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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 253 records · Page 14
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NASA NTRS1985

Gamma-ray bursts from remnant neutron star disks

The consequences of a disk of matter orbiting an old neutron star are examined. When the inner edge of the disk approaches close to the star, due to internal viscous drag, runaway ionization of the disk occurs and the resulting plasma is precipitated to the surface of the neutron star, thereby producing a gamma-ray burst. Rough numerical estimates of the occurrence rate are given and found to be consistent with gamma-ray burst observations. The estimates indicate that energies of 10 to the 39th ergs or more could be released with rise times as fast as 0.3 ms. Consideration is given to explanations of the March 5, 1979 event (Cline et al., 1980). Some possible observational searches for optical or IR emission from such disks at the locations of known burst sources and pulsars are discussed.

Michel, F. C.↗
NASA NTRS1983

A fireball model for the March 25, 1978 gamma ray burst

It is suggested that the bulk of the gamma-ray emission in the March 25, 1978 burst comes from a fireball of photons and electron-positron pairs expanding from a jet off the polar cap of a neutron star, and that the duration of the burst is defined by the expansion and dissipation times of the fireball. This model makes it possible to determine the total energy, about 10 to the 39th ergs, and the distance, about 1kpc, of the burst and should be applicable to a large class of gamma-ray bursts.

Hueter, G. J.↗
NASA NTRS1985

An X-ray study of the Centaurus Cluster of galaxies using Einstein

Einstein Imaging Proportional Counter observations of the core of the Centaurus Cluster of galaxies have been analyzed to map the 0.5-3.5 keV surface brightness and temperature of the intracluster gas. The emission is centered on NGC 4696, the elliptical galaxy believed to be at or near the dynamical center of the cluster. Because the X-ray-emitting gas responds to the gravitational potential of the cluster, the observations may be used to measure the total mass distribution around the central region. It is shown that the gas is very likely in hydrostatic equilibrium. It is found that surrounding NGC 4696, like M87 at the center of the Virgo Cluster, is a dark, massive halo, with a gravitating mass of about 2 x 10 to the 13th M out to a radius of about 20 arcmin (or 200 kpc for H(o) = 50 km/s Mpc). The elliptical galaxy NGC 4709, at the core of a more distant cluster, is also detected with a luminosity of 2 x 10 to the 40th ergs per sec.

Matilsky, T.↗
NASA NTRS1985

A sun-aligned arc observed by DMSP and AE-C

On May 12, 1977, 8 minutes after a DMSP satellite photographed a sun-aligned auroral arc in the southern polar cap, AE-C crossed over the same arc. Precipitating electrons were observed with a peak energy flux of 0.94 erg per sq cm s but with no clear monoenergetic beam, and coincident ion precipitation was measured at energies of a few keV. A very sharp ion convection reversal was found coincident with the particle precipitation and embedded in a region of constant antisunward flow. Magnetic field data indicate that the arc occurred during the recovery phase of a weak magnetic storm, with the IMF in a toward sector and the Z component nearly zero. The data are consistent with a source of particles at altitudes of the order of 5-8 R(E) on field lines containing the plasma sheet boundary layer. The electron spectra do not indicate the existence of an electrostatic potential along the magnetic field lines projecting from the arc. The electrodynamic properties associated with the arc appear to be consistent with a simple model in which field-aligned currents are required along any boundary where the horizontal ionospheric current diverges.

Hoffman, R. A.↗
NASA NTRS1985

Thermonuclear reactions in cool accreting neutron stars and burst phenomena

The ignition of accreting materials on neutron stars is explored using strongly coupled plasma analytical techniques. The calculations cover the ignition temperature and density at the bottom of the accreted envelope of a neutron star. Emphasis is placed on low-temperature ignitions which take place at high densities. The investigation is extended to the accretion of material from a white dwarf in the form of pure He, C + O, or O + Ne + Mg. It is shown that electrons are strongly degenerate in low-temperature flashes, where the ignition is more dependent on density than on temperature. Precursor flashes 0.4-0.7 the intensity of the main burst will appear before the main bursts. The intensity relationship indicates that the appropriate model for an X-ray burst from a neutron star accreting from a white dwarf is a He shell flash in the presence of a hydrogen-rich atmosphere. The flash will have a maximum energy of 2 x 10 to the 43 ergs and could last as long as 40,000 sec.

