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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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NASA NTRS2007

The ROSSI X-Ray Timing Explorer: Capabilities, Achievements and Aims

The prime scientific objectives of the Rossi X-Ray Timing Explorer (RXTE) were the study of astrophysical compact objects: black holes (galactic and extragalactic), many types of neutron stars, and accreting white dwarfs. RXTE was successful in achieving its original observing objectives of large area and high time resolution observations with broadband (2-200 keV) spectra, scheduled flexibly enough to enable observations of targets of opportunity on any timescale greater than a few hours. These capabilities enabled qualitatively new discoveries about dynamical timescale phenomena related to neutron stars and black holes, phenomena which probe basic physics in the most extreme environments of gravity, density, and magnetic fields. RXTE has extended its lifetime by applying the proportional counter area selectively and maintains schedule flexibility by making use of the distribution of targets around the sky. Proposed future observations emphasize opportunity to discover and study additional millisecond pulsars, pursue the high frequency quasi-periodic oscillations in black hole transients, and connect high frequency phenomena with longer term characteristics. RXTE will continue to strongly support, for both galactic and extragalactic targets, combining RXTE observations with other wavelengths (from IR to TeV) or with other capabilities, such as high spectral resolution.

Swank, J. H.↗
NASA NTRS1992

The origin of planets orbiting millisecond pulsars

A model for the formation of planets around millisecond pulsar which no longer have stellar companions is suggested. Detailed hydrodynamical models are presented which suggest that planet formation can occur either in a low-mass X-ray binary progenitor to a progenitor of a star-vaporizing millisecond pulsar when the neutron star is accreting material driven off its companion by X-ray irradiation or after a pulsar has formed and is vaporizing its companion. In both cases a circumbinary disk is created in which planets can form on a timescale of 10 exp 5 to 10 exp 6 yrs and the planets can survive a second phase in which the companion star moves toward the pulsar and is completely vaporized.

Tavani, Marco↗
NASA NTRS1994

Soft X-ray observations of the interacting galaxies NGC 1808 and NGC 1792

The soft X-ray emission from both galaxies NGC 1808 and NGC 1792, which we investigated using the ROSAT HRI and Position Sensitive Proportional Counter (PSPC), comes most probably from X-ray binaries and/or from hot ionized gas in powerful supernovae and supernova remnants. The distribution of the soft X-ray emission in NGC 1808, which is very well correlated with the distribution of 'radio knots' in the central starburst, suggests that hot gas dominates the emission in the ROSAT band. This is consistent with the results of PSPC observations by Junkes et al. The total soft X-ray luminosity in the ROSAT band of NGC 1808 of 1.2 x 10(exp 41) ergs/s is relatively high compared with other nearby starburst galaxies. Soft X-ray emission of diffuse hot ionized gas that is associated with the outflow traced by the conspicuous dust filaments protruding from the plane has been detected. Its luminosity in the ROSAT band is greater than or equal to 3 x 10(exp 39) ergs/s, i.e., several percent of the total soft X-ray luminosity. Thus, NGC 1808 is another example for a 'superwind' galaxy. The soft X-ray radiation from NGC 1792 is more likely to be dominated by a population of high-mass X-ray binaries or young powerful supernovae which are associated with the high-level star formation going on in the very prominent H II regions along its spiral arms, with possibly an additional contribution of diffuse hot ionized gas. The soft X-ray luminosities of individual sources lie in the range of 5 x 10(exp 38) to 2.7 x 10(exp 39) ergs/s, thus exceeding by far the Eddington luminosity of an accreting neutron star. The peaks of some of these soft X-ray luminous sources are offset with respect to the H II regions by a few hundred parsecs. Accordingly, if the soft X-ray sources should originate from the H II regions, their relative velocities with respect to the ambient medium have to be as high as approximately 100 km/s.

Dahlem, Michael↗
NASA NTRS2006

Why Do T Tauri Disks Accrete?

