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Results for “Neutron emission”

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 649 records · Page 36
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

Survey of in-situ and remote sensing methods for soil moisture determination

General methods for determining the moisture content in the surface layers of the soil based on in situ or point measurements, soil water models and remote sensing observations are surveyed. In situ methods described include gravimetric techniques, nuclear techniques based on neutron scattering or gamma-ray attenuation, electromagnetic techniques, tensiometric techniques and hygrometric techniques. Soil water models based on column mass balance treat soil moisture contents as a result of meteorological inputs (precipitation, runoff, subsurface flow) and demands (evaporation, transpiration, percolation). The remote sensing approaches are based on measurements of the diurnal range of surface temperature and the crop canopy temperature in the thermal infrared, measurements of the radar backscattering coefficient in the microwave region, and measurements of microwave emission or brightness temperature. Advantages and disadvantages of the various methods are pointed out, and it is concluded that a successful monitoring system must incorporate all of the approaches considered.

Schmugge, T. J.↗
NASA NTRS1985

Gamma-ray bursters

Current theoretical models developed to explain the observational data (from spaceborne detectors) on gamma-ray bursters are summarized and illustrated with drawings, diagrams, graphs, and photographic images. Although the data are fragmentary and often flawed by instrument defects, models involving neutron stars with strong magnetic fields are generally favored, and it is assumed that most observed bursters lie within the Galaxy. The neutron-star origin of the bursts is suggested by their intensity and rapid variability (implying a very compact high-energy source) and the presence in some burster spectra of a line at 420 keV which is explained by the combination of electron-positron annihilation and gravitational reddening. Consideration is also given to optical flashes observed to occur about once per year in the direction of gamma bursters, and the need for further searches for lower-energy emissions from bursters is stressed.

Schaefer, B. E.↗
NASA NTRS1991

Detection of coherent 7.6 Hz oscillations during a burst from Aquila X-1

The results are reported of timing and spectral analysis of the X-ray source Aql X-1 (X1908 + 005) using data obtained with the Einstein SSS and MPC instruments. A classic type I burst was observed from Aql X-1 in both detectors, and a coherent modulation with a period of 131.66 + or - 0.02 (7.6 Hz) and a pulsed fraction of 10 percent was detected in the SSS data (0.64-4.5 keV) during the period of enhanced emission. The signal has a random occurrence probability of less than 5 x 10 to the -5th. The sensitivity of the MPC for high-resolution timing during the burst is greatly reduced because of the low-duty cycle during the burst, and the modulation is not detected. There is no evidence for a loss of coherence during the 80 s or so when it is observable. It is argued that an asymmetrical burst occurring on a neutron star rotating at 7.6 Hz offers a plausible explanation for the oscillation.

Schoelkopf, R. J.↗
NASA NTRS2016

Ground Calibration of the Silicon Drift Detectors for NICER

The Neutron star Interior Composition ExploreR (NICER) is set to be deployed on the International Space Station (ISS) in early 2017. It will use an array of 56 Silicon Drift Detectors (SDDs) to detect soft X-rays (0.2 - 12 keV) with 100 nanosecond timing resolution. Here we describe the e ort to calibrate the detectors in the lab primarily using a Modulated X-ray Source (MXS). The MXS that was customized for NICER provides more than a dozen emission lines spread over the instrument bandwidth, providing calibration measurements for detector gain and spectral resolution. In addition, the fluorescence source in the MXS was pulsed at high frequency to enable measurement of the delay due to charge collection in the silicon and signal processing in the detector electronics. A second chamber, designed to illuminate detectors with either 55Fe, an optical LED, or neither, provided additional calibration of detector response, optical blocking, and effectiveness of background rejection techniques. The overall ground calibration achieved total operating time that was generally in the range of 500-1500 hours for each of the 56 detectors.

