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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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NASA NTRS1992

Nonthermal electron-positron pairs and the break in the hard X-ray spectrum of NGC 4151

The recent observation by the detectors on board Granat of a spectral steepening above about 50 keV imposes a constraint on possible emission models for NGC 4151. This steepening, for example, is not well fitted by an exponential rollover characteristic of a purely thermal model, or by a Compton-downscattered power law. We find that this spectral behavior is consistent with that produced by a photon-starved nonthermal pair plasma with high compactness. This is the first quantitative test of the nonthermal pair plasma model against broad-band X-ray/gamma-ray observations. Above 200 keV or so, the nonthermal pair plasma model predicts an upturn in the spectrum, and a thermal annihilation feature around 511 keV. Such spectral behavior should be looked for with GRO.

Coppi, Paolo S.↗
NASA NTRS1992

Gamma radiation from pulsar magnetospheric gaps

We investigate the production of gamma rays in two pulsar emission models: the 'polar cap' model and the 'outer cap' model. For the former, we have performed detailed simulations of energetic electrons flowing in the vacuum dipole open field line region. In the outer gap case, we generate light curves for various magnetosphere geometries. Using data from radio and optical observations, we construct models for specific viewing angles appropriate to the Crab and Vela pulsars. Phase-resolved spectra are also computed in the polar cap case and provide signatures for testing the models. The calculations have been extended to include millisecond pulsars, and we have been able to predict fluxes and spectra for populations of recycled pulsars, which are compared to COS B data for globular cluster populations.

Chiang, James↗
NASA NTRS1993

X-ray spectra of 12 distant clusters of galaxies

An X-ray spectral analysis of 12 distant clusters of galaxies observed on-axis with the Einstein Observatory imaging proportional counter is presented. Statistically, X-ray spectral data of the highest red shift clusters in the sample are inconsistent with an optically thin plasma emission model plus absorptions only in the Milky Way. Inclusions of excess absorptions in the clusters significantly improve the spectral fits to the data, indicating the possible presence of large amounts of X-ray absorbing cool gas in some of the distant clusters. By using X-ray luminosities determined from observed fluxes only in the 0.8-3.5 keV band and the temperature-to-luminosity correlation for nearby clusters, the estimated temperatures of the hot intracluster medium in these distant clusters constrain the absorption columns in the clusters. The data also show possible positive temperature evolution of the hot ICM with time which, if present, would further increase the absorptions required in fitting the spectra.

Wang, Qingde↗
NASA NTRS1992

Soil moisture verification study of the ESTAR microwave radiometer - Walnut Gulch, AZ 1991

The application of an electronically steered thinned array L-band radiometer (ESTAR) for soil moisture mapping is investigated over the arid rangeland Walnut Gulch Watershed. Antecedent rainfall and evaporation for the flights are very different and result in a wide range of soil moisture conditions. The high spatial variability of rainfall events within this region results in moisture conditions with dramatic spatial patterns. Sensor performance is verified using two approaches. Microwave data are used in conjunction with a microwave emission model to predict soil moisture. These predictions are compared to ground observations of soil moisture. A second verification is possible using an extensive data set. Both tests showed that the ESTAR is capable of providing soil moisture with the same level of accuracy as existing systems.

Jackson, T. J.↗
NASA NTRS1994

Detection of rotational modulation in the coronal extreme-ultraviolet emission from V711 Tauri?

The RS CVn binary V711 Tauri was observed by the Extreme Ultraviolet Explorer satellite (EUVE) twice during the latter half of 1992, for periods lasting several days. Light curves for the waveband 60-180 A derived from the all-sky survey scanning in August and from a pointed calibration observation made in October both exhibit a modulation of about 40%. The modulation in both data sets is very similar, with minimum flux occurring near orbital phase phi = 0.5. Analysis using a two-temperature optically thin plasma emission model reveals that most of the detected extreme ultraviolet (EUV) flux emanates from hot (approximately 10(exp 7) K) coronal plasma. The modulation is probably mostly due to either flare-like activity or to rotational occultation of a long-lived, compact, and especially bright coronal structure on the more active star of the system. The phased data support the latter hypothesis. This coronal structure is then likely to be associated with the presistent spot patterns seen on V711 Tau when using Doppler and photometric surface imaging techniques. Comparison with contemporaneous Stroemgren b-band photometry indicates that the optical minimum light leads the EUV maximum light by 90 deg in phase.

