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At least 775 records · Page 43
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NASA NTRS1993

Quasar X-ray spectra revisited

A sample of 45 quasars observed by the IPC on the Einstein satellite is used to reexamine the relationship of the soft X-ray energy index with radio properties and the optical Fe II emission. The tendency for radio-loud quasars to have systematically flatter X-ray energy indices than radio-quiet quasars is confirmed with the soft X-ray excess having negligible effect. There is a tendency for the flatness of the X-ray slope to correlate with radio core dominance for radio-loud quasars, suggesting that a component of the X-ray emission is relativistically beamed. For the radio-quiet quasars, the soft X-ray energy indices with a mean of about 1.0 are consistent with the indices found at higher energies, although steeper than those observed for Seyfert 1 galaxies where the reflection model gives a good fit to the data. The correlation of Fe II emission line strength with X-ray energy index is confirmed for radio-quiet quasars using a subset of 18 objects. The radio-loud quasars show no evidence for a correlation. This relation suggests a connection between the ionizing continuum and line emission from the broad emission-line region (BELR) of radio-quiet quasars, but in the opposite sense to that predicted by current photoionization models. The correlations of X-ray slope with radio core dominance and Fe II equivalent width within the radio-loud and radio-quiet subclasses, respectively, imply that the observed wide range of X-ray energy indices is real rather than due to the large measuring uncertainties for individual objects.

Shastri, P.↗
NASA NTRS2022

Solar Flare Heating with Turbulent Suppression of Thermal Conduction

During solar flares plasma is typically heated to very high temperatures, and the resulting redistribution of energy via thermal conduction is a primary mechanism transporting energy throughout the flaring solar atmosphere. The thermal flux is usually modeled using Spitzer’s theory, which is based on local Coulomb collisions between the electrons carrying the thermal flux and those in the background. However, often during flares, temperature gradients become sufficiently steep that the collisional mean free path exceeds the temperature gradient scale size, so that thermal conduction becomes inherently non-local. Further, turbulent angular scattering, which is detectable in nonthermal widths of atomic emission lines, can also act to increase the collision frequency and so suppress the heat flux. Recent work by Emslie & Bian (2018) extended Spitzer’s theory of thermal conduction to account for both non-locality and turbulent suppression. We have implemented their theoretical expression for the heat flux (which is a convolution of the Spitzer flux with a kernel function) into the RADYN flare-modeling code and performed a parameter study to understand how the resulting changes in thermal conduction affect flare dynamics and hence the radiation produced. We find that models with reduced heat fluxes predict slower bulk flows, less intense line emission, and longer cooling times. By comparing features of atomic emission lines predicted by the models with Doppler velocities and nonthermal line widths deduced from a particular flare observation, we find that models with suppression factors between 0.3 to 0.5 relative to the Spitzer value best reproduce observed Doppmission lines forming over a wide range of temperatures. Interestingly, the model that best matches observed nonthermal line widths has a kappa-type velocity distribution function.

Joel C. Allred↗
NASA NTRS2004

Black Hole Demographics in and Nuclear Properties of Nearby Low Luminosity Radio Galaxies; Connections to Radio Activity?

We combine the results of an HST STIS and WFPC study of a complete sample of 21 nearby UGC low luminosity radio galaxies with the results of a radio VLA and VLBA study of the same sample. We examine the relationship between the stellar and gaseous properties of the galaxies on tens to hundreds of parsec scale with the properties of the radio jets on the same scale. From the VLA and VLBA data we constrain the physics of the outflowing radio plasma from the tens of parsecs to hundreds of kiloparsec scales. From the WFPC2 H alpha and dust images and the STIS kinematics of the near nuclear gas we obtain constraints on the orientation of near nuclear disks of gas and measures of the nuclear stellar, continuum point source, and line emission fluxes. Under the statistically supported assumption that the radio jet issues perpendicular to the disk, we use the orientation of the optical (large scale accretion?) disks to constrain the three-dimensional orientation of the radio ejection. From HST/STIS spectroscopy of the near-nuclear emission line gas we obtain measures/limits on the black hole masses. We examine correlations between the VLBA and VLA-scale radio emission, the nuclear line emission, and the nuclear optical and radio continuum emission. Though our sample is relatively small, it is uniquely well defined, spans a narrow range in redshift and we have a consistent set of high resolution data with which to carefully examine these relationships. We use the combined radio and optical data to: 1) Constrain the orientation, physics, and bulk outflow speed of the radio plasma; 2) Put limits on the mass accretion rate and study the relationship between black hole mass, radio luminosity, and near nuclear gaseous content; 3) Provide insight into the relationship between BL Lac objects and low luminosity radio galaxies.