Silicon Drift Detectors↗
NASA NTRS2018

SEXTANT X-Ray Pulsar Navigation Demonstration: Initial On-Orbit Results

Millisecond pulsars (MSPs) are rapidly rotating neutron stars that appear to pulsate across the electromagnetic spectrum. Some MSPs have long-term timing stability that rivals that of atomic clocks. Pulse arrival phase can be predicted with great accuracy at any reference point in the Solar System through use of a pulsar timing model on a spacecraft. Comparing observed phase to predictions gives information that may be used in a navigation process. Why X-rays? Some stable MSPs have conveniently detectable X-ray emissions. X-rays are immune to interstellar dispersion effects thought to limit radio pulsar timing models. Highly directional compact detectors possible.

Navigatio↗
NASA NTRS2014

Gamma-Ray Emission in Dissipative Pulsar Magnetospheres: from Theory to Fermi Observations

We compute the patterns of gamma-ray emission due to curvature radiation in dissipative pulsar magnetospheres. Our ultimate goal is to construct macrophysical models that are able to reproduce the observed gamma-ray light curve phenomenology recently published in the Second Fermi Pulsar Catalog. We apply specific forms of Ohm's law on the open field lines using a broad range for the macroscopic conductivity values that result in solutions ranging, from near-vacuum to near-force-free. Using these solutions, we generate model gamma-ray light curves by calculating realistic trajectories and Lorentz factors of radiating particles under the influence of both the accelerating electric fields and curvature radiation reaction. We further constrain our models using the observed dependence of the phase lags between the radio and gamma-ray emission on the gamma-ray peak separation. We perform a statistical comparison of our model radio-lag versus peak-separation diagram and the one obtained for the Fermi standard pulsars. We find that for models of uniform conductivity over the entire open magnetic field line region, agreement with observations favors higher values of this parameter. We find, however, significant improvement in fitting the data with models that employ a hybrid form of conductivity, specifically, infinite conductivity interior to the light cylinder and high but finite conductivity on the outside. In these models the gamma-ray emission is produced in regions near the equatorial current sheet but modulated by the local physical properties. These models have radio lags near the observed values and statistically best reproduce the observed light curve phenomenology. Additionally, they also produce GeV photon cut-off energies.

stars↗
NASA NTRS1983

Fast pulsars with disks

The observed properties of the pulsar PSR1937+214 are compared with predictions of the disk model. It is assumed that an isolated magnetized rotating neutron star is ringed by a fluid disk with a 0.00001 solar mass, and relative rotations of the star and the disk produce potential differences across the disk. A Faraday disk dynamo is also formed between the disk and the star, and allows the polar cap current to return from the disk to the star through auroral arcing. Preferential regions of the star are recipients of a return current controlled by the surface magnetic field structure, which configures the pulsing emissions. The disk model predicts the average luminosity to be 10 to the 31st erg/sec, and an emission of 3 x 10 to the 30th erg/sec was detected. Only one-millionth of the output of the emissions is in the radio region, and the X and gamma ray emissions are in the normal range for pulsars. It is concluded that PSR1937+214 behaves within the predictions of the disk model and is not a new kind of object.

Michel, F. C.↗
NASA NTRS1989

The Crab-like supernova remnants

Recent observational and theoretical investigations of Galactic and extragalactic SNRs suspected of having neutron central stars are reviewed, with an emphasis on Einstein X-ray observations. Sections are devoted to the Crab Nebula and its twin SNR 0540-69.3, SNR with observationally well determined and indistinct central X-ray nebulae, SNR with weak and strong unresolved internal X-ray sources, binary systems, and the relationship between X-ray and radio luminosity and pulsar energy loss. Consideration is given to radio pulsars, SNR with filled-center X-ray emission attributed to thermal processes, radio plerions and composite SNRs, MSH 15-56, and SNRs in the LMC. Extensive contour maps, sample optical images, and tables of numerical data are included.

