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At least 649 records · Page 36
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NASA NTRS1994

Simultaneous soft and hard X-ray spectroscopy of AM Herculis with EXOSAT: Discovery of photospheric absorption features

We present 0.1-10 keV spectroscopic observations of AM Herculis obtained with the Transmission Grating Spectrometers and Medium Energy experiments on EXOSAT, taken when the object was in its 'reversed X-ray mode.' The observation covers over six binary orbits without interruption, enabling us to analyze the phase and intensity dependence of both the hard and the soft spectrum simultaneously. We resolve the optically thick soft X-ray spectrum, and find definite evidence for time- and phase-dependent photospheric absorption structure arising in the white dwarf atmosphere. We present a simple empirical analysis of the combined soft and hard X-ray spectra, to examine whether the effect of a better determination of the column density of neutral absorbing material, afforded by our data, would solve the problem of the large relative soft X-ray overluminosity previously observed in AM Her. We find that a single absorbing column fits the entire spectrum, and that the column densities implied are indeed substantially lower than previously estimated. However, during half the binary orbit we still determine a strong lower limit to the soft-to-hard luminosity ratio of L(sub soft)/L(sub hard) is greater than or approximately equal to 10, in conflict with the simple radiative shock models for the accretion region. We argue that this indicates the need to reexamine the luminosity problem using explicit models for the emission spectrum based on a full solution of the atmospheric radiative transfer problem.

Paerels, Frits↗
NASA NTRS1995

Discovery of candidate cataclysmic variables in the post-core-collapse globular cluster NGC 6397

A photometric search for objects with H alpha emission in the globular cluster NGC 6397 has been carried out with the Hubble Space Telescope (HST) Planetary Camera. Images were obtained through the F675W (similar to Johnson R) and F656N (H alpha) filters, and photometry carried out on approximately 900 stars detected in the central approximately 38 x 35 arcseconds of the cluster. Limiting magnitudes of R approximately 21 and 19.5 were reached in the F675W and F656N images, respectively. Three H alpha-bright stars are found which have apparent magnitudes in the range R = 17.8-19.5. The corresponding absolute magnitudes (M(sub R) = 5.6-7.3) and inferred H alpha emission-line strengths (EW(H alpha) greater than or approximately equal to 15-26 A) make these objects good candidates to be cataclysmic variables (CVs) in this nearby, post-collapse cluster. All three H alpha-bright objects are found to be UV-bright in a photometric comparison of the F675W images with archival HST Faint Object Camera (FOC) images of the cluster center through the F220W and F346M filters. Such UV excesses further support the identification of these objects as candidates to be CVs in the cluster. The H alpha-bright stars are within the error circles of three X-ray sources previously found with the ROSAT High Resoltion Imager (HRI) and are likley to be the dominant source of the X-ray emission. The implied X-Ray to optical flux ratios (f(sub x)/f(sub v) approximately 0.8-4.2) are consistent with CVs known elsewhere in the Galaxy. Two additional stars with possible H alpha emission are also discussed, along the stars identified on the basis of UV emisssion. Preliminary completeness estimates suggest that these observations should be sensitive to approximately one-third to two-thirds of most varieties of CVs in the cluster, and that therefore greater than or approximately 5-10 CVs are likely to be present in the observed section of NGC 6397. Scaling these numbers to the cluster as a whole would increase these numbers by up to a factor of 2. A total CV population on the order of 5-20 is in reasonable with previous estimates made on the basis of X-ray observations, and with theoretical predictions scaled to this cluster.

