Engineering topics
Publications and source records attributed to Bowyer, S..
In the following we present material in tabular and graphical form, with the aim to allow the non specialist to obtain a realistic estimate of the diffuse night sky brightness over a wide range of wavelengths from the far UV longward of Ly to the far-infrared.
An observation of M 87, the central galaxy of the Virgo cluster, was performed in the 0.065 to 0.245 keV energy band by the Extreme Ultraviolet Explorer (EUVE). A central source and an extended emission halo are visible which represent the first detection of a cluster gas emission in the extreme ultraviolet. A gas component with a temperature of between 5 x 10(exp 5) and 10(exp 6) K has to be introduced in order to explain this emission. The rapid cooling of plasmas at such temperatures implies a mass accretion rate of greater than 300 solar mass/yr. Data from Rosat observations provide evidence of an extended central emission at 10(exp 6) K.
The optical counterpart of the new extreme ultraviolet source EUVE J1016-053 (=RE 1016-053) is known to show intermittent sharp Balmer and HE I emission and traces of an M dwarf associated with a white dwarf optical spectrum. We present extensive optical spectroscopy showing the emission lines vary in velocity and intensity on a period of 0d.78929 +/- 0d.00003. The phase of the emission-line strengths lags that of the velocities by 0.237 +/- 0.013 cycle, consistent with the quarter-cycle offset expected if emission arises from the side of the secondary facing the hot white dwarf. EUVE J1016-053 is another example of an extreme ultraviolet illumination-effect binary, which only recently emerged from a common-envelope phase of binary evolution. Based on spectroscopic measurements and new BVR photometry, we find that the M dwarf secondary's contribution to the combined light is smaller than previously reported, probably because of the difficulty of avoiding contamination from a third star 3".2 distant. The velocity of the weak H II absorption is in antiphase to the emission, suggesting that it originates in the white dwarf photosphere. The mass function implied by the emission-line motion is f(M) = 0.28 +/- 0.08 M solar. Assuming the He II absorption does follow the white dwarf, the mass ratio MWD/MdM is 1.8 +/- 0.5, and the gravitational redshift is 45 +/- 14 km s-1. The modulation of the emission lines suggests the inclination i > 40 degrees, but plausible masses demand this inclination be well above the value.
Using the Berkeley spectrometer of the Orbiting Retrievable Far and Extreme Ultraviolet Spectrometer (ORFEUS) we observed the 87-117 nm UV spectrum of the BL Lac object PKS 2155-304 with about 0.5 A resolution. In addition to the expected interstellar lines we detected higher quantum number counterparts of the intergalactic Lyman alpha lines discovered earlier with IUE and the Hubble Space Telescope (HST) in the direction of PKS 2155-304. The Lyman discontinuities indicate for three of the redshifted clouds a combined H I column density of 2-5 x 10(exp 16)/sq cm, while the column density for another cloud appears to be well below 5 x 10(exp 15)/sq cm. No siginificant O VI absorption in the galactic halo toward PKS 2155-304 could be detected from our data. Assuming that saturation effects are negligible for these weak features, we obtain for the O VI column density toward PKS 2155-304 a 3 sigma upper limit of 2.7 x 10(exp 14)/sq cm.
We present a few scientific highlights from the Extreme Ultraviolet Explorer (EUVE) all-sky and deep surveys, from the EUVE Righ Angle Program, and from the EUVE Guest Observer Program. The First EUVE Source Catalog includes 410 extreme ultraviolet (EUV) sources detected in the initial processing of the EUVE all-sky data. A program of optical identification indicates that counterparts include cool star coronae, flare stars, hot white dwarfs, central stars of planetary nebulae, B star photospheres and winds, an X-ray binary, extragalactic objects (active galactic nuclei, BL Lacertae), solar system objects (Moon, Mars, Io,), supernova remnants, and two novae.