Seward, Frederick D.↗
NASA NTRS2000

Testing Models of Resonant Compton Scattering in X-Ray Pulsars

Over the performance period covered by the grant, the principal investigator modified a Monte Carlo Compton scattering code to model the propagation of x-rays through the magnetosphere of accreting neutron stars. These modifications were made to enable the author to compare the observations of x-ray pulsars to theoretical models of the system. The original code was designed to study relativistic plasmas with one of two geometries: a plane parallel plasma with a differential relativistic bulk velocity, and a static spherically symmetric plasma.- This code did not treat gravitational bending or bulk motion in the magnetosphere of a neutron star. Under the grant, the author incorporated code to trace light paths in a Schwarzschild metric. The code was modified to keep track of the photon polarization during propagati on. The investigator also modified the code so that bulk motion in an axisymmetric system is treated properly. An approximate treatment for resonant Compton scattering was added to the code. Finally, code was added that creates model observables that can be compared to observations, such as projected x-ray emission maps and energy-dependent light curves. Comparison to observations is now commencing.

Brainerd, Jerome J.↗
NASA NTRS2013

Upper Bounds on r-Mode Amplitudes from Observations of Low-Mass X-Ray Binary Neutron Stars

We present upper limits on the amplitude of r-mode oscillations and gravitational-radiation-induced spin-down rates in low-mass X-ray binary neutron stars, under the assumption that the quiescent neutron star luminosity is powered by dissipation from a steady-state r-mode. For masses <2M solar mass we find dimensionless r-mode amplitudes in the range from about 1×10(exp−8) to 1.5×10(exp−6). For the accreting millisecond X-ray pulsar sources with known quiescent spin-down rates, these limits suggest that approx. less than 1% of the observed rate can be due to an unstable r-mode. Interestingly, the source with the highest amplitude limit, NGC 6440, could have an r-mode spin-down rate comparable to the observed, quiescent rate for SAX J1808−3658. Thus, quiescent spin-down measurements for this source would be particularly interesting. For all sources considered here, our amplitude limits suggest that gravitational wave signals are likely too weak for detection with Advanced LIGO. Our highest mass model (2.21M solar mass) can support enhanced, direct Urca neutrino emission in the core and thus can have higher r-mode amplitudes. Indeed, the inferred r-mode spin-down rates at these higher amplitudes are inconsistent with the observed spin-down rates for some of the sources, such as IGR J00291+5934 and XTE J1751−305. In the absence of other significant sources of internal heat, these results could be used to place an upper limit on the masses of these sources if they were made of hadronic matter, or alternatively it could be used to probe the existence of exotic matter in them if their masses were known.

gravitational waves↗
NASA NTRS2023

Heavy Ion Acceleration in 3he-Rich Solar Energetic Particle Events: New Insights From Solar Orbiter

We present Solar Orbiter energetic particle observations of two 3 He-rich events with features more clearly observed than in prior studies. The event of 2022 November 9 observed from 0.59 au contained hundreds of ultra-heavy ions (UH, mass >78 amu) whereas previous observations at 1 au have shown only an occasional count or two. The event of 2023 April 8 observed from 0.29 au fortuitously had very low ambient activity, making it possible to observe spectra from the 3 He acceleration mechanism without contamination, revealing extremely low H and 4 He intensities arriving simultaneously with other ions observed in typical 3 He-rich events. Taken together with previous studies we believe these data show that 3 He-rich events have a single acceleration mechanism that is responsible for the unique abundance features of 3 He, heavy ions, and UH ions. Considering the acceleration model of Roth and Temerin (1997) that heats the ions over a broad range of gyrofrequencies away from those damped by H and 4 He, we calculate reasonable fits to the observed abundances O-Fe. A key result is that high values of, e.g., Fe/O typical of such events is not due to preferential Fe heating, but on the contrary is due mainly to the depletion of O which at elevated temperatures has a charge-to-mass ratio in the region of the waves damped by 4He. The model also naturally incorporates features of high ionization states and neutron-rich isotope enhancements that have been long-standing puzzles in observations of this type of flare.

