Engineering topics

White, Nicholas E.

Publications and source records attributed to White, Nicholas E..

At least 145 records · Page 8

NASA NTRS2001

X-ray Properties of the Central kpc of AGN and Starbursts: The Latest News from Chandra

The X-ray properties of 15 nearby (v less than 3,000 km/s) galaxies that possess AGN (active galactic nuclei) and/or starbursts are discussed. Two-thirds have nuclear extended emission on scales from approx. 0.5 to approx. 1.5 kpc that is either clearly associated with a nuclear outflow or morphologically resembles an outflow. Galaxies that are AGN-dominated tend to have linear structures while starburst-dominated galaxies tend to have plume-like structures. Significant X-ray absorption is present in the starburst regions, indicating that a circumnuclear starburst is sufficient to block an AGN at optical wavelengths. Galaxies with starburst activity possess more X-ray point sources within their central kpc than non-starbursts. Many of these sources are more luminous than typical X-ray binaries. The Chandra results are discussed in terms of the starburst-AGN connection, a revised unified model for AGN, and possible evolutionary scenarios.

Weaver, Kimberly A.↗

NASA NTRS2000

Mapping the X-Ray-Emitting Ejecta in Cassiopeia A with Chandra

We present X-ray emission-line equivalent width images of the bright Galactic supernova remnant Cassiopeia A for the elements Si, S, Ar, Ca, and Fe using a 50,000 s observation with the Advanced CCD Imaging Spectrometer on the Chandra X-Ray Observatory. The images essentially map the bulk of detectable ejecta of these elements over a wide range of surface brightnesses and show morphologies distinctly different from that of the broadband X-ray emission and the 4-6 keV continuum emission. The Si, S, Ar, and Ca maps, while different in turn from those of Fe, are similar to the distribution of fast optical ejecta knots in that they clearly delineate the X-ray counterpart of the northeast optical jet. Low surface brightness regions just outside the bright shell in the north and west are also shown to have strong line emission. The strong Fe emission is exterior to that of other elements in the east, as previously noted, but is generally coincident elsewhere. The projected interior has relatively little emission traced by high line equivalent widths.

Hwang, Una↗

NASA NTRS2000

Iron K Line Variability in the Low-Luminosity AGN NGC 4579

We present results of new ASCA observations of the low-luminosity AGN (LLAGN) NGC 4579 obtained in 1998 December 18 and 28, and we report on detection of variability of an iron K emission line. The X-ray luminosities in the 2-10 keV band for the two observations are nearly identical, L(sub X) approximately = 2 x 10(exp 4l) ergs/s, but they are approximately 35% larger than that measured in 1995 July by Terashima et al. (1998). An Fe K emission line is detected at 6.39 +/- 0.09 keV (source rest frame) which is lower than the line energy 6.73(sup +0.13, sub -0.12) keV in the 1995 observation. If we fit the Fe lines with a blend of two Gaussians centered at 6.4 keV and 6.73 KeV, the intensity of the 6.7 keV line decreased, while the intensity of the 6.4 keV line increased, within an interval of 3.5 years. This variability rules out thermal plasmas in the host galaxy as the origin of the ionized Fe line in this LLAGN. The detection and variability of the 6.4 keV line suggest that an optically thick standard accretion disk is present and subtends a large solid angle viewed from the nucleus at the Eddington ratio of L(sub Bol)/L(sub Eddington) approximately 2 x 10(exp -3) (Ho 1999). A broad disk-line profile is not clearly seen and the structure of the innermost part of accretion disk remains unclear.

