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At least 361 records · Page 20

Evidence of circumstellar matter surrounding the Hercules X-1 system

We analyze data from two eclipse ingresses of Her X-1 observed with Ginga on 1989 April 30 and May 19. These observations occur, respectively, during the MAIN HIGH and SHORT HIGH states in the 35 day modulation of Her X-1 intensity. We find significant residual X-ray flux during eclipse, with a gradual decrease in flux following the occultation of the neutron star by the atmosphere of HZ Her. During the central part of the eclipse the count rate becomes nearly constant, at 0.5 mCrab in the energy range 1.7-36.8 keV. From a spectral analysis of the residual emission during the total eclipse of the central source in the MAIN HIGH state, we determine the energy spectral index, alpha = 0.8, similar to that before eclipse. A remarkable feature of the eclipse spectrum is that it does not show a significant iron line feature in contrast to massive wind-fed pulsars, such as Vela X-1 and Cen X-3. From a timing analysis of the same eclipse data, we show that there are no pulses. These results imply that the emission comes from the scattering of continuum X-rays by material in a region considerably larger than the companion star. An extended accretion disk corona may be responsible for this scattering. However, partial eclipse of an extended accretion disk corona may be responsible for this scattering. However, partial eclipse of an extended accretion disk corona is insufficient to account for the count rates in mid-eclipse, when known parameters of the binary system are used. Based on the present results, we suggest that scattering occurs not only in the accretion disk corona but also in the circumstellar matter surrounding the system of Her X-1/HZ Her.

Choi, C. S.↗

Prospects for experimental research on black holes in binary systems

Cygnus X-1, the single widely accepted example of a black hole in a binary system, is characterized by unusual X-ray properties. The X-ray spectrum of Cygnus X-1 is not cut off above 20 keV, as in most strong X-ray sources. Recent scintillation counter measurements reveal a power law spectrum extending from 40 to 200 keV with a photon spectral index of approximately 2.2. However, it is not clear that these and other X-ray properties of the system are related to the black-hole nature of Cygnus X-1. It is suggested that without a direct test to show that the mass of the compact object in other systems similar to Cygnus X-1 (Circinus X-1 and GX339-4) exceeds the limit of the neutron star mass, a better understanding of the accretion disk phenomenon must be achieved to demonstrate how the properties peculiar to these systems are related to the black hole nature of the compact object. Current accretion disk models are examined, including the alpha-accretion disk and two-temperature accretion disk models.

Long, K. S.↗

The Soft State of Cygnus X-1 Observed with NuSTAR: A Variable Corona and a Stable Inner Disk

We present a multi-epoch hard X-ray analysis of Cygnus X-1 in its soft state based on four observations with the Nuclear Spectroscopic Telescope Array (NuSTAR). Despite the basic similarity of the observed spectra, there is clear spectral variability between epochs. To investigate this variability, we construct a model incorporating both the standard disk-corona continuum and relativistic reflection from the accretion disk, based on prior work on Cygnus X-1, and apply this model to each epoch independently. We find excellent consistency for the black hole spin and the iron abundance of the accretion disk, which are expected to remain constant on observational timescales. In particular, we confirm that Cygnus X-1 hosts a rapidly rotating black hole, 0.93 < approx. a* < approx. 0.96, in broad agreement with the majority of prior studies of the relativistic disk reflection and constraints on the spin obtained through studies of the thermal accretion disk continuum. Our work also confirms the apparent misalignment between the inner disk and the orbital plane of the binary system reported previously, finding the magnitude of this warp to be approx.10deg-15deg. This level of misalignment does not significantly change (and may even improve) the agreement between our reflection results and the thermal continuum results regarding the black hole spin. The spectral variability observed by NuSTAR is dominated by the primary continuum, implying variability in the temperature of the scattering electron plasma. Finally, we consistently observe absorption from ionized iron at approx. 6.7 keV, which varies in strength as a function of orbital phase in a manner consistent with the absorbing material being an ionized phase of the focused stellar wind from the supergiant companion star.

black hole physics↗

The evolution of young stellar object disks and their environment

The main efforts were directed towards determining the frequency of disk occurrence and the timescales for disk evolution for solar-type and intermediate mass stars. The results of the investigation showed that optically thick disks are accretion disks. The projected accomplishments are also discussed.

