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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Low-mass X-ray binaries and gamma-ray bursts

More than twenty years after their discovery, the nature of gamma-ray burst sources (GRBs) remains mysterious. The results from BATSE experiment aboard the Compton Observatory show however that most of the sources of gamma-ray bursts cannot be distributed in the galactic disc. The possibility that a small fraction of sites of gamma-ray bursts is of galactic disc origin cannot however be excluded. We point out that large numbers of neutron-star binaries with orbital periods of 10 hr and M dwarf companions of mass 0.2-0.3 solar mass are a natural result of the evolution of low-mass X-ray binaries (LMXBs). The numbers and physical properties of these systems suggest that some gamma-ray burst sources may be identified with this endpoint of LMXB evolution. We suggest an observational test of this hypothesis.

Lasota, J. P.↗
NASA NTRS1994

Large area soft x-ray collimator to facilitate x-ray optics testing

The first objective of this program is to design a nested conical foil x-ray optic which will collimate x-rays diverging from a point source. The collimator could then be employed in a small, inexpensive x-ray test stand which would be used to test various x-ray optics and detector systems. The second objective is to demonstrate the fabrication of the x-ray reflectors for this optic using lacquer-smoothing and zero-stress electroforming techniques.

Espy, Samuel L.↗
NASA NTRS1994

The optical and X-ray content of the 1992 May 1 gamma-ray burst error box

A gamma-ray burst which occurred on 1992 May 1 was observed by three spacecraft in the third interplanetary network, and rapidly localized to a small error box. The coordinates were promptly circulated to a wide astronomical community, and radio, optical, and X-ray counterpart searches were carried out. A weak X-ray source was found in the error box, and two radio sources are discovered outside the error box, but in alignment with the X-ray source. The X-ray source position contains approximately 25 optical objects down to 23d magnitude. We discuss the prospects for identifying the burster counterpart.

Hurley, K.↗
NASA NTRS1994

The isotopic composition of cosmic-ray beryllium and its implication for the cosmic ray's age

We report a new measurement of the cosmic-ray isotopic composition of beryllium in the low-energy range from 35 to 113 MeV per nucleon. This measurement was made using the High Energy Telescope of the CRS experiment on the Voyager 1 and 2 spacecraft during the time period from 1977 to 1991. In this overall time period of 14 years the average solar modulation level was about 500 MV. The cosmic-ray beryllium isotopes were completely separated with an average mass resolution sigma of 0.185 amu. The isotope fractions of Be-7, Be-9, and Be-10 obtained are 52.4 +/- 2.9%, 43.3 +/- 3.7%, and 4.3 +/- 1.5%, respectively. The measured cosmic-ray abundances of Be-7 and Be-9 are found to be in agreement with calculations based on standard Leaky-Box model for the interstellar propagation of cosmic-ray nuclei using the recent cross sections of the New Mexico-Saclay collaboration. From our observed ratio Be-10/Be = 4.3 +/- 1.5% we deduce an average interstellar density of about 0.28 (+0.14, -0.11) atoms/cu cm, and acosmic-ray lifetime for escape of 27 (+19, -9) x 10(exp 6) years. The surviving fraction of Be-10 is found to be 0.19 +/- 0.07. Modifications to the conclusions of the Leaky-Box model when a diffusion + convection halo model for propagation is used are also considered.

Lukasiak, A.↗
NASA NTRS1994

Constraints on hot star X-ray source characteristics from combinded analysis of X-ray and UV observations

Results from wind ionization calculations are presented which show how the P-Cygni profiles of 'superionized' species such as O VI can provide information about the X-ray source characteristics of early-type stars. Using detailed radiative and atomic physics models, we find that a significant source of X-ray emission from zeta Pup comes from a region in the wind located within rougly 1 to 2 stellar radii of the photosphere. Our results suggest that X-rays sources in which emission occurs exclusively at large radii (r greater than or approximately equal to a few R(sub *)) are inconsistent with UV P-Cygni profiles for O VI. Instead, we find that X-ray emission from shocks distributed throughout the lower regions of the wind (r approximately equal to 1-2 R(sub *)) is consistent with both X-ray and UV data, as well as mass loss rates deduced from radio and H-alpha observations.

