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At least 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

A rotating, expanding disk in the Wolf-Rayet star EZ Canis Majoris?

The discovery of linear polarization changes across the extended wings of He II lines, mainly the strong 4-3 transition at 4686 A, in the WN5 star EZ CMa, is reported. When the polarization across the line profiles is plotted in the Stokes parameters plane, it traces loops clockwise from the blue wing through line center to the red, rather than straight lines. Such polarization loops are reminiscent of what is observed in the Balmer lines of Be stars. The continuum polarization in EZ CMa can be understood by an axisymmetric, electron-scattering envelope, with the decrease in polarization in He II being caused by an increase in absorptive opacity in the lines and dilution by unpolarized line emission, while the variations in position angle are due to the Doppler-shifted absorptive opacity and/or scattered line photons. As the sense of rotation in the loops is also independent of phase of this alleged Wolf-Rayet + compact binary, the polarized line profiles are the signature of a rotating, expanding wind geometry around a single star.

Schulte-Ladbeck, R. E.↗
NASA NTRS1992

Radiation dynamics in X-ray binaries. I - Type 1 bursts. II - Type 2 bursts. III - Extremely compact objects

Equations describing the evolution of a thin, axisymmetric, viscous, relativistic, irradiated accretion disk are presented, as well as numerical solutions of these equations in the case where irradiation results from a thermonuclear flash on the surface of the accreting neutron star. These calculations verify the notion that the radiation torque induces a substantial increase in the accretion rate, during a type 1 X-ray burst, and provide insight into the factors which influence the dynamical response of the disk. A new model for the source XBT 1730-335, the rapid burster, is presented. Temporal and spectral properties are calculated. The rapid burster is found to be a nonmagnetic, 'critically compact', slowly rotating neutron star in a highly eccentric binary system with a period of 6 mo. The spectral modifications which arise from the scattering of photons by accretion disks around nonmagnetic neutron stars are calculated. The 'black hole candidates' are interpreted as extremely compact stars.

Walker, Mark A.↗
NASA NTRS1993

Dynamic processes in Be star atmospheres. I - 'Dimple' formation in the He I lambda 6678 line of lambda Eridani

Several examples of weakenings of the C IV and N V resonance lines are found to coincide with the appearance of lambda 6678 dimples. The absence of variations in other UV lines and in the UV continuum at the same time or nearly the same time argues against dimples being caused by thermal variations from the underlying star. It is instead suggested that the resonance line weakenings are caused by non-LTE effects associated with the condensation of high density structures at some elevation over the star. A simple model of an opaque, essentially stationary slab which backscatters lambda 6678 line radiation into a surrounding 'penumbral' region is presented. Lambda 6678 photons are scattered a second time in this region back into the observer's line of sight and in the process acquire the local projected Doppler shift from rotation. Slabs would probably produce too little emission to be easily detected in the H alpha profile. Their detection in strong He I lines seems the best strategy among early Be stars.

Smith, Myron A.↗
NASA NTRS1995

Reionization and the cosmic microwave background in an open universe

If the universe was reionized at high reshift (z greater than or approximately equal to 30) or never recombined, then photon-electron scattering can erase fluctuations in the cosmic microwave background at scales less than or approximately equal to 1 deg. Peculiar motion at the surface of last scattering will then have given rise to new anisotropy at the 1 min level through the Vishniac effect. Here the observed fluctuations in galaxy counts are extrapolated to high redshifts using linear theory, and the expected anisotropy is computed. The predicted level of anisotropies is a function of Omega(sub 0) and the ratio of the density in ionized baryons to the critical density and is shown to depend strongly on the large- and small-scale power. It is not possible to make general statements about the viability of all reionized models based on current observations, but it is possible to rule out specific models for structure formation, particularly those with high baryonic content or small-scale power. The induced fluctuations are shown to scale with cosmological parameters and optical depth.

Persi, Fred M.↗
NASA NTRS1995

Theoretical overview and modeling of the sodium and potassium atmospheres of mercury