Strom, Stephen E.↗

The dusty envelopes of FU Orionis variables

The spectral energy distributions of five FU Orionis variables are discussed using new ground-based infrared photometry and IRAS Addscan measurements. V1057 Cyg, V1515 Cyg, and Z CMa have much larger far-infrared fluxes than can be explained by the simple accretion disk models that match their spectral energy distributions less than 10 microns. The 10-20-micron excess emission in V1057 has decreased proportionately with the decline in optical light, which demonstrates that the far-infrared flux arises in a dust shell reprocessing light from the central object. It is suggested that the far-infrared excesses arise in somewhat flattened dusty envelopes with a covering factor in a solid angle of roughly one-half, which is consistent with the fraction of FU Orionis variables that are heavily embedded. It is proposed that the dusty envelope falls onto the outer disk at a rate of about 5 x 10 exp -6 solar mass/yr. The rate is sufficient to replenish disk material accreted during a FU Orionis outburst and allow repetitive eruptions on time scales of several thousand years.

Kenyon, Scott J.↗

Research on the Nature of Accretion in Active Galactic Nuclei

he purpose of this grant was to support theoretical research on the nature of accretion in active galactic nuclei. In the brief time of the award (one year), four papers that appeared in refereed journals were written, as well as two invited reviews in conference proceedings These papers significantly advanced our understanding of the structure of the most important parts of bright accretion disks around accreting black holes, such as active galactic nuclei.

Krolik, J. H.↗

What is the Nature of Accretion in Active Galactic Nuclei?

The purpose of this grant was to support theoretical research on the nature of accretion in active galactic nuclei. In the brief time of the award, four papers that appeared in refereed journals were written, as well as two invited reviews in conference proceedings. These papers significantly advanced our understanding of the structure of the most important parts of bright accretion disks around accreting black holes, such as active galactic nuclei.

Krolik, Julian H.↗

FRESIP: A Discovery Mission Concept To Find Earth-Sized Planets Around Solar Like Stars

The current nebular theory postulates that planets are. a consequence of the formation of stars from viscous accretion disks. Condensation from the accretion disk favors the formation of small rocky planets in the hot inner region, and the formation of gas giants in the cool outer region. Consequently, terrestrial-type planet in inner orbits should be commonplace. From geometrical considerations , Borucki and Summers have shown that 1% of planetary systems resembling our solar system should show transits for Earth-sized (or larger) planets. Thus a photometric satellite that uses a wide field of view telescope and a large detector array to simultaneously monitor 5000 target stars should detect 50 planetary systems. To differentiate regularly recurring transits from statistical fluctuations of the stellar flux, one must observe over several orbital periods so that the false positive rate can be reduced to one event or less. A one-meter aperture telescope placed in a halo orbit about either the L1 or L2 Lagrange points and viewing perpendicular to both the orbital and ecliptic planes can view continuously for the required period because neither the Sun, Earth, or Moon would enter the field of view. Model calculations show that the observations should provide statistically significant estimates of the distributions of planetary size, orbital radius, coplanarity, and the frequency of planetary systems that have Earth-sized planets in inner orbits. Because approximately one half of the star systems observed will be binary systems, the frequency of planetary systems orbiting either one or both of the stars can also be determined.

Borucki, William↗

General Relativistic Radiative Transfer: Applications to Black-Hole Systems

We present general relativistic radiation transfer formulations which include opacity effects due to absorption, emission and scattering explicitly. We consider a moment expansions for the transfer in the presence of scattering. The formulation is applied to calculation emissions from accretion and outflows in black-hole systems. Cases with thin accretion disks and accretion tori are considered. Effects, such as emission anisotropy, non-stationary flows and geometrical self-occultation are investigated. Polarisation transfer in curved space-time is discussed qualitatively.

