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At least 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

EGRET detection of high-energy gamma radiation from the OVV quasar 3C 454.3

High-energy gamma radiation has been observed from the optically violent variable quasar 3C 454.3 (PKS 2251 + 158) by the Energetic Gamma-Ray Experiment Telescope on the Compton Observatory. During the 1992 January-February observation, the emission showed a power-law photon spectrum with an exponent of -2.18 +/- 0.08. The flux density (greater than 100 MeV) was observed to vary within the range (0.4-1.4) x 10 exp -6 photon/sq cm s on a time scale of less than a week. Lower sensitivity observations during 1992 April and May also detected emission within that range, but with lower statistical significance. An earlier low-sensitivity exposure during 1991 August gave a 95 percent confidence upper limit of 0.26 x 10 exp 6 photon/sq cm s. The similarity of the gamma-ray emission of 3C 454.3 to that of 3C 279 parallels the similarity of these two objects at lower frequencies.

Hartman, R. C.↗
NASA NTRS1999

Origin of Gamma-Ray Emissions from the MeV Blazars

More than sixty gamma-ray emitting blazars have been detected by the Compton Gamma Ray Observatory (CGRO). A sub-class of these blazars is known as MeV blazars that are most luminous at the MeV energies. Spectra of these sources show a break somewhere between 1 MeV and 30 MeV with break amplitude as large as DELTA-GAMMA = 1.5. Different models like Compton cooling jet models, synchrotron self-Compton models, etc., have been suggested to explain the observed properties of the MeV luminous blazars. However these models are unable to account for spectral break amplitudes in excess of 0.5. Here we suggest an inverse-Compton model based on recent published experimental data where MeV gamma rays were produced via Compton backscattering of UV photons with 500 MeV electrons. We show that the high energy electrons of the relativistic jet can collide with the copious ultraviolet photons of the big blue bump of the blazar, and produce photons peaked in the MeV region. Also we show that the X rays and the soft gamma rays in these sources are produced through the synchrotron self-Compton process. Flux computed from our model have been used to compare with the observed X-ray/gamma-ray spectra of the MeV luminous blazars. Details of the model are discussed in the present paper.

Ghosh, K. K.↗
NASA NTRS1998

Origin of Gamma-Ray Emissions from the MeV Blazars

More than sixty gamma-ray emitting blazars have been detected by the Compton Gamma Ray Observatory (CGRO). A sub-class of these blazars is known as MeV blazars that are most luminous at the MeV energies. Spectra of these sources show a break somewhere between 1 MeV and 30 MeV with break amplitude as large as Delta(Gamma) = 1.5. Different models like Compton cooling jet models, synchrotron self-Compton models, etc., have been suggested to explain the observed properties of the MeV luminous blazars. However these models are unable to account for spectral break amplitudes in excess of 0.5. Here we suggest an inverse-Compton model based on recent published experimental data where MeV gamma rays were produced via Compton backscattering of UV photons with 500 MeV electrons. We show that the high energy electrons of the relativistic jet can collide with the copious ultraviolet photons of the big blue bump of the blazar, and produce photons peaked in the MeV region. Also we show that the X rays and the soft gamma rays in these sources are produced through the synchrotron self-Compton process. Flux computed from our model have been used to compare with the observed X-ray/ gamma-ray spectra of the MeV luminous blazars. Details of the model are discussed in the present paper.

