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NASA NTRS1994

Multiwavelength observations of Hercules X-1/HZ Herculis

We present first results from a multiwavelength campaign to observe the binary pulsar system Her X-1/ HZ Her. The campaign spanned 4 days during 1993 August: observations were taken with five satellites (IUE, EUVE, ROSAT, ASCA, and CGRO/BATSE) and several ground-based facilities. A substantial, unexpected drop in flux at X-ray energies, with no change in absorbing column density, was observed. The pulse period has increased from the previous measurement, contrary to the usual spin-up, and no pulsed emission is detected above 0.9 keV in the low state. The optical and UV fluxes continued to show 1.7 day modulation attributed to X-ray heating of the companion star. However, the significant reduction in UV flux observed around eclipse implies an absence of the normally observed excess attributed to X-ray heating of the disk. We conclude that we have observed an anomalous low state, seen only once before (Parmar et al. 1985), in which the X-ray flux is not redistributed but obscured. We suggest explanations for the behavior of the flux at different wavelengths.

Vrtilek, S. D.↗
NASA NTRS1994

Solar activity and coronal mass ejections on the western hemisphere of the Sun in mid-August 1989: Association with interplanetary observations at the ICE and IMP 8 spacecraft

During the 10-day period from August 12 to 21, 1989, a sequence of coronal mass ejections (CMEs) was observed above the west limb of the Sun by the Solar Maximum Mission (SMM) coronagraph. Most of these CMEs apparently originated in the vicinity of one particularly active region during its passage from near central meridian to behind the west limb of the Sun. We present observations made at 1 AU during this period by the International Cometary Explorer (ICE) (formerly International Sun Earth Explorer-3 (ISEE 3)) and Interplanetary Monitoring Platform (IMP 8) spacecraft which were separated by approximately 75 deg in heliolongitude. Following CMEs on August 12 associated with solar events at approximately W40 deg, IMP 8 (in Earth orbit) detected a strong shock followed by signatures in magnetic field, solar wind plasma, and energetic ion data which suggest that CME-related material ('ejecta') forming the shock driver engulfed the spacecraft. This spacecraft only observed weak shocks, and no ejecta, from later CMEs originating further west of the spacecraft. In contrast ICE, off the west limb at approximately W75 deg, observed the shock from the W40 deg event but failed to encounter the shock driver, whereas clear ejecta signatures were observed following events further west, closer to the spacecraft heliolongitude. The disappearance of these signatures (which include bidirectional energetic ion flows, bidirectional solar wind heat fluxes, quiet, enhanced magnetic fields and anomalously cool plasma) at IMP 8 and their emergence at ICE as the solar source region moved westward supports the association of such signatures with ejecta related to CMEs. The dual-spacecraft observations are also consistent with the conclusion of Richardson and Cane (1993) that ejecta at 1 AU typically extend approximately 50 degs in longitude from the solar source. Some plausible associations between particular intervals of ejecta signatures at ICE and individual CMEs are made. However, these associations are complicated by the large number of CMEs present, by intermittent ICE data coverage, and by uncertainties in the ejecta propagation speeds to the spacecraft.

Richardson, I. G.↗
NASA NTRS1994

The value of NDBC observations during March 1993's 'Storm of the Century'

National Data Buoy Center (NDBC) observations from the western Gulf of Mexico Provided several early indications of more rapid cyclogenesis on 12 March 1993 than was forecast by numerical weather prediction (NWP) models. Observations demostrated a closed circulation with gale-force winds shortly after the storm entered the Gulf of Mexico. Pressure measurements at two buoys off the Texas coast were 4-6 hPa lower than the 12-h NWP forecasts, a significant forecast error. Observations from NDBC's moored buoys and Coastal-Marine Automated Network stations revealed that the developing storm was significantly deeper than was indicated on the National Meteorological Center's automated surface analyses. Ocean wave observations reveal some of the steepest waves NDBC has ever measured, indicating phenomenonal wave growth and a high potential for damage to vessels and structures. A warm eddy caused sea surface temperatures (SST) to be several degrees above normal under the track of the storm, creating a strong SST gradient to the north. This provided ample energy and strengthened the baroclinity. NDBC observations showed the eddy to be somewhat larger and warmer than indicated by the most recent National Hurricane Center analysis. This event demonstrates the tremendous value of NDBC marine observations for accurately detecting the occurrence and strength of coastal cyclogenesis events.

