the density in the flourescence region. Our estimate of the flux incident on the cloud at 1000 A is a factor of 2 to 3 lower than the value required to produce the measured flourescence intensity. Most mechanisms which might reconcile the discrepancy reduce the derived albedo and phase factor.

Hurwitz, M.↗
NASA NTRS1994

Hard X-rays from NGC 4151: A thermal origin?

We present a model for explaining the recent combined X-ray and low-energy gamma-ray observations of the Seyfert galaxy NGC 4151. According to this model, soft photons become Comptonized in a hot spot producing simultaneously the low-energy power law as observed by Ginga and the high-energy cutoff observed by the Oriented Scintillation Spectrometer Experiment (OSSE). Implementing recently developed theoretical calculations toward a generalized theory of Comptonization, we were able to find fits to the observations using only two parameters which characterize the physical quantities of the emission region: the plasma cloud optical depth and its temperature. We find that there is no need for additional nonthermal, reflection, or higher temperature thermal components to fit the aforementioned OSSE and Ginga observations. We derive in addition the size of the photon region and the temperature of the upscattered soft photons. We should emphasize, also, that any attempt at fitting only the high-energy parts of the spectrum (photon energies greater than 60 keV) by the Sunyaev & Titarchuk (1980) nonrelativistic Comptonization model leads to an underestimate of the Comptonization parameter y (or, equivalently, to an overestimation of the X-ray power-law spectral slope) and leads, as a result, to incorrect proportions between the low-energy and high-energy parts of the spectrum.

Titarchuk, Lev↗
NASA NTRS1994

Modeling and interpretation of the optical and HST UV spectrum of SN 1993J

We present spherically symmetric, non-local thermodynamic equilibrium (non-LTE), expanding model atmosphere synthetic spectra of SN 1993J and compare them to the ultraviolet and optical spectra obtained simultaneously on 1993 April 15 by the Hubble Space Telescope (HST) and at Lick Observatory. We are able to fit the optical and HST near-UV spectra with a variety of compositions, but find that our best fits are obtained with an enhanced helium abundance (Y = 0.8) and significant nonthermal ionization due to gamma rays produced by the decay of Ni-56. We find the effects of X-rays produced by thermal bremsstrahlung in the circumstellar material to have little effect on the spectrum. Our current models are unable to fit the HST far-UV region, predicting too little flux and strong, unseen Fe II features.

Baron, E.↗
NASA NTRS1994

Fe XXV temperatures in flares from the Yohkoh Bragg crystal spectrometer

Studies by Doschek et al. using P78-1 and Solar Maximum Misson (SMM) data have shown that the ratio of intensities of the Fe XXV and Ca XIX resonance lines can be expressed as a function of Fe XXV temperature. Using a more recent data set consisting of 13 flares observed by the Bragg crystal spectrometer (BCS) experiment on board Yohkoh, we find a nearly identical functional relationship between the same resonance line ratios and Fe XXV temperatures. We use this functional relationship to obtain resonance line ratio temperatures (T(sub RLR)) for each flare in our data set, and compare them with temperatures resulting from application of a simple spectral fitting method. (T(sub SSF)) to individal Fe XXV spectra. We also use a more involved free-parameter spectral fitting method to deduce temperatures (T(sub FSF)) from some of these spectra. On average, agreement between T(sub RLR) and T(sub SSF) improves as a flare progresses in time, with average agreements of 10.0% +/- 5.2%, 6.4% +/- 5.4%, and 5.0% +/- 3.9% over the rise, peak, and decay phases, respectively. Deviations between T(sub RLR) and T(sub FSF) are about the same or smaller. Thus, for most analysis purposes, all three methods yield virtually identical temperatures in flares. The somewhat poorer agreement between T(sub SSF) and T(sub RLR) during the earlier phases may be partially a result of difficulties in obtaining precise values for temperatures from spectral fits when blueshifts and large nonthermal broadenings are present in the spectra. Because of the high sensitivity of the Yohkoh BCS compared to that of BCS experiments on earlier spacecraft, we can for the first time consistently observe the heating phase of flares in Fe XXV.