A general theoretical overview for the sources, sinks, gas-surface interactions, and transport dynamics of sodium and potassium in the exospheric atmsophere of Mercury is given. Information for these four factors, which control the spatial distribution of these two alkali-group gases about the planet, is incorporated in numerical models. The spatial nature and relative importance of the initial source atom atmosphere and the ambient (ballistic hopping) atom atmosphere are then examined and are shown to be controlled and coupled to a great extent by the extremely large and variable solar radiation acceleration experienced by sodium and potassium as they resonantly scatter solar photons. The lateral (antisunward) transport rate of thermally accommodated sodium and potassium ambient atoms is shown to be driven by the solar radiation acceleration and, over a significant portion of Mercury's orbit about the Sun, is sufficiently rapid to be competitive with the short photoionization lifetimes for these atoms when they are located on the summit surface near or within about 30 deg of the terminator. The lateral transport rate is characterized by a migration time determined by model calculations for an ensemble of atoms initially starting at a point source on the surface (i.e., a numerical spacetime dependent Green's function). Four animations for the spacetime evolution of the sodium (or potassium) atmosphere produced by a point source on the surface are presented on a videotape format. For extended surface sources for sodium and potassium, the local column density is determined by competition between the photoionization lifetimes and the lateral transport times of atoms originating from different surface source locations. Sodium surface source fluxes (referenced to Mercury at perihelion) that are required on the sunlit hemisphere to reproduce the typically observed several megarayleighs of D2 emission-line brightness and the inferred column densities of 1-2 x 10(exp 11) atoms per sq cm range from approximately 2-5 x 10(exp 7) atoms/sq cm/sec. The sodium model is applied to study observational data that document an anticorrelation in the average sodium column density and solar radiation acceleration. Lateral transport driven by the solar radiation acceleration is shown to produce this behavior for combinations of different sources and surface accomodation coefficients. The best fit model fits to the observational data require a significant degree of thermal accommodation of the ambient sodium atoms to the surface and a source rate that decreases as an inverse power of 1.5 to 2 in heliocentric distance.

Smyth, William H.↗
NASA NTRS1995

Theoretical overview and modeling of the sodium and potassium atmospheres of the moon

A general theoretical overview for the sources, sinks, gas-surface interactions, and transport dynamics of sodium and potassium in the exospheric atmosphere of the Moon is given. These four factors, which control the spatial distribution of these two alkali-group gases about the Moon, are incorporated in numerical models. The spatial nature and relative importance of the initial source atoms atmosphere (which must be nonthermal to explain observational data) and the ambient (ballistic hopping) atom atmosphere are examined. The transport dynamics, atmospheric structure, and lunar escape of the nonthermal source atoms are time variable with season of the year and lunar phase because of their dependence on the radiation acceleration experienced by sodium and potassium atoms as they resonantly scatter solar photons. The dynamic transport time of fully thermally accomodated ambient atoms along the surface because of solar radiation acceleration (only several percent of surface gravity) is larger than the photoionization lifetimes and hence unimportant in determining the local density, although for potassium the situation is borderline. The sodium model was applied to analyze sodium observations of the sunward brightness profiles acquired near last quarter by Potter & Morgan (1988b) extending from the surface to an altitude of 1200 km, and near first quarter by Mendillo, Baumgardner, & Flynn (1991), extending in altitude from approximately 1430 to approximately 7000 km. The observations at larger altitudes could be fitted only for source atoms having a velocity distribution with a tail that is mildly nonthermal (like an approximately 1000 K Maxwell-Boltzmann distribution). Solar wind sputtering appears to a be a viable source atom mechanism for the sodium observations with photon-simulated desorption also possible but highly uncertain, although micrometeoroid impact vaporization appears to have a source that is too small and too hot, with likely an incorrect angular distribution about the Moon.

Smyth, William H.↗
NASA NTRS1999

Coronally Fluoresced Stellar Photospheric X-Ray Spectra

Monte Carlo computations of photospheric fluorescent x-ray spectra are described. X-rays emitted from a hot corona incident on an underlying photosphere can undergo either scattering events by Compton scattering or photoabsorption events through the ionization of atoms or weakly ionized species. Through scattering events, photons can be reflected back in a direction towards the stellar surface where they have a finite chance of escape. Photoionization events involving x-ray photons directed toward the photosphere are predominantly inner shell interactions, Observable fluorescent lines can arise as a result of the finite escape probabilities of photons emitted in outward directions by hole transitions in these atoms. For a given stellar composition, fluorescent line strengths are dependent on the height of the fluorescing source above the photosphere and on the angle between the source and the observer. Although the only fluorescent lines likely to be detected in stellar coronae are Fe Ka and perhaps O Ka, these lines could provide powerful and direct spectroscopic diagnostics of coronal geometry.

Drake, Jeremy J.↗
NASA NTRS1999

Probing Blazar Structure Through Multiwavelength Variability

This was a project to understand the multiwavelength variability of blazars, via monitoring with the XTE satellite and many other space- and ground-based observatories. Blazars are dominated by strong nonthermal emission, we now know, due to relativistic beaming in an aligned jet. Thus they are ideal laboratories for studying jet physics. The physical scales of the inner, energetically dominant jets are generally too small to resolve with current or next-generation instruments, so we used variability to probe the balance between acceleration/injection of energetic particles and radiative/other losses, and to infer jet structure. Our work has shown that the radio through UV-X-ray emission is produced via synchrotron radiation, and that at the peak frequency and above, the emission region is approximately homogeneous. A separate gamma-ray component, which is often energetically dominant, varies in a correlated way but with larger amplitude, suggesting it is produced when the synchrotron-emitting electrons Compton scatter soft photons (though other possibilities remain viable). With RXTE we carried out two intensive monitoring campaigns on the blazar PKS2155-304, in May 1996 and November 1996. Analysis and interpretation of these data constituted the main activities under this grant. The results have been presented at several meetings, and a number of publications have resulted from this work. In addition, a final paper for a refereed publication is in final draft form, as are companion papers based on simultaneous Rosat and EUVE results.