Cool, Adrienne M.↗
NASA NTRS1995

Discovery of a transient MeV range gamma-ray source

The University of California, San Diego (UCSD)/MIT hard X-ray and gamma-ray instrument on the HEAO 1 surveyed the region near the Galactic center 3 times during its lifetime in 1977-1979. During the 1977 September-October scan, a gamma-ray source was detected south of the Galactic center. The source was below the threshold sensitivity in the spring and fall of 1978. The source was detected with the medium energy phoswich scintillation counters which operated over the 80 keV-2 MeV range, had an area of 42 sq cm each, and a 17 deg FWHM aperture. The error box for the source is centered on l = 2.4 deg, b = -12.2 deg, with a 90% confidence error circle of approximately 3.5 deg radius. The flux in the 333-635 keV range was (1.89 +/- 0.29) x 10(exp -5) photons/(sq cm s keV) and was constant within statistics during the 1 month period the source was in the field of view. The spectrum can be characterized as a Gaussian in the range 300 less than or = E less than or = 650 keV, with a FWHM of 249 +/- 51 keV centered on 461 +/- 22 keV. The flux of this broad Gaussian is (6.6 +/- 1.1) x 10(exp -3) photons/(sq cm s). The source is tentatively identified with the 5.57 hr period low-mass X-ray-emitting binary system 1H 1822-371. Assuming this is correct, the ratio of gamma-ray to X-ray luminosity during the outburst was about 5; at a distance of 8 kpc, the gamma ray luminosity is 4 x 10(exp 37) ergs. The emission may be interpreted as a positron-pair plasma ejected from a compact object, possibly a black hole, and annihilating in a thick accretion disk surrounding the object.

Briggs, M. S.↗
NASA NTRS1995

Discovery and imaging of a Galactic cirrus cloud with the far ultraviolet space telescope

We present new far-ultraviolet (1400-1800 A) data concerning a Galactic cirrus cloud G251.2+73.3 near the north Galactic pole obtained with the space-borne imaging telescope FAUST (Far Ultraviolet Space Telescope). We obtain a good correlation between the far-ultraviolet (FUV) and IRAS 100 micrometers surface brightnesses, their relation being I(sub FUV) = (128 +/- 3) I(sub 100 micrometers) - (264 +/- 9), where the I(sub FUV) flux is given in units of photon/s/sq cm/A/sr and I(sub 100 micrometers) in MJy/sr. Using uvbyH-beta photometry, we get a distance of 120 pc and a visual extinction in the center of the cloud of 0.39 mag corresponding to an extinction of 1.0 mag at 1565 A. We have performed a multiple scattering calculation for the scattered light using the Monte Carlo method. These calculations provide restrictions on the FUV scattering properties of the interstellar dust.

Haikala, Lauri K.↗
NASA NTRS1995

Discovery of a candidate old, isolated neutron star in the field of a galactic cirrus cloud

New x-ray optical, and radio observations of the bright, unidentified x-ray source MS 0317.7-6647 are presented suggesting that this source is a likely compact stellar remnant. As part of the Einstein Extended Medium Sensivity Survey, this source was discovered serendipitously in the Einstein Imaging Proportional Counter pointing towards the nearby (4.5 Mpc) spiral galaxy NGC 1313. The very high x-ray to optical flux ratio, pointlike ROSAT HRI detection, and extremely soft component in the x-ray spectrum of MS 0317.7-6647 eliminate all the usual classes of optical counterparts to faint x-ray sources except for a very massive x-ray binary (and black hole candidate) in NGC 1313, a nearby, companionless pulsar similar to Geminga, or a very nearby ( approximately 100 pc) isolated, old neutron star slowly accreting interstellar matter onto its magnetic poles. The presence of an IR cirrus cloud which shadows the approximately 0.25 keV x-ray background in this direction supports the latter possibilty.

Stocke, John T.↗
NASA NTRS1994

Discovery of intense gamma-ray flashes of atmospheric origin

Observations have been made of a new terrestrial phenomenon: brief (approx. millisecond), intense flashes of gamma rays, observed with space-borne detectors. These flashes must originate at altitudes in the atmosphere above at least 30 km in order to be observable by orbiting detectors aboard the Compton Gamma-Ray Observatory (CGRO). At least a dozen events have been detected over the past 2 years. The photon spectra from the events are very hard and are consistent with bremsstrahlung emission from energetic (MeV) electrons. The most likely origin of these high energy electrons, while speculative at this time, is a rare type of high altitude electrical discharge above thunderstorm regions.

