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NASA NTRS1993

Magnetic field power density spectra during 'scatter-free' solar particle events

We have examined interplanetary magnetic field power spectral density during four previously identified 3He-rich flare periods when the about 1 MeV nucleon-1 particles exhibited nearly scatter-free transport from the sun to 1 AU. Since the scattering mean free path A was large, it might be expected that interplanetary turbulence was low, yet the spectral density value was low only for one of the four periods. For the other three, however, the spectral index q of the power density spectrum was near 2.0, a value at which quasi-linear theories predict an increase in the scattering mean free path. Comparing the lambda values from the energetic particles with that computed from a recent quasi-linear theory which includes helicity and the propagation direction of waves, we find lambda(QLT)/lambda(SEP) = 0.08 +/- 0.03 for the four events. Thus, the theory fits the q-dependence of lambda; however, as found for previous quasi-linear theories, the absolute value is low.

Tan, L. C.↗
NASA NTRS1993

The EGRET detection of quasar 1633 + 382

In the course of a full-sky survey, the EGRET instrument aboard the Compton Observatory detected an intense, high Galactic latitude source, which is identified as the OVV (optically violent variable) quasar 1633 + 382. The spectrum is represented by a power law with an energy spectral index of 0.9 between 30 MeV and 30 GeV. The gamma-ray power per decade is about 100 times larger than typical values at any longer wavelength. Significant variation of the gamma-ray flux density on a time scale as short as two days was observed, limiting the size of the region in which the gamma-rays are produced. Assuming the X-rays detected by the Einstein Observatory from this quasar are produced in the same region as the gamma-rays, and a similar X-ray flux density was extant at the time of the EGRET observation, the emission must be beamed to avoid a pair-production optical depth of 10 exp 4 which would exist otherwise. If beaming arises from bulk relativistic motion, the minimum Doppler factor required is 7.6.

Mattox, J. R.↗
NASA NTRS1993

ROSAT observations of the LMC pulsar PSR 0540-69

Observations of the LMC pulsar PSR 0540-69 acquired during June-July 1990 and February 1991 with the Position Sensitive Proportional Counter on board ROSAT are reported, as are the results of timing and spectral analyses. The timing data are consistent with contemporaneous optical ephemerides, suggesting that the optical and X-ray pulses are in phase. The timing properties are used to ascertain the spectral characteristics of the pulsar and nebula separately. The spectral index of the pulsed component is determined using solely X-ray data. While many of the characteristics of the LMC pulsar are found to be similar to those of the Crab, the differences have yet to be explained.

Finley, John P.↗
NASA NTRS1993

HST observations of NGC 4395, the least luminous Seyfert 1 nucleus - Evidence against the starburst hypothesis for broad-lined active galactic nuclei

The starburst hypothesis for broad-line AGN is examined here using ultraviolet spectra and optical images of NGC 4395, the least luminous Seyfert 1 nucleus. From the evidence it is concluded that the apparent activity in the Seyfert 1 nucleus of NGC 4395 probably cannot be explained in the context of the starburst hypothesis of Terlevich et al. (1992). The strongest piece of evidence is the absence of stellar absorption lines, especially in the UV and NIR spectra. Other relevant characteristics are the UV spectral index of roughly -1, small physical size, low luminosity, and lack of obvious variability.

Filippenko, Alexei V.↗
NASA NTRS1993

A high-sensitivity survey of radio continuum emission from Herbig Ae/Be stars

Results of a high-sensitivity VLA/Australia Telescope survey of radio continuum emission from the 57 Herbig Ae/Be stars and candidates in the 1984 catalog of Finkenzeller and Mundt are presented. Twelve stars were detected at the primary observing wavelength of 3.6 cm, on the basis that not less than 4 sigma radio sources lie within 1 arcsec of the optical positions. It is suggested that the radio emission is predominantly thermal and in many cases wind-related. The unusual eclipsing binary TY CrA is an exception and is classified as a nonthermal radio source on the basis of its decidedly negative spectral index (alpha = -1.2). A simple spherically symmetric free-fall accretion model is used to show that the predicted radio fluxes due to accretion at rates, estimated in the literature, of about 10 exp -6 to 10 exp -5 solar mass/yr are one to four orders of magnitude larger than observed.

