Engineering topics

White, Nicholas E.

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

At least 37 records · Page 2

NASA NTRS2003

Investigation of Spectral Lag and Epeak as Joint Luminosity Indicators in GRBs

Models for gamma-ray bursts which invoke jetted, colliding shells would appear to have at least two determinants for luminosity, e.g., observer viewing angle and Lorentz factor, or possibly shell mass. The latter two internal physical parameters may vary from pulse to pulse within a burst, and such variation might be reflected in evolution of observables such as spectral lag and peak in the spectral energy distribution. We analyze bright BATSE bursts using the 16-channel medium energy resolution (MER) data, with time resolutions of 16 and 64 ms, measuring spectral lags and peak energies for significant pulse structures within a burst, identified using a Bayesian block algorithm. We then explore correlations between the measured parameters and total flux for the individual pulse structures.

White, Nicholas E.↗

NASA NTRS2003

Limitations on Space-based Air Fluorescence Detector Apertures obtained from IR Cloud Measurements

The presence of clouds between an airshower and a space-based detector can dramatically alter the measured signal characteristics due to absorption and scattering of the photonic signals. Furthermore, knowledge of the cloud cover in the observed atmosphere is needed to determine the instantaneous aperture of such a detector. Before exploring the complex nature of cloud-airshower interactions, we examine a simpler issue. We investigate the fraction of ultra-high energy cosmic ray events that may be expected to occur in volumes of the viewed atmosphere non-obscured by clouds. To this end, we use space-based IR data in concert with Monte Carlo simulated 10(exp 20) eV airshowers to determine the acceptable event fractions. Earth-observing instruments, such as MODIS, measure detailed cloud configurations via a CO2-slicing technique that can be used to determine cloud-top altitudes over large areas. Thus, events can be accepted if their observed 3-dimensional endpoints occur above low clouds as well as from areas of cloud-free atmosphere. An initial analysis has determined that by accepting airshowers that occur above low clouds, the non-obscured acceptance can be increased by approximately a factor of 3 over that obtained using a cloud-free criterion.

White, Nicholas E.↗

NASA NTRS2003

What is your Cosmic Connection to the Elements?

This information and activity booklet describes the roles of the Big Bang, types of stars, supernovae, cosmic ray interactions, and radioactive decay in the formation of the elements. The booklet includes instructions for the following classroom activities, intended for students in Grades 9-12: Grandma's Apple Pie; Cosmic Shuffle; Nickel-odeon; Kinesthetic Big Bang; Elemental Haiku; Cosmic Ray Collisions; Cosmic Abundances; and What's Out There.

White, Nicholas E.↗

NASA NTRS2002

Characteristics of Transients in the Galactic Bulge

Biweekly X-ray observations of the Galactic Bulge with the Rossi X-Ray Timing Explorer Proportional Counter Array for 5 years have built up statistics for different kinds of source behavior for low-mass binaries with neutron stars. The relation between the characteristic behaviors is not known. A typical transient may rise fast and decay exponentially in brightness and repeat this cycle after short (1/2 yr) or long intervals (20 yr). But an X-ray source also may be steady for an extended period (years) before the disk can become quiescent. It fades in the same way as do the transients which immediately decay. These sources have been reported to be very cool in quiescence; either not substantially heated by outbursts, or cooled very quickly. There are 3 sources active during the last 5 years which were long lasting transients which turned off and 1 which turned on. We will compare their X-ray characteristics in outburst, and in so far as available with Chandra, in quiescence.

Swank, Jean H.↗

NASA NTRS2002

Bulk X-ray Doppler Velocities in the Supernova Remnant Cassiopeia A

We map the average line-of-sight velocities of the X-ray-emitting mass in the supernova remnant Cassiopeia A using measurements of the centroid of the Si-Healpha blend with the Advanced CCD Imaging Spectrometer on the Chandra X-Ray Observatory. This 4 inch scale map confirms the broad trends noted in previous studies, namely, that the line-of-sight velocity scale is roughly 2000-3000 kilometers per second with relatively more blueshifted material in the southeastern region of the remnant than in the northwest; new details are that the northwestern region consists of two arcs, with the southernmost one representing the most redshifted X-ray-emitting material in Cas A. These results are consistent with contemporaneous results from XMM-Newton. The X-ray patterns resemble the complex velocity patterns measured at optical wavelengths for much denser ejecta, and they support the growing body of evidence that the explosion and subsequent evolution of Cas A were highly asymmetrical.