Gilhousen, David B.↗
NASA NTRS1995

HST observations of the ring around SN 1987A

Hubble Space Telescope observations of SN 1987A show that the circumstellar material near the supernova forms a clumpy elliptical ring. We analyze six epochs of images made with the Faint Object Camera (FOC) imaging the lines of (O III) and H beta over the interval 1990 August-1993 October, and three epochs of observations with the Planetary Camera in the continuum and (N II) line over the interval 1992 April-1993 May. The elliptical ring in the (O III) images has a semimajor axis of 858 +/- 11 milliarcsec and a semiminor axis of 621 +/- 11 milliarsce (6.4 x 10(exp 17) and 4.6 x 10(exp 17) +/- 8.1 x 10(exp 15) cm, respectively, for a distance of 50 kpc), which implies an inclination angle of 44 +/- 1, if the ring is a circle inclined to our line of sight. The apparent width of the ring in (O III) is 121 +/- 22 milliarcsec (9.0 x 10(exp 16) +/- 1.6 x 10(exp 16) cm) based on our deconvolved image, which represents an upper limit to the width of the of the ring. The position angle of the major axis on the sky is 89 deg +/- 3 deg. One of the 'clumps' on the ring is a superposed star, whose energy distribution is consistent with an A-type dwarf (V = 20.2 mag) in the Large Magellanic Cloud (LMC) The radial profile of the ring is not symmetric, with the inner boundary sharper than the outer. Based on the radial profiles and the flux interior to the ring, we conclude that the ring is indeed a tilted ring, and not a spheroidal shell. While the observed width of the ring is approximately 10(exp 17) cm, we find that the average path length through the emitting gas is approximately 4 x 10(exp 15) cm, so the filling factor is small. The ring has faded by a factor of approximately equals 3.7 in (O III) from 1990 August to 1993 October. The characteristic electron density from the (O III) lambda 5007 emission line averages 1.2 x 10exp 4)/cu cm, based on recombination models of the emission. Because the fading rate is uncorrelated with the surface brightness, the density appears to be independent of the brightness, which implies significant path length differences through the ring. By assuming the current structure observed in optical lines applies to the early structure in ultraviolet lines, we construct light curve models to compare with early IUE observations. The clumpy structure of the ring has relatively little effect on the light curves. The (O III) images show faint arclike nebulosity probably associated with the bipolar nebula around SN 1987A and projected inside the ring, which may have affected previous observations of the ring. Finally, we find that the fading rate provides evidence, though weak, favoring an expanding ring over a contracting one.

Plait, Philip C.↗
NASA NTRS1994

The HST quasar absorption line key project. 4: HST faint-object spectrograph and ground-based observations of the unusual low-redshift broad absorption-line quasi-stellar object PG 0043+039