Ghosh, K. K.↗
NASA NTRS2001

Possible Evidence for an Event Horizon in Cyg XR-1

The X-ray emitting component in the Cyg XR-1/HDE226868 system is a leading candidate for identification as a stellar-mass sized black hole. The positive identification of a black hole as predicted by general relativity requires the detection of an event horizon surrounding the point singularity. One signature of such an event horizon would be the existence of dying pulse trains emitted by material spiraling into the event horizon from the last stable orbit around the black hole. We observed the Cyg XR-1 system at three different epochs in a 1400 - 3000 A bandpass with 0.1 ms time resolution using the Hubble Space Telescope's High Speed Photometer. Repeated excursions of the detected flux by more than three standard deviations above the mean are present in the UV flux with FWHM 1 - 10 ms. If any of these excursions are pulses of radiation produced in the system (and not just stochastic variability associated with the Poisson distribution of detected photon arrival times), then this short a timescale requires that the pulses originate in the accretion disk around Cyg XR-1. Two series of pulses with characteristics similar to those expected from dying pulse trains were detected in three hours of observation.

Dolan, Joseph F.↗
NASA NTRS2004

Discovery of a Transient Magnetar: XTE J1810-197

We report the discovery of a new X-ray pulsar, XTE J1810-197, that was serendipitously discovered on 2003 July 15 by the Rossi X-Ray Timing Explorer (RXTE) while observing the soft gamma repeater SGR 1806-20. The pulsar has a 5.54 s spin period, a soft X-ray spectrum (with a photon index of approx. = 4). and is detectable in earlier RXTE observations back to 2003 January but not before. These show that a transient outburst began between 2002 November 17 and 2003 January 23 and that the source's persistent X-ray flux has been declining since then. The pulsar exhibits a high spin-down rate P approx.= l0(exp -11) s/s with no evidence of Doppler shifts due to a binary companion. The rapid spin-down rate and slow spin period imply a supercritical characteristic magnetic field B approx. = 3 x l0(exp 14) G and a young age tau less than or = 7600 yr. Follow-up Chandra observations provided an accurate position of the source. Within its error radius, the 1.5 m Russian-Turkish Optical Telescope found a limiting magnitude R(sub c) = 21.5. All such properties are strikingly similar to those of anomalous X-ray pulsars ad soft gamma repeaters, providing strong evidence that the source is a new magnetar. However, archival ASCA and ROSAT observations found the source nearly 2 orders of magnitude fainter. This transient behavior and the observed long-term flux variability of the source in absence of an observed SGR-like burst activity make it the first confirmed transient magnetar and suggest that other neutron stars that share the properties of XTE 51810- 197 during its inactive phase may be unidentified transient magnetars awaiting detection via a similar activity. This implies a larger population of magnetars than previously surmised and a possible evolutionary connection between magnetars and other neutron star families. Subject headings: pulsars: general -pulsars: individual (XTE 51810- 197) - stars: magnetic fields -

Ibrahim, Alaa I.↗
DOE OSTI2022

Isotope-selective radiography and material assay using high-brilliance, quasi-monochromatic, high-energy photons

We demonstrate the possibility to directly detect microgram amounts of the isotope 7 Li using a quasi-monochromatic high-energy photon beam. The isotope selective detection is based on a witness scatterer absorbing and re-emitting photons via nuclear resonance fluorescence. This enables the detection of isotopes with microgram accuracy at long distances from the actual sample. Further, we demonstrate that the technique can deliver quantitative information without specific knowledge of the photon flux and no spectral capabilities or knowledge of the resonance fluorescence cross section. Detection of low-atomic-weight isotopes screened by heavy shielding is also shown. The techniques described are applicable to all next-generation, ultrahigh brilliance, laser-Compton light sources currently under construction.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

Performance Analysis and Radiation Damage Estimation of the Stopping Target Monitor Detectors in Mu2e