Skinner, Stephen L.↗
NASA NTRS1992

Faint Object Camera observations of M87 - The jet and nucleus

UV and optical images of the central region and jet of the nearby elliptical galaxy M87 have been obtained with about 0.1 arcsec resolution in several spectral bands with the Faint Object Camera (FOC) on the HST, including polarization images. Deconvolution enhances the contrast of the complex structure and filamentary patterns in the jet already evident in the aberrated images. Morphologically there is close similarity between the FOC images of the extended jet and the best 2-cm radio maps obtained at similar resolution, and the magnetic field vectors from the UV and radio polarimetric data also correspond well. We observe structure in the inner jet within a few tenths arcsec of the nucleus which also has been well studied at radio wavelengths. Our UV and optical photometry of regions along the jet shows little variation in spectral index from the value 1.0 between markedly different regions and no trend to a steepening spectrum with distance along the jet.

Boksenberg, A.↗
NASA NTRS1993

Radio observations of peculiar emission-line Algol binary stars

Twelve peculiar emission-line Algol (PELA) binaries were observed at cm wavelengths. Six of these objects, WW And (tentative), KX And, AM Aur, V644 Mon, DN Ori, and AY Per were detected at the 100 to 400 micro-Jy level, corresponding to luminosity densities of order 10 exp 17 erg/s/Hz. These luminosity densities, combined with nearly flat cm spectra and high brightness temperatures, are consistent with the nonthermal gyrosynchrotron radiation model proposed as the result of observations from the previous PELA survey. Also, the star V367 Cyg was reobserved at thirteen different orbital phases; no variability was found within the errors. V367 Cyg's lack of variability and well-determined average spectral index, however, imply the possibility of a different emission mechanism, thermal bremsstrahlung, although the nonthermal model cannot be entirely ruled out. The characteristic brightness temperature of thermal bremsstrahlung is only about 10,000 K, which means that the radius of the emission region would have to be much larger than previously assumed, about 10 AU. Suggestions for further observations of all previously observed PELAS are presented in order to determine the correct emission mechanism.

Elias, Nicholas M., II↗
NASA NTRS1993

Spectrophotometry of Wolf-Rayet stars. I - Continuum energy distributions

All available low-resolution IUE spectra are assembled for Galactic, LMC, and SMC W-R stars and are merged with ground-based optical and NIR spectra in order to collate in a systematic fashion the shapes of these energy distributions over the wavelength range 0.1-1 micron. They can be consistently fitted by a power law of the form F(lambda) is approximately equal to lambda exp -alpha over the range 1500-9000 A to derive color excesses E(B-V) and spectral indices by removing the 2175-A interstellar absorption feature. The WN star color excesses derived are found to be in good agreement with those of Schmutz and Vacca (1991) and Koesterke et al. (1991). Significant heterogeneity in spectral index values was generally seen with any given subtype, but the groups consisting of the combined set of Galactic and LMC W-R stars, the separate WN and WC sequences, and the Galactic and LMC W-R stars all showed a striking and consistent Gaussian-like frequency distribution of values.

Morris, Patrick W.↗
NASA NTRS1993

Pulsed high-energy gamma rays from PSR 1055-52

The Energetic Gamma Ray Experiment Telescope (EGRET) aboard the Compton Gamma Ray Observatory has detected a high-energy gamma-ray source at a position coincident with that of the radio pulsar PSR 1055-52. Analysis of the EGRET data at the radio pulsar period of 197 ms has revealed pulsed gamma-radiation at energies above 300 MeV, making PSR 1055-52 the fifth detected high-energy gamma-ray pulsar. The pulsed radiation from PSR 1055-52 has a very hard photon spectral index of -1.18 +/- 0.16 and a high efficiency for converting its rotational energy into gamma-rays. No unpulsed emission was observed.

Fierro, J. M.↗
NASA NTRS1993

Soft X-ray spectra of Seyfert 2 galaxies

The paper presents soft X-ray spectra of three Seyfert 2 galaxies obtained with the ROSAT PSPC. For two of these objects, NGC 5506 and MCG -5-23-16, most of the flux occurs at energies greater than 1 keV and probably represents a low-energy extension of the source seen at higher (greater than 2 keV) energies. The third source, Mrk 348, has very little flux in the ROSAT band, suggesting a photoelectric cutoff at energies above the ROSAT band, in agreement with Ginga observations. In all three galaxies, X-rays are detected at energies below these photoelectric cutoffs with flux above that expected from a model in which the source has the same spectral index as at higher energies (2-10 keV) and is covered by a uniform, cold, photoelectric absorber. This result indicates the presence of an additional component at soft energies, which is referred to as a 'soft excess'. The monochromatic flux of the soft excess measured at 1 keV is about 1-2 percent of the directly viewed hard X-ray continuum in all three galaxies.