Sterling, Alphonse C.↗
NASA NTRS1994

ROSAT X-ray observations of late-type evolved stars: On the relationship between coronal temperatures and luminosities

We present ROSAT Position Sensitive Proportional Counters (PSPC) X-ray observations of three near-solar-mass stars, in different evolutionary phases beyond the main sequence: eta Sco (F3 III-IV), iota Vir (F6 III), and HD 74772 (G5 III). All three of these nearby, presumably single stars have been detected, and we have collected enough counts to perform a detailed analysis of their soft X-ray spectra. While the X-ray spectra of eta Sco and HD 74772 can be fitted with Raymond-Smith thermal models with temperatures around 2 x 10(exp 6) K, the high signal-to-noise spectrum of iota Vir provides unambiguous evidence of a multitemperature plasma, with a two-temperature best-fit model with components at approximately 2 x 10(exp 6) K and 8 x 10(exp 6) K. Evidence of some hot plasma (T approximately 10(exp 7) K) has been also found for HD 74772. The present data, compared with spectral fitting results for other late-type stars observed with the Einstein Observatory, indicate that the low X-ray luminosity giants (L(sub x) is less than 5 x 10(exp 28) ergs/s) do not share with the higher X-ray luminosity stars of the same class the property of having substantial amount of 10(exp 7) K plasma. Moreover, our results confirm the trend of increasing X-ray luminosities with increasing coronal temperatures.

Maggio, A.↗
NASA NTRS1994

Calibration of the COBE FIRAS instrument

The Far-Infrared Absolute Spectrophotometer (FIRAS) instrument on the Cosmic Background Explorer (COBE) satellite was designed to accurately measure the spectrum of the cosmic microwave background radiation (CMBR) in the frequency range 1-95/cm with an angular resolution of 7 deg. We describe the calibration of this instrument, including the method of obtaining calibration data, reduction of data, the instrument model, fitting the model to the calibration data, and application of the resulting model solution to sky observations. The instrument model fits well for calibration data that resemble sky condition. The method of propagating detector noise through the calibration process to yield a covariance matrix of the calibrated sky data is described. The final uncertainties are variable both in frequency and position, but for a typical calibrated sky 2.6 deg square pixel and 0.7/cm spectral element the random detector noise limit is of order of a few times 10(exp -7) ergs/sq cm/s/sr cm for 2-20/cm, and the difference between the sky and the best-fit cosmic blackbody can be measured with a gain uncertainty of less than 3%.

Fixsen, D. J.↗
NASA NTRS1994

Day 640 infrared line and continuum measurements: Dust formation in SN 1987A

We have measured day 640-645 line and continuum spectra of (Ni II) 6.6 micrometer (Ne II) 12.8 micrometer (line emission was not detected), and (Fe II) 17.9 and 26.0 micrometer from SN 1987A. The high velocity feature at v(sub HVF) approximately 3900 km/sec found in both of our day 410 (Fe II) spectra is again detected in the day 640 (Ni II) spectrum, although the signal-to-noise of the day 640 (Fe II) spectra is insufficient to show this feature. The continuum fluxes provide clear evidence for the formation of dust between day 410 and day 640 and are best fitted by a graybody spectrum with a temperature of 342 +/- 17 K at day 640 and a surface area corresponding to a minimum dust velocity v(sub dust) = 1910 +/- 170 km/sec. Optically thin dust emissivity laws proportional to lambda(exp -1) or lambda(exp -2) are inconsistent with the data. Either the dust grains are large (radius a much greater than 4 micrometer and radiate like individual blackbodies, or else they are located in clumps optically thick in the 6-26 micrometer range. The (Ni II) 6.6 micrometer line flux yields a minimum Ni(+) mass of 5.8 +/- 1.6 x 10(exp -4) solar mass and a Ni/Fe abundance ratio of 0.06 +/- 0.02, equal to the solar value. The ratio of the two (Fe II) line profiles implies a gas temperature 2600 +/- 700 K, a drop of 1800 +/- 800 K from our day 410 measurement. The (Fe II) 26.0 micrometer line flux has decreased by a factor of 2 and the day 640 (Ni II) profile is blueshifted by -440 +/- 270 km/sec, relative to observations before day 500. We show that the decrease in the (Fe II) flux and the blueshift are not produced by a decrease in electron scattering optical depth, electron density, or temperature, but rather are probably due to obscuration by the same dust which produces the infrared continuum. This supports the interpretation that the dust spectrum is produced by optically thick clumps. We discuss possible explanations for the discrepancy between the mass of Fe(+) detected and the total iron mass required to power the light curve. The decrease in the (Fe II) fluxes relative to the decrease required to account for the blueshifts of optical lines from non-iron-group elements and the similarity between v(sub dust) and the Ni(+) expansion velocity imply a spatial association between the dust clumps and the iron-group elements. In addition, the larger blueshift observed for the near and far-infrared, heavy metal transitions relative to non-iron-group lines suggests that the iron-group elements are somewhat segregated from lighter elements such as the Mg(sup 0) and O(sup 0) responsible for shorter wavelength lines. We speculate that FeS may be an important constituent of the dust. A comparison of our line profiles with radiative transfer models shows that while power law and exponential density distributions yield reasonable fits to the data, polytrope distributions provided significantly worse agreement. The best fits require a substantial fraction of the iron to be undetectable, and are consistent with maximum expansion velocities of v(sub max) approximately 3000 km/sec.