The Mu2e experiment aims to search for the charged lepton flavour violating (CLFV) process of a coherent, neutrinoless, conversion of a muon into an electron within the proximity of an aluminium nucleus. Mu2e seeks to measure the ratio (R μe ) of the rate of this conversion process, relative to that of ordinary muon capture. Mu2e will achieve world-leading sensitivity, improving the current limit of Rμe < 7 ×10 -13 (90% C.L.),set by the SINDRUM-II experiment, by an order of 104. This corresponds to a single-event sensitivity of Rμe < 2.87 × 10 -17 (90% C.L.). Many beyond Standard Model (BSM) theories require CLFV to occur at a rate accessible by Mu2e. Any observation of CLFV at Mu2e would have profound implications on particle physics. The Stopping Target Monitor (STM) will be comprised of both a HPGe and a LaBr3 detector which will monitor the signals of photons produced in stopped-muon processes to a required accuracy of 10%. To achieve the desired sensitivity, it is imperative that the detectors perform at their optimal level. The STM detectors are placed within a harsh radiation environment in the form of a high energy ‘flash’ of gamma radiation as well as a flux of fast neutrons. Estimations of radiation damage to HPGe in literature is limited. This thesis reports an approach which utilizes the Kinetic-Energy Released in Matter (KERMA) parameter that considers the energy dependence of the incident flux and the cross-section information for the incident particle. Resampling schemes have been adopted to give a realistic rate over the experiment run time. The results have then been normalized to the radiation damage caused by a 252 Cf source. The estimated time before annealing of the detector is necessary is found to be 75 and 78 months in the case of neutron damage and τ ~ 24 years, 11 months in the case of electron/positron induced damage. This shows significant improvement on the previous estimations.

36 MATERIALS SCIENCE↗
NASA NTRS1993

A study of the impact of the Space Shuttle environment on faint far-UV geophysical and astronomical phenomena

FAUST is a far ultraviolet (1400-1800 A) photon-counting imaging telescope featuring a wide field of view (7.6 deg) and a high sensitivity to extended emission features. During its flight as part of the ATLAS-1 payload aboard the STS-45 mission in March 1992, 19 deep-space nighttime viewing opportunities were utilized by FAUST. Here we report the observed fluxes and their time and space variations, and identify the signatures of postsunset airglow phenomena and Orbiter Vernier attitude control thruster firing events. We find that the Space Shuttle nighttime environment at 296 km altitude is often sufficiently dark to permit geophysical and astronomical UV observations down to levels on the order of 1000 photons/sq cm sr A sec, or 0.01 Rayleighs/A. We also find evidence for occasional geophysical fluxes of some tens or hundreds of Rayleighs in the upward-looking direction.

Lampton, Michael↗
NASA NTRS1999

ASCA Observations of W44

We report the detection, using data from the Advanced Satellite for Cosmology and Astrophysics (ASCA), of a hard X-ray source in the vicinity of the radio pulsar PSR B1853+01, which is located within the supernova remnant (SNR) W44. PSR B1853+01, a 267 ms pulsar, has to date been detected only in the radio band. Previous observations at soft X-ray energies (e.g., with ROSAT HRI) have failed to detect any significant X-ray emission (pulsed or unpulsed) from the pulsar. In addition, no high-energy emission (approx. > 4 keV) has been detected previously from W44. Over the 0.5-4.0 keV band, the ASCA data show soft thermal emission from W44, with a morphology very similar to that observed earlier by Einstein and ROSAT. In the high-energy band (4.0-9.5 keV), the SNR is, for the most part, invisible, although a source coincident with the position of PSR B1853+01 is evident. The observed ASCA spectra are consistent with a power-law origin (photon index approx. 2.3) for the X-ray emission from this source at a flux level (flux density approx. 0.5 micro Jy at I keV) consistent with previous upper limits. The maximum allowed size for the source is determined directiv from the ASCA data (<5 min.), while the minimum size is derived from the nondetection of a point source in the ROSAT HRI data (approx. > 30 sec.). Timing analysis of the hard X-ray source failed to detect pulsations at the pulsar's period. Based on these lines of evidence, we conclude that the new hard source in W44 represents an X-ray synchrotron nebula associated with PSR B1853+01, rather than the beamed output of the pulsar itself. This discoverv adds W44 to the small group of previously known plerionic SNRs This nebula lies at the low end of, but is consistent with, the correlation between X-ray luminosity and pulsar spin-down energy loss found for such objects, lending further support to our interpretation.