Terashima, Yuichi↗

NASA NTRS2000

The Ultimate Mountaintop: Astronomy Aboard Stratospheric Balloons

As funding, for astronomy dwindles and the competition for observation time heats up, more astronomers may turn to balloons. Far above the Keck telescope on Hawaii's Mauna Kea, higher still than the hostile snowcapped peaks of Mt. Everest, there exists a 40-kilometer summit that will place their telescopes above 99% of the atmosphere. With the prospect of 100-day and even 1,000-day balloons, the climb to the summit is more and more tempting. Surely, given enough cash, most astronomers would opt for a lunar base or a platform beyond the Earth. Until then, many seem happy to settle for a stratospheric mountaintop.

Wanjek, Christopher↗

NASA NTRS2000

High Energy Astrophysics Mission

The nature of gravity and its relationship to the other three forces and to quantum theory is one of the major challenges facing us as we begin the new century. In order to make progress we must challenge the current theories by observing the effects of gravity under the most extreme conditions possible. Black holes represent one extreme, where the laws of physics as we understand them break down. The Universe as whole is another extreme, where its evolution and fate is dominated by the gravitational influence of dark matter and the nature of the Cosmological constant. The early universe represents a third extreme, where it is thought that gravity may somehow be unified with the other forces. NASA's "Cosmic Journeys" program is part of a NASA/NSF/DoE tri-agency initiative designed to observe the extremes of gravity throughout the universe. This program will probe the nature of black holes, ultimately obtaining a direct image of the event horizon. It will investigate the large scale structure of the Universe to constrain the location and nature of dark matter and the nature of the cosmological constant. Finally it will search for and study the highest energy processes, that approach those found in the early universe. I will outline the High Energy Astrophysics part of this program.

White, Nicholas E.↗

NASA NTRS2000

Tape Measure to the Stars

Calculating distances is one of the thorniest issues of astronomy. But now there are two new, promising methods that may serve as useful yardsticks. The Chandra X-ray Observatory has demonstrated a way to measure distances within the Milky Way and perhaps to nearby galaxies by measuring the delays of X-ray photons as they scatter off intervening matter. The late Compton Gamma-Ray Observatory, through its exhaustive archive of 2,700 gamma-ray bursts, has revealed patterns in these flashes of light that correspond to cosmological distances.

Wanjek, Christopher↗

NASA NTRS2000

The X-Ray Lightcurve of Eta Carinae: Refinement of the Orbit and Evidence for Phase Dependent Mass Loss

We solve the RXTE X-ray lightcurve of the extremely luminous and massive star eta Carinae with a colliding wind emission model to refine the ground-based orbital elements. The sharp decline to X-ray minimum at the end of 1997 fixes the date of the last periastron passage at 1997.95 +/- 0.05, not 1998.13 as derived from ground-based radial velocities. This helps resolve a discrepancy between the ground-based radial velocities and spatially-resolved velocity measures obtained by STIS. The X-ray data are consistent with a mass function f(M) approx. = 1.5, lower than the value f(M) approx. = 7.5 previously reported, so that the masses of eta Carinae and the companion are M(sub eta) greater than or = 80 solar mass and M(sub c) approx. 30 solar mass respectively. In addition the X-ray data suggest that the mass loss rate from eta Carinae is generally less than 3 x 10(exp -4) solar mass/yr, about a factor of 5 lower than that derived from some observations in other wavebands. We could not match the duration of the X-ray minimum with any standard colliding wind model in which the wind is spherically symmetric and the mass loss rate is constant. However we show that we can match the variations around X-ray minimum if we include an increase of a factor of approx. 20 in the mass loss rate from eta Carinae for approximately 80 days following periastron. If real, this excess in M would be the first evidence of enhanced mass flow off the primary when the two stars are close (presumably driven by tidal interactions). Our interpretation of the X-ray data suggest that the ASCA and RXTE X-ray spectra near the X-ray minimum are significantly contaminated by unresolved hard emission (E greater than or = 2 keV) from sonic other nearby source, probably associated with scattering of tile colliding wind emission by circumstellar dust. Based on the X-ray fluxes the distance to n Carinae is 2300 pc with formal uncertainties of only approx. 10%.