Baum, S. A.↗
NASA NTRS1988

The spatial distribution of ultraviolet line and continuum emission in Herbig-Haro objects

Archival IUE data and monochromatic CCD images in the optical range are used to compare the spatial distribution of UV and optical emission in HH 1, HH 2, HH 24, HH 32, HH 43, and HH 47. For all six objects, the observed UV radiation is shown to originate in the objects themselves. The results indicate that the C IV and semiforbidden emission-line regions are small. Although the continuum in the IUE short-wavelength range displays a distribution that is broader than that of any measured line emission in the UV or optical range, the continuum distribution in the IUE long-wavelength range is quite narrow.

Lee, M. G.↗
NASA NTRS1992

On the nature of the symbiotic binary AX Persei

Photometric and spectroscopic observations of the symbiotic binary AX Persei are presented. This system contains a red giant that fills its tidal lobe and transfers material into an accretion disk surrounding a low-mass main-sequence star. The stellar masses - 1 solar mass for the red giant and about 0.4 solar mass for the companion - suggest AX Per is poised to enter a common envelope phase of evolution. The disk luminosity increases from L(disk) about 100 solar luminosity in quiescence to L(disk) about 5700 solar luminosity in outburst for a distance of d = 2.5 kpc. Except for visual maximum, high ionization permitted emission lines - such as He II - imply an EUV luminosity comparable to the disk luminosity. High-energy photons emitted by a hot boundary layer between the disk and central star ionize a surrounding nebula to produce this permitted line emission. High ionization forbidden lines form in an extended, shock-excited region well out of the binary's orbital plane and may be associated with mass loss from the disk.

Mikolajewska, Joanna↗
NASA NTRS1992

Chemistry in circumstellar disks - CS toward HL Tauri

High spatial and spectral resolution dust continuum and CS line-emission maps of the material immediately surrounding HL Tauri are presented. The 3.06-thermal dust map gives an upper limit to the source size of roughly 200 AU. When combined with previous millimeter continuum measurements, a beta value of 1.2 +/-0.3 is derived along with a disk mass of 0.1 solar mass, in agreement with previous disk radiative transfer models of the IR and submillimeter flux from HL Tau. The low dust emissivity index as compared to particles in the diffuse ISM is best explained by grain growth or composition evolution. The present observations of the CS and CO emission lines forming the molecular cloud surrounding HL Tauri are consistent with the 'canonical' temperatures, densities, and abundances derived in other Taurus cloud cores. In contrast, the CS emission in aperture synthesis maps at 650-AU spatial resolution is most consistent with CS/CO ratios at least 25-50 times lower in the gas immediately surrounding HL Tauri if the CS emission is optically thin. The continuum and spectral line results obtained are strong indicators of conditions thought to be similar to those in the presolar nebula some 4.5 AE ago.

Blake, Geoffrey A.↗
NASA NTRS2008

The First Detection of [O IV] from an Ultraluminous X-ray Source with Spitzer: Evidence of High Unbeamed Luminosity in Holmberg II ULX

We present the first Spitzer Infrared Spectrograph (IRS) observations of the [O IV] 25.89 micron emission line detected from the ultraluminous X-ray source (ULX) in Holmberg II. This line is a well established signature of high excitation, usually associated with AGN. Its detection suggests that the ULX has a strong impact on the surrounding gas. A Spitzer high resolution spectral map shows that the [O IV] is coincident with the X-ray position of the Holmberg II ULX. We find that the luminosity and the morphology of the line emission is consistent with photoionization by the soft X-ray and far ultraviolet (FUV) radiation from the accretion disk of the binary system and inconsistent with narrow beaming. We show that the emission nebula is radiation bounded both in the line of sight direction and to the west, and probably matter bounded to the east. Evidence for a massive black hole (BH) in this ULX is mounting. Detailed photoionization models favor an intermediate mass black hole of at least 85 Solar Mass as the ionization source for the [OIV] emission. We find that the spectral type of the companion star strongly affects the expected strength of the [O IV] emission. This finding could explain the origin of [O IV] in some starburst galaxies containing black hole binaries.