Drake, Jeremy J.↗
NASA NTRS1994

ASCA observations of cooling flows in clusters of galaxies

ASCA spectra of the central regions of three cooling flows, in the Perseus, Centaurus, and A1795 clusters, together with the spectrum of the center of the Coma Cluster, are studied. Absorbed, cooler and/or cooling components are required for the cooling flow spectra but not for that of the Coma Cluster. Problems have been encountered with the basic plasma emission models in the energy range of the iron L complex, which preclude further detailed analysis at present. Our results show the potential of ASCA data for revealing the structure of cooling flows.

Fabian, A. C.↗
NASA NTRS1995

Resonant Compton scattering and gamma-ray burst continuum spectra

The Thomson limit of resonant inverse Compton scattering in the strong magnetic fields of neutron stars is considered as a mechanism for producing gamma-ray burst continuum spectra. Photon production spectra and electron cooling rates are presented using the full magnetic Thomson cross-section. Model emission spectra are obtained as self-consistent solutions of a set of photon and electron kinetic equations, displaying spectral breaks and other structure at gamma-ray energies.

Baring, M. G.↗
NASA NTRS1997

New COMPTEL results on pulsar studies at MeV energies

The Compton telescope (COMPTEL) onboard the Compton Gamma Ray Observatory (CGRO) detected the pulsar PSR B1951-32 at MeV energies, and found indications of a signal from PSR B0656+14. In the combined spectra from COMPTEL and the energetic gamma ray experiment telescope (EGRET) onboard CGRO, it can be seen that the maximum luminosities of these objects are reached in the COMPTEL energy range. These spectra can be compared with those from four other pulsars observed in MeV energies with COMPTEL. The spectral properties of five of the six pulsars, Vela, PSR B1509-58, PSR B1951-32 and PSR B0656+14, require breaks and bends at MeV energies. The sixth pulsar, the Crab pulsar, approximately follows a power law flux relation from keV to GeV energies. It is concluded that this spectral behavior may play a role in the discrimination between current gamma ray emission models.

Hermsen, W.↗
NASA NTRS1997

Determining the Composition of the Vela Pulsar's Jet

The pulsar jet is significant in explaining how the Vela pulsar's rotational energy is transported outward to the rest of the SNR, since direct radiation from the pulsar only accounts for a small percentage of the total power. Our previous ROSAT observations presented the first evidence that the pulsar is driving a narrow, collimated, and remarkably symmetrical jet into the SNR (Markwardt, C. and Oegelman, H., 1995, Nature, 375, p. 40) which we interpret to be from a 'cocoon' of hot gas surrounding the jet itself. We obtained an ASCA exposure of the jet in order to determine whether the spectrum is thermal or power-law continuum. The jet cocoon is detected with ASCA at approximately 2-3 x 10(exp -3) ct/s. The X-ray spectrum of the jet is remarkably similar to the surrounding supernova remnant spectrum and extends to X-ray energies of at least 7 keV, with a total flux of approximately 2 x 10(exp -13) erg/s sq cm sq arcmin. The only strong emission line is from He-like Neon at approx. 0.9 keV; otherwise the spectrum is quite smooth. The spectrum cannot be fit by any one standard plasma emission model, so we used models with two-components. The lower energy component is thermal and has a temperature of 0.29 +/- 0.03 keV; the higher energy portion can either be fit by a thermal component of temperature approx. 4 keV or a power law with photon index approx. 2.0. If the observed spectrum is of a 'traditional' jet cocoon, then we estimate the speed of the jet to be at least 800 km/s, depending on the angle of inclination of the jet axis to our line of sight. The mechanical power driving the jet is greater than or equal to 10(exp 36) erg/s which is comparable to the pulsar's spin-down luminosity of 7 x 10(exp 36) erg/s. and the mass flow rate at the head is greater than or equal to 10(exp -6) solar radius/yr. We conclude that the jet must be entraining material all along its length in order to generate such a large mass flow rate.