Hughes, John P.↗
NASA NTRS1997

Environmental modification of yield and nutrient composition of 'Waldmann's Green' leaf lettuce

Leaf number, dry weight, and nutrient composition of Lactuca sativa L. cv. Waldmann's Green leaves were compared following 9 days of treatment in a controlled environment room under various combinations of photosynthetic photon flux (PPF:350 vs 800 micromoles m-2 s-1), atmospheric CO2 level (ambient vs 1500 micromoles mol-1), and single-strength (1X:15 mM) vs double-strength (2X:30 mM) nitrogen (N) as NO3- alone or as NH4(+) + NO3- (1:5 molar ratio). CO2 enrichment greatly enhanced leaf number under all PPF and N conditions, but increased leaf dry weight only at high PPF. Conditions favoring high photosynthesis enhanced leaf starch content 3-fold, and protein content increased as much as 64% with 2X NH4(+)+NO3-. Free sugar content was 6 to 9% of leaf dry weight for all treatment combinations, while fat was 1.5 to 3.5%. Ash content varied from 15 to 20% of leaf dry weight. Modified controlled environments can be used to enhance the nutritional content as well as the yield of crops to be used for life support in space-deployed, self-sustaining human habitats. Leaf lettuce is a useful model crop for demonstrating the potential of nutritional value added by environmental manipulation.

Non-NASA Center↗
DOE OSTI2021

Radiation Effects Related to Repaired BWR Core Shrouds

The material properties of the core shroud and supporting tie rod components are important factors for Boiling Water Reactor (BWR) life extension considerations. Given the variability of nickel and boron content found in 304 stainless steel shroud plates, and XM-19 and Inconel X-750 tie rod materials, this paper combines the latest spectral information in the vicinity of the core shroud to calculate hydrogen and helium gas production limits as well as atomic displacement damage spanning 80 full-power years (FPY) using the SPECTER code. Results indicate that, whereas boron has little effect on the atomic displacement damage, helium production from boron can be important and must be included in weldability assessments. Since the boron concentration is uncertain, helium generation is calculated assuming a composition range based on specifications and reported measurements. The results show that helium production caused by transmutations in the 304 SS shroud material are dominated by the concentration of boron and will have a significant impact on weldability after only a few years of service. For tie rods, the susceptibility to irradiation-assisted stress corrosion cracking (IASCC) has been assessed by determining helium production in Inconel X-750 IASCC specimens irradiated in the Advanced Test Reactor (ATR) reactor and correlating with the helium produced in a BWR. As a tie rod material in a high thermal neutron flux environment, Inconel X-750 is more likely to be IASCC sensitive during service in a BWR due to helium than the XM-19 stainless steel alloy because of a greater nickel content and boron concentration. Weldability of the core shroud plate material, IASCC of the tie rod materials, and the gamma photon contribution to radiation damage are discussed.

radiation damage in materials, BWR↗
NASA NTRS1992

Radiative shocks and hydrogen molecules in pregalactic gas - The effects of postshock radiation

Previous calculations of radiative shocks in pregalactic gas are generalized and substantially improved. The hydrodynamical conservations are solved, along with the rate equations for nonequilibrium ionization, recombination, and molecule formation, and the equation of radiative transfer, for steady-state shocks in a gas of primordial compositions. This calculation self-consistently includes the effects of the diffuse postshock emission as well as a possible external radiation flux on the postshock flow and the preshock ionization levels. A previous result is confirmed that the shocked gas cools faster than it can recombine and, as a result, is able to form an H2 concentration as high as 0.003 via the formation of H(-) and H2(+) intermediaries due to the enhanced nonequilibrium ionization fraction at 10,000 K. It is shown that, as the level of the external radiation flux is increased, the formation of and cooling by H2 molecules can be inhibited and delayed. The detailed radiation spectra and ionizing photon number fluxes emergent from these shocks are presented.