Corcoran, M. F.↗

NASA NTRS2000

The X-ray Spectral Evolution of eta Carinae as Seen by ASCA

Using data from the ASCA X-ray observatory, we examine the variations in the X-ray spectrum of the supermassive star nu Carinae with an unprecedented combination of spatial and spectral resolution. We include data taken during the recent X-ray eclipse in 1997-1998, after recovery from the eclipse, and during and after an X-ray flare. We show that the eclipse variation in the X-ray spectrum is apparently confined to a decrease in the emission measure of the source. We compare our results with a simple colliding wind binary model and find that the observed spectral variations are only consistent, with the binary model if there is significant high-temperature emission far from the star and/or a substantial change in the temperature distribution of the hot plasma. If contamination in the 2-10 keV band is important, the observed eclipse spectrum requires an absorbing column in excess of 10(exp 24)/sq cm for consistency with the binary model, which may indicate an increase in the first derivative of M from nu Carinae near the time of periastron passage. The flare spectra are consistent with the variability seen in nearly simultaneous RXTE observations and thus confirm that nu Carinae itself is the source of the flare emission. The variation in the spectrum during the flare seems confined to a change in the source emission measure. By comparing 2 observations obtained at the same phase in different X-ray cycles, we find that the current, X-ray brightness of the source is slightly higher than the brightness of the source during the last cycle perhaps indicative of a long-term increase in the first derivative of M, not associated with the X-ray cycle.

Corcoran, M. F.↗

NASA NTRS2000

Constraints on Cosmological Parameters From The Sunyaev-Zel'dovich Effect and Thermal Bremsstrahlung

We discuss the possibility of constraining cosmological parameters using the Sunyaev-Zel'dovich (SZ) effect and thermal bremsstrahlung caused by intra-cluster gas in clusters of galaxies. The new generation of X-ray satellites and ground based interferometers dedicated to SZ observations will enable one to reduce uncertainties in these measurements, and thus make this method potentially quite promising in the near future. The importance of this method is that, unlike most other methods, it is based on physical principles, no 'standard candles' or 'rulers' needed. We estimate the accuracy achievable in the determination of the matter density, OMEGA(sub m), the cosmological constant, OMEGA(sub LAMBDA), and the Hubble constant, h, using the redshift dependence of the angular diameter distance derived from observations in the near future. We demonstrate that constraints from the angular diameter distance are orthogonal to those from Cosmic Microwave Background (CMB) fluctuations in the parameter space defined by OMEGA(sub m), OMEGA(sub LAMBDA), and h. Assuming a statistical error of five percent in the angular diameter distance for each cluster in a sample of five hundred clusters, we show that the redshift dependence of the angular diameter distance combined with constraints from CMB fluctuations can put stringent constraints on OMEGA(sub m) (+/- 0.03), OMEGA(sub LAMBDA) (+/- 0.03) and h (+/- 0.03, 3(sigma) errors). We also show that, with as few as 50 clusters between redshifts 0.01 and 1.5 with an assumed 10% statistical error in the angular diameter distance determination, one can distinguish between models with zero cosmological constant and spatially flat models with a cosmological constant with high confidence level (independently from the supernova results) and put meaningful constraints on OMEGA(sub m) (+/- 0.01), OMEGA(sub LAMBDA) (+/- 0.01) and h (+/- 0.01, 3(sigma) errors). With the expected advances in observational technology, we will be limited by systematic errors. We discuss the sources of systematic errors and how they can be reduced.