Urry, C. Megan↗
NASA NTRS2001

Probing the Early Universe with the SZ Effect

The Cosmic Microwave Background Radiation (CMBR) which we observe today is relic radiation which last interacted with matter more than 10 billion years ago, when the expanding universe cooled to the point that free electrons and ionized nuclei recombined to form atoms. Prior to recombination, scattering between photons and free electrons was a very frequent occurrence, and the distance light could penetrate was small; afterwards, with free electrons out of circulation, the universe became largely transparent to light. Thus, the CMBR photons we observe today give us a clear view of the state of the early universe. Measured deviations in the intensity of the CMBR trace the small perturbations in the primordial matter density, which have been amplified by gravitational forces to form the magnificent, complex structures which comprise the present-day universe.

Joy, M. K.↗
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 NTRS2004

The X-ray Polarimetry Explorer (XPE)

We present details of a Thomson scattering polarimeter suitable for a small-explorer- class mission. Termed XPE, for X-Ray Polarimetry Explorer, the instrument consists of a beryllium scattering cone surrounded by an annular multiwire proportional counter which registers the energy and position of scattered source photons. The polarimeter is sensitive over an energy range of 7-27 keV and will achieve 3% minimum detectable polarization of Her X-1 in a 1/2-day observation, and cover a 30-target core observation program, at the same sensitivity level, in approximately 8 months.

Ramsey, Brian↗
NASA NTRS2004

THOR: Cloud Thickness from Off beam Lidar Returns

Conventional wisdom is that lidar pulses do not significantly penetrate clouds having optical thickness exceeding about tau = 2, and that no returns are detectable from more than a shallow skin depth. Yet optically thicker clouds of tau much greater than 2 reflect a larger fraction of visible photons, and account for much of Earth s global average albedo. As cloud layer thickness grows, an increasing fraction of reflected photons are scattered multiple times within the cloud, and return from a diffuse concentric halo that grows around the incident pulse, increasing in horizontal area with layer physical thickness. The reflected halo is largely undetected by narrow field-of-view (FoV) receivers commonly used in lidar applications. THOR - Thickness from Off-beam Returns - is an airborne wide-angle detection system with multiple FoVs, capable of observing the diffuse halo, detecting wide-angle signal from which physical thickness of optically thick clouds can be retrieved. In this paper we describe the THOR system, demonstrate that the halo signal is stronger for thicker clouds, and validate physical thickness retrievals for clouds having z > 20, from NASA P-3B flights over the Department of Energy/Atmospheric Radiation Measurement/Southern Great Plains site, using the lidar, radar and other ancillary ground-based data.

Cahalan, Robert F.↗
NASA NTRS2005

Strong-field Gravitational Effects in the Timing and Spectroscopy of X-Ray Binaries

Using the ray-tracing code described, we developed additional features to the hot spot model to explain the broadening of the quasi-periodic oscillation (QPO) peaks as well as the damping of higher frequency harmonics in the power spectrum. We derived a number of analytic results that agree closely with more detailed numerical calculations. Three primary pieces are developed: the superposition of multiple hot spots with finite lifetimes and random phases, a width in the radial distribution of geodesic orbits, and the scattering of photons from the hot spot through a corona around the black hole. The complete model is used to fit the observed power spectra of both type A and type B QPOs seen in XTE 51550-564, giving confidence limits on each of the model parameters. These results were presented in a conference paper and in a refereed publication. We also explored the possibility of using higher-order statistics, particularly the bispectrum and bicoherence, to distinguish between competing QPO models. These methods were applied to the hot spot model and a method was proposed to map out the spacetime metric around Kerr black holes in Ref. 3 below.