Miyaji, S.↗
NASA NTRS1985

EUV observations of subflares and surges

EUV observations of two subflares and associated surges have been analyzed. At maximum brightness, the emission measures and radiative outputs of the subflares were approximately 20 percent of the corresponding values for the active region. Multiple EUV surges were observed during and following each subflare, with surge material being ejected in a variety of directions, including toward a coronal bright point located outside of the active region. The total energy of the surges appears to be comparable to that radiated by the subflares, a few times 10 to the 28th erg. As reported in previous studies of surges, it is found that there was no significant emission from these features in spectral lines formed at temperatures T greater than 1,000,000 K. The ejection of surges in several different directions and nearly simultaneous flaring of various areas of the active region suggest that the primary site of the subflares was magnetically connected to a variety of different areas in the active region and the surrounding quiet region.

Withbroe, G. L.↗
NASA NTRS1986

Search for ultraviolet emission lines from a hot gaseous halo in the edge-on galaxy NGC 4244

Short and long wavelength IUE spectra of the halo region in the edge-on galaxy NGC 4244 are analyzed in order to identify evidence of line emission at the level of 0.000001 ergs per cu cm sr/s. Features are found at 1245 A and 1402 A, having peaks four times greater than the rms intensity fluctuations of nearby spectra. The spectral features are identified with semi-forbidden N V, semi-forbidden S IV at 1240 A, and Si IV and semi-forbidden O IV multiplets at 1400 A, respectively. The appearance of high-peak features and the lack of astrophysically important lines in the sample are evidence of a gas near T = 10 exp 5.2 and emission measure (EM) equal to about 0.000001 pc. However, the case for the existence of such a gas is weakened due to the existence of two other similarly sized features with no identifiable astrophysical origin and the extremely faint nature of the candidate features. The assumed upper limit for the line intensities in NGC 4244 leads to the conclusion that at T less than 100,000 K any emitting gas is either highly clumped or has a p/k value of less than 1000 per cu cm K. It is suggested that if the observed low level features in the short wavelength spectrum are real, the temperature and emission measures allow for a single component gas in the halo of NGC 4244, and are in agreement with those derived by Paresce et al. (1983).

Deharveng, J.-M.↗
NASA NTRS1986

Detection of an X-ray flare in the RS CVn binary Sigma Coronae Borealis

The detection of an X-ray flare in the RS CVn binary Sigma Coronae Borealis with the Monitor Proportional Counter on the Einstein Observatory is described. During the 513 min of observation, an X-ray flare of 208 min duration was detected at a significance level of 26 sigma in the 1.19-10.16 keV band. The rise time of the flare is between 25 and 70 min and the decay time is greater than or equal to 34 min. The X-ray luminosity at the flare maximum is found to be 6 x 10 to the 30th erg/s and the total energy radiated in X-rays during the flare is 2 x 10 to the 34th erg. The energy spectrum in the flaring state is found to be harder (temperature T about 2.5 x 10 to the 7th K) compared to the one observed in the quiescent state (T about 6 x 10 to the 6th K). Applying the coronal loop model, the loop parameters are calculated and compared for the X-ray flares observed in the various RS CVn binaries and the sun. The significance of the differences in the observed and derived parameters of the X-ray flares is briefly discussed.