Fishman, G. J.↗
NASA NTRS1993

Discovery of Orbital Decay in SMC X-1

We report on the results of three observations of the binary X-ray pulsar SMC X-1 with the Ginga satellite. Timing analyses of the 0.71 s X-ray pulsations yield Doppler delay curves which, in turn, enable the most accurate determination of the SMC X-1 orbital parameters available to date. Epochs of phase zero for the 3.9 day orbit were determined for 1987 May, 1988 August, and 1989 August with accuracies of 13, 0.6, and 3 s, respectively. These epochs are combined with two previous determinations of the orbital epoch to yield the rate of change in the orbital period dot-P(orb)/P(orb) = ( 3.36 +/- 0.02) x 10(exp -6) yr(exp -1). An interpretation of the orbital decay is made in the context of tidal evolution, with consideration of the influence of the increasing moment of inertia of the companion star due to its nuclear evolution. We find that, while the orbital decay is probably driven by tidal interactions, the asynchronism between the orbit and the rotation of the companion star is most likely maintained by the evolutionary expansion of the companion star (Sk 160) rather than via the Darwin instability. In this case Sk 160 is likely to be in the hydrogen shell burning phase of its evolution. Finally, a discussion is presented of the relation among the time scales for stellar evolution (less than 10(exp 7) yr), orbital decay (3 x 10(exp 5) yr), and neutron-star spin-up in the SMC X-1 system (2000 yr). In particular, we present the result of a self-consistent calculation for the histories of the spin of the neutron star and the mass transfer in this system. A plausible case can be made for the spin-up time scale being directly related to the lifetime of the luminous X-ray phase which will end in a common-envelope phase within a time of less than approx. 10(exp 4) yr.

Levine, A.↗
NASA NTRS1994

Discovery of Intense Gamma-Ray Flashes of Atmospheric Origin

Detectors aboard the Compton Gamma Ray Observatory have observed an unexplained terrestrial phenomenon: brief, intense flashes of gamma rays. These flashes must originate in the atmosphere at altitudes above at least 30 kilometers in order to escape atmospheric absorption and reach the orbiting detectors. At least a dozen such events have been detected over the past 2 years. The photon spectra from the events are very hard (peaking in the high-energy portion of the spectrum) and are consistent with bremsstrahlung emission from energetic (million-electron volt) electrons. The most likely origin of these high- energy electrons, although speculative at this time, is a rare type of high-altitude electrical discharge above thunderstorm regions.

Fishman, G. J.↗
NASA NTRS1994

The Discovery of Hot Stars in the Vicinity of the Thermal Filaments

We have found hot stars within the central 40 parsecs of the Galactic Center. They may at least partially ionize the thermal radio emission regions. A cluster of such stars in the vicinity of the Arched Filaments and individual stars scattered about the region are discussed. From comparisons with spectra of optically classified stars, the new stars appear to be B[e] and WN7-9 stars.

Cotera, Angela S.↗
NASA NTRS1996

Discovery of 0.5 million K gas in the center of galaxy clusters

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.

Lieu, R.↗
NASA NTRS1998

Discovery of the 198 Second X-Ray Pulsar GRO J2058+42

GRO J2058+42, a transient 198 s X-ray pulsar, was discovered by the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO) during a "giant" outburst in 1995 September-October. The total flux peaked at about 300 mcrab (20-50 keV) as measured by Earth occultation. The pulse period decreased from 198 to 196 s during the 46 day outburst. The pulse shape evolved over the course of the outburst and exhibited energy-dependent variations. BATSE observed five additional weak outbursts from GRO J2058 + 42, each with a 2 week duration and a peak-pulsed flux of about 15 mcrab (20-50 keV), that were spaced by about 110 days. An observation of the 1996 November outburst by the Rossi X-Ray Timing Explorer (RXTE) proportional counter array (PCA) localized the source to within a 4 s radius error circle (90% confidence) centered on R.A. = 20h 59m.0, decl. = 41 deg 43 s (J2000). Additional shorter outbursts with peak-pulsed fluxes of about 8 mcrab were detected by BATSE halfway between the first four 15 mcrab outbursts. The RXTE All-Sky Monitor detected all eight weak outbursts with approximately equal durations and intensities. GRO J2058 + 42 is most likely a Be/X-ray binary that appears to outburst at periastron and apastron, No optical counterpart has been identified to date, and no X-ray source was present in the error circle in archival ROSAT observations.

Wilson, Colleen A.↗
NASA NTRS1999

Discovery of a Magnetar Associated with the Soft Gamma Repeater SGR 1900+14

The soft gamma repeater SGR 1900+14 became active again in June 1998 after a long period of quiescence; it remained at a low state of activity until August 1998, when it emitted a series of extraordinarily intense outbursts. We have observed the source with the Rossi X-Ray Timing Explorer twice, during the onset of each active episode. We confirm the pulsations at the 5.16 s period reported earlier from SGR 1900+14. Here we report the detection of a secular spin-down of the pulse period at an average rate of 1.1 x 10(exp -10)s/s. In view of the strong similarities between SGRs, we attribute the spin-down of SGR 1900+14 to magnetic dipole radiation, possibly accelerated by a quiescent flux, as in the case of SGR 1806-20. This allows an estimate of the pulsar dipolar magnetic field, which is (2-8) x 10(exp 14) G. Our results confirm that SGRs are magnetars.

