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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 217 records · Page 12
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NASA NTRS1986

Comets, X-ray bursts, and gamma-ray bursts

The proposal, revived by Tremaine and Zytkow (1985), that accretion of comets by neutron stars may be the origin of gamma-ray bursts is considered. This mechanism has difficulty accounting for the observed gamma-ray spectrum and optical counterparts of the bursts. The survival of comets near supernovae is investigated. Ablation rates and the thermal structure of an ablating surface layer are calculated. In some circumstances, mechanical disruption will erode a comet more rapidly than evaporation. The accretion of comets by neutron stars may produce a class of X-ray burst sources with novel properties.

Katz, J. I.↗
NASA NTRS1986

Very energetic gamma-rays from the June 3, 1982 solar flare

The Gamma-Ray Spectrometer on the SMM satellite has recorded high-energy gamma-ray and neutron emission from the flare on June 3, 1982. During the 65-s impulsive phase the gamma rays above 10 MeV contain emissions from both primary electron bremsstrahlung and nuclear pion decay. Hence the impulsive phase acceleration process must produce both primary electrons with energies above 60 MeV and ions above 500 MeV. This flare also has a extended emission phase lasting more than 1000 s which is most easily observed at gamma-ray energies above 10 MeV. After removing the counting rates from the more slowly moving neutrons produced at earlier times, the resulting gamma ray spectrum can be entirely explained by nuclear pion production. Over 70 percent of the pions were produced in the extended emission phase. In contrast, more than 70 percent of the high-energy primary electron bremsstrahlung and the nuclear line emission produced by ions below 30 MeV occurred in the 65-s impulsive phase. This represents the first clear observation of a new acceleration process which produces an electron-deficient, very hard ion spectrum extending beyond 1000 MeV.

Forrest, D. J.↗
NASA NTRS1989

Search for a narrow annihilation feature correlated with the MeV emission of Cygnus X-1

Strong gamma-ray emission near 1 MeV from Cygnus X-1 observed in the fall of 1979 during the 'gamma 1' period has been interpreted by Liang and Dermer (1988) as thermal emission from a relativistic, pair-dominated region surrounding the black hole. The observed gamma-rays should interact with each other and with X-rays to form electron/positron pairs inside and outside the accretion disk. Because the Eddington limit is reduced by a factor of 1836 in the presence of positrons, some pairs formed inside the disk may also escape the system under the influence of radiation pressure. If these pairs stop in the cold surrounding medium, a narrow 511 keV annihilation feature should be produced. The high resolution gamma 1 spectrum has been examined for such a feature, and some evidence at 1.9 sigma statistical significance for the line is observed with a flux which is generally consistent with theoretical expectations.

Ling, J. C.↗
NASA NTRS1989

Signposts to the future

The major scientific issues raised in this meeting about Active Galactic Nuclei (AGN) are summarized. The X-ray background and the capabilities of missions to be launched in the next 5 years (GRANAT, ROSAT, BBXRT, GRO, Astro-D, SAX, Spectrum-X-gamma, XTE) are discussed. Each one of these missions is considerably more powerful than its predecessor and will add to the knowledge of AGN and the X-ray background. By the end of the 1990's both of the X-ray astronomy Great Observatories, Advanced X-ray Astrophysics Facility (AXAF) and X-ray Multi mirror Mission (XMM), should be flying, each one of them more powerful than the intermediate scale missions of the early 1990's.

Mushotzky, R. F.↗
NASA NTRS1991

Gamma-ray bursts from sheared Alfven waves

The physical process by which sheared Alfven waves can accelerate electrons to a Lorentz factor of 10,000 to 100,000 within 5 km of the stellar surface is applied to a study of gamma-ray bursts, taking both resonant and nonresonant scattering into account. Several very encouraging features of the model are discussed. Although the field is oscillatory, virtually all the charges are ejected from the system, resulting in very little backheating of the stellar surface. The particle number density is accounted for naturally in terms of BA0 and m, which in principle are known from the physical manifestation of the agent causing the crustal disturbance. The resulting gamma-ray spectrum compares very favorably with the observation. The model restricts the geometry of the emission region, in the sense that only the Compton upscattering of soft photons from a warm polar cap can produce the correct spectral shape.

Melia, Fulvio↗
NASA NTRS1992

Effects of material mixing on planetary gamma ray spectroscopy

The effect of mixing of rocks and soil on the gamma-ray signal emitted by Martian surface was investigated with special consideration given to the effect of the geometry of mixing. It is shown that, under certain circumstances, this scale can have a significant effect on the gamma-ray spectrum observed. The techniques that can be used to address this problem are discussed, showing that, using these techniques, rock compositions can be inferred from the signal produced by a rock-soil mixtures of unknown rock concentration and mixing geometry.