Observations of T Tauri stars and young brown dwarfs suggest that the accretion rates of their disks scales roughly with the square of the central stellar mass. No dependence of accretion rate on stellar mass is predicted by the simplest version of the Gammie layered disk model, in which nonthermal ionization of upper disk layers allows accretion to occur via the magnetorotational instability. We show that a minor modification of Gaminie's model to include heating by irradiation from the central star yields a modest dependence of accretion on the mass of the central star. A purely viscous disk model could provide a strong dependence of accretion rate on stellar mass if the initial disk radius (before much viscous evolution has occurred) has a strong dependence on stellar mass. However, it is far from clear that at least the most massive pre-main-sequence disks can be totally magnetically activated by X-rays or cosmic rays. We suggest that a combination of effects are responsible for the observed dependence, with the lowest mass stars having the lowest mass disks, which can be thoroughly magnetically active, while the higher mass stars have higher mass disks that have layered accret,ion and relatively inactive or "dead" central zones at some radii. In such dead zones, we suggest that gravitational instabilities may play a role in allowing accretion to proceed. In this connection, we emphasize the uncertainty in disk masses derived from dust emission and argue that T Tauri disk masses have been systematically underestimated by conventional analyses. Furtlier study of accretion rates, especially in the lowest mass stars, would help to clarify the mechanisms of accretion in T Tauri stars.

Hartmann, Lee↗
NASA NTRS2008

Thermonuclear Burning as a Probe of Neutron Star

Thermonuclear fusion is a fundamental process taking place in the matter transferred onto neutron stars in accreting binary systems. The heat deposited by nuclear reactions becomes readily visible in the X-ray band when the burning is either unstable or marginally stable, and results in the rich phenomenology of X-ray bursts, superbursts, and mHz quasiperiodic oscillations. Fast X-ray timing observations with NASA's Rossi X-ray Timing Explorer (RXTE) over the past decade have revealed a wealth of new phenomena associated with thermonuclear burning on neutron stars, including the discovery of nuclear powered pulsations during X-ray bursts and superbursts. I will briefly review our current observational and theoretical understanding of these new phenomena, with an emphasis on recent findings, and discuss what they are telling us about the structure of neutron stars.

Strohmayer, Tod↗
NASA NTRS2012

Effect of Population III Multiplicity on Dark Star Formation

We numerically study the mutual interaction between dark matter (DM) and Population III (Pop III) stellar systems in order to explore the possibility of Pop III dark stars within this physical scenario. We perform a cosmological simulation, initialized at z approx. 100, which follows the evolution of gas and DM. We analyze the formation of the first mini halo at z approx. 20 and the subsequent collapse of the gas to densities of 10(exp 12)/cu cm. We then use this simulation to initialize a set of smaller-scale 'cut-out' simulations in which we further refine the DM to have spatial resolution similar to that of the gas. We test multiple DM density profiles, and we employ the sink particle method to represent the accreting star-forming region. We find that, for a range of DM configurations, the motion of the Pop III star-disk system serves to separate the positions of the protostars with respect to the DM density peak, such that there is insufficient DM to influence the formation and evolution of the protostars for more than approx. 5000 years. In addition, the star-disk system causes gravitational scattering of the central DM to lower densities, further decreasing the influence of DM over time. Any DM-powered phase of Pop III stars will thus be very short-lived for the typical multiple system, and DM will not serve to significantly prolong the life of Pop III stars.

Stacy, Athena↗
NASA NTRS2018

NICER Discovers mHz Oscillations in the "Clocked" Burster GS 1826-238

We report the discovery with the Neutron Star Interior Composition Explorer (NICER) of mHz X-ray brightness oscillations from the "clocked burster" GS 1826−238. NICER observed the source in the periods 2017 June 20–29, July 11–13, and September 9–15, for a total useful exposure of 34 ks. Two consecutive dwells obtained on 2017 September 9 revealed highly significant oscillations at a frequency of 8 mHz. The fractional, sinusoidal modulation amplitude increases from 0.7% at 1 keV to ≈2% at 6 keV. Similar oscillations were also detected at lower significance in three additional dwells. The oscillation frequency and amplitude are consistent with those of mHz QPOs reported in other accreting neutron star systems. A thermonuclear X-ray burst was also observed on 2017 June 22. The burst properties and X-ray colors are both consistent with GS 1826 being in a soft spectral state during these observations, findings that are confirmed by ongoing monitoring with MAXI and SWIFT-BAT. Assuming that the mHz oscillations are associated with blackbody emission from the neutron star surface, modeling of the phase-resolved spectra shows that the oscillation is consistent with being produced by modulation of the temperature component of this emission. In this interpretation, the blackbody normalization, proportional to the emitting surface area, is consistent with being constant through the oscillation cycle. We place the observations in the context of the current theory of marginally stable burning and briefly discuss the potential for constraining neutron star properties using mHz oscillations.