Wu, Kinwah↗

Reflection Modeling of the Black Hole Binary 4U 1630–47: The Disk Density and Returning Radiation

We present the analysis of X-ray observations of the black hole binary 4U 1630−47 using relativistic reflection spectroscopy. We use archival data from the Rossi X-ray Timing Explorer, Neil Gehrels Swift Observatory, and Nuclear Spectroscopic Telescope Array observatories, taken during different outbursts of the source between 1998 and 2015. Our modeling includes two relatively new advances in modern reflection codes: high-density disks, and returning thermal disk radiation. Accretion disks around stellar-mass black holes are expected to have densities well above the standard value assumed in traditional reflection models (i.e., n(e) ∼ 10^(15) per cu. cm). New high-density reflection models have important implications in the determination of disk truncation (i.e., the disk inner radius). This is because one must retain self-consistency in the irradiating flux and corresponding disk ionization state, which is a function of disk density and system geometry. We find that the disk density is n(e) ≥ 10^(20) per cu. cm across all spectral states. This density, combined with our constraints on the ionization state of the material, implies an irradiating flux impinging on the disk that is consistent with the expected theoretical estimates. Returning thermal disk radiation—the fraction of disk photons that bend back to the disk producing additional reflection components—is expected predominantly in the soft state. We show that returning radiation models indeed provide a better fit to the soft-state data, reinforcing previous results that show that in the soft state, the irradiating continuum may be blackbody emission from the disk itself.

Riley M. T. Connors↗

An instability associated with a magnetosphere-disk interaction

The evolution of a thin accretion disk surrounding a rapidly rotating magnetosphere is considered. By taking account of the variations of the magnetospheric boundary in response to the conditions at the inner edge of the disk, we find from linear analysis and numerical computation that the accretion disk can become unstable. Mass can be accreted by the central object in a cyclic fashion, with the cycle involving the storage and release of mass in the inner parts of the disk. The physical origin of the instability is associated with the variations of the magnetospheric boundary about corotation. The recurrence time scale of the cycle can vary by several orders of magnitude depending on the details of the conditions at the magnotosphere. The possible applicability of this instability process to the 'rapid burster' MXB 1730-335 is briefly discussed.

Spruit, H. C.↗

Magnetocentrifugally driven flows from young stars and disks. 1: A generalized model

We propose a generalized model for stellar spin-down, disk accretion, and truncation, and the origin of winds, jets, and bipolar outflows from young stellar objects. We consider the steady state dynamics of accretion of matter from a viscous and imperfectly conducting disk onto a young star with a strong magnetic field. For an aligned stellar magnetosphere, shielding currents in the surface layers of the disk prevent stellar field lines from penetrating the disk everywhere except for a range of radii about pi = R(sub x), where the Keplerian angular speed of rotation Omega(sub x) equals the angular speed of the star Omega(sub *). For the low disk accretion rates and high magnetic fields associated with typical T Tauri stars, R(sub x) exceeds the radius of the star R(sub *) by a factor of a few, and the inner disk is effectively truncated at a radius R(sub t) somewhat smaller than R(sub x). Where the closed field lines between R(sub t) and R(sub x) bow sufficiently inward, the accreting gas attaches itself to the field and is funneled dynamically down the effective potential (gravitational plus centrifugal) onto the star. Contrary to common belief, the accompanying magnetic torques associated with this accreting gas may transfer angular momentum mostly to the disk rather than to the star. Thus, the star can spin slowly as long as R(sub x) remains significantly greater than R(sub *). Exterior to R(sub x) field lines threading the disk bow outward, which makes the gas off the mid-plane rotate at super-Keplerian velocities. This combination drives a magnetocentrifugal wind with a mass-loss rate M(sub w) equal to a definite fraction f of the disk accretion rate M(sub D). For high disk accretion rates, R(sub x) is forced down to the stellar surface, the star is spun to breakup, and the wind is generated in a manner identical to that proposed by Shu, Lizano, Ruden, & Najita in a previous communication to this journal. In two companion papers (II and III), we develop a detailed but idealized theory of the magnetocentrifugal acceleration process.