Squyres, Steven W.↗
NASA NTRS1990

Observatories in earth orbit and beyond

The present volume on observations in earth orbit and beyond discusses current and future missions, launch vehicles, the relative merits of various observatories, and long-term future issues. Attention is given to the Granat automatic spacecraft, the prospects of the Hipparcos mission, EUV and FUV astronomy from Voyagers 1 and 2, and the X-ray Timing Explorer. Topics addressed include the SAX mission for X-ray astronomy, the Space Infrared Telescope Facility, the Ulysses mission in the high-latitude heliosphere, and science operations for future space astrophysics missions. Also discussed are science observations with the IUE using the one-gyro mode, new methods of determining spacecraft attitude, cryogenic testing of optics for ISOCAM, and the stellar X-ray polarimeter for the Spectrum-X-Gamma mission.

Kondo, Yoji↗
NASA NTRS1992

Performances of the Imaging Proportional Counter of the Stellar X-ray Polarimeter

The Stellar X-ray Polarimeter (SXRP) will be mounted on the focal plane of one of the two SODART telescopes aboard the Russian mission Spectrum-X-gamma. The SXRP employs the same Imaging Proportional Counters for both the Bragg and the scattering stages. An account is presently given of the main characteristics of these detectors and their performance, on the basis of tests at the technical-model and engineering model levels of development.

Costa, E.↗
NASA NTRS1993

Evidence for a spatially extended component of gamma rays from solar flares

We present gamma-ray measurements of a large solar flare that occurred beyond the western solar limb on September 29, 1989. The gamma-ray spectrum shows a neutron capture line which is much stronger than predicted by limb-darkening curves that fit the measurements for flares on the visible hemisphere. We show that this strong neutron capture line can be explained if, in addition to the compact impulsive phase component that normally dominates the total fluence, there is a spatially extended component. For the September 29, 1989 flare we find that such a spatially extended region must subtend more than about 30 deg on the solar surface. We suggest that the extended component could be powered by particles that diffuse from a compact impulsive region or by particles that diffuse into the lower solar atmosphere from a large-scale acceleration site.

Vestrand, W. T.↗
NASA NTRS1992

X-ray scattering measurements from thin foil X-ray mirrors

Thin foil X-ray mirrors are to be used as the reflecting elements in the telescopes of the X-ray satellites Spectrum-X-Gamma (SRG) and ASTRO-D. High resolution X-ray scattering measurements from the Au coated and dip-lacquered Al foils are presented. These were obtained from SRG mirrors positioned in a test quadrant of the telescope structure and from ASTRO-D foils held in a simple fixture. The X-ray data is compared with laser data and other surface structure data such as STM, atomic force microscopy (AFM), TEM, and electron micrography. The data obtained at Cu K-alpha(1), (8.05 keV) from all the mirrors produced on Al foils shows a scatter which limits the obtainable half-power width to above 1.5 arcmin. Mirrors based on electroformed Ni foils, however, show local regions with a factor of 4 better performance, and they are being developed for future applications.

Christensen, F. E.↗
NASA NTRS1993

EGRET observations of active galactic nuclei - 0836 + 710, 0454 - 234, 0804 + 499, 0906 + 430, 1510-089, and 2356 + 196

The Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory observed high-energy gamma rays (50 - 2000 MeV) from quasar 0836 + 710 (z = 2.16) during observations in 1992 January, near the time of an optical fare (von Linde et al., 1993). The gamma-ray spectrum can be fitted with a power law with photon number index 2.4 +/- 0.2. EGRET identifies quasars 0454 - 234, 0804 + 499, 0906 + 430, 1510 - 089, and 2356 + 196 at a statistical significance of between 4 and 5 standard deviations.

Thompson, D. J.↗
NASA NTRS1995

An upper limit to the infrared background from observations of TeV gamma rays

An upper bound to the energy density of infrared background radiation is derived from considering the effect of gamma-gamma interactions on the observed TeV gamma-ray spectrum of the active galaxy Markarian 421. This upper bound proves to be the most restrictive for the wavelength range of 10-12 micrometers. These constraints are presently limited by the uncertainty of extrapolating the source spectrum from 5 to 500 GeV. Observations in the regime less than 100 GeV would significantly improve these limits, allowing for a wide range of IR production models to be constrained.