Kouveliotou, C.↗
NASA NTRS1999

Discovery of a Magnetar Associated with the Soft Gamma Repeater SGR 1900 + 14

The soft-gamma repeater (SGR) 1900 + 14 became active again on June 1998 after a long period of quiescence; it remained at a low state of activity until August 1998, when it emitted a series of extraordinarily intense outbursts. We have observed the source with RXTE twice, during the onset of each active episode. We confirm the pulsations at the 5.16 s period reported earlier from SCR 1900 + 14 . Here we report the detection of a secular spindown of the pulse period at an average rate of 1.1 x 10(exp -10) s/s. In view of the strong similarities between SGRs, we attribute the spindown of SGR 1900 + 14 to magnetic dipole radiation, possibly accelerated by a quiescent flux, as in the case of SGR 1806 - 20. This allows an estimate of the pulsar dipolar magnetic field, which is 2 - 8 x 10(exp 14) G. Our results confirm that SGRs are magnetars.

Kouveliotou, Chryssa↗
NASA NTRS1996

Parallel Knowledge Discovery from Large Complex Databases

NASA is focusing on grand challenge problems in Earth and space sciences. Within these areas of science, new instrumentation will be providing scientists with unprecedented amounts of unprocessed data. Our goal is to design and implement a system that takes raw data as input and efficiently discovers interesting concepts that can target areas for further investigation and can be used to compress the data. Our approach will provide an intelligent parallel data analysis system.

Cook, Diane J.↗
NASA NTRS1999

Knowledge Discovery Process for Characterization of Materials Failure Mechanism

It is the intent of this project to provide a platform to visualize the various data collected from stress-strain testing of composite ceramic matrix materials. The data collected from the stress-strain tests are acoustic emissions (AE). As a material is subjected to a stress-strain test, various failure mechanisms occur in the material. The recorded sounds emitted during the test may correspond to various failure mechanisms. This project, thus, will give a possible way to visualize the data and data derived from the recorded AE. The stress-strain testing was performed on several composite matrix material combinations. Each of these tests produced anywhere from 1000 to 10,000+ AE events. For each AE event recorded, several characteristics in both the time and frequency domains are created. This project has two goals. First, this project will provide a summation page for a selected waveform. This page will include all of the characteristics determined from the AE event waveform along with graphs of the AE event waveform and its corresponding Power Spectrum. The other function of this project is to retrieve and display selected AE event waveforms for comparison.

Cios, Krzysztof J.↗
NASA NTRS1997

Discovery of the 198 s X-Ray Pulsar GRO J2058+42

GRO J2058+42, a transient 198 second x-ray pulsar, was discovered by the Burst and Transient Source Experiment (BATSE) on the Compton Gamma-Ray Observatory (CGRO), during a "giant" outburst in 1995 September-October. The total flux peaked at about 300 mCrab (20-50 keV) as measured by Earth occultation. The pulse period decreased from 198 s to 196 s during the 46-day outburst. The pulse shape evolved over the course of the outburst and exhibited energy dependent variations. BATSE observed five additional weak outbursts from GRO J2058+427 each with two week duration and peak pulsed flux of about 15 mcrab (20-50 keV), that were spaced by about 110 days. An observation of the 1996 November outburst by the Rossi X-ray Timing Explorer (RXTE) Proportional Counter Array (PCA) localized the source to within a 4' radius error circle (90% confidence) centered on R.A. = 20 h 59 m.0, Decl. = 41 deg 43 min (J2000). Additional shorter outbursts with peak pulsed fluxes of about 8 mCrab were detected by BATSE halfway between the first four 15 mCrab outbursts. The RXTE All-Sky Monitor detected 8 weak outbursts with approximately equal durations and intensities. GRO J2058+42 is most likely a Be/X-ray binary that appears to outburst at periastron and apastron. No optical counterpart has been identified to date and no x-ray source was present in the error circle in archival ROSAT observations.

Wilson, Colleen A.↗