Shu, Frank↗

Kinematic Dynamo In Turbulent Circumstellar Disks

Many circumstellar disks associated with objects ranging from protoplanetary nebulae, to accretion disks around compact stars allow for the generation of magnetic fields by an (alpha)omega dynamo. We have applied kinematic dynamo formalism to geometrically thin accretion disks. We calculate, in the framework of an adiabatic approximation, the normal mode solutions for dynamos operating in disks around compact stars. We then describe the criteria for a viable dynamo in protoplanetary nebulae, and discuss the particular features that make accretion disk dynamos different from planetary, stellar, and galactic dynamos.

Stepinski, T.↗

Accretion flows in binary X-ray systems

Recent developments in the study of accretion flows in binary X-ray systems are reviewed, concentrating on issues concerning the gas flows rather than the radiation mechanisms. A model for Her X-1 is presented which illustrates the main structures expected to exist in the case where mass transfer is by Roche-lobe overflow and the X-ray source is a rotating magnetized neutron star. A model for Cyg X-1 is given which shows the accretion-flow structures to be expected if the compact object is a black hole, including an accretion wake and a very small accretion disk around the black hole. Physical parameters of the accretion flows in these two instances are examined along with some scale lengths that may be derived from these parameters by dimensional analysis. Detailed attention is given to neutron-star magnetospheres (for slow, intermediate, and fast rotators), accretion-disk structure, accretion from stellar winds, and the relationship between X-rays and the gas dynamics of accretion flows. It is suggested that accretion flows in binary X-ray systems be observed directly by means of high-resolution UV and X-ray spectroscopy.

Mccray, R.↗

IUE observations of cataclysmic variable

Twenty two approved International Ultraviolet Explorer (IUE) programs were studied over a 14 year period. These programs are listed. The observations and subsequent analysis centered on cataclysmic variables (close binaries with a late main sequence star transferring material to a primary white dwarf via an accretion disk). The early studies highlighted the flux distribution of the accretion disk at outburst and quiescence, while later studies accomplished time-resolved observations throughout the orbital cycles, the study of the outflowing winds present at outburst, the study of the white dwarf in those systems with low accretion rate. There are 39 publications resulting from this work which are listed. These results include those for individual systems (Stepanian's star, Lanning 10, AM Her, MV Lyr, TV Col, VW Hyi, T Leo, IR Gem, TT Ari, Z Cam, BV Pup, IP Peg, PG1030+590, V1315 Aql, SW UMa, V426 Oph, WZ Sge, BY Cam, and U Gem) as well as review articles in journals and publications from reviews at meetings that summarize the impact of IUE on the study of accretion disks, white dwarfs, and hot spots resulting from stream impact as well as magnetic accretion columns.

Szkody, Paula↗

Radiation-Driven Warping: Nonisothermal Disks - 2

Recent work by Pringle and by Maloney, Begelman, & Pringle has shown that geometrically thin, optically thick, accretion disks are unstable to warping driven by radiation torque from the central source. This work was confined to isothermal (i.e., surface density Sigma varies as R(sup -3/2) disks. In this paper we generalize the study of radiation-driven warping to include general power-law surface density distributions, Sigma varies as R(sup -delta).We consider the range from Delta = 3/2 (the isothermal case) to Delta = -3/2, which corresponds to a radiation-pressure-supported disk; this spans the range of surface density distributions likely to be found in real astrophysical disks. In all cases there are an infinite number of zero-crossing solutions (i.e., solutions that cross the equator), which are the physically relevant modes if the outer boundary of the disk is required to lie in a specified plane. However, unlike the isothermal disk, which is the degenerate case, the frequency eigenvalues for Delta does not equal 3/2 are all distinct. In all cases the location of the zero moves outward from the steady state (pure precession) value with increasing growth rate; thus, there is a critical minimum size for unstable disks. Modes with zeros at smaller radii are damped. The critical radius and the steady state precession rate depend only weakly on Delta. An additional analytic solution has been found for Delta = 1. The case Delta = 1 divides the solutions into two qualitatively different regimes. For Delta greater than or equal to 1, the fastest growing modes have maximum warp amplitude, close to the disk outer edge, and the ratio of Beta(sub max) to the warp amplitude at the disk inner edge, Beta(sub o), is much greater than 1. For Delta less than 1, Beta(sub max/Beta(sub o) approximately equals 1, and the warp maximum steadily approaches the origin as Delta decreases. This implies that nonlinear effects must be important if the warp extends to the disk inner edge for Delta greater than or equal to 1, but for Delta less than 1 nonlinearity will be important only if the warp amplitude is large at the origin. Because of this qualitative difference in the shapes of the warps, the effects of shadowing of the central source by the warp will also be very different in the two regimes of Delta. This has important implications for radiation-driven warping in X-ray binaries, for which the value of Delta characterizing the disk is likely to be less than unity. In real accretion disks the outer boundary condition is likely to be different from the zero-crossing condition that we have assumed. In accretion disks around massive black holes in active galactic nuclei, the disk will probably become optically thin before the outer disk boundary is reached, whereas in X-ray binaries there will be an outer disk region (outside the circularization radius) in which the inflow velocity is zero but angular momentum is still transported. We show that in both these cases the solutions are similar to the zero-crossing eigenfunctions.

