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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 91 records · Page 5
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NASA NTRS1978

A hydrodynamic study of a slow nova outburst

The paper reports use of a Lagrangian implicit hydrodynamics computer code incorporating a full nuclear-reaction network to follow a thermonuclear runaway in the hydrogen-rich envelope of a 1.25 solar-mass white dwarf. In this evolutionary sequence the envelope was assumed to be of normal (solar) composition and the resulting outburst closely resembles that of the slow nova HR Del. In contrast, previous CNO-enhanced models resemble fast nova outbursts. The slow-nova model ejects material by radiation pressure when the high luminosity of the rekindled hydrogen shell source exceeds the local Eddington luminosity of the outer layers. This is in contrast to the fast nova outburst where ejection is caused by the decay of the beta(+)-unstable nuclei. Nevertheless, radiation pressure probably plays a major role in ejecting material from the fast nova remnants. Therefore, the sequence from slow to fast novae can be interpreted as a sequence of white dwarfs with increasing amounts of enhanced CNO nuclei in their hydrogen envelopes, although other parameters such as the white-dwarf mass and accretion rate probably contribute to the observed variation between novae.

Sparks, W. M.↗
NASA NTRS1980

Spectrophotometric observations of P/Schwassmann-Wachmann 1 during outburst

Spectra of Comet Schwassmann-Wachmann 1 were obtained on November 26-27, 1978 and February 15, 1979 using the Cassegrain Digicon spectrograph, and on February 21-23, 1979 using the IDS spectrograph of McDonald Observatory. All spectra were obtained while the comet was in an extended series of outburst (12th-13th mag). The spectra were predominantly reflected solar light, indicating the outburst was very rich in dust. Weak emission between 4000 and 4050 A, and between 4240 and 4290 A was detected in November 1978 and is probably due to CO(+). These outbursts, and the detection of CO(+), indicate that comets are not inactive at distances as great as 6 AU.

Cochran, A. L.↗
NASA NTRS1983

Ultraviolet photometry of dwarf novae in outburst

Broad-band photometry from the Astronomical Netherlands Satellite of five dwarf novae at 1550, 1800, 2200, 2500, and 3300 A is presented. EM Cyg in standstill exhibits a flux excess at 3300 A. The lack of a strong extinction feature at 2200 A is seen as suggesting that the reddening is small toward all five systems. Using an independent estimate of the distances, mass flux rates are derived by fitting steady-state, optically thick accretion disk models to the observed UV luminosity. The rate is found to be 3 x 10 to the 17th g/s for Z Cam and EM Cyg in outburst and a factor of 3 smaller of Z Cam in standstill. The models, however, do not reproduce the slope of the Balmer continuum. Of special interest are the observations of outburst rises of SS Aur, SU UMa, and EM Cyg. SS Aur exhibits an exponential increase at a rate of 3.5 mag in 0.5 d. The outburst in EM Cyg also reveals an exponential rise, but at a rate of only 1.2 mag per day.

Wu, C.-C.↗
NASA NTRS1986

Z Camelopardalis - Outburst P Cygni profiles and quiescent continuum

The first orbital study of the C IV P Cygni profile in the dwarf nova Z Cam, a system of intermediate inclination. A modulation of the absorption equivalent width is apparent, with phases suggesting an effect from the mass-transfer stream. Over the outburst cycle, the strength of the P Cygni absorption and emission components is greater after outburst than during the standstill configuration, while the terminal velocities and central absorption wavelengths are similar. The continuum flux distributions during the decline from outburst and during quiescence are discussed.

Szkody, P.↗
NASA NTRS1986

Modeling of dust halo formation following comet outbursts Preliminary results

Evolution of gas and dust distributions following a spatially and temporally localized comet outburst was calculated using a hybrid kinetic - hydrodynamic method. It was found that a comet outburst resulted in significantly increased dust terminal velocities. As these higher terminal velocity values result in larger apex distances, therefore the outburst generates distinct dust envelopes in front of the regular dust coma. Such envelopes were observed at several comets (cf. comet Donati).