Colgan, Sean W. J.↗
NASA NTRS1994

The evolution of X-ray clusters in a cold plus hot dark matter universe

We present the first self-consistently computed results on the evolution of X-ray properties of galaxy clusters in a cold + hot dark matter (CHDM) model. We have performed a hydrodynamic plus N-body simulation for the COBE-compatible CHDM model with standard mass components: Omega(sub hot) = 0.3, Omega (sub cold) = 0.6 and Omega(sub baryon) = 0.1 (h = 0.5). In contrast with the CDM model, which fails to reproduce the observed temperature distribution function dN/dT (Bryan et al. 1994b), the CHDM model fits the observational dN/dT quite well. Our results on X-ray luminosity are less firm but even more intriguing. We find that the resulting X-ray luminosity functions at redshifts z = 0.0, 0.2, 0.4, 0.7 are well fit by observations, where they overlap. The fact that both temperatures and luminosities provide a reasonable fit to the available observational data indicates that, unless we are missing some essential physics, there is neither room nor need for a large fraction of gas in rich clusters: 10% (or less) in baryons is sufficient to explain their X-ray properties. We also see a tight correlation between X-ray luminosity and gas temperature.

Bryan, Greg L.↗
NASA NTRS1994

Inverse Compton X-ray emission from the superluminal quasar 3C 345

In quasars with strong radio cores, the inverse-Compton process is believed to be the dominant source X-ray emission. For objects with parsec-scale radio jets, simple models have predicted that components in the jet emerging from the quasar nucleus generate the observed X-ray emission. We have tested this hypothesis in detail for the quasar 3C 345 using a ROSAT X-ray observation in 1990 July, together with quasi-simultaneous very long base interferometry (VLBI) imaging of the parsec-scale jet at five frequencies. The ROSAT spectrum is well fitted by a power law with index alpha = -0.96 +/- -0.13, consistent with models in which the X-ray emission results from inverse-Compton scattering of radio radiation from high-energy electrons in compact components. We show that the radio properties of brightest `knot' in the jet (`C5') can be fitted with a homogeneous sphere model whose parameters require bulk relativistic motion of the emitting material; otherwise the predicted model whose parameters require bulk relativistic motion of the emitting material; otherwise the predicted inverse-Compton X-ray emission exceeds the observed flux. If C5 is the origin of the X-ray emission, then it has a Doppler factor delta = 7.5((sup +3 sub -2)). If the nucleus or other components contribute to the X-ray emission, then this becomes a firm lower limit to delta. The inhomogeneous jet model of Koenigl is a good fit both to the barely resolved (less than 1 pc) flat-spectrum nucleus in the radio, and also to the ROSAT X-ray spectrum. The synchrotron and inverse-Compton emitting fluid moves down a narrow cone (opening angle 2 phi approximately 5 deg) nucleus relativistically, with delta approximately 4.6. Doppler factors for the nucleus and C5, derived from our ROSAT observation, provide evidence for bulk relativistic motion in the jet. By combining these constraints with well-known superluminal motion of jet components, we can deduce geometry. For epoch 1990.5 we infer the Lorentz factor gamma = 7.5 ((sup +1.0 sub -1.5)) and angle to the line of sight theta = 8((sup +2 deg sub -3 deg)) for H(sub 0) = 100 km/s/Mpc. These values are the most reliable yet derived using this method, because of the near-simultaneity of our X-ray and VLBI observations and the quality of the multifrequency of VLBI images and component radio spectra.