Maloney, Philip R.↗

Large Solar-Rejection Filter

NASA utilized Image Intensified Video Cameras for ATV data acquisition from a jet flying at 12.8 km. Afterwards the video was digitized and then analyzed with a modified commercial software package, Image Systems Trackeye. Astrometric results were limited by saturation, plate scale, and imposed linear plate solution based on field reference stars. Time-dependent fragment angular trajectories, velocities, accelerations, and luminosities were derived in each video segment. It was evident that individual fragments behave differently. Photometric accuracy was insufficient to confidently assess correlations between luminosity and fragment spatial behavior (velocity, deceleration). Use of high resolution digital video cameras in future should remedy this shortcoming. A scenically accurate description of matter interpreted as a substance made up of corpuscular constituents was established during the course of the 19th century. In this description, atoms--the building blocks of the matter--form molecules. The properties of the molecules were described by chemistry or thermodynamics depending on what characteristics of the matter were investigated. In both theories, the molecules can dissociate to atoms when the kinetic energies of the atoms exceed the strength of the chemical bonds. The number of atoms is always preserved in a closed system. This is not true, however, when the matter takes up much higher energies at relativistic scales. New particles can be produced at the expense of the kinetic energy. The number of particles is no longer preserved. There are other conserved quantities, however, these quantities, the charge, baryon number, lepton number, are associated with particles that are considered elementary today. The properties and behavior of these elementary particles is the subject of Particle Physics or High Energy Physics. Practice Page: A crewmember touching a positively charged surface was thought to be galvanically isolated from the vehicle ground analogous to a bird on a high voltage power wire. Recent analysis confirms that positive floating potentials, ionospheric currents to the EVA suit, can be hazardous. The analysis is wrong in that the ionospheric plasma itself can close the circuit. Parametric analysis of very low voltage exposures (2 to 15 volts) could cause pain and/or involuntary muscle tetani or spinal cord shock. NASA worked with the Naval Health Research Center Detachment Directed Energy Bioeffects Laboratory to examine the affects electrical hazards could have on extravehicular activity using two models. The results of the two computational models were combined to predict areas of the body in which neurons of different diameters would be excited. They predicted that physiologically active current could be conducted across the crew member causing catastrophic hazards. Future work to analyze additional current paths was proposed. The FUSE spectrum of BB Dor, observed in a high state, is modeled with an accretion disk with a very low inclination (possibly lower than 10 degrees). Assuming an average WD mass of 0.8 solar mass leads to a distance of the order of approximately 650pc, consistent with the extremely low galactic reddening in its direction, and a mass accretion rate of 10 (exp -9) solar mass a year. The spectrum presents some broad and deep silicon and sulfur absorption lines, indicating that these elements are over-abundant: silicon is 3 times solar, and sulfur is 20 times solar. The FUSE spectrum of BB Dor, observed in a high state, is modeled with an accretion disk with a very low inclination (possibly lower than 10 degrees). Assuming an average WD mass of 0.8 solar mass leads to a distance of the order of approximately 650pc, consistent with the extremely low galactic reddening in its direction, and a mass accretion rate of 10 (exp -9) solar mass a year. The spectrum presents some broad and deep silicon and sulfur absorption lines, indicating that these elements are over-abundant: silicon is times solar, and sulfur is 20 times solar. From its previously observed optical emission lines, P831-57 (WD 0334 6400 or Ret 1 in A Catalog and Atlas of Cataclysmic Variables: Living Edition) has been suspected to contain an accretion disk associated with a companion star in orbit around a subdwarf star with a temperature T is greater than 21,000K. P831-57 has therefore been classified as a nova-like. However, our present observations show it to be a DA + dMe