Solar flares↗
NASA NTRS2016

A Soft X-Ray Spectral Episode for the Clocked Burster, GS 1826-24 as Measured by Swift and NuSTAR

We report on NuSTAR and Swift observations of a soft state of the neutron star low-mass X-ray binary GS 1826-24, commonly known as the "clocked" burster. The transition to the soft state was recorded in 2014 June through an increase of the 2-20 keV source intensity measured by MAXI, simultaneous with a decrease of the 15-50 keV intensity measured by Swift/BAT. The episode lasted approximately two months, after which the source returned to its usual hard state. We analyze the broadband spectrum measured by Swift/XRT and NuSTAR and estimate the accretion rate during the soft episode to be approximately equal to 13% m(sub Edd), within the range of previous observations. However, the best-fit spectral model, adopting the double Comptonization used previously, exhibits significantly softer components. We detect seven type-I X-ray bursts, all significantly weaker (and with shorter rise and decay times) than observed previously. The burst profiles and recurrence times vary significantly, ruling out the regular bursts that are typical for this source. One burst exhibited photospheric radius expansionand we estimate the source distance as (5.7 +/- 0.2) xi(sub b)(exp -1/2) kpc, where xi(sub b) parameterizes the possible anisotropy of the burst emission. The observed soft state may most likely be interpreted as a change in accretion geometry at about similar bolometric luminosity as in the hard state. The different burst behavior can therefore be attributed to this change in accretion flow geometry, but the fundamental cause and process for this effect remain unclear.

accretion↗
NASA NTRS1994

Detection of X-ray emission from the PSR 1259-63/SS 2883 binary system

Nonpulsed but variable X-ray emission has been detected from the binary system containing the radio pulsar PSR 1259-63 during two pointed ROSAT observations, taken 5 months apart. This 47.7 ms radio pulsar is in a highly eccentric (epsilon approximately 0.85) binary system with the 10-15 solar mass Be star SS 2883. It is the first radio pulsar found to be in a binary system with a massive main-sequence companion; it is also the most highly eccentric binary system known to contain a neutron star. The level of X-ray flux detected in the ROSAT observations has increased with orbital phase by a factor of at least 10 between 1992 February and 1993 February. The X-ray flux is significantly greater than expected from the Be star's corona and seems likely to originate either from low-level stellar wind accretion onto the neutron star or from the shock between the stellar wind and the relativistic pulsar wind. The system may be the progenitor of the more slowly rotating Be X-ray binary pulsar systems.

Cominsky, Lynn↗
NASA NTRS2019

Physical Models for Accreting Pulsars at High Luminosity

A new window for better understanding the accretion onto strongly magnetized neutron stars in X-ray binaries is opening. In these systems the accreted material follows the magnetic field lines as it approaches the neutron star, forming accretion columns above the magnetic poles. The plasma falls toward the neutron star surface at near-relativistic speeds, losing energy by emitting X-rays. The X-ray spectral continua are commonly described using phenomenological models, i.e., power laws with different types of curved cut-offs at higher energies. Here we consider high luminosity pulsars. In these systems the mass transfer rate is high enough that the accreting plasma is thought to be decelerated in a radiation-dominated radiative shock in the accretion columns. While the theory of the emission from such shocks had already been developed by 2007, a model for direct comparison with X-ray continuum spectra in xspec or isis has only recently become available. Here we analyze the broadband X-ray spectra of the accreting pulsars Centaurus X-3 and 4U1626-67 obtained withNuSTAR. We present results from traditional empirical modeling as well as successfully apply the radiation-dominated radiative shock model. We also fit the energy-dependent pulse profiles of 4U 1626-67 using a new relativistic light bending model.