Biller, S. D.↗
NASA NTRS1998

Intergalactic Extinction of High Energy Gamma-Rays

We discuss the determination of the intergalactic pair-production absorption coefficient as derived by Stecker and De Jager by making use of a new empirically based calculation of the spectral energy distribution of the intergalactic infrared radiation field as given by Malkan and Stecker. We show that the results of the Malkan and Stecker calculation agree well with recent data on the infrared background. We then show that Whipple observations of the flaring gamma-ray spectrum of Mrk 421 hint at extragalactic absorption and that the HEGRA observations of the flaring spectrum of Mrk 501 appear to strongly indicate extragalactic absorption. We also discuss the determination of the y-ray opacity at higher redshifts, following the treatment of Salamon and Stecker. We give a predicted spectrum, with absorption included for PKS 2155-304. This XBL lies at a redshift of 0.12, the highest redshift source yet observed at an energy above 0.3 TeV. This source should have its spectrum steepened by approx. 1 in its spectral index between approx. 0.3 and approx. 3 TeV and should show an absorption cutoff above approx. 6 TeV.

Stecker, F. W.↗
NASA NTRS2002

A Focal Plane Stellar X-Ray Polarimeter for SXG

The Stellar X-Ray Polarimeter (SXRP) is the only X-ray polarimeter designed to view astrophysical sources that is currently scheduled to be flown. The SXRP is one of eight astronomical X-ray instruments that intended to be flown at the focal plane of the two SODART large-area metal-foil grazing-incidence X-ray telescopes on the Russian Spectrum-Roentgen-Gamma (SRG) mission. The engineering model of the SXRP was delivered to Russia in February 1994. Construction of the flight model (FM) is complete. The SXRP-FM is currently in storage at the Smithsonian Astrophysical Observatory and will be delivered when required.

Kaaret, P.↗
NASA NTRS2002

A Focal Plane Stellar X-Ray Polarimeter for SXG

The Stellar X-Ray Polarimeter (SXRP) is the only X-ray polarimeter designed to view astrophysical sources that is currently scheduled to be flown. The SXRP is one of eight astronomical X-ray instruments intended to be flown at the focal plane of the two SODART large-area metal-foil grazing-incidence X-ray telescopes on the Russian Spectrum-Roentgen-Gamma (SRI) mission. The engineering model of the SXRP was delivered to Russia in February 1994. Construction of the flight model (FM) is complete. The SXRP-FM is currently in storage at SAO.

Kaaret, P.↗
NASA NTRS2003

A Focal Plane Stellar X-Ray Polarimeter for SXG

This report covers progress on a grant for the Stellar X-Ray Polarimeter (SXRP). SXRP is the only X-ray polarimeter designed to view astrophysical sources that is currently scheduled to be flown. The SXRP is one of eight astronomical X-ray instruments that intended to be flown at the focal plane of the two SODART large-area metal-foil grazing-incidence X-ray telescopes on the Russian Spectrum-Roentgen-Gamma (SRG) mission.

Christensen, Larry↗
NASA NTRS2004

Spectral Index and Quasi-Periodic Oscillation Frequency Correlation in Black Hole (BH) Sources: Observational Evidence of Two Phases and Phase Transition in BHs

Recent studies have shown that strong correlations are observed between the low frequencies (1-10 Hz) of quasiperiodic oscillations (QPOs) and the spectral power law index of several Black Hole (BH) candidate sources, in low hard states, steep power-law (soft) states and in transition between these states. The observations indicate that the X-ray spectrum of such state (phases) show the presence of a power-law component and are sometimes related to simultaneous radio emission indicated the probable presence of a jet. Strong QPOs (less than 20% rms) are present in the power density spectrum in the spectral range where the power-law component is dominant ( i.e. 60-90% ). This evidence contradicts the dominant long standing interpretation of QPOs as a signature of the thermal accretion disk. We present the data from the literature and our own data to illustrate the dominance of power-law index-QPO frequency correlations. We provide a model, that identifies and explains the origin of the QPOs and how they are imprinted on the properties of power-law flux component. We argue the existence of a bounded compact coronal region which is a natural consequence of the adjustment of Keplerian disk flow to the innermost sub-Keplerian boundary conditions near the central object and that ultimately leads to the formation of a transition layer (TL) between the adjustment radius and the innermost boundary. The model predicts two phases or states dictated by the photon upscattering produced in the TL: (1) hard state, in which the TL is optically thin and very hot (kT approx. greater than 50 keV) producing photon upscattering via thermal Componization; the photon spectrum index Gamma appprox.1.5 for this state is dictated by gravitational energy release and Compton cooling in an optically thin shock near the adjustment radius; (2) a soft state which is optically thick and relatively cold (approx. less than 5 keV); the index for this state, Gamma approx. 2.8 is determined by soft-photon upscattering and photon trapping in converging flow into BH. In the TL model for corona the QPO frequency vnu(sub high) is related to the gravitational (close to Keplerian) frequency nu(sub K) at the outer (adjustment) radius and nu(sub low) is related to the TL s normal mode (magnetoacoustic) oscillation frequency nu(sub MA). The observed correlations between index and low and high QPO frequencies are readily explained in terms of this model. We also suggest a new method for evaluation of the BH mass using the index-frequency correlation.

Titarchuk, Lev↗