binary. The analysis of its Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum (continuum and lines) reveal an average mass white dwarf (Log(g) approximately equals 7.8 plus or minus 0.1)with a temperature T approximately equals 37,000 plus or minus 500K, an extremely low projected rotational velocity, and a distance of about 115 plus or minus 5pc. The photosphere contains C, N, Si, and S (at about 1% of solar abundances). The dMe star is seen as a flux excess in near-infrared photometry and appears to show occasional flaring of about one magnitude as seen in the Harvard plates. There is no evidence of periodic variability in the spectroscopic or photometric data. We find no evidence of a an accretion disk, instead we find evidence of wind accretion as the stellar carbon abundance (N(C)/N(H) = 2.5 x 10 (exp -6) is about ten times larger than predicted by radiative levitation for such a gravity and temperature. The power needs and solutions for the space exploration and lunar mobility program are discussed. Long term missions in space and on the lunar surface require high energy batteries. Rechargeable batteries for mobility systems and portable utility pallet are needed for successful exploration missions. Nanomaterial usage increases the energy density of the cells apart from increasing the power density. The symptoms and threats from acute mountain sickness (AMS) are discussed. The underlying assumptions concerning spacecraft atmosphere mean there is a potential risk to astronauts. The baseline worst case scenario is about 25% based on direct ascent to 8.0 psia with 32% O2. The practical mitigation approach is a staged depressurization scheme. Due to the uncertainty about potential AMS risk the flight surgeon should prepare, focused research should proceed, and current analytical efforts should continue. This slide presentation reviews the threat astronauts face from acute mountain sickness (AMS). The incidence of AMS is highly variable and the risk to astronauts is unclear. However, the potential risk is higher than current EAAs suggest and precautions should be implemented. Due to the uncertain nature of the risk flight surgeons should prepare, and focused research should proceed. Quasiparticle tunneling can be understood at low temperatures using a non-equilibrium kinetic theory. Low-temperature tunneling can be suppressed using existing techniques: island-lead gap engineering, lead cooling with SIN junctions, and quasiparticle traps. The slide presentation reviews the science goals and mission planning for the proposed Jupiter System Observer (JSO). The spacecraft would accommodate 9 instruments making the instrument payload more capable than any previously flown to Jupiter. The would-be mission includes study programs for Jupiter's satellites, interior, magnetosphere and atmosphere. The power needs and solutions for the space exploration and lunar mobility program are discussed. Long term missions in space and on the lunar surface require high energy batteries. Rechargeable batteries for mobility systems and portable utility pallet are needed for successful exploration missions. Nanomaterial usage increases the energy density of the cells apart from increasing the power density. This slide presentation reviews the power requirements for the space exploration and the lunar surface mobility programs. It includes information about the specifications for high energy batteries and the power requirements for lunar rovers,

Roberts, William↗

Effects of inclination angle on the spectra of X-ray binaries

Iron emission and absorption features from nine X-ray binaries are compared with a model that predicts characteristics of iron K-shell features corresponding to different viewing angles for the accretion disk. In this model, emission and absorption features arise from an accretion disk corona whose structure is computed under the assumptions of ionization, thermal, and hydrostatic balance of gas illuminated by the central continuum source. Hence the equivalent widths of the emission and absorption features are predicted without the introduction of any free parameters to control the physical conditions in the disk corona. Our data/model comparison suggests an explanation for the equivalent widths of the iron K-alpha emission and the depth of the absorption edge in terms of (1) the angle of inclination between the observer's line of sight and the plane of the accretion disk and (2) the shape of the incident source spectrum.

Vrtilek, S. D.↗