Ogelman, Hakki↗
NASA NTRS1997

SS 433: Total Coverage of 162-Day Precession Phase in Four Years

The observations prior to AO-4 covered a number of precession phases, leaving a gap at phase 0.8. In addition, ASCA and previous observations of SS 433 did not observe the spectrum above approx. 10 keV, and consequently the continuum underlying the spectral lines was poorly constrained. Therefore RXTE observations were scheduled for April 1997 to extend the observed spectrum to higher energies; these observations were planned to sample the X-ray lightcurve during the 13.08 day binary period, concentrating on the eclipse of the compact object which emits the jets. We proposed and were awarded ASCA observations simultaneous with the RXTE observations; the purpose of the ASCA observations was to provide greater spectral resolution at the low end of the spectrum observed by RXTE, and to complete the phase coverage of SS 433. As a result of scheduling difficulties early in the mission the RXTE observations were confined to a much shorter time range than originally planned, April 18-91 1997. Optical observations of SS 433 were performed at a number of observatories. The ASCA observations occurred from April 18 13:10 (UT) to April 21 13:20 (UT) for a total effective exposure of 120 ks. The continuum X-ray light curve shows that the ASCA observations started shortly before the ingress into the X-ray partial eclipse, and ended approximately at the time of the egress. Light curves were also obtained for the prominent Fe emission lines in the blue-shifted frame (approaching jet), red-shifted frame (receding jet), and the stationary frame (fluorescent line from the ambient matter). Through the eclipse mapping technique using the light curves, the parameters of the jet emission model were constrained, showing that the kinetic power in the jet exceeds 104? erg s-l. If the energy source is gravitational accretion, as is commonly believed, the derived l;inetic power implies extremely supercritical accretion even for a black; hole with 10M. These results will be described more fully in a major presentation of all the ASCA observations of SS 433.

Band, David L.↗
NASA NTRS1998

Magnetospheric Accretion in PSR 1259-63

We report the results from X-ray and hard X-ray observations of the PSR B1259-63/SS 2883 system with the ASCA and the CGRO satellites, performed between 1995 February and 1996 January when the pulsar was near apastron. The system was clearly detected in each of the two ASCA observations with luminosity in the 1-10 keV band of L(sub X) = (9 +/- 3) x 10(exp 32) (d/2 kpc) erg/ s, while OSSE did not detect significant hard X-rays from the system. X-ray spectra obtained with ASCA show a single power-law spectrum with a photon index of 1.6 +/- 0.3. No pulsations were detected in either the ASCA nor the OSSE data. Combined with previous results from X-ray and hard X-ray observations, we obtained binary modulation in X-ray luminosity and photon index of high energy emission from the system for the entire orbit. The results are in agreement with predictions based on synchrotron emission model from relativistic particles in a shocked pulsar wind interacting with the gaseous outflow from the Be star.

Cominsky, Lynn R.↗
NASA NTRS2000

The Low Energy Effective Area of the Chandra Low Energy Transmission Grating Spectrograph

The Chandra X-ray Observatory was successfully launched on July 23, 1999, and subsequently began an intensive calibration phase. We present the preliminary results from the in-flight calibration of the low energy response of the High Resolution Camera spectroscopic readout (HRC-S) combined with the Low Energy Transmission Grating (LETG) aboard Chandra. These instruments comprise the Low Energy Transmission Grating Spectrograph (LETGS). For this calibration study, we employ a pure hydrogen non-LTE white dwarf emission model (T = 25000 K and log g = 9.0) for comparison with the Chandra observations of Sirius B. The pre-flight calibration of the LETGS effective area only covered wavelengths shortward of 44 A (E less than 277 eV). Our Sirius B analysis shows that the HRC-S quantum efficiency (QE) model assumed for longer wavelengths leads to an overestimate of the effective area by an average factor of about 1.6. We derive a correction to the low energy HRC-S QE model to match the predicted and observed Sirius B spectra over the wavelength range of 44-185 A. We make an independent test of our results by the comparison of a Chandra LETGS observation of HZ 43 with pure hydrogen model atmosphere predictions and find good agreement.

Pease, D.↗
NASA NTRS2000

A Search for CMB Decrements Towards Distant Cluster Candidates PC 1643+4631 and VLA 1312+4237 at 28.5 GHz

Recently, Jones et al. used the Ryle telescope, operating at a frequency of 15 GHz, to detect a flux decrement in the direction of the quasar pair PC 1643+461A,B. They interpreted this signal as the Sunyaev-Zel'dovich effect (SZE) produced by a distant cluster of galaxies. In the course of an effort to measure CMB anisotropies using the VLA at 8.4 GHz, Richards et al. (1997) detected a similar, but smaller, decrement which we refer to as VLA 1312+4237. They also proposed that this signal might be explained as the SZE signal of a distant galaxy cluster. We report observations in the direction of these claimed sources with the Berkeley Illinois Maryland Association (BIMA) interferometer operating at 28.5 GHz. We find no evidence for SZE emission in the direction of either of the claimed sources. In the case of PC 1643+4631, the BIMA data are inconsistent with the cluster emission model proposed by Jones et al. at greater than 99.99% confidence. Together with published x-ray and optical searches, these results make a compelling case against the existence of a massive cluster in the direction of PC 1643+4631. Because of the different scales to which the VLA and BIMA instruments are sensitive, the BIMA observations are not as constraining for the VLA 1312+4237 source. The BIMA data are inconsistent with the cluster model proposed by Richards et al. (1997) at approximately 80% confidence.