Unwin, S. C.↗
NASA NTRS1994

Analysis of small-scale microwave background radiation anisotropy in the presence of foreground contamination

Many of the current round of experiments searching for anisotropies in the microwave background radiation (MBR) are confronting the problem of how to disentangle the cosmic signal from contamination due to Galactic and intergalactic foreground sources. Here we show how commonly used likelihood function techniques can be generalized to account for foreground. Specifically we set some restrictions on the spectrum of foreground contamination but allow the amplitude to vary arbitrarily. The likelihood function thus generalized gives reasonable limits on the MBR anisotropy which, in some cases, are not much less restrictive than what one would get from more detailed modeling of the foreground. Furthermore, the likelihood function is exactly the same as one would obtain by simply projecting out foreground contamination and looking at the reduced data set. We apply this generalized analysis to the recent medium-angle data sets of ACME-HEMT (Gaier et al. 1992; Schuster et al. 1993) and MAX (Meinhold et al. 1993; Gunderson et al. 1993). The resulting analysis constrains the one free parameter in the standard cold dark matter theory to be Q(sub rms-ps) = 18(sub -5 sup +8) microKelvin. This best fit value, although in striking agreement with the normalization from Cosmic Background Explorer (COBE), is not a very good fit, with an overall chi-squared/degrees of freedom = 208/168. We also argue against three commonly used methods of dealing with foreground: (1) ignoring it completely; (2) subtracting off a best-fit foreground and treating the residuals as if uncontaminated; and (3) culling data which appears to be contaminated by foreground.

Dodelson, Scott↗
NASA NTRS1994

Statistical uncertainties in temperature diagnostics for hot coronal plasma using the ASCA SIS

Statistical uncertainties in determining the temperatures of hot (0.5-10 keV) coronal plasmas are investigated. The statistical presicion of various spectral temperature diagnostics is established by analyzing synthetic ASCA solid-state imaging spectrometer (SIS) CCD spectra. The diagnostics considered are the ratio of hydrogen-like to helium-like line complexes of Z greater than or = 14 elements, line-free portions of the continuum, and the entire spectrum. While fits to the entire spectrum yield the highest statistical precision, it is argued that fits to the line-free continuum are less susceptible to atomic data uncertainties but lead to a modest increase in statistical uncertainty over full spectral fits. Temperatures deduced from line ratios can have similar accuracy, but only over a narrow range of temperatures. Convenient estimates of statistical accuracies for the various temperature diagnostics are provided which may be used in planning ASCA SIS observations.

Swartz, Douglas A.↗
NASA NTRS1994

An improved solution to the 'Rankine-Hugoniot' problem

This paper presents an extension of the nonlinear least squares fitting technique of Vinas and Scudder (1986) (VS), which finds the physical and geometrical properties of nondissipational magnetohydrodynamic (MHD) shocks. The new method incorporates plasma temperature observations in the form of normal momentum flux and energy density flux conservation as well as plasma density, velocity, and magnetic field data. The new technique is capable of using known standard deviations in the individual measurement points to properly weight the fitting procedure. The new fitting code is validated through the analysis of synthetic shocks with known physical and geometrical properties. Finally, it is compared to the original VS method and the preaveraged velocity coplanarity technique.

Szabo, A.↗
NASA NTRS1995

Ultraviolet extinction to 10.8 inverse microns

UV extinction curves that represent observed variations i diffuse clouds (Zeta Oph, Xi Per, and Sigma Sco) have been derived by combining IUE and Voyager data and have been modeled by a multicomponent size distribution of silicate and carbonaceous giants. We have used theoretical model atmospheres as our reddening-free standards, and, in the case of Zeta Oph, also compare with the use of a stellar standard. An attempt was made to classify the UV spectra via theoretical spectral indices, but still missing line opacity appears to limit the use of this method at the present. The modeling includes five possible grain ingrediants, largge (0.015-0.25 micrometers) and small (0.005 micrometers) silicate grains, similarly large and small carbonaceous grains, and polycyclic aromatic hydrocarbons (PAHs). The observed extinction curves are fitted by the model via nonlinear Chi-square-minimization, varying the fractional amount of cosmic Si and C that are locked up in each igredient. The observed variation from 'steep' to 'flat' UV extinction curves is found to correspond to a modest model variation in the lower limit of the power-law size distribution of the large grains. Graphite is found to be necessary grain ingredient that cannot be replaced by the laboratory amorphous carbon materials. Carbonaceous mantles on the larger silicate grains provide no good fits to the observed curves. Using absorption properties of neutral laboratory PAHs limits PAHs to a few percent of the cosmic C abundance for acceptable fits that also give values for R(sub V) close to the observed values.