Pottschmidt, Katja↗
NASA NTRS1994

Discovery of Be/x-ray stars in two supernova remnants in the Small Magellanic Cloud

We present ROSAT high resolution x-ray images of two previously cataloged supernova remnants in the Small Magellanic Cloud (SMC): 0101-72.4 and 0104-72.3. These remnants were known to show optical, x-ray, and radio emission based on earlier observations: the present data show the first evidence for arcsecond-scale x-ray structure. There is no diffuse x-ray emission associated with the optically emitting shell in 0101-72.4; we set a 3 sigma upper limit of 7 x 10(exp 34) erg s(exp -1) on the 0.02-2 keV luminosity from the region. The x-ray emission comes instead from a weak pointlike object near the limb of the remnant. Optical observations of this source reveal a m(sub v) = 14.8 blue star with H alpha and H beta in emission; we identify this as a Be star in the SMC. No evidence for variability down to time scales of about 1 s was found in the ROSAT data; however, a comparison of the ROSAT and Einstein fluxes indicates possible long term variability by a factor of approximately 2 over several years. The other SNR, 0104-72.3, also contains a pointlike x-ray source with a blue optical counterpart (m(sub v) = 16.7) and H alpha emission. We tentatively identify this as a Be star as well. In addition to the point source there is weak diffuse x-ray emission from 0104-72.3 (L(sub x) approximately 1.4 x 10(exp 35) erg s(exp -1)), but the remnant's appearance in the x-ray band is considerably different from that in either the radio or optical band. We argue for a physical association between the supernova remnants (SNRs) and Be/x-ray stars. A large space velocity (greater than or approximately 100 km s(exp -1)) for the Be/x-ray binaries is required if the explosions that produced the remnants also formed the neutron stars in the binaries. Alternatively, the associations could be the result of common membership in OB associations in the SMC.

Hughes, John P.↗
NASA NTRS2008

Magnetar-Like Emission from the Young Pulsar in Kes 75

We report the detection of magnetar-like x-ray bursts from the young pulsar PSR J1846-0258, at the center of the supernova remnant Kes 75. This pulsar, long thought to be exclusively rotation-powered, has an inferred surface dipolar magnetic field of 4.9deg 10(exp 13) gauss, which is higher than those of the vast majority of rotation-powered pulsars, but lower than those of the approximately 12 previously identified magnetars. The bursts were accompanied by a sudden flux increase and an unprecedented change in timing behavior. These phenomena lower the magnetic and rotational thresholds associated with magnetar-like behavior and suggest that in neutron stars there exists a continuum of magnetic activity that increases with inferred magnetic field strength.

Gavriil, F. P.↗
NASA NTRS1986

IUE observations of V Sagittae

Observations of the nova-like variable V Sge are discussed, and the ground-based light curve is analyzed, compensating for the component interaction effect. Low-dispersion IUE spectra indicate that interstellar reddening is smaller than suggested from visible-band observations. The UV and visible continuum are modeled by a He-rich model and by a simple accretion disk, and the UV spectrum emission lines are used to determine ionization temperatures and a lower N(e) limit. The present data are consistent with the picture of the companion to the He star being embedded in an accretion disk, with the disk being large (of dimension comparable to that of the embedding Roche lobe as a result of vigorous mass transfer) at the time of the ground-based data, and with the disk having been diminished to a radius of the order of 200 times of the neutron star, at the time of the IUE spectra.

Koch, R. H.↗
NASA NTRS1991

Dynamic effects on cyclotron scattering in pulsar accretion columns

A resonant scattering model for photon reprocessing in a pulsar accretion column is presented. The accretion column is optically thin to Thomson scattering and optically thick to resonant scattering at the cyclotron frequency. Radiation from the neutron star surface propagates freely through the column until the photon energy equals the local cyclotron frequency, at which point the radiation is scattered, much of it back toward the star. The radiation pressure in this regime is insufficient to stop the infall. Some of the scattered radiation heats the stellar surface around the base of the column, which adds a softer component to the spectrum. The partial blocking by the accretion column of X-rays from the surface produces a fan beam emission pattern. X-rays above the surface cyclotron frequency freely escape and are characterized by a pencil beam. Gravitational light bending produces a pencil beam pattern of column-scattered radiation in the antipodal direction, resulting in a strongly angle-dependent cyclotron feature.

Brainerd, J. J.↗