Strohmayer, T. E.↗
NASA NTRS2010

Spinning-Up: The Case of the Symbiotic X-Ray Binary 3A 1954+319

We present a timing and spectral analysis of the variable X-ray source 3A 1954+319, Our analysis is mainly based on an outburst serendipitously observed during INTEGRAL Key Program observations of the Cygnus region in 2008 fall and on the Swift/BAT longterm light curve, Previous observations, though sparse, have identified the source to be one of only nine known symbiotic X-ray binaries, i.e., systems composed of an accreting neutron star orbiting in a highly inhomogeneous medium around an M-giant companion. The spectrum of 3A 1954+319 above 20 keY can be best described by a broken power law model. The extremely long pulse period of approx.5.3 hours is clearly visible in the INTEGRAL/ISGRI light curve and confirmed through an epoch folding period search. Furthermore, the light curve allows us to determine a very strong spin up of -2x10(exp 4)h/h during the outburst. This spin up is confirmed by the pulse period evolution calculated from Swift/BAT data. The Swift/BAT data also show a long spin-down trend prior to the 2008 outburst. which is confirmed in archival INTEGRAL/ISGRI data. We discuss possible accretion models and geometries allowing for the transfer of such large amounts of angular momentum and investigate the harder spectrum of this outburst compared to previously published results.

Fuerst, F.↗
NASA NTRS2011

Spinning-Up: the Case of the Symbiotic X-Ray Binary 3A 1954+319

We present a timing and spectral analysis of the variable X-ray source 3A 1954+319. Our analysis is mainly based on an outburst serendipitously observed during INTEGRAL Key Program observations of the Cygnus region in 2008 fall and on the Swift/BAT longterm light curve. Previous observations, though sparse, have identified the source to be one of only nine known symbiotic X-ray binaries, i.e., systems composed of an accreting neutron star orbiting in a highly inhomogeneous medium around an M-giant companion. The spectrum of3A 1954+319 above > 20 keV can be best described by a broken power law model. The extremely long pulse period of approx.5.3 hours is clearly visible in the INTEGRAL/ISGRI light curve and confirmed through an epoch folding period search. Furthermore, the light curve allows us to determine a very strong spin up of -2 x 10(exp -4) h/h during the outburst. This spin up is confirmed by the pulse period evolution calculated from Swift/BAT data. The Swift/BAT data also show a long spin-down trend prior to the 2008 outburst, which is confirmed in archival INTEGRAL/ISGRI data. We discuss possible accretion models and geometries allowing for the transfer of such large amounts of angular momentum and investigate the harder spectrum of this outburst compared to previously published results.

Fuerst, F.↗
NASA NTRS2014

A Chandra-Swift View of Point Sources in Hickson Compact Groups: High AGN Fraction but a Dearth of Strong AGNs

We present Chandra X-ray point source catalogs for 9 Hickson Compact Groups (HCGs, 37 galaxies) at distances of 34-89 Mpc. We perform detailed X-ray point source detection and photometry and interpret the point source population by means of simulated hardness ratios. We thus estimate X-ray luminosities (L(sub x)) for all sources, most of which are too weak for reliable spectral fitting. For all sources, we provide catalogs with counts, count rates, power-law indices (gamma), hardness ratios, and L(sub X), in the full (0.5-8.0 keV), soft (0.5-2.0 keV), and hard (2.0-8.0 keV) bands. We use optical emission-line ratios from the literature to re-classify 24 galaxies as star-forming, accreting onto a supermassive black hole (AGNs), transition objects, or low-ionization nuclear emission regions. Two-thirds of our galaxies have nuclear X-ray sources with Swift/UVOT counterparts. Two nuclei have L(sub X),0.5-8.0 keV > 10(exp 42) erg s−1, are strong multi-wavelength active galactic nuclei (AGNs), and follow the known alpha OX-νLν (nearUV) correlation for strong AGNs. Otherwise, most nuclei are X-ray faint, consistent with either a low-luminosity AGN or a nuclear X-ray binary population, and fall in the 'non-AGN locus' in alpha OX-νLν (nearUV) space, which also hosts other normal galaxies. Our results suggest that HCG X-ray nuclei in high specific star formation rate spiral galaxies are likely dominated by star formation, while those with low specific star formation rates in earlier types likely harbor a weak AGN. The AGN fraction in HCG galaxies with MR (is) less than −20 and L(sub X),0.5-8.0 keV (is) greater than 10(exp 41) erg s−1 is 0.08+0.35 −0.01, somewhat higher than the ∼5% fraction in galaxy clusters.