Hubble Space Telescope Faint Object Spectrograph (HST FOS) observations have shown that the spectrum of the low-redshift (z(sub em) approximately equal to 0.384) QSO PG 0043+039 exhibits weak broad absorption lines (BALs). The BALs were discovered during the course of UV spectrophotometry made for the HST Quasar Absorption Line Key Project. The HST data are analyzed along with ground-based optical and IUE spectrophotometry. The object is found to have a number of atypical properties relative to normal non-BAL QSOs. The observed continuum is atypical in the sense that it is much weaker than that of a normal optically selected QSO at rest wavelengths approximately less than 2200 A. Intrinsic reddening of E(B-V) approximately equal to 0.11 mag by dust similar to that found in the SMC at the redshift of PG 0043+039 conservatively accounts for the observed continuum shape moderately well. These observed characteristics are typical of low-ionization BAL QSOs, but convincing evidence for BALs due to low-ionization transitions of Mg II, Al III, Al II, or C II does not exist. Therefore, this object may be a misaligned BAL QSO having many of the characteristics of low-ionization BAL QSOs with the sight line passing through a putative dusty region, but evidently missing clouds of high enough column density to produce observable low-ionization BALs. If the intrinsic dust-extinction model is correct, the observations suggest that the dust is not confined to the presumably higher density, low-ionization BAL clouds, but that it has drifted to nearby high-ionization BAL regions. We also consider other possible mechanisms for producing the shape of the continuous energy distribution which cannot be ruled out. We compare the Fe II emission in PG 0043+039 with that in another Key Project QSO, NGC 2841-UB 3, which has optical Fe II emission comparable in strength to that in PG 0043+039, but has anomalously weak UV Fe II emission. In addition, from an analysis of UV and optical spectrophotometric data at 5 epochs over approximately 11 yr, there is tentative evidence that PG 0043+039 has varied in brightness by as much as 1.1 mag during this time interval. Two different interpretations of PG 0043+039 and the low-ionization BAL QSOs are considered. Various model scenarios for explaining the weak narrow-line (O III) emission are considered, but there is no definitive explanation.

Turnshek, David A.↗
NASA NTRS1995

Observations of the X-ray Nova GRO J0422+32. 1: Outburst and the decay to quiescence

We present optical photometry and spectroscopy and Burst and Transient Source Experiment (BATSE) observations of the X-ray nova GRO J0422+32, obtained during outburst and its subsequent decay to quiescence. Although the X-ray and optical properties of GRO J0422+32 are broadly similar to those of other X-ray novae, it is unique in several respects. The unusually protracted decay to quiescence of the optical light curve has been punctuated by at least two minioutbursts of approximately 4 mag. The BATSE and optical outbursts are each separated by approximately 120 days. We find that the optical luminosity of GRO J0422+32 during the primary outburst is dominated by reprocessing of E greater than 10 keV X-rays. In contrast, the optical minioutbursts are most likely generated by an intrinsically bright disk rather than X-ray reprocessing: they do not appear to have any X-ray counterparts. Extremely broad (up to 6000 km/s FWZI) absorption lines have also been observed during both primary outbursts and minioutbursts. During the second minioutburst, H-alpha and H-beta emission was observed superposed on redshifted absorption features. We find that the interoutburst light curve of GRO J0422+32 may be inconsistent with an accretion disk instability as the origin of the minioutbursts. Finally, a transient 5.1/10.2 hr modulation, which may be related to the orbital period, has been observed during roughly half of our observations. However, confirmation of the orbital period must await observations in quiescence.

Callanan, Paul J.↗
NASA NTRS1994

A method for correcting aspect solution errors in ROSAT HRI observations of compact sources

X-ray point sources observed with the ROSAT High Resolution Imager (HRI) often appear elongated over scales of approximately 5 sec-10 sec from the image core. This elongation has been attributed to errors in the attitude correction as the satellite is wobbled during the observations, and affects sources with both soft and hard X-ray spectra. In this paper, I report the results of an attempt to rid a high signal-to-noise observation of the soft X-ray point source HZ 43 of its characteristic elongation. I divided the observation into 181 separate images, each containing photons from only a small region on the detector through which the source passed during the satellite's wobble. By measuring the positions of the individual image centroids, I found clear evidence for systematic offsets from a common mean by up to approximately +/- 3 sec in both right ascension and declination as a function of phase in the satellite wobble. Shifting the subimages to a common center and then restacking them into a single image measurably improved the symmetry of the point-spread function. HRI observations are wobbled primarily to smooth out variations in the pixel-to-pixel sensitivity of the detector and also to extend the lifetime of the microchannel plates in the detector since these decay at a given location as a function of the number of photons detected. However, the elongations introduced by the aspect errors inhibit the identification of possible extended X-ray emission associated with sources such as pulsars and active galactic nuclei. In light of these results, I suggest that until the aspect errors are understood, observations of compact sources, where this effect may be important, should not be wobbled.