Hwang, Una↗

NASA NTRS2002

The Origin of Soft X-rays in DQ Herculis

DQ Herculis (Nova Herculis 1934) is a deeply eclipsing cataclysmic variable containing a magnetic white dwarf primary. The accretion disk is thought to block our line of sight to the white dwarf at all orbital phases due to its extreme inclination angle. Nevertheless, soft X-rays were detected from DQ Her with ROSAT PSPC. To probe the origin of these soft X-rays, we have performed Chandra ACIS observations. We confirm that DQ Her is an X-ray source. The bulk of the X-rays are from a point-like source and exhibit a shallow partial eclipse. We interpret this as due to scattering of the unseen central X-ray source, probably in an accretion disk wind. At the same time, we detect weak extended X-ray features around DQ Her, which we interpret as an X-ray emitting knot in the nova shell.

White, Nicholas E.↗

NASA NTRS2002

Diffuse X-ray Emission from M101

The total 0.45-2.0 keV luminosity of M101 is 3.1 x 10(exp 39) ergs/s, of which 2.2 x 10(exp 39) ergs/s is due to diffuse emission. Of the diffuse emission, no more than 6% can be due to unresolved point sources such as X-ray binaries, and approx. 11% is due to dwarf stars. The diffuse emission traces the spiral arms and is roughly correlated with the H alpha and FUV (far ultraviolet) emission. The radial distribution closely follows the optical profile. The bulk of the diffuse emission is characterized by a two thermal component spectrum with kT = 0.20,0.75 keV, and the ratios of the emission measures of the two components is roughly constant as a function of both radius and surface brightness. The softer component has a sufficiently large covering factor that the bulk of the emission is likely extra-planar. We find no evidence of an extended axisymmetric X-ray halo, suggesting that any such halo has a strength much smaller than current predictions.

Kuntz, K. D.↗

NASA NTRS2002

Thermal Comptonization and Disk Thermal Reprocessing in NGC3516

We present an application of the thermal Comptonization/disk reprocessing model recently proposed by Zdziarski, Lubiniski, and Smith. We show that the absence of strong optical variations in the presence of strong concurrent X-ray variations, similar to those found by Hubble Space Telescope (HST)/Rossi X-Ray Timing Explorer (RXTE) monitoring observations of NGC 3516, can be explained by changing the geometry of the Comptonizing plasma rather than the accretion disk itself. The total X-ray luminosity of the Comptonizing plasma must decrease as its spatial extent increases. In contrast, the disk inner radius must be roughly fixed in order not to produce optical/ultraviolet color variations stronger than observed. By including emission due to internal viscous dissipation in the disk, we can roughly match the optical and X-ray flux levels and variability amplitudes seen from NGC 3516 during the HST/RXTE campaign.

Chiang, James↗

NASA NTRS2002

Pulsar Polar Cap Heating and Surface Thermal X-ray Emission: Curvature Radiation Pair Fronts - 1

We investigate the effect of pulsar polar cap (PC) heating produced by positrons returning from the upper pair formation front. Our calculations are based on a self-consistent treatment of the pair dynamics and the effect of electric field screening by the returning positrons. We calculate the resultant X-ray luminosities and discuss the dependence of the PC heating efficiencies on pulsar parameters, such as characteristic spin-down age, spin period, and surface magnetic field strength. In this study we concentrate on the regime where the pairs are produced in a magnetic field by curvature photons emitted by accelerating electrons. Our theoretical results are not in conflict with the available observational x-ray data and suggest that the effect of PC heating should significantly contribute to the thermal x-ray fluxes from middle-aged and old pulsars. The implications for current and future x-ray observations of pulsars are briefly outlined.