Macfarlane, J. J.↗
NASA NTRS1996

The Integrated X-Ray Spectrum of Galactic Populations of Luminous Supersoft X-Ray Sources

We compute the composite X-ray spectrum of a population of unresolved SSS's in a spiral galaxy such as our own or M31. The sources are meant to represent the total underlying population corresponding to all sources which have bolometric luminosities in the range of 10(exp 37) - 10(exp 38) ergs/s and kT on the order of tens of eV. These include close-binary supersoft sources, symbiotic novae, and planetary nebulae, for example. In order to determine whether the associated X-ray signal would be detectable, we also 'seed' the galaxy with other types of X-ray sources, specifically low-mass X-ray binaries (LMXB's) and high-mass X-ray binaries (HMXB's). We find that the total spectrum due to SSS's, LMXB's, and HMXB's exhibits a soft peak which owes its presence to the SSS population. Preliminary indications are that this soft peak may be observable.

DiStefano, R.↗
NASA NTRS1997

Low-state gamma-ray emission from blazars and the gamma-ray background

The contribution of flat spectrum radio sources (FSRS) or blazars to the diffuse gamma ray background is examined. The basic assumptions of the investigation are: the existence of steady state gamma ray emission at the energetic gamma ray experiment telescope (EGRET) instrument energy band from the entire population of the FSRS; a proportionality between the FSRS' gamma ray luminosities and radio luminosities; and the production of the diffuse gamma ray background by the ensemble of blazars. Under these assumptions, the estimated average value of the proportionality constant in the luminosity relationship (vF(sub v))(sub 100 MeV) = f(vF(sub v))(sub 5 GHz) is approximately 70, compared to a mean observed value of 750. The implications of this result for the active galactic nuclei models are considered.

Kazanas, Demosthenes↗
NASA NTRS1997

Cosmic Ray Induced Degradation in X-Ray Detectors On Board the Near Earth Asteroid Rendezvous Spacecraft

The Near-Earth Asteroid Rendezvous (NEAR) mission is the first in NASA's new Discovery Program to explore the solar system. Launched in February 1996, the NEAR spacecraft will take a long cruise flight arriving at the asteroid 433 Eros in January 1999 for a one year orbiting survey operation. This long exposure to the space environment has already proven to be an additional complication for the x-ray spectrometer. The asteroid pointing detectors for the x-ray spectrometer are three gas-filled proportional counters with resolving power in the range of 1 keV. Therefore, to resolve the important but closely spaced magnesium, aluminum and silicon k alpha lines( 1.255, 1.487, 1.739 keV respectively), magnesium and aluminum balanced filters are used on two of the detectors. The x-ray florescence from the surface of Eros is stimulated by solar x-rays. A proportional counter and a silicon PIN detector are used to monitor the solar incident x-ray flux. The proportional counters are single wire gas filled beryllium lined steel tubes operating at about 1100 volt. The 25 cm(exp 2) optical window is one mil. thick beryllium. To define the detector's active region, two boron nitride disks were incorporated in the tube just outside window area. It appears from the space flight data that the space environment is creating a charge on these boron nitride disks which ultimately distorts the tube gain and resolution. This broadening of the photo peak makes it more difficult to identify weak peaks and so degrades the statistical accuracy for some very important elements such as sulfur, calcium and iron (2.307, 3.690, 6.403 keV respectively). If the broadening is severe enough in the low energy region, counts will be lost as the photo peak spreads below the lower level discriminator (0.7 keV).

Floyd, S. R.↗
NASA NTRS1998

Measuring the Radius of a Neutron Star; Origin of High X-Ray Luminosities in Optically Passive Galaxies; Resolving the Source of X-Rays in IC 1613"

This recently expired grant has supported the work of the PI, his students, and his collaborators on a variety of ROSAT projects over the past three years. Annual reports have summarized much of the work accomplished; here we provide a brief review of the work resulting from this effort, and a summary of the personnel who have benefited from its support. A high resolution ROSAT HRI X-ray image of the Local Group dwarf IC1613 revealed that the principal source of X-ray emission in this direction arises in a background cluster of galaxies, as first suggested by Eskridge (1995). In addition, however, we found a bright X-ray source coincident with the only known supernova remnant in this galaxy, S # 8. Extensive ground-based follow-up observations in the radio and optical regimes were conducted. We confirmed the nonthermal radio spectral index of the source and measured its extent to be approx. 3 sec at 20 cm. Imaging spectrophotometric observations taken with the multi-pupil spectrograph of the Special Astrophysical Observatory in the FSU allowed us to determine the density and velocity distribution of the gas in the remnant. The simultaneous presence of luminous X-ray and optical emission suggests a relatively young remnant in which the outward-moving shock has recently encountered dense material. Many of this object's properties are similar to those of the brightest optical remnant in the Large Magellanic Cloud, N49. Another potential source of X-rays in this galaxy which featured prominently in our original proposal, an Oxygen Wolf-Rayet star with a large surrounding wind-blown bubble, was not detected.