Morse, Jon A.↗
NASA NTRS1994

ROSAT observations of the Jupiter aurora

Roentgen satellite (ROSAT) high-resolution imager (HRI) and position sensitive proportional counter (PSPC) observations of Jupiter obtained in April 1991 and May 1992 reveal soft X-ray emissions apparently associated with Jupiter's aurora and similar to X-ray emssions observed earlier by the Einstein Observatory. The HRI images show emission mainly from Jupiter's northern hemisphere at all Jovian longitudes observed, and there is some indication of a longitudinal modulation of the emission in phase with well-known ultraviolet modulation of the northern aurora. The PSPC data reveal a very soft spectrum. Comparison of the observed spectrum with models for both electron bremsstrahlung radiation and line emission from S and O ions indicates that the line spectrum gives a much better statistical fit to the observed spectrum. The X ray observations presented here therefore support the hypothesis that ion precipitation is the most likely cause of the Jovian X ray emissions, a result first suggested by the Einstein results (Metzger et al., 1983).

Waite, J. H., Jr.↗
NASA NTRS1995

Earth observations from space: History, promise, and reality. Executive summary

In this report the Committee on Earth Studies (CES), a standing committee of the Space Studies Board (SSB) within the National Research Council (NRC), reviews the recent history (nominally from 1981 to 1995) of the U.S. earth observations programs that serve civilian needs. The principal observations programs examined are those of NASA and the National Oceanic and Atmospheric Administration (NOAA). The Air Force' s Defense Meteorological Satellite Program (DMSP) is discussed, but only from the perspective of its relationship to civil needs and the planned merger with the NOAA polar-orbiting system. The report also reviews the interfaces between the earth observations satellite programs and the major national and international environmental monitoring and research programs. The monitoring and research programs discussed are the U.S. Global Change Research Program (USGCRP), the International Geosphere-Biosphere Program (IGBP), the World Climate Research Program (WCRP), related international scientific campaigns, and operational programs for the sharing and application of environmental data. The purpose of this report is to provide a broad historical review and commentary based on the views of the CES members, with particular emphasis on tracing the lengthy record of advisory committee recommendations. Any individual topic could be the subject of an extended report in its own right. Indeed, extensive further reviews are already under way to that end. If the CES has succeeded in the task it has undertaken. This report will serve as a useful starting point for any such more intensive study. The report is divided into eight chapters: ( I ) an introduction, (2) the evolution of the MTPE, (3) its relationship to the USGCRP, (4) applications of earth observations data, (5) the role that smaller satellites can play in research and operational remote sensing, (6) earth system modeling and information systems, (7) a number of associated activities that contribute to the MTPE and the USGCRP, and (8) organizational issues in the conduct of civil earth observations programs. Following the body of the report is a series of appendixes: after a list of acronyms and abbreviations and collected short biographies of CES members, six brief tutorials discuss several scientific topics important to the science and applications of earth observations. Highlights from the eight chapters follow.

Source record↗
NASA NTRS1995

Simultaneous Extreme-Ultraviolet Explorer and Optical Observations of Ad Leonis: Evidence for Large Coronal Loops and the Neupert Effect in Stellar Flares

We report on the first simultaneous Extreme-Ultraviolet Explorer (EUVE) and optical observations of flares on the dMe flare star AD Leonis. The data show the following features: (1) Two flares (one large and one of moderate size) of several hours duration were observed in the EUV wavelength range; (2) Flare emission observed in the optical precedes the emission seen with EUVE; and (3) Several diminutions (DIMs) in the optical continuum were observed during the period of optical flare activity. To interpret these data, we develop a technique for deriving the coronal loop length from the observed rise and decay behavior of the EUV flare. The technique is generally applicable to existing and future coronal observations of stellar flares. We also determine the pressure, column depth, emission measure, loop cross-sectional area, and peak thermal energy during the two EUV flares, and the temperature, area coverage, and energy of the optical continuum emission. When the optical and coronal data are combined, we find convincing evidence of a stellar 'Neupert effect' which is a strong signature of chromospheric evaporation models. We then argue that the known spatial correlation of white-light emission with hard X-ray emission in solar flares, and the identification of the hard X-ray emission with nonthermal bremsstrahlung produced by accelerated electrons, provides evidence that flare heating on dMe stars is produced by the same electron precipitation mechanism that is inferred to occur on the Sun. We provide a thorough picture of the physical processes that are operative during the largest EUV flare, compare and contrast this picture with the canonical solar flare model, and conclude that the coronal loop length may be the most important factor in determining the flare rise time and energetics.