Molnar, S. M.↗

NASA NTRS2000

X-Ray Constraints on the Warm-Hot Intergalactic Medium

Three observational constraints can be placed on a warm-hot intergalactic medium (WHIM) using ROSAT Position Sensitive Proportional Counter (PSPC) pointed and survey data, the emission strength, the energy spectrum, and the fluctuation spectrum. The upper limit to the emission strength of the WHIM is 7.5 +/- 1.0 keV/(s*sq cm*sr*keV) in the 3/4 keV band, an unknown portion of which value may be due to our own Galactic halo. The spectral stape of the WHIM emission can be described as thermal emission with logT = 6.42, although the true spectrum is more likely to come from a range of temperatures. The values of emission strength and spectral shape are in reasonable agreement with hydrodynamical cosmological models. The autocorrelation function in the 0.44 keV < E < 1.21 keV band range, w(theta), for the extragalactic soft X-ray background (SXRB) which includes both the WHIM and contributions due to point sources, is approx. < 0.002 for 10 min < 0 < 20 min in the 3/4 keV band. This value is lower than the Croft et al. (2000) cosmological model by a factor of approx. 5, but is still not inconsistent with cosmological models. It is also found that the normalization of the extragalactic power law component of the soft X-ray background spectrum must be 9.5 +/- 0.9 keV/(s*sq cm*sr*keV) to be consistent with the ROSAT All-Sky Survey.

Kuntz, K. D.↗

NASA NTRS2000

Exploring the Extreme Universe with GLAST

The Gamma-ray Large Area Space Telescope, GLAST, is a satellite-based experiment under development to measure the cosmic gamma-ray flux in the energy range 20 MeV to > 300 GeV. With a sensitivity that is more than a factor 30 greater than that of the EGRET detector on GRO, GLAST will open a new and important window on a wide variety of high energy phenomena, including supermassive black holes and active galactic nuclei, gamma-ray bursts, supernova remnants, and searches for new phenomena such as supersymmetric dark matter annihilations and big bang particle relics. The launch is planned for 2005. This talk will include a discussion of the physics, the scientific capabilities, the novel collaboration of particle physicists and high energy astrophysicists, and the mission status.

Ritz, Steven↗

NASA NTRS2000

Gamma Ray Pulsars: Observations

High-energy gamma rays are a valuable tool for studying particle acceleration and radiation in the magnetospheres of energetic pulsars. The six or more pulsars seen by CGRO/EGRET show that: the light curves usually have double-peak structures (suggesting a broad cone of emission); gamma rays are frequently the dominant component of the radiated power; and all the spectra show evidence of a high-energy turnover. Unless a new pulsed component appears at higher energies, progress in gamma-ray pulsar studies will be greatest in the 1-20 GeV range. Ground-based telescopes whose energy ranges extend downward toward 10 GeV should make important measurements of the spectral cutoffs. The Gamma-ray Large Area Space Telescope (GLAST), now in planning for a launch in 2005, will provide a major advance in sensitivity, energy range, and sky coverage.

Thompson, David J.↗

NASA NTRS2000

Gamma-ray Pulsars: Models and Predictions

Pulsed emission from gamma-ray pulsars originates inside the magnetosphere, from radiation by charged particles accelerated near the magnetic poles or in the outer gaps. In polar cap models, the high energy spectrum is cut off by magnetic pair production above an energy that is, dependent on the local magnetic field strength. While most young pulsars with surface fields in the range B = 10(exp 12) - 10(exp 13) G are expected to have high energy cutoffs around several GeV, the gamma-ray spectra of old pulsars having lower surface fields may extend to 50 GeV. Although the gamma-ray emission of older pulsars is weaker, detecting pulsed emission at high energies from nearby sources would be an important confirmation of polar cap models. Outer gap models predict more gradual high-energy turnovers of the primary curvature emission around 10 GeV, but also predict an inverse Compton component extending to TeV energies. Detection of pulsed TeV emission, which would not survive attenuation at the polar caps, is thus an important test of outer gap models. Next-generation gamma-ray telescopes sensitive to GeV-TeV emission will provide critical tests of pulsar acceleration and emission mechanisms.