Helfand, David J.↗
NASA NTRS1999

X-ray and Gamma-ray Observations of the COS-B Field 2CG 075+00

We present a summary of gamma-ray and X-ray observations of the intriguing COS-B field, 2CG 075+00, in order to search for potential counterparts. The third EGRET (Energetic Gamma Ray Experiment Telescope) (3EG) catalog shows that the COS-B emission corresponds to at least two localized gamma-ray sources, 3EG 2016+3657 and 3EG J2021+3716. We present analyses of archival X-ray fields which overlap error boxes of both the EGRET sources.

Mukherjee, R.↗
NASA NTRS1994

Particle Acceleration, X-Rays, and Gamma-Rays From Winds

The instability of the line-driven winds of hot stars leads to the formation of strong shocks. These shocks not only emit thermal X-rays, but also accelerate a small fraction of the thermal electrons and ions to relativistic energies. Synchrotron radiation from these energetic particles can account for the non-thermal radio emission observed from some hot stars, and can also explain the hard X-rays detected in the Einstein X-ray spectra. Our calculations indicate that the gamma-ray emission from non-thermal particles should be detectable by GRO. The detection (or non-detection) of these emissions over a wide energy range, from the radio to gamma-rays, should provide a great deal of information on the structure of the unstable winds and the physics of particle acceleration by shocks.

White, Richard L.↗
NASA NTRS2002

Guiding the Way to Gamma-Ray Souces: X-Ray Studies of Supernova Remnants

Supernova remnants (SNRs) have long been suggested as a class of potential counterparts to unidentified gamma-ray sources. The mechanisms by which such gamma-rays can arise may include emission from a pulsar associated with a remnant, or a variety of processes associated with energetic particles accelerated by the SNR shock. Imaging and spectral observations in the X-ray band can be used to identify properties of the remnants that lead to gamma-ray emission, including the presence of pulsar-driven nebulae, nonthermal X-ray emission from the SNR shells, and the interaction of SNRs with dense surrounding material.

Slane, Patrick O.↗
NASA NTRS1996

In Situ Mineralogical Analysis of Planetary Materials Using X-Ray Diffraction and X-Ray Fluorescence

Remote observations of Mars have led scientists to believe that its early climate was similar to that of the early Earth, having had abundant liquid water and a dense atmosphere. One of the most fascinating questions of recent times is whether simple bacterial life developed on Mars (as it did on the Earth) during this early element period. Analyses of SNC meteorites have broadened considerably our knowledge of the chemistry of certain types of Martian rocks, underscoring the tantalizing possibility of early hydrothermal systems and even of ancient bacterial life. Detailed analyses of SNC meteorites in Terrestrial laboratories utilize the most sophisticated organic, isotopic and microscopic techniques in existence. Indeed; it is unlikely that the key biogenic indicators used in McKay et al (ibid) could be identified by a remote instrument on the surface of Mars. As a result, it is probable that any robotic search for evidence of an ancient Martian biosphere will have as its focus the identification of key minerals in likely host rocks rather than the direct detection of organic or isotopic biomarkers. Even on a sample return mission, mineralogical screening will be utilized to choose the most likely candidate rocks. X-ray diffraction (XRD) is the only technique that can provide a direct determination of the crystal structures of the phases present within a sample. When many different crystalline phases are present, quantitative analysis is better constrained if used in conjunction with a determination of elemental composition, obtainable by X-ray fluorescence (XRF) using the same X-ray source as for XRD. For planetary surface analysis, a remote instrument combining XRD and XRF could be used for mineralogical characterization of both soils and rocks. We are designing a remote XRD/XRF instrument with this objective in mind. The instrument concept pays specific attention to constraints in sample preparation, weight, volume, power, etc. Based on the geometry of a pinhole camera (transmission geometry, flat two-dimensional detector perpendicular to the direct beam), the instrument (which we call CHEMIN, for Chemistry and Mineralogy) uses an X-ray sensitive CCD detector which will allow concurrent positional and energy-dispersive analysis of collected photons. Thus XRF (energy) and XRD (geometry) analysis of transmitted X-rays will be performed at the same time. Tests performed with single minerals and simple mixtures give promising results. Refinements of the prototype promise interpretable results on complex samples.