Aannestad, Per A.↗
NASA NTRS1995

The spectrum and pulses of 1E 2259+586 from ASCA and BBXRT observations

The 7 s X-ray pulsator 1E 2259+586 was observed for approximately 1 day in 1993 with the Advanced Satellite for Cosmology and Astrophysics (ASCA). Observations were also obtained with Broad Band X-ray Telescope (BBXRT) in 1990 a few months after Ginga had observed 1E 2259+586 to be brighter than normal and the BBXRT data show 1E 2259+586 to be at an intermediate brightness level. By contrast, the ASCA data appear to have been obtained during a more common lower luminosity state. The pulse profiles we obtain are consistent with a connection between flux and pulse shape reported from Ginga data, and the pulsator continues to spin down. We use our high spectral resolution data to search for cyclotron lines in the spectrum that were claimed from observations made with other satellites. We find that the ASCA spectra of 1E 2259+586 cannot be satisfactorily fitted with either a single power law or a combination of two power laws, and that significant residuals occur around 1.5 and 5 keV. However, a combination of a power law and blackbody gives a good fit over the entire ASCA energy band with no evidence of spectral features. We have reanalyzed a Ginga LAC spectrum and find that this is also significantly better fitted by this two-component spectrum than a single power law. A possible explanation for such a two-component spectrum is that the blackbody emission comes from a neutron star and that the power-law component comes, at least in part, from a surrounding nebula. As there has, so far, no direct evidence that 1E 2259+586 is a binary system we consider whether there are other plausible mechanisms that might power the observed X-ray emission.

Corbet, R. H. D.↗
NASA NTRS1995

A study of coronal X-ray emission from short-period Algol binaries

A study of X-ray emission from five short-period Algol-type binaries based on observations with Advanced Satellite for Cosmology and Astrophysics (ASCA) and ROSAT is presented. We have observed RZ Cas with both satellites, and beta Per, U Cep, delta Lib, and TW Dra with ROSAT. Significant intensity variations are seen in the X-ray emission from RZ Cas, U Cep, TW Dra, and delta Lib. These variations seem unrelated to the eclipsing behavior of these systems and are probably due to either rotational modulation of compact active regions on the surfaces of the chromospherically active secondary components or to flaring activity in the systems. The spectra of all but one of the systems require the presence of at least two discrete plasma components with different temperatures (0.6 - 0.7 keV, and approximately 2 keV) and the abundances of the medium-Z elements 20% - 50% of the solar photospheric values. The high resolving power and signal-to-noise ratio of the ASCA spectra allow us to individually constrain the coronal abundances of O, Ne, Mg, Si, S, and Fe in RZ Cas. We demonstrate that, if we use the elemental abundances and temperatures obtained from the analysis of their ASCA spectra as (fixed) inputs, to fit the ROSAT PSPC spectra well requires the presence of a third component (kT approximately 0.2 - 0.3 keV) in RZ Cas and beta Per. A continuous emission measure model of the power-law type (EM(T) variesas (T/T(sub max)(sup alpha)) generally gives a poor fit to the ASCA and ROSAT data on most sources. Circumstellar or circumbinary absorbing matter seems to be present in some of these systems, as indicated by the variable total column density needed to fit their X-ray spectra.

Singh, K. P.↗
NASA NTRS1995

Bringing the cross-correlation method up to date

The cross-correlation (XC) method of Tonry & Davis (1979, AJ, 84, 1511) is generalized to arbitrary parametrized line profiles. In the new algorithm the correlation function itself, rather than the observed galaxy spectrum, is fitted by the model line profile: this removes much of the complication in the error analysis caused by template mismatch. Like the Fourier correlation quotient (FCQ) method of Bender (1990, A&A, 229, 441), the inferred line profiles are, up to a normalization constant, independent of template mismatch as long as there are no blended lines. The standard reduced chi(exp 2) is a good measure of the fit of the inferred velocity distribution, largely decoupled from the fit of the spectral template. The updated XC method performs as well as other recently developed methods, with the added virtue of conceptual simplicity.

Statler, Thomas↗