We present high resolution (lambda/delta lambda = 3000) observations of the Small Magellanic Cloud (SMC) star NGC 346 #1 near the O VI resonance doublet (1031.9, 1037.6 A). The data were collected with the Berkeley extreme/far ultraviolet (EUV/FUV) spectrometer in the ORFEUS telescope aboard the space platform Astro-SPAS during the missiom of space shuttle Discovery flown in September 1993. The O VI features shows broad absorption centered near nu(sub LSR) is approximately equal to +140 km/sec. This is almost certainly associated with the OB cluster NGC 346 itself, which also shows deep absorption at Si IV and C IV at comparable velocities. An absorption feature corresponding to O VI near nu(sub LSR) = -230 km/sec is present in our data, but this feature is caused by contamination from a second star, probably an early B-type main sequence star at lower standard of rest (LSR) velocity, which is also present in the 20 min circular aperture. Between these velocity extrema lies one broad feature (b is approximately equal 70 km/sec) or multiple narrower features, with a combined column density of about 1.4 x 10(exp 14)/sq cm. Assuming an exponential distribution of O VI with the in-plane density of Jenkins, we infer a scale height, H(sub 0), of no more than approximately equal 1.2 kpc toward NGC 346 #1. Our O VI column density is about a factor of 4 below the predictions of cooling fountain models, but our low scale height is consistent with more recent predictions.
Active Galactic Nuclei (AGN) emit a substantial fraction of their total radiated power in the EUV passband, because spectroscopically this band is situated at or near the peak of the 'big blue bump,' which is bounded on either side by the UV and soft X-ray excesses. Owing to the lack of observational data, the shape of AGN spectra in the EUV is not well known. This paper presents broad band (65-190 eV) count rates of 11 AGN detected by the Extreme Ultraviolet Explorer (EUVE). Assuming a power law spectral shape for the EUV and soft X-ray energies, we demonstrate that the photon indices of the sources are considerably steeper than typical AGN indices. For 6 sources where ROSAT data are available to provide a more precise determination of their spectral parameters in the 0.1-2.5 keV range, we have used these parameters to compute predicted EUVE count rates and compared them with the observed values. The two sets of quantities agree to within experimental error. We conclude that there is no evidence of spectral steepening or turn-over of the soft X-ray excess at EUV wavelengths for a sample of AGN which covers a wide range of brightness.
This colloquium reviews findings in the Astrophysics in the Extreme Ultraviolet region. The papers presented are: (1) The Composition and Structure of White Dwarf Atmospheres Revealed by Extreme Ultraviolet Spectroscopy, (2) EUVE Spectrophotometry of QS Tel: The Second Pole Becomes Active, (3) Results from the ROSAT EUV Wide Field Camera, (4) Temporal Behaviour of Sources in the ROSAT Extreme-Ultraviolet All-Sky Survey, (5) A Search for the Signature of the Diffuse Soft X-ray Background in the ROSAT Wide-Field Camera All-Sky Survey, (6) Features of the Soft X-ray Background and Implications for the EUV Background.
We have used the set of point sources detected by the Far Ultraviolet Space Telescope (FAUST) instrument to identify galaxies and study the total galaxy flux in a 250 A wide band peaking at 1650 A. A sample of 144 galaxies has been obtained after cross-reference with the RC3 catalog, elimination of objects confused with stars and various corrections for the photometry. The UV-B color dispersion is found to increase while the galaxies get redder from late to early types. The irregular galaxies appear on average redder and the Sbc galaxies bluer than indicated by the spectral energy distributions currently used for the calculations of K-corrections. Various arguments lead us to make the assumption of a constant dust extinction within each galaxy. The UV flux per unit area decreases on average from late to early type spirals. We find a weak correlation between the UV and far infra-red emission while the infra-red to UV flux ratio gets lower when galaxies get bluer (as measured by the UV to B flux ratio). The UV flux per unit area correlates with the HI gas surface density and the total gas surface density when this quantity is available. The correlation with the molecular gas alone is weak. In the Virgo cluster, the UV flux per unit area does not decrease in direct proportion to the HI deficiency. Galaxy counts per square degree and per magnitude interval have been obtained at high-galactic latitudes. Combined with data at fainter magnitudes, they show a variation as a function of magnitude with a near-euclidean slope over a range of 8 magnitudes.