Sarrazin, P.↗
NASA NTRS2004

Propagation of Cosmic Rays and Diffuse Galactic Gamma Rays

This paper presents an introduction to the astrophysics of cosmic rays and diffuse gamma-rays and discusses some of the puzzles that have emerged recently due to more precise data and improved propagation models: the excesses in Galactic diffuse gamma-ray emission, secondary antiprotons and positrons, and the flatter than expected gradient of cosmic rays in the Galaxy. These also involve the dark matter, a challenge to modern physics, through its indirect searches in cosmic rays. Though the final solutions are yet to be found, I discuss some ideas and results obtained mostly with the numerical propagation model GALPROP. A fleet of spacecraft and balloon experiments targeting these specific issues is set to lift off in a few years, imparting a feeling of optimism that a new era of exciting discoveries is just around the corner. A complete and comprehensive discussion of all the recent results is not attempted here due to the space limitations.

Moskalenko, Igor V.↗
NASA NTRS2005

Chandra Observation of an X-ray Flare at Saturn: Evidence for Direct Solar Control on Saturn's Disk X-ray Emissions

Saturn was observed by Chandra ACIS-S on 20 and 26-27 January 2004 for one full Saturn rotation (10.7 hr) at each epoch. We report here the first observation of an X-ray flare from Saturn s non-auroral (low-latitude) disk, which is seen in direct response to an M6-class flare emanating from a sunspot that was clearly visible from both Saturn and Earth. Saturn s X-ray emissions are found to be highly variable on time scales of tens of minutes to weeks. Unlike Jupiter, X-rays from Saturn s polar (auroral) region have characteristics similar to those from its disk and varies in brightness inversely to the FUV auroral emissions observed by the Hubble Space Telescope. This report establishes that disk X-ray emissions of the giant planets Saturn and Jupiter are directly regulated by processes happening on the Sun. We suggest that these emissions could be monitored to study X-ray flaring from solar active regions when they are on the far side and not visible to Near-Earth space weather satellites.

Bhardwaj, Anil↗
NASA NTRS2006

Observations of Compact X-Ray Binaries with the Chandra X-Ray Observatory

The Chandra X-Ray Observatory Launched on July 23, 1999. The first X-ray photons were detected on August 12 of that same year. Subsequently observations with the Observatory, which features sub-arcsecond angular resolution, have revolutionized our understanding of the X-ray emitting sky providing hosts of spectacular energy-resolved images and high-resolution spectra. Here we present a brief overview of Chandra X-Ray Observatory observations of compact X-ray binaries.

Weisskopf, Martin C.↗
NASA NTRS2007

Eta Carinae: X-ray Line Variations during the 2003 X-ray Minimum, and the Orbit Orientation

The future evolution of Eta Carinae will be as a supernova (or hypernova) and black hole. The evolution is highly contingent on mass and angular momentum changes and instabilities. The presence of a companion can serve to trigger instabilities and provide pathways for mass and angular momentum exchange loss. X-rays can be used a a key diagnostic tool: x-ray temperatures trace pre-shock wind velocities, periodic x-ray variability traces the orbit, and x-ray line variations traces the flow and orientation of shocked gas. This brief presentation highlights x-ray line variations from the HETG and presents a model of the colliding wind flow.

Corcoran, M. F.↗
NASA NTRS2009

Terrestrial Gamma-ray Flash (TGF) Observations with the Gamma-ray Burst Monitor on the Fermi Observatory

Terrestrial Gamma-ray Flashes (TGFs) have now been detected with four different orbiting spacecraft. The latest observations are being made with the scintillation detectors of Gamma-ray Burst Monitor (GBM) on the Fermi Gamma-ray Space Telescope Observatory (Fermi). Although this experiment was designed and optimized for the observation of cosmic gamma-ray bursts (GRBs), it has unprecedented capabilities for TGF observations, surpassing those of the experiment that discovered TGFs, the BATSE experiment on the Compton Gamma-ray Observatory. Launched in June 2008 from the Kennedy Space Center, the Fermi-GBM has been detecting about one TGF every four weeks. The thick bismuth germinate (BGO) scintillation detectors of the GBM have now observed photon energies from TGFs at energies up to approx.40 MeV. Individual photons are detected with an absolute timing accuracy of 2 microsec. Unlike the BATSE instrument, the GBM data system allows higher counting rates to be recorded and deadtime characteristics are well-known and correctable; thus the saturation effects seen with BATSE are avoided. TGF pulses as narrow as approx.0.1ms have been observed with the GBM. Like BATSE (and unlike RHESSI) an on-board trigger is required to detect TGFs. The minimum time window for this trigger is 16ms. A trigger window this wide greatly reduces the number of detected TGFs, since they most often have a much shorter duration than this window, thus reducing the signal-to-background. New on-board trigger algorithms based on detected photon energies are about to be implemented; this should increase the number of TGF triggers. High-energy spectra from TGFs observed with Fermi-GBM will be described.

Fishman, Gerald J.↗