Kang, Hyesung↗
NASA NTRS1990

A comparison of UV surface brightness and HI surface densities for spiral galaxies

Shaya and Federman (1987) suggested that the ambient ultraviolet flux at 1000 A permeating a spiral galaxy controls the neutral hydrogen (HI) surface density in the galaxy. They found that the atomic envelopes surrounding small molecular clouds, because of their great number, provide the major contribution to the HI surface density over the stellar disk. The increase in HI surface density with later Hubble types was ascribed to the stronger UV fields from more high-mass stars in later Hubble types. These hypotheses are based on the observations of nearby diffuse interstellar clouds, which show a sharp atomic-to-molecular transition (Savage et al. 1977), and on the theoretical framework introduced by Federman, Glassgold, and Kwan (1979). Atomic envelopes around interstellar clouds in the solar neighborhood arise when a steady state is reached between photodissociation of H2 and the formation of H2 on grains. The photodissociation process involves photons with wavelengths between 912 A and 1108 A. Shaya and Federman used H-alpha flux as an approximate measure for the far UV flux and made their comparisons based on averages over Hubble type. Here, researchers compare, on an individual basis, UV data obtained with space-borne and balloon-borne instruments for galaxies with measurements of HI surface density (Warmels 1988a, b). The comparisons substantiate the conclusion of Shaya and Federman that the far UV field controls the HI content of spiral galaxies.

Federman, S. R.↗
DOE OSTI2024

Procrustean entanglement concentration in quantum-classical networking

The success of a future quantum internet will rest in part on the ability of quantum and classical signals to coexist in the same optical fiber infrastructure, a challenging endeavor given the orders of magnitude differences in flux of single-photon-level quantum fields and bright classical traffic. Here, we theoretically describe and experimentally implement Procrustean entanglement concentration for polarization-entangled states contaminated with classical light, showing significant mitigation of crosstalk noise in dense wavelength-division multiplexing. Our approach leverages a pair of polarization-dependent loss emulators to attenuate highly polarized crosstalk that results from imperfect isolation of conventional signals copropagating on shared fiber links. We demonstrate our technique both on the tabletop and over a deployed quantum local area network, finding a substantial improvement of two-qubit entangled state fidelity from approximately 75% to over 92%. This local filtering technique could be used as a preliminary step to reduce asymmetric errors, potentially improving the overall efficiency when combined with more complex error-mitigation techniques in future quantum repeater networks.

97 MATHEMATICS AND COMPUTING↗
NASA NTRSDate not supplied

Atmospheric Escape from Magnetized Rocky Exoplanets

Recent Kepler and TESS observations discovered many rocky exoplanets in habitable zones around active main-sequence stars. The upper atmospheres of exoplanets are subject to two important energy sources derived from their host stars. First, the stellar photon flux in the X-ray and XUV bands ionizes and heats the upper atmosphere, driving atmospheric heating, affecting the conductance, and enhancing atmospheric escape. Second, the stellar wind’s interaction with the exoplanet’s intrinsic magnetic field transfers energy to the atmosphere through field aligned currents and Poynting flux. That energy is dissipated in the high latitude cusp and auroral regions through Joule heating which can inflate the atmosphere and also enhance the atmospheric escape rate. This presentation will discuss recent advances in modeling these energy inputs and their consequences for exoplanetary habitability. Also, I will discuss the development of a new model, the (exo) PLANETary Ionosphere-Thermosphere Tool for Research (PLANET-ITTR) and some early results from it.