The Extreme Ultraviolet Explorer (EUVE) has conducted an all-sky survey to locate and identify point sources of emission in four extreme ultraviolet wavelength bands centered at approximately 100, 200, 400, and 600 A. A companion deep survey of a strip along half the ecliptic plane was simultaneously conducted. In this catalog we report the sources found in these surveys using rigorously defined criteria uniformly applied to the data set. These are the first surveys to be made in the three longer wavelength bands, and a substantial number of sources were detected in these bands. We present a number of statistical diagnostics of the surveys, including their source counts, their sensitivites, and their positional error distributions. We provide a separate list of those sources reported in the EUVE Bright Source List which did not meet our criteria for inclusion in our primary list. We also provide improved count rate and position estimates for a majority of these sources based on the improved methodology used in this paper. In total, this catalog lists a total of 410 point sources, of which 372 have plausible optical ultraviolet, or X-ray identifications, which are also listed.
108 +/- 11 photons of the martian He 584-A airglow detected by the Extreme Ultraviolet Explorer (EUVE) satellite during a 2-day exposure (January 22-23, 1993) correspond to the effective disk average intensity of 43 +/- 10 Rayleigh (Ra). Radiative transfer calculations, using a model atmosphere appropriate to the conditions of the observation and having an exospheric temperature of 210 +/- 20 K, result in a He mixing ratio of 1.1 +/- 0.4 ppm in the lower atmosphere. Nonthermal escape of helium is due to electron impact ionization and pickup of He(+) by the solar wind, to collisions with hot oxygen atoms, and to charge exchange with molecular species with corresponding column loss rates of 1.4 x 10(exp 5), 3 x 10(exp 4), and 7 x 10(exp 3)/sq cm/s, respectively. The lifetime of helium on Mars is 5 x 10(exp 4) years. the He outgassing rate, coupled with the Ar-40 atmospheric abundance and with the K:U:Th ratio measured in the surface rocks, is used as input to a single two-reservoir degassing model which is applied to Mars and then to Venus. A similar model with known abundances if K, U, and Th is applied to Earth. The models for Earth and Mars presume loss of all argon accumulated in the atmospheres during the first billion years by large-scale meteorite and planetesimal impacts. The models show that the degassing coefficients for all three planets may be approximated by function delta = delta(sub 0) x (t(sub 0)/t)(exp 1/2) with delta(sub 0) = 0.1, 0.04, and 0.0125 Byr for Earth, Venus, and Mars, respectively. After a R(exp 2) correction this means that outgassing processes on Venus and Mars are weaker than on Earth by factors of 3 and 30, respectively. Mass ratios of U and Th are almost the same for all three planets, while potassiumis depleted by a factor of 2 in Venus and Mars. Mass ratio of helium and argon are close to 5 x 10(exp -9) and 2 x 10(exp -8) g/g in the interiors of all three planets. The implications of these results are discussed.
The extreme ultraviolet wave band (100 to 912 angstroms) was thought until recently to be useless to astronomy, primarily because the opacity of the interstellar medium would prevent observations at these wavelengths. However, the interstellar medium has been found to be markedly inhomogeneous in both density and ionization state and the sun is fortunately located in a region of low extreme ultraviolet opacity. The Extreme Ultraviolet Explorer, launched in June 1992, has surveyed the sky in this wave band and has detected a wide variety of astronomical sources at considerable distances, including some extragalactic objects. Studies in the extreme ultraviolet band have already begun to increase our understanding of the contents of the universe.
The 108 photons of the Martian He 584 A airglow detected by the Extreme Ultraviolet Explorer satellite during a two-day exposure (22-23 Jan. 1993) correspond to the effective disk average intensity of 43 (+/-) 10 Rayleigh. Radiative transfer calculations, using a model atmosphere appropriate to the conditions of the observation and having an exospheric temperature of 210 (+/-) 20 K, result in an He mixing ratio of 1.1 (+/-) 0.4 ppm in the lower atmosphere. Nonthermal escape of helium is due to the following: electron impact ionization and pickup of He(+) by the solar wind; collisions with hot oxygen atoms; and charge exchange with molecular species with corresponding column loss rates of 1.4 x 10(exp 5), 3 x 10(exp 4), and 7 x 10(exp 3) cm(exp -2)s(exp -1), respectively. The lifetime of helium on Mars is 5 x 10(exp 4) yr. The He outgassing rate, coupled with the Ar-40 atmospheric abundance and with the K:U:Th ratio measured in the surface rocks, is used as input to a simple two-reservoir degassing model which presumes the loss of all argon accumulated in the atmosphere during the first Byr by large-scale impacts. The model results in total planet mass ratios of 10(exp -5) g/g for K, 2.3 x 10(exp -9) g/g for U, 8.5 x 10(exp -9) g/g for Th, 4 x 10(exp -10) g/g for He, and 1.5 x 10(exp -9) g/g for Ar-40. The predicted radiogenic heat flux is 2 erg cm(exp -2)s(exp -1). Similar modeling for Venus results in total plant mass ratios of 4.7 x 10(exp -5) g/g for K, 6.7 x 10(exp -9) g/g for U, 2.2 x 10(exp -8) g/g for Th, 1.3 x 10(exp -9) for He, 6.7 x 10(exp -9) g/g for Ar-40, and a radiogenic heat flux of 15 erg cm(exp -2)s(exp -1). The implications of these results are discussed. The modeling shows that the radioactive elements were not distributed uniformly in the protoplanetary nebula, and their relative abundances differ very much in the terrestrial planets.