Berghea, C. T.↗
NASA NTRS2019

A New Class of Changing-look LINERs

We report the discovery of six active galactic nuclei (AGNs) caught "turning on" during the first nine months of the Zwicky Transient Facility (ZTF) survey. The host galaxies were classified as low-ionization nuclear emission-line region galaxies (LINERs) by weak narrow forbidden line emission in their archival SDSS spectra, and detected by ZTF as nuclear transients. In five of the cases, we found via follow-up spectroscopy that they had transformed into broad-line AGNs, reminiscent of the changing-look LINER iPTF16bco. In one case, ZTF18aajupnt/AT2018dyk, follow-up Hubble Space Telescope ultraviolet and ground-based optical spectra revealed the transformation into a narrow-line Seyfert 1 with strong [Fe vii, x, xiv] and He ii λ 4686 coronal lines. Swift monitoring observations of this source reveal bright UV emission that tracks the optical flare, accompanied by a luminous soft X-ray flare that peaks ~60 days later. Spitzer follow-up observations also detect a luminous mid-infrared flare, implying a large covering fraction of dust. Archival light curves of the entire sample from CRTS, ATLAS, and ASAS-SN constrain the onset of the optical nuclear flaring from a prolonged quiescent state. Here we present the systematic selection and follow-up of this new class of changing-look LINERs, compare their properties to previously reported changing-look Seyfert galaxies, and conclude that they are a unique class of transients well-suited to test the uncertain physical processes associated with the LINER accretion state.

Sara Frederick↗
NASA NTRS2020

Imagery and UV Spectroscopy of the LMC Supernova Remnant N103B Using HST

We present Hubble Space Telescope(HST)/Wide Field Camera 3 multiband imagery of N103B, which is the remnant of a SN Ia in the Large Magellanic Cloud, as well as HST/Cosmic Origins Spectrograph(COS)ultraviolet spectroscopy of the brightest radiatively shocked region. The images show a wide range of morphology and relative emission-line intensities, from smooth Balmer-line dominated collisionless shocks that are due to the primary blast wave to clumpy radiative shock filaments that are due to secondary shocks in density enhancements. The COS data show strong FUV line emissions, despite a moderately high extinction along this line of sight. We use the COS data with previous optical spectra to constrain the shock conditions, we refine the abundance analysis, and we find abundances that are typical of the local interstellar medium within the uncertainties. Under an assumption that the material being shocked was shed from the pre-supernova system, this finding places constraints on any significant enrichment in that material, and thus on the non-degenerate star in what was presumably a single-degenerate SN Ia.

William P. Blair↗
NASA NTRS2022

The Case for Comprehensive Spectroscopic Measurements of the Sun: Understanding Solar Flares and Coronal Heating

We discuss spectroscopic capabilities needed to resolve two important unresolved questions in solar physics, namely, ``What heats the solar corona?" and ``What causes the sudden, rapid release of energy that produces flares?" Spectroscopic capabilities needed to answer these questions include: (1) high spectral resolution, to enable measurement of unblended profiles of numerous emission lines; (2) high sensitivity with a wide dynamic range, to enable detection of faint line emission as well as weak wing components; (3) comprehensive temperature coverage, observing lines formed at log T separated by 0.1 or 0.2 dex between at least 20,000 K and 20 MK; (4) an absolute wavelength scale, to enable Doppler velocity measurements accurate to within 1 km/s or better; (5) simultaneous measurements over spatial area comparable in size to active regions (4' x 4'); (6) spatial resolution sufficient to resolve loop strands and the immediate vicinities of reconnection sites (<1''); (7) cadence short enough to monitor evolution of the solar atmosphere during periods of most rapid change (<1 s).