X-rays: galaxies↗
NASA NTRS2012

A Suzaku View of Cyclotron Line Sources and Candidates

Seventeen accreting neutron star pulsars, mostly high mass X-ray binaries with half of them Be-type transients, are known to exhibit Cyclotron Resonance Scattering Features (CRSFs) in their X-ray spectra, with characteristic line energies from 10 to 60 keY. To date about two thirds of them, plus a few similar systems without known CRSFs, have been observed with Suzaku. We present an overview of results from these observations, including the discovery of a CRSF in the transient IA1118-61 and pulse phase resolved spectroscopy of OX 301-2. These observations allow for the determination of cyclotron line parameters to an unprecedented degree of accuracy within a moderate amount of observing time. This is important since these parameters vary - e.g., with orbital phase, pulse phase, or luminosity - depending on the geometry of the magnetic field of the pulsar and the properties of the accretion column at the magnetic poles. We briefly introduce a spectral model for CRSFs that is currently being developed and that for the first time is based on these physical properties. In addition to cyclotron line measurements, selected highlights from the Suzaku analyses include dip and flare studies, e.g., of 4U 1907+09 and Vela X-I, which show clumpy wind effects (like partial absorption and/or a decrease in the mass accretion rate supplied by the wind) and may also display magnetospheric gating effects.

Pottschmidt, K.↗
NASA NTRS2017

A Space-Based Observational Strategy for Characterizing the First Stars and Galaxies Using the Redshifted 21cm Global Spectrum

The redshifted 21 cm monopole is expected to be a powerful probe of the epoch of the first stars and galaxies(10 less than z less than 35). The global 21 cm signal is sensitive to the thermal and ionization state of hydrogen gas and thusprovides a tracer of sources of energetic photonsprimarily hot stars and accreting black holeswhich ionize andheat the high redshift intergalactic medium (IGM). This paper presents a strategy for observations of the globalspectrum with a realizable instrument placed in a low-altitude lunar orbit, performing night-time 40120 MHzspectral observations, while on the farside to avoid terrestrial radio frequency interference, ionospheric corruption,and solar radio emissions. The frequency structure, uniformity over large scales, and unpolarized state of theredshifted 21 cm spectrum are distinct from the spectrally featureless, spatially varying, and polarized emissionfrom the bright foregrounds. This allows a clean separation between the primordial signal and foregrounds. Forsignal extraction, we model the foreground, instrument, and 21 cm spectrum with eigenmodes calculated viaSingular Value Decomposition analyses. Using a Markov Chain Monte Carlo algorithm to explore the parameterspace defined by the coefficients associated with these modes, we illustrate how the spectrum can be measured andhow astrophysical parameters (e.g., IGM properties, first star characteristics) can be constrained in the presence offoregrounds using the Dark Ages Radio Explorer (DARE).

Burns, Jack O.↗
NASA NTRS2012

He II lambda-4686 in Eta Carinae: Collapse of the Wind-Wind Collision Region During Periastron Passage

The periodic spectroscopic events in Eta Carinae are now well established and occur near the periastron passage of two massive stars in a very eccentric orbit. Several mechanisms have been proposed to explain the variations of different spectral features, such as an eclipse by the wind-wind collision boundary, a shell ejection from the primary star or accretion of its wind onto the secondary. All of them have problems explaining all the observed phenomena. To better understand the nature of the cyclic events we performed a dense monitoring of Eta Carinae with 5 Southern telescopes during the 2009 low excitation event, resulting in a set of data of unprecedented quality and sampling. The intrinsic luminosity of the He II lambda-4686 emission line (L approx 310 solar L) just before periastron reveals the presence of a very luminous transient source of extreme UV radiation emitted in the wind-wind collision (WWC) region. Clumps in the primary's wind probably explain the flare-like behavior of both the X-ray and He II lambda-4686 light-curves. After a short-lived minimum, He II lambda-4686 emission rises again to a new maximum, when X-rays are still absent or very weak. We interpret this as a collapse of the WWC onto the "surface" of the secondary star, switching off the hard X-ray source and diminishing the WWC shock cone. The recovery from this state is controlled by the momentum balance between the secondary's wind and the clumps in the primary's wind.