Harding, Alice K.↗

NASA NTRS2002

On the Structure of the Iron K-Edge

It is shown that the commonly held view of a sharp Fe K edge must be modified if the decay pathways of the series of resonances converging to the K thresholds are adequately taken into account. These resonances display damped Lorentzian profiles of nearly constant widths that are smeared to impose continuity across the threshold. By modeling the effects of K damping on opacities, it is found that the broadening of the K edge grows with the ionization level of the plasma, and the appearance at high ionization of a localized absorption feature at 7.2 keV is identified as the Kbeta unresolved transition array.

Palmeri, P.↗

NASA NTRS2002

Discovery of a 450 Hz Quasi-Periodic Oscillation from the Microquasar GRO J1655-40 with the Rossi X-ray Timing Explorer

We report the discovery with the Proportional Counter Array on board the Rossi X-Ray Timing Explorer of a 450 Hz quasi-periodic oscillation (QPO) in the hard X-ray flux from the Galactic microquasar GRO J1655-40. This is the highest frequency QPO modulation seen to date from a black hole. The QPO is detected only in the hard X-ray band above approx. 13 keV. It is both strong and narrow, with a typical rms (root mean square) amplitude of 4.5% in the 13-27 keV range and a width of approx. 40 Hz (FWHM). For two observations in which we detect the 450 Hz QPO, a previously known approx. 300 Hz QPO is also observed in the 2-13 keV band. We show that these two QPOs sometimes appear simultaneously, thus demonstrating the first detection of a pair of high-frequency QPOs in a black hole system. Prior to this, pairs of high-frequency QPOs have been detected only in neutron star systems. GRO J1655-40 is one of only a handful of black hole systems with a good dynamical mass constraint. For a nonrotating black hole with mass between 5.5 and 7.9 solar masses, the innermost stable circular orbit (ISCO) ranges from 45 to 70 km. For any mass in this range the radius at which the orbital frequency reaches 450 Hz is less than the ISCO radius, indicating that, if the modulation is caused by Kepler motion, the black hole must have appreciable spin. If the QPO frequency is set by the orbital frequency of matter at the ISCO, then for this mass range the dimensionless angular momentum lies in the range 0.15 < j < 0.5. Moreover, if the modulation is caused by oscillation modes in the disk or Lense-Thirring precession, then this would also require a rapidly rotating hole. We briefly discuss the implications of our findings for models of X-ray variability in black holes and neutron stars.

Strohmayer, Tod E.↗

NASA NTRS2002

No Compton Reflection In a Chandra/RXTE Observation of Mkn 509: Implications for the Fe-K Line Emission From Accreting X-Ray Sources

We report the results of simultaneous Chandra and RXTE observations of the Seyfert 1 galaxy Mkn 509. We deconvolve the broad and narrow Fe-K emission-line components for which we measure rest-frame equivalent widths of 119+/-18 eV and 57+/-13 eV respectively. The broad line has a FWHM of 57,600((sup 14,400)(sub -21,000)) km/s and the narrow line is unresolved, with an upper limit on the FWHM of 4,940 km/s. Both components must originate in cool matter since we measure rest-frame center energies of 6.36((sup +0.13)(sub -0.12)) keV and 6.42+/-0.01 keV for the broad and narrow line respectively. This rules out He-like and H-like Fe for the origin of both the broad and narrow lines. If, as is widely accepted, the broad Fe-K line originates in Thomson-thick matter (such as an accretion disk), then one expects to observe spectral curvature above approximately 10 keV, (commensurate with the observed broad line), characteristic of the Compton-reflection continuum. However our data sets very stringent limits on deviations of the observed continuum from a power law. Light travel-time delays cannot be invoked to explain anomalies in the relative strengths of the broad Ferry line and Compton-reflection continuum since they are supposed to originate in the same physical location. We are forced to conclude that both the broad and narrow Fe-K lines had to originate in Thomson-thin matter during our observation. This result, for a single observation of just one source, means that our understanding of Fe K line emission and Compton reflection from accreting X-ray sources in general needs to be re-examined. For example, if an irradiated accretion disk existed in Mkn 509 at the time of the observations, the lack of spectral curvature above approximately 10 keV suggests two possibilities. Either the disk was Thomson-thick and highly ionized, having negligible Fe-K line emission and photoelectric absorption or the disk was Thomson-thin producing some or all of the broad Fe-K line emission. In the former case, the broad Fe-K line had to have produced in a Thomson-thin region elsewhere. In both cases the predicted spectral curvature above approximately 10 keV is negligible. An additional implication of our results is that any putative obscuring torus in the system, required by unification models of active galaxies, must also be Thomson-thin. The same applies to the optical broad line region (BLR) if it has a substantial covering factor.