Mulchaey, John S.↗
NASA NTRS1994

Correlative analysis of hard and soft x ray observations of solar flares

We have developed a promising new technique for jointly analyzing BATSE hard X-ray observations of solar flares with simultaneous soft X-ray observations. The technique is based upon a model in which electric currents and associated electric fields are responsible for the respective heating and particle acceleration that occur in solar flares. A useful by-product of this technique is the strength and evolution of the coronal electric field. The latter permits one to derive important flare parameters such as the current density, the number of current filaments composing the loop, and ultimately the hard X-ray spectrum produced by the runaway electrons. We are continuing to explore the technique by applying it to additional flares for which we have joint BATSE/Yohkoh observations. A central assumption of our analysis is the constant of proportionality alpha relating the hard X-ray flux above 50 keV and the rate of electron acceleration. For a thick-target model of hard X-ray production, it can be shown that cv is in fact related to the spectral index and low-energy cutoff of precipitating electrons. The next step in our analysis is to place observational constraints on the latter parameters using the joint BATSE/Yohkoh data.

Zarro, Dominic M.↗
NASA NTRS1993

A nonthermal superbubble in the irregular galaxy IC 10

We present synthesis radio continuum observations of the nearby irregular galaxy IC 10. These observations, at 6, 20, and 49 cm, allow us to measure the flux and spectral index of a number of resolved sources in IC 10. While most of these are easily identified as thermal emission from H II regions and a few are nonthermal background sources, one extended, nonthermal source appears to be a superbubble in IC 10. Its large size (about 250 pc) implies that it is most likely the product of several supernovae. Comparisons of these radio observations with Halpha, H I, and optical imaging observations reveal that the large nonthermal superbubble is associated with a region of star formation containing two of the most luminous H II regions and the most massive H I cloud in IC 10. We tentatively identify a stellar cluster with two Wolf-Rayet stars in the center of the superbubble. We propose that this superbubble in IC 10 represents a bridge between the giant H II regions and the H I shells and supershells observed in our Galaxy and external galaxies.

Yang, Hui↗
NASA NTRS1994

High-energy gamma-ray emission from solar flares: Constraining the accelerated proton spectrum

Using a multi-component model to describe the gamma-ray emission, we investigate the flares of December 16, 1988 and March 6, 1989 which exhibited unambiguous evidence of neutral pion decay. The observations are then combined with theoretical calculations of pion production to constrain the accelerated proton spectra. The detection of pi(sup 0) emission alone can indicate much about the energy distribution and spectral variation of the protons accelerated to pion producing energies. Here both the intensity and detailed spectral shape of the Doppler-broadened pi(sup 0) decay feature are used to determine the spectral form of the accelerated proton energy distribution. The Doppler width of this gamma-ray emission provides a unique diagnostic of the spectral shape at high energies, independent of any normalisation. To our knowledge, this is the first time that this diagnostic has been used to constrain the proton spectra. The form of the energetic proton distribution is found to be severely limited by the observed intensity and Doppler width of the pi(sup 0) decay emission, demonstrating effectively the diagnostic capabilities of the pi(sup 0) decay gamma-rays. The spectral index derived from the gamma-ray intensity is found to be much harder than that derived from the Doppler width. To reconcile this apparent discrepancy we investigate the effects of introducing a high-energy cut-off in the accelerated proton distribution. With cut-off energies of around 0.5-0.8 GeV and relatively hard spectra, the observed intensities and broadening can be reproduced with a single energetic proton distribution above the pion production threshold.

Alexander, David↗
NASA NTRS1994

On determining the spectrum of primordial inhomogeneity from the COBE DMR sky maps: Results of two-year data analysis

A new technique of Fourier analysis on a cut sky has been applied to the two-year Cosmic Background Explorer (COBE) Differential Microwave Radiometer (DMR) 53 and 90 GHz sky maps. The Bayesian power spectrum estimation results are consistent with the Harrison-Zel'dovich n = 1 model. The maximum likelihood estimates of the usual parameters defining the power spectrum of primordial perturbations are n = 1.22 (1.02) and Q(sub rms-PS) = 17 (20) microK including (excluding) the quadrupole. A spectral-index-independent normalization is naturally expressed for the two-year maps in terms of the multipole amplitude a(sub 9) = 8.2 (8.3) microK (to approximately 12 sigma significance). The marginal likelihood function on n obtained by intergration with respect to a(sub 9) renders n = 1.17 +/- 0.31 (0.96 +/- 0.36).