Holzapfel, W. L.↗
NASA NTRS2000

X-Ray, EUV, UV and Optical Emissivities of Astrophysical Plasmas

This grant primarily covered the development of the thermal X-ray emission model code called APEC, which is meant to replace the Raymond and Smith (1977) code. The new code contains far more spectral lines and a great deal of updated atomic data. The code is now available (http://hea-www.harvard.edu/APEC), though new atomic data is still being added, particularly at longer wavelengths. While initial development of the code was funded by this grant, current work is carried on by N. Brickhouse, R. Smith and D. Liedahl under separate funding. Over the last five years, the grant has provided salary support for N. Brickhouse, R. Smith, a summer student (L. McAllister), an SAO predoctoral fellow (A. Vasquez), and visits by T. Kallman, D. Liedahl, P. Ghavamian, J.M. Laming, J. Li, P. Okeke, and M. Martos. In addition to the code development, the grant supported investigations into X-ray and UV spectral diagnostics as applied to shock waves in the ISM, accreting black holes and white dwarfs, and stellar coronae. Many of these efforts are continuing. Closely related work on the shock waves and coronal mass ejections in the solar corona has grown out of the efforts supported by the grant.

Raymond, John C.↗
NASA NTRS1998

Comparison of the X-Ray and Radio Light Curves of Quasar PKS 1510--089

We present results for the X-ray-bright superluminal AGN PKS 1510-089 (z=0.36) monitored weekly with the Rossi X-Ray Timing Explorer for the past four years in order to study the origin of X-ray emission from this extremely variable blazer. These RXTE data are compared with weekly cm-band flux and polarization observations from the Michigan Diameter telescope, to identify correlated activity and associated frequency-dependent time delays for constraining X-ray emission models; and bimonthly 7mm VLBA total and linearly polarized intensity imaging to identify temporal associations between X-ray events and the ejection of superluminal components and disturbances in the magnetic field, to test if the X-ray energy release is related to changes in the inner jet flow. Both the X-ray (2-20 keV) and radio flux are highly variable on timescales of weeks. The VLBA mas structure is dominated by a bright core with a weak jet; both the ejection of very fast superluminal knots and changes in the fractional polarization and EVPA of the core on timescales of one to four months are identified. Two outbursts in 1997 are well-resolved in both the centimeter and X-ray bands. Both the strong temporal association and the similar outburst shape support a causal relation, and a discrete cross-correlation analysis identifies that the X-ray lags the radio by 16 days during the bursts. Starting in 1998 the behavior changes: the correlation is weaker with the X-ray possibly leading the radio by six days. During the full time window there is a correlation between bands as expected if the radio photons are upscattered to X-ray energies. The time correlations and difference between the flat X-ray spectral index (0.0 <= alpha <= 0.5 where F(sub v) is proportional to v(exp -alpha)), and the mm-wave synchrotron spectrum (alpha = 0.8) are discussed within the framework of viable SSC models.

Aller, M. F.↗
NASA NTRS2002

Cometary Science After Hale-Bopp

Comets and the chondritic porous interplanetary dust particles (CP IDPs) that they shed in their comae are reservoirs of primitive solar nebula materials. The high porosity and fragility of cometary grains and CP IDPs, and anomalously high deuterium contents of pyroxene-rich CP IDPs imply these aggregate particles contain significant interstellar grain components. Spectrophotometry of comets at thermal IR wavelengths (3-40 microns) reveal the presence of a warm (near-IR) featureless emission modeled by amorphous carbon grains, and mid-IR and far-IR broad and narrow resonances modeled by chondritic (50% Fe and 50% Mg) amorphous and Mg-rich crystalline silicate minerals, respectively. Cometary amorphous silicate resonances are well matched by IR spectra of CP IDPs dominated by 0.1 micron spherules of Glass with Embedded Metal and Sulfides (GEMS) that are thought to be the interstellar Fe-bearing amorphous silicates produced in the cooling outflows of Asymptotic Giant Branch (AGB) stars. Acid-etched microtomed CP IDP samples, however, show that both the carbon phase (aliphatic) and the amorphous silicate phase (Mg-rich) are not optically absorbing while the embedded Fe nanoparticles make the IDPs dark. The CP IDPs suggest either significant processing has occurred in the ISM or that the AGB amorphous silicates have Mg-rich stoichiometry and possibly grew on Fe particle condensates. Cometary crystalline silicate resonances are well matched by IR spectra of laboratory submicron Mg-rich olivine crystals, [Mg(sub y),Fe(sub 1-y)]2SiO4 with y/ge0.85, and in the case of Hale-Bopp at r(sub h) less than or equal to approx. 1.5 AU, by Mg-rich pyroxene crystals, [Mg(sub x),Fe(sub 1-x)]SiO3 with x/ge0.85. While a fraction of AGB stardust (less than or equal to 15%) are Mg-rich crystals, this interstellar star dust component is insufficient to account for the deduced abundance of crystalline minerals in comet dust. An insufficient source of ISM Mg-rich crystals leads to the inference that Mg-rich crystals in comets may be hot, early solar nebula condensates that traveled large radial distances out to the comet-forming zone.