Hawley, Suzanne L.↗
NASA NTRS1995

Observations of Gamma-Ray Bursts

Some basic observed properties of gamma-ray bursts are reviewed. Although some properties were known 25 years ago, new and more detailed observations have been made by the Compton Observatory in the past three years. The new observation with the greatest impact has been the observed isotropic distribution of bursts along with a deficiency of weak bursts which would be expected from a homogeneous burst distribution. This is not compatible with any known Galactic population of objects. Gamma-ray bursts show an enormous variety of burst morphologies and a wide spread in burst durations. The spectra of gamma-ray bursts are characterized by rapid variations and peak power which is almost entirely in the gamma-ray energy range. Delayed gamma-ray burst photons extending to GeV energies have been detected for the first time. A time dilation effect has also been reported to be observed in gamma-ray, bursts. The observation of a gamma-ray burst counterpart in another wavelength region has yet to be made.

Fishman, G. J.↗
NASA NTRS1996

BATSE Observations of the Large-Scale Isotropy of Gamma-Ray Bursts

We use dipole and quadrupole statistics to test the large-scale isotropy of the first 1005 gamma-ray bursts observed by the Burst and Transient Source Experiment (BATSE). In addition to the entire sample of 1005 gamma-ray bursts, many subsets are examined. We use a variety of dipole and quadrupole statistics to search for Galactic and other predicted anisotropies and for anisotropies in a coordinate-system independent manner. We find the gamma-ray burst locations to be consistent with isotropy, e.g., for the total sample the observed Galactic dipole moment (cos theta) differs from the value predicted for isotropy by 0.9 sigma and the observed Galactic quadrupole moment (sin(exp 2) b - 1/3) by 0.3 sigma. We estimate for various models the anisotropies that could have been detected. If one-half of the locations were within 86 deg of the Galactic center, or within 28 deg of the Galactic plane, the ensuing dipole or quadrupole moment would have typically been detected at the 99% confidence level. We compare the observations with the dipole and quadrupole moments of various Galactic models. Several Galactic gamma-ray bursts models have moments within 2 sigma of the observations; most of the Galactic models proposed to date are no longer in acceptable agreement with the data. Although a spherical dark matter halo distribution could be consistent with the data, the required core radius is larger than the core radius of the dark matter halo used to explain the Galaxy's rotation curve. Gamma-ray bursts are much more isotropic than any observed Galactic population, strongly favoring but not requiring an origin at cosmological distances.

Briggs, Michael S.↗
NASA NTRS1997

Astrometric Observations of Comets and Asteroids and Subsequent Orbital Investigations

An earlier series of photographic observations was made with the 1.5-m reflector from 1972 to 1989. The start of the series to which this report refers occurred shortly before the conversion from photographic to CCD operation in August 1989, at which point there was a dramatic increase in the productivity of the program. This is evident gives a month-by-month summary of the observations; the earlier data refer to the measurement or remeasurement of photographic plates previously taken with the same telescope. The total number of observations made was 24,423, of which 1338 were of comets. Of the 23,085 observations of asteroids, 21,529 referred to asteroids that were unnumbered when the observations were made. Since an important emphasis of the program was to improve knowledge of the orbits to the point where asteroids can be numbered, the fact that only 4262 of the observations refer to asteroids that are still unnumbered is a measure of the program's success, with 30-35 percent of all the new numberings being habitually made solely because of the recent data from the Oak Ridge program, which even at the time of McCrosky's retirement was still the fourth largest comet-asteroid astrometric program in the world.