Jeffrey W Brosius↗
DOE OSTI2024

Filterscope Techniques for Parasitic Signal Screening

Here, the filterscope diagnostic uses bandpass filters and photomultiplier tubes (PMTs) to detect specific spectral emission lines. A filterscope was used to measure the W I 400.88 nm line emission as a function of ion energy on the Radio Frequency Plasma Interaction Experiment (RF PIE) for the purpose of assessing W erosion on plasma-facing components (PFCs). Different filter techniques are being explored and compared in order to effectively screen out nearby impurity lines, like the Ar II 401.39 nm line. The effectiveness of these techniques is determined by comparing the measurements to a high-resolution 1.0 m Czerny–Turner spectrometer with 0.012 nm spectral resolution. The ability to filter out nearby impurity emissions is useful when imaging PFCs in fusion devices including divertor and antenna guard limiters. Initial results with a helium plasma show little difference between the two techniques at higher bias voltages. In a helium plasma at lower dc bias voltages, a two-filter technique with filters at two separate wavelengths was shown to be more effective at screening out background signals. Data collected with an argon plasma however show the technique with overlapping filters on the line of interest is a closer match to spectrometer data at lower dc bias voltages.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
NASA NTRS1984

Spectral formation in compact X-ray sources

The gas flows responsible for X-ray spectral emission in compact X-ray sources are complex and likely to be optically thick. In galactic sources these flows include: stellar wind, accretion disk and corona, magnetosphere, and shock at the compact object surface. Active galactic nuclei are surrounded by many dense clouds responsible for the broad optical and UV emission lines. The nature of these flows can be discovered through observations of their X-ray spectra and time variability. Theoretical models for the formation of line and continuum spectra in these sources are described, emphasizing atomic photoabsorption, line emission, and Comptonization. Current observations of X-ray and UV spectra of compact galactic sources and active galactic nuclei are interpreted in terms of these models. Great advances in the understanding of these systems will result from future missions capable of observing their spectra at higher resolution. Observations of the complex of iron K-alpha lines at 6.4-6.7 keV will be particularly valuable.

Mccray, R.↗
NASA NTRS2018

New Constraints on the Geometry and Kinematics of Matter Surrounding the Accretion Flow in X-Ray Binaries from Chandra High-Energy Transmission Grating X-Ray Spectroscopy

The narrow, neutral Fe Kα fluorescence emission line in X-ray binaries (XRBs) is a powerful probe of the geometry, kinematics, and Fe abundance of matter around the accretion flow. In a recent study it has been claimed, using Chandra High-Energy Transmission Grating (HETG) spectra for a sample of XRBs, that the circumnuclear material is consistent with a solar-abundance, uniform, spherical distribution. It was also claimed that the Fe Kα line was unresolved in all cases by the HETG. However, these conclusions were based on ad hoc models that did not attempt to relate the global column density to the Fe Kα line emission. We revisit the sample and test a self-consistent model of a uniform, spherical X-ray reprocessor against HETG spectra from 56 observations of 14 Galactic XRBs. We find that the model is ruled out in 13/14 sources because a variable Fe abundance is required. In two sources a spherical distribution is viable, but with nonsolar Fe abundance. We also applied a solar-abundance Compton-thick reflection model, which can account for the spectra that are inconsistent with a spherical model, but spectra with a broader bandpass are required to better constrain model parameters. We also robustly measured the velocity width of the Fe Kα line and found FWHM values of up to approx. 5000 km/s. Only in some spectra was the Fe Kα line unresolved by the HETG.