Harding Alice K.↗

NASA NTRS2000

Galactic Diffuse Gamma Ray Emission Is Greater than 10 Gev

AGILE and Gamma-ray Large Area Telescope (GLAST) are the next high-energy gamma-ray telescopes to be flown in space. These instruments will have angular resolution about 5 times better than Energetic Gamma-Ray Experiment Telescope (EGRET) above 10 GeV and much larger field of view. The on-axis effective area of AGILE will be about half that of EGRET, whereas GLAST will have about 6 times greater effective area than EGRET. The capabilities of ground based very high-energy telescopes are also improving, e.g. Whipple, and new telescopes, e.g. Solar Tower Atmospheric Cerenkov Effect Experiment (STACEE), Cerenkov Low Energy Sampling and Timing Experiment (CELESTE), and Mars Advanced Greenhouse Integrated Complex (MAGIC) are expected to have low-energy thresholds and sensitivities that will overlap the GLAST sensitivity above approximately 10 GeV. In anticipation of the results from these new telescopes, our current understanding of the galactic diffuse gamma-ray emission, including the matter and cosmic ray distributions is reviewed. The outstanding questions are discussed and the potential of future observations with these new instruments to resolve these questions is examined.

Hunter, Stanley D.↗

NASA NTRS2000

Neutrinos from AGN

The great penetrating power of neutrinos makes them ideal probe of astrophysical sites and conditions inaccessible to other forms of radiation. These are the centers of stars (collapsing or not) and the centers of Active Galactic Nuclei (AGN). It has been suggested that AGN presented a very promising source of high energy neutrinos, possibly detectable by underwater neutrino detectors. This paper reviews the evolution of ideas concerning the emission of neutrinos from AGN in view of the more recent developments in gamma-ray astronomy and their implications for the neutrino emission from these class of objects.

Kazanas, Demosthenes↗

NASA NTRS2000

The Axisymmetric Pulsar Magnetosphere

We present the structure of the axisymmetric force-free magnetosphere of an aligned rotating magnetic dipole, in the case in which there exists a sufficiently large charge density (whose origin we do not question) to satisfy the ideal MHD condition, E (raised dot) B = 0, everywhere. The unique distribution of electric current along the open magnetic field lines which is required for the solution to be continuous and smooth is obtained numerically. We expect that our solution will be useful as the starting point for detailed studies of pulsar magnetospheres under more general conditions, namely when either the force-free and/or the ideal MHD condition E (raised dot) B = 0 are not valid in the entire magnetosphere. Based on our solution, we consider that the most likely positions of such an occurrence are the polar cap, the crossings of the zero space charge surface by open field lines, and the return current boundary, but not the light cylinder.

Contopoulos, J.↗

NASA NTRS2000

Could the Unidentified EGRET Sources in the Gould Belt be of Pulsar Origin?

We explore the possibility, within the polar cap model, that at least some of the recently identified new population of Energetic Gamma-Ray Experiment Telescope (EGRET) sources in the Gould Belt sources are gamma-ray pulsars, viewed at large angles to the magnetic axis, such that the line-of-sight misses both the bright gamma-ray double cone and the radio beam. We estimate that the detectability of such off-beam gamma-ray sources is about a factor of 4-5 times higher than that of the on-beam sources. The off-beam gamma-ray pulsars also have a faint, soft nature, which is consistent with the observed properties of the Gould Belt sources. We conclude that at least some of the middle latitude sources might be such off-beam gamma-ray pulsars if they are indeed in the Gould Belt.

Zhang, Bing↗

NASA NTRS2000

Cosmic Ray and Tev Gamma Ray Generation by Quasar Remnants

Results from new broadband (radio to X-ray) high-resolution imaging studies of the dormant quasar remnant cores of nearby giant elliptical galaxies are now shown to permit the harboring of compact dynamos capable of generating the highest energy cosmic ray particles and associated curvature radiation of TeV photons. Confirmation would imply a global inflow of interstellar gas all the way to the accretion powered supermassive black hole at the center of the host galaxy.

Boldt, Elihu↗