Marsden, Brian G.↗
NASA NTRS1997

Mars Telescopic Observations Workshop II

Mars Telescopic Observations Workshop E convened in Tucson, Arizona, in October 1997 by popular demand slightly over two years following the first successful Mars Telescopic Observations Workshop, held in Ithaca, New York, in August 1995. Experts on Mars from the United Kingdom, Japan, Germany, and the United States were present. Twenty-eight oral presentations were made and generous time allotted for useful discussions among participants. The goals of the workshop were to (1) summarize active groundbased observing programs and evaluate them in the context of current and future space missions to Mars, (2) discuss new technologies and instrumentation in the context of changing emphasis of observations and theory useful for groundbased observing, and (3) more fully understand capabilities of current and planned Mars missions to better judge which groundbased observations are and will continue to be of importance to our overall Mars program. In addition, the exciting new discoveries presented from the Pathfinder experiments and the progress report from the Mars Global Surveyor infused the participants with satisfaction for the successes achieved in the early stages of these missions. Just as exciting was the enthusiasm for new groundbased programs designed to address new challenges resulting from mission science results. We would like to thank the National Aeronautics and Space Administration as well as Dr. David Black, director of the Lunar and Planetary Institute, and the staff of the Institute's Publications and Program Services Department for providing logistical, administrative, and publication support services for this workshop.

Sprague, A. L.↗
NASA NTRS1997

Contemporaneous IUE, EUVE, and High-Energy Observations of 3C 273

We present the results of our 1994 January and 1995 January observations of the quasar 3C 273 obtained with the International Ultraviolet Explorer (IUE) and the Extreme-Ultraviolet Explorer (EUVE). These observations were part of a large multiwavelength campaign to observe 3C 273 from radio through gamma-rays. Our 1995 January photometric observations with the EUVE Lexan/B Deep Survey (DS) instrument indicate strong evidence for variability, at a 99% confidence level, during the 12 day observing period. We have utilized ROSAT PSPC soft X-ray power-law models to correlate with EUVE count rates. Besides variations in the normalization level between both observations, our EUV count rates are consistent with a simple power-law model with spectral index alpha approx. 1.77 (F(sub upsilon) proportional to upsilon(sup -alpha) that can be extrapolated from the soft X-rays to the EUV range. The active galactic nucleus 3C 273 is an important blazar to study because in our picture it reveals the presence of both disk and relativistic beam spectral contributions.

Ramos, E.↗
NASA NTRS1997

A Survey of Large-Scale Variations in Thermospheric Oxygen Column Density with Magnetic Activity as Inferred from Observations of the FUV Dayglow

Brightness of the terrestrial far-ultraviolet (FUV) dayglow is dominated by the 130.4-nm emission of neutral atomic oxygen, OI, and variations in the brightness observed from altitudes high above the emitting region reflect variations in thermospheric oxygen density. This paper summarizes the results of an initial survey of the Dynamics Explorer 1 observations of the FUV dayglow through a presentation of 13 representative events selected to demonstrate the spatial extent and short-term temporal stability of the brightness perturbations. The emphasis here is on the morning sector of local time and the polar cap for observations obtained in the time interval from September 23, 1981, through January 19, 1982. An analytic expression is derived for the average response of the FUV photometer to the dayglow during periods of high-latitude magnetic quiescence. The remaining observations in this time interval are then analyzed for their deviations from the established quiet time values. Deviations of -40% to +30% are found following intervals of increased magnetic activity. The most significant decreases (-30% to -40%) are observed equatorward of the instantaneous aurora] oval only after sustained periods (approx. 6 hours) of intense magnetic activity (average AE greater than approx. 700 nT). Decreases extend equatorward from the aurora to geographic latitudes as low as approx. 30 deg N. Decreases of lesser magnitude that do not extend as far equatorward are associated with sustained periods of more moderate activity in which the average value of AE is smaller (approx. 300-400 nT). Also, the spatial extent and magnitude of the decreases in the morning sector appear greater when the IMF B(sub y) component is positive. In both cases, decreases are readily observed within the polar cap. Localized enhancements of +20% to +30% occur much less frequently and are detected at the middle latitudes, well equator-ward of the auroral oval.