William Danchi↗
NASA NTRS2004

NGC 4388- Spectral Studies of the First Seyfert 2 Seen by INTEGRAL

We present first INTEGRAL and XMM-Newton ob- servations of a Seyfert galaxy, the type 2 AGN NGC 4388. Several INTEGRAL observations performed in 2003 allow us to study the spectrum in the 20 - 300 keV range. In addition two XMM-Newton observations give detailed insight into the 0.2 - 10 keV emission. Comparison with previous observations by BeppoSAX, SIGMA and CGROIOSSE show that the overall spectrum for soft X-rays up to the gamma-rays can be described by a highly absorbed and variable non-thermal component in addition to constant non-absorbed thermal emission of low abundance (2 - 5%Za), plus a constant Fe K alpha line. The hard X-ray component is well described by a simple power law with a mean photon index of 1 = 1.7. During the INTEGRAL observations the flux at 100 keV increased by a factor of 1.5. The analysis of XMM-Newton data implies that the emission below 3 keV is decoupled from the AGN and probably due to extended emission as seen in Chandra observations. The constant iron line emission is apparently also decoupled from the direct emission of the central engine and likely to be generated in the obscuring material, e.g. in the molecular torus or even further away.

Beckman, V.↗
DOE OSTI2022

The throughput calibration of the VERITAS telescopes

Context. The response of imaging atmospheric Cherenkov telescopes to incident γ -ray-initiated showers in the atmosphere changes as the telescopes age due to exposure to light and weather. These aging processes affect the reconstructed energies of the events and γ -ray fluxes. Aims. This work discusses the implementation of signal calibration methods for the Very Energetic Radiation Imaging Telescope Array System (VERITAS) to account for changes in the optical throughput and detector performance over time. Methods. The total throughput of a Cherenkov telescope is the product of camera-dependent factors, such as the photomultiplier tube gains and their quantum efficiencies, and the mirror reflectivity and Winston cone response to incoming radiation. This document summarizes different methods to determine how the camera gains and mirror reflectivity have evolved over time and how we can calibrate this changing throughput in reconstruction pipelines for imaging atmospheric Cherenkov telescopes. The implementation is validated against seven years of observations with the VERITAS telescopes of the Crab Nebula, which is a reference object in very-high-energy astronomy. Results. Regular optical throughput monitoring and the corresponding signal calibrations are found to be critical for the reconstruction of extensive air shower images. The proposed implementation is applied as a correction to the signals of the photomultiplier tubes in the telescope simulation to produce fine-tuned instrument response functions. This method is shown to be effective for calibrating the acquired γ -ray data and for recovering the correct energy of the events and photon fluxes. At the same time, it keeps the computational effort of generating Monte Carlo simulations for instrument response functions affordably low.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1997

The High-Energy Continuum Emission of the Gamma-Ray Blazar PKS 0528+134

We present Advanced Satellite for Cosmology and Astrophysics (ASCA) observations of the gamma-ray blazar PKS 0528 + 134, obtained at two separate epochs in 1994 August and 1995 March. These data represent the first measurement of the X-ray continuum emission of this source in the medium-hard X-ray band. Both ASCA spectra are consistent with a single power law with photon index GAMMA approximate 1.7-1.8 and column density N(sub H) approximately 5 x 10(exp 21) /sq cm, higher than Galactic. The X-ray flux increased by a factor of 4 in approximately 7 months without appreciable change of the spectral shape. During the lower state of 1994 August, PKS 0528 + 134 was observed simultaneously in the optical, X-rays, and at gamma-ray energies with EGRET. The gamma-ray intensity is the faintest detected thus far in the source, with a steep spectrum (GAMMA approximately 2.7). The extrapolation of the X-ray continuum to the gamma-ray range requires a sharp spectral break at approximately 10(exp 22) Hz. We discuss the radio through gamma-ray spectral energy distribution of PKS 0528 + 134, comparing the low state of 1994 August with the flare state of 1993 March. We show that in PKS 0528 + 134, a non-negligible contribution from the external radiation field is present and that, although synchrotron self-Compton scenarios cannot be ruled out, inverse Compton upscattering of thermal seed photons may be the dominant cooling process for the production of the high-energy continuum in this blazar.

Sambruna, Rita M.↗