Tzanavaris, P.↗
DOE OSTI2021

Long duration x-ray source development for x-ray diffraction at the National Ignition Facility

We present the results of experiments to produce a 10 ns-long, quasi-monochromatic x-ray source. This effort is needed to support time-resolved x-ray diffraction (XRDt) measurements of phase transitions during laser-driven dynamic compression experiments at the National Ignition Facility. To record XRDt of phase transitions as they occur, we use high-speed (∼1 ns) gated hybrid CMOS detectors, which record multiple frames of data over a timescale of a few to tens of ns. Consequently, to make effective use of these imagers, XRDt needs the x-ray source to be narrow in energy and uniform in time as long as the sensors are active. The x-ray source is produced by a laser irradiated Ge foil. Our results indicate that the x-ray source lasts during the whole duration of the main laser pulse. Both time-resolved and time-integrated spectral data indicate that the line emission is dominated by the He-α complex over higher energy emission lines. Time-integrated spectra agree well with a one-dimensional Cartesian simulation using HYDRA that predicts a conversion efficiency of 0.56% when the incident intensity is 2 × 1015 W/cm2 on a Ge backlighter.

Werellapatha, K. (ORCID:0000000293154054)↗
DOE OSTI2020

Periodic Eruptive Variability of the Isolated Pre-main-sequence Star V347 Aurigae

V347 Aurigae is associated with the small dark cloud L1438 and appears to be an isolated pre-main-sequence star located at distance d ≈ 200 pc. Multi-epoch, archival photometry reveals periodic brightness variations with amplitude V ≈ 2.0 mag occurring on timescales of ~160 days that have persisted for decades. Regular-cadence, optical imaging of the source with the Zwicky Transient Facility shows that a small reflection nebula illuminated by V347 Aur also fluctuates in brightness, at times fading completely. Multi-epoch, Keck/HIRES data suggests the presence of two distinct spectral components: a prominent emission-line-dominated spectrum with a heavily veiled continuum correlated with the bright photometric state, and an M-type absorption line spectrum associated with quiescence. All spectra exhibit strong Balmer and He i line emission, consistent with accretion, as well as high velocity emission arising from the forbidden transitions of [O i], [N ii], and [S ii] that are generally associated with collimated jets and disk winds. There is no evidence in existing high-dispersion spectroscopy or high-resolution imaging for binarity of V347 Aur. The repeating outburst events are possibly linked to accretion instabilities induced by an undetected companion or a structure within the circumstellar disk that periodically increases the mass accretion rate. V347 Aur is perhaps analogous to an EXor-type variable, though more regularly recurring.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1973

The OSO-7 year of discovery.

The OSO 7 scientific findings reveal hitherto unknown processes in solar flares and some remarkable properties of the corona. Gamma-ray emission lines provide evidence of nuclear reactions in solar flares. The measurements were made during the large flares of Aug. 4 and 7, 1972. The data clearly show line emission at energies of about 0.5 MeV (million electron volts) and 2.2 MeV, with weaker enhancements at 4.4 MeV and 6.1 MeV. It appears that the nuclear processes in flares as revealed by OSO 7 are probably thermal reactions. Some flare electron streams also produce bursts of hard X rays. Holes and poles in the corona, and coronal streams and the interplanetary blast were studied.

Maran, S. P.↗
NASA NTRS1978

C III density diagnostics in nonequilibrium plasmas

Mass flow through a temperature gradient can produce substantial departures of the ionization states from their equilibrium values and can change the character of line emission. The effect of such flows on extreme ultraviolet intensities in the solar atmosphere is evaluated for the beryllium-sequence ion C III, which is an easily observed and powerful diagnostic of electron density in a collisionally dominated plasma. A downflow of material in a transition region can lead to an underestimate of the electron density; outflow of material, as in a stellar or solar wind passing from chromospheric to coronal temperatures, yields a reliable density diagnostic. Ambiguities in the analysis of ultraviolet emission lines can be circumvented by measurement of several transitions.

Raymond, J. C.↗
NASA NTRS1980

Detection of a new extended soft X-ray source H1538-32 - A possible old supernova remnant

The discovery in the Lupus region of a new, extended soft X-ray source, H1538-32, is reported, having a distance of approximately 340 pc, and a luminosity of 1 to 2 x 10 to the 34th ergs/sec. The observed energy spectrum of the source is well fitted either by a thermal bremsstrahlung spectrum with Gaunt factor but without line emission, or by a coronal plasma model which includes the X-ray emission lines of various elements and the continuum as outlined by Raymond and Smith (1977). On the basis of the extended nature of the source and its thermal spectrum, it is suggested that H1538-32 may be an old supernova remnant.

Riegler, G. R.↗