Gombosi, T. I.↗
NASA NTRS1986

Is CO2 responsible for the outbursts of comet Halley?

Spectroscopic data from the IUE are reported which show a strong correlation between a visible outburst from the near-nuclear region of comet Halley on 18.8 March 1986 and the sudden appearance of a large column abundance of CO2(+) ions in the tail. These data suggest that CO2 may play a significant role in the outburst process. This outburst may have been the source of the detached clumps of CO(+) emission recorded by ground-based observers on 19 and 20 March, 1986.

Feldman, P. D.↗
NASA NTRS1988

The classical nova outburst

The astrophysics of outbursts in classical and recurrent novae is explored, summarizing and analyzing the results of ground-based and space observations obtained in different regions of the electromagnetic spectrum. The stages of fast and slow outbursts are reviewed; the thermonuclear processes involved are described; and some observations of recent novae are listed and briefly characterized. Particular attention is then given to the specific contributions of optical, UV, IR, radio, and X-ray studies of nova outbursts.

Starrfield, Sumner↗
NASA NTRS1990

The impact of IUE on studies of the nova outburst: 1986 to 1990

Developments in understanding the nova outbursts that have occurred since the IUE (International Ultraviolet Explorer) meetings in 1986 and 1988 are outlined. Those developments that have occurred as a direct result of studies with the IUE satellite are emphasized. These involve the four outbursts that occurred in the large Magellanic cloud, the numerous recurrent novae outbursts, and a number of archival studies. As a direct result of IUE studies done since 1986, it is shown that fast novae become super Eddington at maximum, that many novae decline at the same rate, and that neon novae may be more numerous than previously thought. In addition, it is shown that there are differences in the ourburst characteristics between recurrent novae with giant secondaries and those with compact secondaries.

Starrfield, Sumner G.↗
NASA NTRS1994

Observation of GX 339-4 hard state outbursts in 1991 and 1992

Two hard state (X-ray low state) outbursts in the GX 339-4 system in 1991 June-October and 1992 September-1993 January were detected and monitored by the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO). The outbursts were remarkably similar in both temporal and spectral aspects. The spectrum in the 20-300 keV band evolves from a hard, Comptonized shape to a steeper (alpha approximately equals -3) power law during each outburst.

Harmon, B. A.↗
NASA NTRS1995

Infrared observations of an outburst of small dust grains from the nucleus of Comet P/Halley 1986 III at perihelion

A close examination of the 0.7- to 23-micron infrared data base acquired by Gehrz and Ney (1992), suggests that the nucleus of Comet P/Halley 1986 III emitted a burst of small dust grains during a 3-day period commencing within hours of perihelion passage on 1986 February 9.46 UT. The outburst was characterized by significant increases in the coma's grain color temperature T(sub obs), temperature excess (superheat: S = T(sub obs)/T(sub BB)), infrared luminosity, albedo, and 10-micron silicate emission feature strength. These changes are all consistent with the sudden ejection from the nucleus of a cloud of grains with radii of approximately 0.5 micron. This outburst may have produced the dust that was responsible for some of the tail streamers photographed on 1986 February 22 UT. The peak of the dust outburst occurred about 3 days before a pronounced increase in the water production rate measured by the Pioneer Venus Orbiter Ultraviolet Spectrometer. We suggest that jets that release large quantities of small particles may be largely responsible for some of the variable infrared behavior that has been reported for P/Halley and other comets during the past two decades. Such jets may also account for some of the differences IR Type I and IR Type II comets.

Gehrz, R. D.↗
NASA NTRS1996

Periodic outbursts from the black hole candidate 4U 1630-47

Observational data from several observatories, including Rosat, are used to identify two previously unreported outbursts from the ultrasoft X-ray transient 4U 1630-47. These outbursts occurred in March 1979 and September 1993, which is consistent with a previously reported 600-day outburst recurrence period. This source is a member of the class of transient X-ray binary systems that are thought to contain black holes. The 601.0 +/- 2.8 day period is the shortest observed so far for this class.