Nicholas, A. C.↗
NASA NTRS1998

Contrast Between 1992 and 1997 High Latitude Spring Haloe Observations of Lower Stratospheric HCl

HCl measurements from HALOE in the northern hemisphere during mid-May 1997 revealed vortex fragments in which the chlorine reservoir partitioning was strongly pushed toward HCl (approx. 90% HCl, approx. 10% ClONO2), similar to partitioning previously observed in the Antarctic vortex region. In contrast, observations of ClONO2 and HCl in the northern polar spring, 1992, and in other years, show these species established the balance typical for gas phase photochemical reactions in this region (approx. 60% HCl, approx. 40% ClONO2). Annually, chlorine reservoirs in the winter lower stratosphere polar vortex are converted to chlorine radicals via heterogeneous reactions on particle surfaces at very cold temperatures (less than about 200 K). As temperatures warm in spring, the heterogeneous processes become insignificant compared with gas phase reactions, and the chlorine reservoirs are reformed. Measurements through the northern winter/spring in 1992 show rapid formation of ClONO2, followed by steady loss of ClONO2 and increasing HCl. Although ClONO2 measurements are not available for 1997, the HCl increase in 1997 is observed to be much more rapid and the eventual HCl mixing ratio is about 50% greater than that of 1992. The observations are examined through comparison with the Goddard three-dimensional chemistry and transport model. This model utilizes winds and temperatures from the Goddard Earth Observing System Data Assimilation System and a complete integration scheme for stratospheric photochemistry. Analysis of the evolution of HCl and ClONO2 shows that the observed difference in the overall rate of HCl formation is explained by the sensitivity of the gas-phase chemistry to the ozone mixing ratio and the temperature. The results show that the model accurately simulates HCl and ClONO2 evolution during these two winters. Model validity is further supported by comparisons with O3 and reactive nitrogen species NO and NO2. This analysis provides a sensitive test of the lower stratospheric chlorine photochemistry, particularly because the analysis considers constituent evolution at a time when the HCl and ClONO2 are far from a photochemical stationary state.

Douglass, A. R.↗
NASA NTRS1998

RXTE Observation of Cygnus X-1: TIming Analysis - Report 2

We present timing analysis for a Rossi X-ray Timing Explorer (RXTE) observation of Cygnus X-1 in its hard/low state. This was the first RXTE observation of Cyg X-1 taken after it transited back to this state from its soft/high state. RXTE's large effective area, superior timing capabilities, and ability to obtain long, uninterrupted observations have allowed us to obtain measurements of the power spectral density (PSD), coherence function, and Fourier time lags to a decade lower in frequency and half a decade higher in frequency than typically was achieved with previous instruments. Notable aspects of our observations include a weak 0.005 Hz feature in the PSD coincident with a coherence recovery; a 'hardening' of the high-frequency PSD with increasing energy; a broad frequency range measurement of the coherence function, revealing rollovers from unity coherence at both low and high frequency; and an accurate determination of the Fourier time lags over two and a half decades in frequency. As has been noted in previous similar observations, the time delay is approximately proportional to f(exp -0.7), and at a fixed Fourier frequency the time delay of the hard X-rays compared to the softest energy channel tends to increase logarithmically with energy. Curiously, the 0.01-0.2 Hz coherence between the highest and lowest energy bands is actually slightly greater than the coherence between the second highest and lowest energy bands. We carefully describe all of the analysis techniques used in this paper, and we make comparisons of the data to general theoretical expectations. In a companion paper, we make specific comparisons to a Compton corona model that we have successfully used to describe the energy spectral data from this observation.

Nowak, Michael A.↗