Yaqoob, Tahir↗

NASA NTRS2002

Exploring the Extreme Universe! (2nd Edition)

A large array of web sites devoted to the science that the NASA Goddard Space Flight Center Laboratory for High Energy Astrophysics (LHEA) studies have been frozen in time and included on this CD. Featured are five of our 'Understanding the Extreme Universe' sites, which offer explanations about cosmic ray, gamma ray, and X-ray science and satellites, as well as cosmic distances. They also include images and engaging activities that are great for helping both children and adults to learn more about science and basic physical principles. The 'Missions That Take Us There' section contains web sites on the satellite, balloon-borne, International Space Station, and rocket missions in our Laboratory that study X-rays, gamma rays, and cosmic rays. There are also two multi-mission sites and an experiment that measures Earth's UV light in preparation for a future cosmic ray mission. Most of the sites on this CD contain high-resolution images that are great for scientific presentations, study, or just your own enjoyment. This CD shows our web sites as they existed in April of 2001. We have made sure to include the WWW address for every site, so you will know where to go to access the most current versions of them.

White, Nicholas E.↗

NASA NTRS2002

Pulsar Polar Cap Heating and Surface Thermal X-ray Emission: Inverse Compton Radiation Pair Fronts - 2

We investigate the production of electron-positron pairs by inverse Compton scattered (ICS) photons above a pulsar polar cap (PC) and calculate surface heating by returning positrons. This paper is a continuation of our self-consistent treatment of acceleration, pair dynamics, and electric field screening above pulsar PCs. We calculate the altitude of the inverse Compton pair-formation fronts, the flux of returning positrons, and present the heating efficiencies and X-ray luminosities. We revise pulsar death lines implying cessation of pair formation, and present them in surface magnetic field-period space. We find that virtually all known radio pulsars are capable of producing pairs by resonant and nonresonant ICS photons radiated by particles accelerated above the PC in a pure star-centered dipole field, so that our ICS pair death line coincides with empirical radio pulsar death. Our calculations show that ICS pairs are able to screen the accelerating electric field only for high PC surface temperatures and magnetic fields. We argue that such screening at ICS pair fronts occurs locally, slowing but not turning off acceleration of particles until screening can occur at a curvature radiation (CR) pair front at higher altitude. In the case where no screening occurs above the PC surface, we anticipate that the pulsar gamma-ray luminosity will be a substantial fraction of its spin-down luminosity. The X-ray luminosity resulting from PC heating by ICS pair fronts is significantly lower than the PC heating luminosity from CR pair fronts, which dominates for most pulsars. PC heating from ICS pair fronts is highest in millisecond pulsars, which cannot produce CR pairs, and may account for observed thermal X-ray components in the spectra of these old pulsars.

Harding, Alice K.↗

NASA NTRS2002

The Seyfert-Starburst Connection in X-rays: The Data - 1

We analyze X-ray spectra and images of a sample of Seyfert 2 galaxies that unambiguously contain starbursts, based on their optical and UV characteristics. Although all sample members contain active galactic nuclei (AGNs), supermassive black holes or other related processes at the galactic centers alone cannot account for the total X-ray emission in all instances. Eleven of the 12 observed galaxies are significantly resolved with the ROSAT High Resolution Imager, while six of the eight sources observed with the lower resolution Position Sensitive Proportional Counter also appear extended on larger scales. The X-ray emission is extended on physical scales of 10 kpc and greater, which we attribute to starburst-driven outflows and supernova heating of the interstellar medium. Spectrally, a physically motivated composite model of the X-ray emission that includes a heavily absorbed (N(sub H) greater than 10(exp 23)/sq cm) nuclear component (the AGN), power-law-like scattered AGN flux, and a thermal starburst describes this sample well. Half the sample exhibit iron K(alpha) lines, which are typical of AGNs.