Bertschinger, Edmund↗
NASA NTRS2018

Lyman-Alpha Comes of Age

The Lyman-alpha (Lyα) line is the strongest recombination line of hydrogen, which constitutes the dominant share of normal matter in the Universe. Fifty-one years ago, it was predicted that Lyα emission should appear prominently in the spectra of young primordial galaxies. For the next 30 years, this prediction was tested by many surveys, but none found the predicted galaxies at the predicted luminosities. The blame for these non-detections was squarely cast on the resonant nature of the Lyα line, which requires Lyα photons to scatter off many hydrogen atoms before escaping from a galaxy. This can hugely increase the path that they traverse, and consequently also the probability that they will be absorbed by dust. This remained the dominant explanation of the missing high-redshift lines for decades, despite some dissenting opinions about the effect of dust. The spectra of nearby galaxies from the International Ultraviolet Explorer are inconclusive about what allows galaxies to be Lyα emitters. The ‘missing’ lines were finally detected 20 years ago, at flux levels much below the original predictions - a difference that we understand to be predominantly due to the hierarchical growth of galaxies, such that star-forming objects at high redshift are much less massive than anticipated in the original prediction. The discovery and study of Lyα galaxies has blossomed into an invaluable method for understanding the youngest stages of galaxy formation, the circumgalactic medium, and ultimately the epoch of reionization. The high equivalent widths of Lyα galaxies from the earliest high-redshift surveys show that they are low-mass galaxies dominated by young stellar populations. The Sakura CLAW workshop (which used the Twitter hashtag #SakuraCLAW) gathered more than 110 participants (Fig. 1) who showcased progress that pushed the frontiers of Lyα galaxy science in sample size, sensitivity, redshifts and physical understanding..

Malhotra, Sangeeta↗
NASA NTRS2019

Cyclotron Lines in Highly Magnetized Neutron Stars

Cyclotron lines, also called cyclotron resonant scattering features are spectral features, generally appearing in absorption, in the X-ray spectra of objects containing highly magnetized neutron stars, allowing the direct measurement of the magnetic field strength in these objects. Cyclotron features are thought to be due to resonant scattering of photons by electrons in the strong magnetic fields. The main content of this contribution focusses on electron cyclotron lines as found in accreting X-ray binary pulsars (XRBP) with magnetic fields on the order of several 1012 Gauss. Also, possible proton cyclotron lines from single neutron stars with even stronger magnetic fields are briefly discussed. With regard to electron cyclotron lines, we present an updated list of XRBPs that show evidence of such absorption lines. The first such line was discovered in a 1976 balloon observation of the accreting binary pulsar Hercules X-1, it is considered to be the first direct measurement of the magnetic field of a neutron star. As of today (end 2018), we list 35 XRBPs showing evidence of one ore more electron cyclotron absorption line(s). A few have been measured only once and must be confirmed (several more objects are listed as candidates). In addition to the Tables of objects, we summarize the evidence of variability of the cyclotron line as a function of various parameters (especially pulse phase, luminosity and time), and add a discussion of the different observed phenomena and associated attempts of theoretical modeling. We also discuss our understanding of the underlying physics of accretion onto highly magnetized neutron stars. For proton cyclotron lines, we present tables with seven neutron stars and discuss their nature and the physics in these objects.

Staubert, R.↗
NASA NTRS1986

Some atmospheric scattering considerations relevant to BATSE: A model calculation

The orbiting Burst and Transient Source Experiement (BATSE) will locate gamma ray burst sources by analysis of the relative numbers of photons coming directly from a source and entering its prescribed array of detectors. In order to accurately locate burst sources it is thus necessary to identify and correct for any counts contributed by events other than direct entry by a mainstream photon. An effort is described which estimates the photon numbers which might be scattered into the BATSE detectors from interactions with the Earth atmosphere. A model was developed which yielded analytical expressions for single-scatter photon contributions in terms of source and satellite locations.

Young, John H.↗
DOE OSTI2022

Towards a direct measurement of the quantum-vacuum Lagrangian coupling coefficients using two counterpropagating super-intense laser pulses

In this paper we will show that photon–photon collision experiments using extreme lasers can provide measurable effects giving fundamental information about the essence of QED, its Lagrangian. A possible scenario with two counterpropagating ultra-intense lasers for an experiment to detect scattering between optical photons is analyzed. We discuss the importance of the pulse widths and waists, the best scenario for overlapping the beams and signal detection, as well as ways to distinguish the signal from the noise. This would need a high-precision measurement, with control of temporal jitter and noise. We conclude that such experiment is barely feasible at 10 23 W cm -2 and very promising at 10 24 W cm -2

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2024

Probing axion-like particles at the Electron-Ion Collider

Abstract The Electron-Ion Collider (EIC), a forthcoming powerful high-luminosity facility, represents an exciting opportunity to explore new physics. In this article, we study the potential of the EIC to probe the coupling between axion-like particles (ALPs) and photons in coherent scattering. The ALPs can be produced via photon fusion and decay back to two photons inside the EIC detector. In a prompt-decay search, we find that the EIC can set the most stringent bound form a ≲ 20 GeV and probe the effective scales Λ ≲ 10 5 GeV. In a displaced-vertex search, which requires adopting an EM calorimeter technology that provides directionality, the EIC could probe ALPs withm a ≲ 1 GeV at effective scales Λ ≲ 10 7 GeV. Combining the two search strategies, the EIC can probe a significant portion of unexplored parameter space in the 0.2 a< 20 GeV mass range.

Physics↗