Agrawal, P. C.↗
NASA NTRS1986

X-ray and optical observations of a dMe flare star in the T Tauri field

Observations of a newly discovered dMe flare star within 20 pc of the sun are presented. An EXOSAT observation of this source shows an almost complete X-ray flare light curve with rise time about 600 s, decay time about 1500 s, and peak luminosity 6.1 x 10 to the 29th erg/s, a 40-fold increase in brightness over the quiescent level. The total energy in the flare is between 2.7 x 10 to the 32nd and 3.3 x 10 to the 33rd erg. Results of two Einstein IPC observations are also presented; the source is quiescent in the first and shows evidence for a luminosity enhancement, possibly the tail of another flare, in the second. The source is identified with a hitherto unsteadied nearby star with magnitude V about 13 and spectral type roughly dM5e, at a distnce of about 13 pc. For the flare, a variety of different physical assumptions about the cooling mechanism is explored and possible values for loop lengths and electron densities are derived.

Smale, A. P.↗
NASA NTRS1986

Research relative to high resolution camera on the advanced X-ray astrophysics facility

The HRC (High Resolution Camera) is a photon counting instrument to be flown on the Advanced X-Ray Astrophysics Facility (AXAF). It is a large field of view, high angular resolution, detector for the x-ray telescope. The HRC consists of a CsI coated microchannel plate (MCP) acting as a soft x-ray photocathode, followed by a second MCP for high electronic gain. The MCPs are readout by a crossed grid of resistively coupled wires to provide high spatial resolution along with timing and pulse height data. The instrument will be used in two modes, as a direct imaging detector with a limiting sensitivity of 10 to the -15 ergs sq cm sec in a 10 to the 5th second exposure, and as a readout for an objective transmission grating providing spectral resolution of several hundreds to thousands.

Source record↗
NASA NTRS1986

Where do flares occur in RS CVn systems? Analysis of the October 3, 1981 flare on V711 Tau = HR 1099 observed by IUE

A set of IUE observations of V711 Tau = HR 1099 (K1 IV and G2 V) in which high-resolution spectra with the SWP and LWR cameras were obtained during the luminous flare of October 3, 1981 is discussed. Multigaussian fits to the Mg II k and C IV 1548 A lines are consistent with the flare radial velocity being identical to that of the K star. Thus, the flare probably occurred on the K star. There is no evidence for flows as seen during a flare on UX Ari. The strongest emitters of UV radiation during the flare were the L alpha and Mg II lines, which emitted seven times as much energy as all the transition region lines combined. The peak luminosity of the flare in the UV emission lines is 1.5 times 10 to the 31st power ergs/sec, and the total emission in these lines during the flare was about 4 times 10 to the 35th power ergs.

Linsky, J. L.↗
NASA NTRS1987

Shock heated dust in L1551: L(IR) greater than 20 solar luminosities

The infrared bolometric luminosity of the extended emission from the L1551 flow exceeds 20 solar luminosities. Ultraviolet radiation from the shock associated with the flow appears to heat the dust requiring shock temperatures from 10,000 to 90,000 K in L1551, velocities of approximately 50 km/s near the end of the flow, and a minimum mechanical luminosity of approximately 40 solar luminosities. The total energy requirement of the infrared emission over a 10,000 year lifetime is 10 to the 46th to 47th ergs, two orders of magnitude higher than previous estimates for L1551. Infrared radiation offers a method of probing interstellar shocks, by sampling the untraviolet halo surrounding the shock. At least one current model for bipolar flows is capable of meeting the energetic requirements.

Clark, F. O.↗
NASA NTRS1987

IRAS observations of BL Lac objects

IRAS data was analyzed for 35 BL Lac objects selected from a complete 5 GHz radio sample, using the coadded survey database. The detection rate is 50% with more than 40% detected in more than one band. This compares with only 15% of these sources that are included in the IRAS Point Source Catalog. High luminosity BL Lac objects generally have smooth energy spectra over four or five decades in frequency, consistent with incoherent synchrotron emission from 1 cm to 1 micron. However, many low luminosity BL Lac objects have discontinuous spectra, with a large range in the spectral index at IRAS wavelengths. For BL Lacs with a total luminosity of less than 10 to the 44th power ergs/-s, most of the far infrared energy probably originates from dust heated near the galaxy nucleus. The energy budget shows that the majority of the power per unit bandwidth emerges in the infrared (1 to 100 microns).