Wooden, Diane H.↗
NASA NTRS2002

Comet Grains: Their IR Emission and Their Relation to ISM Grains

Comets and the chodritic, porous interplanetary dust particles (CP IDPs) that they shed in their comae are reservoirs of primitive solar nebula materials. The high porosity and fragility of cometary grains and CP IDPs, and anomalously high deuterium contents of highly fragile, pyroxene-rich Cluster IDPs imply these aggregate particles contain significant abundances of grains from the interstellar medium (ISM). IR spectra of comets (3 - 40 micron) reveal the presence of a warm (nearIR) featureless emission modeled by amorphous carbon grains. Broad and narrow resonances near 10 and 20 microns are modeled by warm chondritic (50% Fe and 50% Mg) amorphous silicates and cooler Mg-rich crystalline silicate minerals, respectively. Cometary amorphous silicates resonances are well matched by IR spectra of CP IDPs dominated by GEMS (0.1 micron silicate spherules) that are thought to be the interstellar Fe-bearing amorphous silicates produced in AGB stars. Acid-etched ultramicrotomed CP IDP samples, however, show that both the carbon phase (amorphous and aliphatic) and the Mg-rich amorphous silicate phase in GEMS are not optically absorbing. Rather, it is Fe and FeS nanoparticles embedded in the GEMS that makes the CP IDPs dark. Therefore, CP IDPs suggest significant processing has occurred in the ISM. ISM processing probably includes in He' ion bombardment in supernovae shocks. Laboratory experiments show He+ ion bombardment amorphizes crystalline silicates, increases porosity, and reduces Fe into nanoparticles. Cometary crystalline silicate resonances are well matched by IR spectra of laboratory submicron Mg-rich olivine crystals and pyroxene crystals. Discovery of a Mg-pure olivine crystal in a Cluster IDP with isotopically anomalous oxygen indicates that a small fraction of crystalline silicates may have survived their journey from AGB stars through the ISM to the early solar nebula. The ISM does not have enough crystalline silicates (<5%) , however, to account for the deduced abundance of crystalline silicates in comet dust. An insufficient source of ISM Mg-rich crystals leads to the inference that most Mg-rich crystals in comets are primitive grains processed in the early solar nebula prior to their incorporation into comets. Mg-rich crystals may condense in the hot (approx. 1450 K), inner zones of the early solar nebula and then travel large radial distances out to the comet-forming zone. On the other hand, Mg-rich silicate crystals may be ISM amorphous silicates annealed at approx. 1000 K and radially distributed out to the comet-forming zone or annealed in nebular shocks at approx. 5 - 10 AU. Determining the relative abundance of amorphous and crystalline silicates in comets probes the relative contributions of ISM grains and primitive grains to small, icy bodies in the solar system. The life cycle of dust from its stardust origins through the ISM to its incorporation into comets is discussed.

Wooden, Diane H.↗
NASA NTRS2002

A Search for Evidence of Non-Thermal Emission from the Supernova Remnants 37A/B

The ADP grant NAG5-9211 entitled 'A Search for Evidence of Non-Thermal Emission from the Supernova Remnants 37 A/B' was not used to support an analysis of the ASCA data for these two remnants because the ASCA mission ended before the remnants were observed. The grant was used to support similar research on two remnants in the Large Magellanic Cloud, N132D and N 103B. An analysis of the Chandra data for these two remnants exhibits some evidence of non-thermal emission from small regions in the remnants. The X-ray spectra for these regions can not be adequately described by a single thermal X-ray emission model. However, if an X-ray synchrotron component is also included, the spectral data can be well described by the model and the values of the fit parameters are consistent with the values expected. These results were presented at the 199th Meeting of the American Astronomical Society. In summary, the grant has enabled us to strengthen the evidence that supernova remnants outside our Galaxy can also accelerate electrons to very-high energies. The results of this analysis will be published soon in the Astrophysical Journal,

Oliversen, R.↗