Parmar, A. N.↗
NASA NTRS1998

Recent Outbursts from the Transient X-Ray Pulsar Cep X-4 (GS 2138+56)

We report on X-ray observations of the 66 s period transient X-ray pulsar Cep X-4 (GS 2138+56) with the Burst and Transient Source Experiment (BATSE) on the Compton Gamma-Ray Observatory (CGRO) and with the Rossi X-ray Timing Explorer (RXTE). Two outbursts from Cep X-4 were observed with BATSE in 1993 June-July and 1997 July. Spin periods of P(sub spin) = 66.2510 +/- 0.0009 s on 1993 June 25 (MJD 49,163) and P(sub spin) = 66.27 68 +/- 0.0007 s on 1997- July 12 (MJD 50,641) were each measured from 2 day spans of BATSE data near each outburst's peak. Cep X-4 showed an average spin down rate of P/P = (8.7 +/- 0.2) x 10(exp -5)/ yr between the 1993 and 1997 outbursts. After BATSE could no longer detect Cep X-4, public observations were performed on 1997 July 18 & 25 with the Proportional Counter Array (PCA) on RXTE. A spin period Of P(sub spin) = 66.28 +/- 0.02 s was measured from observations on 1997 July 18 (MJD 50,647). Significant aperiodic noise, with an rms variance of approx. 18% in the frequency range 0.01 - 1.0 Hz was observed on both days. Energy and intensity dependent pulse shape variations were also seen in these data. Recently published optical observations associate Cep X-4 with a Be companion star.

Wilson, Colleen A.↗
NASA NTRS1999

GeV Outbursts in Mrk 501

Mrk 501 is the third TeV blazar with a known GeV component. Previous multiwavelength campaigns on Mrk 501 showed well correlated outbursts at x-ray and TeV energies with no significant activity at GeV energies. We present here new evidence suggesting GeV outbursts in Mrk 501 when the spectrum appears to be extremely hard. However, this outburst appears uncorrelated with emission at x-ray energies. The resulting spectral energy distribution suggests a sharp cut off in the high-energy emission beyond a few hundred GeV.

Sreekumar, P.↗
NASA NTRS1999

Optical Variability of BL Lacertae During the Major Outburst of 1997

We have undertaken an investigation of recent flux variability in BL Lac. We present optical observations taken over 12 nights documenting major as well as minor outbursts. This has been combined, for purposes of multifrequency analysis, with published X-ray and gamma ray data taken for an additional single night. On two nights in particular, which includes the night of the X-ray observations, a major outburst of about a full magnitude of variation was recorded. All the data have been analyzed with theoretical models. Attempts were made to use synchrotron self-Compton and external Comptonization models to explain the data; however both classes of models were found lacking. More satisfactory results were obtained using an analytical model proposed by Wang et al. that involves the evolution of synchrotron spectra in a homogeneous jet due to the injection of relativistic electrons, taking into account radiation losses during the outbursts. It is hoped that the results of this study of BL Lac, an archetype for the class of blazars in general, represent a more generic phenomenon applicable to the entire class.

Ghosh, K. K.↗
NASA NTRS2000

The Timing Evolution of 4U 1630-47 During Its 1998 Outburst

We report on the evolution of the timing of 4U 1630-47 during its 1998 outburst using data obtained with the Rossi X-ray Timing Explorer (RXTE). The count rate and position in hardness intensity, color-color diagrams, and simple spectral fits are used to track the concurrent spectral changes. The source showed seven distinct types of timing behavior, most of which show differences with the canonical black hole spectral/timing states. In marked contrast to previous outbursts, we find quasi-periodic oscillation (QPO) signals during nearly all stages of the outburst with frequencies between 0.06 and 14 Hz and a remarkable variety of other characteristics. In particular, we find large (up to 23% rms) amplitude QPOs on the early rise. Later, slow 0.1 Hz semiregular short (-5 s), 9%-16% deep dips dominate the light curve. At this time there are two QPOs, one stable near 13.5 Hz and the other whose frequency drops from the 6-8 Hz range to -4.5 Hz during the dips. BeppoSAX observations during the very late declining phase show 4U 1630-47 in a low state.