Impey, C.↗
NASA NTRS1986

The soft X-ray background as a supernova blast wave viewed from inside - Solar abundance models

A model of the soft X-ray background is presented in which the sun is assumed to be inside an active supernova blast wave. The blast wave evolves in a preexisting cavity. The broad band surface brightnesses is explained by such a blast wave with an explosion energy of E sub approximately 5 x 10 to the 50th power ergs and radius 80 to 100 pc, using solar abundances. An approach to treating the problem of large anisotropies in the ambient medium is also explored, accommodating the observed anticorrelation between the soft X-ray surface brightness and the 21 cm column density. It is found that only for post shock temperatures below 10 to the 6 power K a shock propagating into a density enhancement will be dimmer than a similar shock in a lower density region.

Edgar, R. J.↗
NASA NTRS1986

Gamma-ray bursters at cosmological distances

It is proposed that some, perhaps most, gamma-ray bursters are at cosmological distances, like quasars, with a redshift of about 1 or 2. This proposition requires a release of supernova-like energy of about 10 to the 51st ergs within less than 1 s, making gamma-ray bursters the brightest objects known in the universe, many orders of magnitude brighter than any quasars. This power must drive a highly relativistic outflow of electron-positron plasma and radiation from the source. It is proposed that three gamma-ray bursts, all with identical spectra, detected from B1900 + 14 by Mazets, Golenetskii, and Gur'yan and reported in 1979, were all due to a single event multiply imaged by a gravitational lens. The time intervals between the successive bursts, 10 hr to 3 days, were due to differences in the light travel time for different images.

Paczynski, B.↗
NASA NTRS1986

Cosmological shells and blast waves

It is argued that cosmic explosions could have produced significant amounts of large-scale structure in the cosmic microwave background (CMB). Observations appear to indicate the presence of bubblelike structures with radii suggesting that positive energy perturbations were more prevalent than negative energy perturbations, since the latter would produce a prevalence of clusterlke irregularities. Energy input from processes occurring during galaxy formation and releasing about 10 to the 61st-62nd ergs per event would not overly disturb the CMB. The merging of bubbles typically resulting in bubbles with radii of roughly (10-20)/h x Mpc also would not affect the CMB. If much larger bubbles of radii 50-100 Mpc exist, a different energy input is likely to be the cause.

Ostriker, Jeremiah P.↗
NASA NTRS1987

Preflare energy build-up in a filament circuit

The May 16, 1981 flare observed at Debrecen is studied by extending to a fully three-dimensional model the two-dimensional Van Tend and Kuperus (1978) scenario for preflare energy build-up. It is shown that there are 10 to the 33rd ergs of free energy available to explain the subsequent large two-ribbon flare. As a result of the three-dimensional character of the present model, this estimate is an order of magnitude larger than that made by Van Tend. It is confirmed that the global form of the preflare circuit is highly important in determining the amount of energy stored in the preflare configuration. The present model gives correct predictions for the independently observed photospheric flow velocity and current strength in filaments.

Martens, P. C. H.↗
NASA NTRS1987

Nonpotential features observed in the magnetic field of an active region

A unique coordinated data set consisting of vector magnetograms, H-alpha photographs, and high-resolution ultraviolet images of a solar active region is used, together with mathematical models, to calculate potential and force-free magnetic field lines and to examine the nonpotential nature of the active region structure. It is found that the overall bipolar magnetic field of the active region had a net twist corresponding to net current of order 3 x 10 to the 12th A and average density of order 4 x 10 to the -4th A/sq m flowing antiparallel to the field. There were three regions of enhanced nonpotentiality in the interior of the active region; in one the field had a marked nonpotential twist or shear with height above the photosphere. The measured total nonpotential magnetic energy stored in the entire active region was of order 10 to the 32nd ergs, about 3 sigma above the noise level.

Gary, G. A.↗