Gorski, K. M.↗
NASA NTRS1994

Limits on three-point correlations in the COBE DMR first-year anisotropy maps

We compute the three-point temperature correlation function of the Cosmic Background Explorer (COBE) Differential Microwave Radiometer (DMR) first-year sky maps to search for non-Gaussian temperature fluctuations. The level of fluctuations seen in the computed correlation function are too large to be attributable solely to instrument noise. However the fluctuations are consistent with the level expected to result from a superposition of istrument noise and sky signal arising from a Gaussian power-law model of initial fluctuations, with a quadrupole normalized amplitude of 17 micro K and a power-law spectral index n = 1. We place limits on the amplitude of intrinsic three-point correlations with a variety of predicted functional forms.

Hinshaw, G.↗
NASA NTRS1994

Electromagnetic and corpuscular emission from the solar flare of 1991 June 15: Continuous acceleration of relativistic particles

Data on X-, gamma-ray, optical and radio emission from the 1991 June 15 solar flare are considered. We have calculated the spectrum of protons that produces gamma-rays during the gradual phase of the flare. The primary proton spectrum can be described as a Bessel-function-type up to 0.8 GeV and a power law with the spectral index approximately equal to 3 from 0.8 up to 10 GeV or above. We have also analyzed data on energetic particles near the Earth. Their spectrum differed from that of primary protons producing gamma-ray line emission. In the gradual phase of the flare additional pulses of energy release occurred and the time profiles of cm-radio emission and gamma-rays in the 0.8-10 MeV energy band and above 50 MeV coincided. A continuous and simultaneous stochastic acceleration of the protons and relativistic electrons at the gradual phase of the flare is considered as a natural explanation of the data.

Kocharov, L. G.↗
NASA NTRS1994

Steepened structures in equatorial spread F. 1: New observations

Sounding rocket data from the 1990 Combined Release and Radiation Effects Satellite (CRRES)/EQUIS equatorial spread F campaign on Kwajalein Atoll are presented. Two Terrier Malamute sounding rockets were launched into active spread F conditions on July 30 and August 2, respectively, and achieved apogee slightly below 500 km, just above the F peak. Plasma frequency probes aboard both rockets showed that the unstable nighttime F region is characterized by propagating steepened structures. Power density spectra for the structures typically exhibit two regions that obey k(exp -n) scaling, where n is approximately equal to 2 at wavelengths greater than 80-100 m and approximately equal to 5 at shorter wavelengths. These spectral indices are quiet variable, and the long-wavelength spectral index in particular seems to decrease with increasing amplitude of density fluctuations. The rocket payloads also measured vector electric fields in the plane perpendicular to B over wavelengths ranging from over 60 km to less than 6 m. Both vector components of the perpendicular electric field are proportional to delta n/n at wavelengths longer than 300 m but assume a Boltzmann relationship (with square of the absolute value of delta E approximately equal to (k(exp2))(square of the absolute value of (delta n/n)) at smaller scales. The perturbed zonal electric field, delta E(sub x), dominates the vertical field, delta E(sub z), at long wavelenghts, but the situation is reversed at smaller scales.

Hysell, D. L.↗
NASA NTRS1994

Evidence of circumstellar matter surrounding the Hercules X-1 system

We analyze data from two eclipse ingresses of Her X-1 observed with Ginga on 1989 April 30 and May 19. These observations occur, respectively, during the MAIN HIGH and SHORT HIGH states in the 35 day modulation of Her X-1 intensity. We find significant residual X-ray flux during eclipse, with a gradual decrease in flux following the occultation of the neutron star by the atmosphere of HZ Her. During the central part of the eclipse the count rate becomes nearly constant, at 0.5 mCrab in the energy range 1.7-36.8 keV. From a spectral analysis of the residual emission during the total eclipse of the central source in the MAIN HIGH state, we determine the energy spectral index, alpha = 0.8, similar to that before eclipse. A remarkable feature of the eclipse spectrum is that it does not show a significant iron line feature in contrast to massive wind-fed pulsars, such as Vela X-1 and Cen X-3. From a timing analysis of the same eclipse data, we show that there are no pulses. These results imply that the emission comes from the scattering of continuum X-rays by material in a region considerably larger than the companion star. An extended accretion disk corona may be responsible for this scattering. However, partial eclipse of an extended accretion disk corona may be responsible for this scattering. However, partial eclipse of an extended accretion disk corona is insufficient to account for the count rates in mid-eclipse, when known parameters of the binary system are used. Based on the present results, we suggest that scattering occurs not only in the accretion disk corona but also in the circumstellar matter surrounding the system of Her X-1/HZ Her.

Choi, C. S.↗