Dieters, S. W.↗
NASA NTRS2000

Optical Variability of BL Lacertae During the Major Outburst of 1997

We have undertaken an investigation of recent flux variability in BL Lac. We present optical observations taken over 22 nights documenting major as well as minor outbursts. This has been combined, for purposes of multifrequency analysis, with published X-ray and T-ray data taken for an additional single night, On two nights in particular, including the night of the X-ray observations, a major outburst of about a full magnitude of variation was recorded. All the data have been analyzed with theoretical models. Attempts were made to use synchrotron self-Compton and external Comptonization models to explain the data; however, both classes of models were found lacking. More satisfactory results were obtained using an analytical model proposed by Wang et al. that involves the evolution of synchrotron spectra in a homogeneous jet due to the injection of relativistic electrons, taking into account radiation losses during the outbursts. It is hoped that the results of this study of BL Lac, an archetype for the class of blazars in general, represent a more generic phenomenon applicable to the entire class.

Ghosh, K. K.↗
NASA NTRS2001

High-Frequency Quasi-Periodic Oscillations in the 2000 Outburst of the Galactic Microquasar XTE J1550-564

We present an analysis of the high-frequency timing properties of the April-May 2000 outburst of the black hole candidate and Galactic microquasar XTE J1550-564, measured with the Rossi X-ray Timing Explorer, The rapid X-ray variability we measure is consistent with the source being in either the "very high" or "intermediate" canonical black hole state. A strong (5-8% RMS) quasi-periodic oscillation (QPO) is found between 249-278 Hz; this may represent the first recurrence of the same high-frequency QPO in subsequent outbursts of a transient black hole candidate. We also present possible evidence for a lower-frequency QPO at approximately 187 Hz, also reported previously and likely present simultaneously with the higher-frequency QPO. We discuss these findings within the context of the 1998 outburst of XTE J1550-564, and comment on implications for models of QPOs, accretion flows, and black hole spin.

Miller, J. M.↗
NASA NTRS2001

Dust Trails of SP/Tuttle and the Unusual Outbursts of the Ursid Shower

Halley-type comets tend to have a series of dust trails that remain spatially correlated for extended periods of time, each dating from a specific return of the comet. Encounters with 1 - 9 revolution old individual dust trails of 55P/Tempel-Tuttle have led to well recognized Leonid shower maxim, the peak time of which was well predicted by recent models. Now. we used the same model to calculate the position of dust trails of comet Shuttle, a Halley-type comet in an (approximately) 13.6 year orbit passing just outside of Earth's orbit. We discovered that the meteoroids tend to be trapped in the 14:12 mean motion resonance with Jupiter, while the comet librates in a slightly shorter period orbit around the 13:15 resonance. It takes six centuries to change the orbit enough to intersect Earth's orbit. During that time, the meteoroids and comet separate in mean anomaly by six years. thus explaining the unusual aphelion occurrences of Ursid outbursts. The resonances also prevent dispersion, so that the dust trail encounters (specifically, from dust trails of AD 1378 - 1405) occur only in one year in each orbit. We predicted enhanced activity on December 22, 2000, at around 7:29 and 8:35 UT (universal time) from dust trails dating to the 1405 and 1392 return, respectively. This event was observed from California using video and photographic techniques. At the same time, five Global-MS-Net stations in Finland, Japan and Belgium counted meteors using forward meteor scatter. The outburst peaked at 8:06:07 UT, December 22, at Zenith Hourly Rate (approx.) 90 per hour. The Ursid rates were above half peak intensity during 4.2 hours. This is only the second Halley type comet for which a meteor outburst can be dated to a specific return of the parent comet, and traces their presence back form 9 to at least 45 revolutions of the comet. New orbital elements of Ursid meteoroids are presented. We find that most orbits do scatter around the anticipated positions, confirming the link with comet Shuttle and the epoch of ejection. The 1405 and.1392 dust trails appear to have contributed similar amounts to the activity profile. Some orbits provide a hint of much older debris being present as well. Some of the dispersion in the radiant position may reflect a true variation in inclinations, with two groupings at low and high values, which is not understood at present.

Jenniskens, Peter↗