Data obtained during the first five months of calibration and science operation of the Extreme Ultraviolet Explorer (EUVE) are presented. Spectra of an extragalactic object were obtained; the object is detectable to wavelenghts longer than 100 A, demonstrating that extragalactic EUV astronomy is possible. Spectra of a hot white dwarf, and a late-type star in quiescence and flaring are shown as examples of the type of spectrographic data obtainable with EUVE. Other objects for which broad band photometric mode data have been obtained and analyzed include an RS CVn star and several late-type stars. The backgrounds in the EUVE detectors are quite low and the character of the diffuse astronomical EUV background has been investigated using these very low rates. Evidence is presented showing that, contrary to previously published reports, EUVE is about three times more sensitive than the English Wide Field Camera in the short wavelength bandpass covered by both instruments. Only limited information has been extracted from the longer bandpasses coered only by EUVE. Nonetheless, the brightest EUV source in the sky, a B star, has been discovered and is detected only in these longer bandpasses.
Diffuse astrophysical emission at wavelengths between 900 and 1200 A (FUV(sub 900-1200 A)) contains unique information about the nature and extent of the poorly understood hot phase of the interstellar medium. Observations of the diffuse background emission in this bandpass are fraught with difficulty, and previous measurements have provided only upper limits to this flux. The data discussed in this paper were obtained with the Far Ultraviolet Coronal Rocket (FU-VCR), which had important advantages over previous instruments: intense geocoronal emission was rejected, eliminating potentially overwhelming contamination of the data; the data were spectroscopic with resolution sufficient for robust modeling of both astrophysical and instrumental data components; and point sources were imaged, allowing for the elimination of these critical contamination sources. The superior quality of these data permits the determination of new bounds to the physical properties of the hot interstellar medium.
We present early results from the Far Ultraviolet Space Telescope (FAUST), which flew in March 1992 with the ATLAS space shuttle mission. The telescope provides wide-field images in the far ultraviolet (1400-1800 A). Studies underway using the data obtained on this mission include establishing the brightness and distribution of far ultraviolet stars in the halo of our Galaxy, establishing the far ultraviolet properties of nearby galaxies and nearby clusters of galaxies, analyzing the diffuse galactic light, and searching for the origin of the extragalactic ultraviolet light. We discuss the instrument performance, and early results from these observations.
We report observations of two RS CVn systems, II Peg and AR Lac, which were calibration targets during the in-orbit checkout phase of the Extreme Ultraviolet Explorer (EUVE). We have determined count rates, and, based on these count rates, preliminary emission measure analyses have been done. The Lexan/boron band count rates were 0.0849 +/- 0.0023 counts/s for AR Lac and 0.218 +/- 0.031 counts/s for II Peg during its quiescent state. Temperatures and emission measures derived from the EUVE data are consistent with previously published values. We also report a flare seen during the II Peg observations which has characteristics similar to previously observed flares on II Peg. During the flare the Lexan/boron count rate increased by at least a factor of 2.5, and the minimum total duration was about 34,000 s (about 9.4 hr).
The history of extreme ultraviolet (EUV) astronomy is briefly reviewed, and an overview of the Extreme Ultraviolet Explorer mission, launched into a near-earth (550 km) orbit on June 7, 1992, is presented. First, the principal objective of the mission are summarized. The instrumentation and operation of the mission are then described, with particular attention given to the sky survey instruments, the deep survey instrument, and the spectrometers. The discussion also covers the current view of the interstellar medium, early results from the mission, and future prospects for EUV astronomy.