Levenson, N. A.↗

NASA NTRS2002

Imagine the Universe! (6th Edition)

Contained on this CD-ROM you will find three astronomy and space science learning centers, individually captured from the World Wide Web in December of 2001. Each site contains its own learning adventure full of facts, fun, beautiful images, movies, and excitement!

Lochner, James↗

NASA NTRS2002

A Chandra X-Ray Study of Cygnus A: The Cluster of Galaxies - 3

We present an analysis of the Chandra Advanced CCD Imaging Spectrometer (ACIS) observation of the intracluster gas associated with the cluster of galaxies surrounding Cygnus A. The dominant gaseous structure is a roughly elliptical (presumably prolate spheroidal in three dimensions) feature with semimajor axis approximately equal to 1.(prime)1 (approximately equal to 100 kpc). This structure apparently represents intracluster gas that has been swept up and compressed by a cavity inflated in this gas by relativistic material that has passed through the ends of the radio jets. The X-ray-emitting gas shows this prolate spheroidal morphology to approximately equal to 1.(prime)2 (110 kpc) from the radio galaxy but is spherical on larger scales. The X-ray emission from the intracluster gas extends to at least 8 prime (approximately equal to 720 kpc) from the radio galaxy, and a second, extended source of X-ray emission (probably associated with a second cluster of galaxies) is seen some 12 prime (approximately equal to 1 Mpc) to the northwest of Cygnus A. The X-ray spectrum of the integrated intracluster gas imaged on the S3 chip (dimensions 8 prime x 8 prime = 720 x 720 kpc), excluding the contribution from the radio galaxy and other compact sources of X-ray emission, has a gas temperature, metallicity, and unabsorbed 2-10 keV rest-frame luminosity of 7.7 keV, 0.34 times solar, and 3.5 x 10(exp 44) ergs per second, respectively. We have projected the X-ray spectra taken from 12 elliptical and circular annuli in order to derive a run of temperature, metallicity, density, and pressure as a function of radius. The temperature of the X-ray-emitting gas drops from approximately equal to 8 keV more than 100 kpc from the center to approximately equal to 5 keV some 80 kpc from the center, with the coolest gas immediately adjacent to the radio galaxy. "Belts" of slightly cooler (approximately equal to 4 keV) X-ray-emitting gas run around the minor dimension of the cavity created by the radio source, while the limb-brightened edges of the cavity are slightly hotter (approximately equal to 6 keV), perhaps as a result of heating by a bow shock driven by the probably expanding cavity into the intracluster gas. There is a metallicity gradient in the X-ray-emitting gas, with the highest metallicities (approximately solar) found close to the center, decreasing to approximately 0.3 times solar in the outer parts. We have used the assumption of hydrostatic equilibrium to derive a total cluster mass within 500 kpc of 2.0 x 10(exp 14) solar mass and 2.8 x 10(exp 14) solar mass for constant and centrally decreasing temperature profiles, respectively. The total mass of X-ray-emitting gas within the same radius is 1.1 x 10(exp 13) solar mass. Thus, the gas fraction of the cluster within 500 kpc is 0.055 and 0.039 for the constant and centrally decreasing temperature profiles, respectively.

Smith, David A.↗

NASA NTRS2002

Particle Astrophysics and Cosmology: Cosmic Laboratories for New Physics (Summary of the Snowmass 2001 P4 Working Group)

The past few years have seen dramatic breakthroughs and spectacular and puzzling discoveries in astrophysics and cosmology. In many cases, the new observations can only be explained with the introduction of new fundamental physics. Here we summarize some of these recent advances. We then describe several problems in astrophysics and cosmology, ripe for major advances, the resolution of which will likely require new physics.

Akerib, Daniel S.↗