Teodoro, M.↗
NASA NTRS2014

The Broad-Band X-Ray Spectrum of IC 4329A from a Joint NuSTAR/Suzaku Observation

We have obtained a deep, simultaneous observation of the bright, nearby Seyfert galaxy IC 4329A with Suzaku and NuSTAR (Nuclear Spectroscopic Telescope Array). Through a detailed spectral analysis, we are able to robustly separate the continuum, absorption, and distant reflection components in the spectrum. The absorbing column is found to be modest (approximately 6 times 10 (sup 21) per square centimeter), and does not introduce any significant curvature in the Fe K band. We are able to place a strong constraint on the presence of a broadened Fe K alpha line (E (sub rest) equals 6.46 (from plus 0.08 to minus 0.07) kiloelectronvolts with sigma equals 0.33 (from plus 0.08 to minus 0.07) kiloelectronvolts and EW equals 34 (from plus 8 to minus 7) electronvolts), though we are not able to constrain any of the parameters of a relativistic reflection model. These results highlight the range in broad Fe K line strengths observed in nearby, bright, active galactic nuclei (roughly an order of magnitude), and imply a corresponding range in the physical properties of the inner accretion disk in these sources. We have also updated our previously reported measurement of the high-energy cutoff of the hard X-ray emission using both observatories rather than just NuSTAR alone: E (sub cut) equals 186 plus or minus 14 kiloelectronvolts. This high-energy cutoff acts as a proxy for the temperature of the coronal electron plasma, enabling us to further separate this parameter from the plasmas optical depth and to update our results for these parameters as well. We derive kT 50 (from plus 6 to minus 3) kiloelectronvolts with tau equals 2.34 (from plus +0.16 to minus 0.11) using a spherical geometry, kT equals 61 plus or minus1 kiloelectronvolt with tau equals 0.68 plus or minus 0.02 for a slab geometry, with both having an equivalent goodness-of-fit.

Dynamics - stars↗
NASA NTRS1974

A new measurement of the Hercules X-1 X-ray pulse profile

Report on measurements of a triple-peaked 1.24-sec pulse profile in a 1-minute rocket-borne exposure to Her X-1, in contrast to the double-peaked profiles expected from models which maximize the X-ray emission at the magnetic equator of an accreting neutron star. The profile exhibits statistically significant energy dependence, with the emission greater than about 12 keV having narrower peaks which appear to lag (by approximately 5% of the pulse period) the corresponding peaks at lower energies. Approximately one-third of the total emission from the source is nonpulsed.

Holt, S. S.↗
NASA NTRS1974

A new measurement of the Her X-1 X-ray pulse profile

A triple peaked 1.24 sec. pulse profile in a 1-minute rocket borne exposure to Her X-1 was measured, in contrast to the doublepeaked profiles expected from models which maximize the X ray emission at the magnetic equator of an accreting neutron star. The profile exhibits statistically significant energy dependence, with the emission approximately greater than 12 keV having narrower peaks which lag (by approximately 5% of the pulse period) the corresponding peaks at lower energies. Approximately one third of the total emission from the source is nonpulsed.

Holt, S. S.↗
NASA NTRS1975

The SAS-3 X-ray observatory

The experiment section of the Small Astronomy Satellite-3 (SAS-3) launched in May 1975 is an X-ray observatory intended to determine the location of bright X-ray sources to an accuracy of 15 arc-seconds; to study a selected set of sources over a wide energy range, from 0.1 to 55 keV, while performing very specific measurements of the spectra and time variability of known X-ray sources; and to monitor the sky continuously for X-ray novae, flares, and unexpected phenomena. The improvements in SAS-3 spacecraft include a clock accurate to 1 part in 10 billion, rotatable solar panels, a programmable data format, and improved nutation damper, a delayed command system, improved magnetic trim and azimuth control systems. These improvements enable SAS-3 to perform three-axis stabilized observations of any point on the celestial sphere at any time of the year. The description of the experiment section and the SAS-3 operation is followed by a synopsis of scientific results obtained from the observations of X-ray sources, such as Vela X-1 (supposed to be an accreting neutron star), a transient source of hard X-ray (less than 36 min in duration) detected by SAS-3, the Crab Nebula pulsar, the Perseus cluster of galaxies, and the Vela supernova remnant.

Mayer, W. F.↗
NASA NTRS1979

Highly compact binary X-ray sources

It is proposed that many of the X-ray sources associated with the galactic bulge are accreting neutron stars or black holes with masses of at least about 1 solar mass in ultrashort-period binary systems with very low-mass (no more than about 0.5 solar mass) stellar companions. It is demonstrated that this hypothesis may provide a coherent picture of the evolutionary history and observational properties of these sources, including the apparent lack of conspicuous optical counterparts and the apparent absence of X-ray eclipses.

Joss, P. C.↗