Disk Flux Research Articles

Temporal Variations of the White Dwarf and Disk in OY Carinae Following the 1992 Superoutburst

Hubble Space Telescope observations of the eclipsing dwarf nova OY Carinae after its 1992 April superoutburst are used to isolate ultraviolet spectra (1150-2500 A at 9.2 A FWHM resolution) of the white dwarf, the accretion disk, and the bright spot. The white dwarf spectra have a Stark-broadened photospheric Lyα absorption feature but are veiled by a forest of absorption features that we attribute to absorption by intervening disk material (a curtain). All the spectral fits required supersonic turbulence in the curtain material with Mach numbers of 6-8. All curtain temperatures were between 10,000 and 11,000 K. There was a curtain temperature increase ~3 months after the superoutburst. We find that the white dwarf temperature changed from 19,700 K just 27 days after the end of the superoutburst to 18,000 K roughly 3 months after the superoutburst; the exponential (e-folding) decay time of the white dwarf temperature was 66 days. We present evidence that the heating of the white dwarf was more extensive during the superoutburst than the normal outburst. The thermal response of the OY Car white dwarf to outburst heating is compared with WZ Sagittae, VW Hydri (the most similar dwarf nova to OY Car), and the cooling timescales of other dwarf novae after superoutburst. The measured cooling timescales of the five systems with superoutbursts appear to be shorter the longer the orbital period (accretion rate). Possible implications are discussed. There is evidence of a disk flux variation, independent of the effect of white dwarf cooling, which suggests a possible contradiction of the disk instability model. To establish this, however, data are required throughout a quiescent cycle.

Correlations between Low‐Frequency Quasi‐periodic Oscillations and Spectral Parameters in XTE J1550−564 and GRO J1655−40

Utilizing observations obtained with the Rossi X-Ray Timing Explorer, we examine correlations between the properties of 0.08-22 Hz variable-frequency quasi-periodic oscillations (QPOs) and the X-ray spectral parameters for the black hole candidates XTE J1550-564 and GRO J1655-40. The spectra were fitted to a model including a multitemperature blackbody disk and a power-law component. We find that the QPO frequency and amplitude are well correlated with the spectral parameters, although the correlations found for XTE J1550-564 are generally opposite to those for GRO J1655-40. There is one exception: both sources exhibit a general increase in the QPO frequency as the disk flux increases (or as the rate of mass accretion through the disk increases). In addition, these QPOs are observed only when the power-law component contributes more than 20% of the 2-20 keV flux, which indicates that both the disk and the power-law components are linked to the QPO phenomenon.

The Circumstellar Disk of HD 141569 Imaged with NICMOS.

Coronagraphic imaging with the Near-Infrared Camera and Multiobject Spectrometer on the Hubble Space Telescope reveals a large, approximately 400 AU (4&arcsec;) radius, circumstellar disk around the Herbig Ae/Be star HD 141569. A reflected light image at 1.1 µm shows the disk oriented at a position angle of 356&j0;+/-5&j0; and inclined to our line of sight by 51&j0;+/-3&j0;; the intrinsic scattering function of the dust in the disk makes the side inclined toward us, the eastern side, brighter. The disk flux density peaks 185 AU (1&farcs;85) from the star and falls off to both larger and smaller radii. A region of depleted material, or a gap, in the disk is centered 250 AU from the star. The dynamical effect of one or more planets may be necessary to explain this morphology.

The Quiescent Accretion Disk in IP Pegasi at Near‐Infrared Wavelengths

We present near-infrared, H-band (1.45-1.85 μm) observations of an eclipsing dwarf nova, IP Peg, in quiescence. The light curves are composed of ellipsoidal variations from the late-type secondary star and emission from the accretion disk and the bright spot. The light curves have two eclipses: a primary eclipse of the accretion disk and the bright spot by the companion star, and a secondary eclipse of the companion star by the disk. The ellipsoidal variations of the secondary star were modeled and subtracted from the data. The resulting light curve shows a pronounced double-hump variation. The double-hump profile resembles those seen in the light curves of WZ Sge and AL Com and likely originates in the accretion disk. The primary eclipse was modeled using maximum entropy disk mapping techniques. The accretion disk has a flat intensity distribution and a cool brightness temperature (Tbr 3000 K) in the near-infrared. Superimposed on the face of the disk is the bright spot (Tbr 10,000 K); the position of the bright spot is different from the observed range of visible bright spot positions. The near-infrared accretion disk flux is dominated by optically thin emission. The secondary eclipse indicates the presence of some occulting medium in the disk, but the eclipse depth is too shallow to be caused by a fully opaque accretion disk.

The optical light curves of Cygnus X-2 (V1341 Cyg) and the mass of its neutron star

We present U, B and V light curves (taken from the literature) of the low-mass X-ray binary Cygnus X-2. We show that the most significant photometric periods seen in the B and V light curves are consistent with half of the orbital period found from spectroscopy (P=9.8444 d). The `lower envelopes' of the light curves folded on the orbital period are ellipsoidal (i.e. they have two maxima and two minima per orbital cycle). We fit an ellipsoidal model to the lower envelopes of the B and V light curves to derive inclination constraints. This model includes light from an accretion disc, and accounts for eclipses and X-ray heating. Using the extreme assumption that there is no disc light, we derive a lower limit on the inclination of i≥ 49°. If we assume the accretion disc is steady-state where its radial-temperature profile goes as T(r)∝ r-3/4, we find an inclination of i=62°.5 ± 4°. However, the predicted ratio of the disc flux to the total flux in B (the `disc fraction') is larger than what is observed (≈0.55 compared with ≤0.3). If we use a flatter radial-temperature profile of the disc expected for strongly irradiated discs [T(r) ∝ r-3/7], then we find an inclination of i = 54° 6 and a disc fraction in B of ≈ 0.30. However, in this case the value of χ2 is much larger (48.4 with 36 degrees of freedom compared with 40.9 for the steady-state case). Adopting i=62°.5 ±4° and using a previous determination of the mass ratio (q=Mc/Mx=0.34 ± 0.04) and the optical mass function [f(M)=0.69 ± 0.03 M⊙], we find that the mass of the neutron star is Mx=1.78 ± 0.23 M⊙ and the mass of the secondary star is Mc=0.60 ± 0.13 M⊙. We derive a distance of d=7.2 ± 1.1 kpc, which is significantly smaller than a recent distance measurement of d=11.6 ± 0.3 kpc derived from an observation of a type I radius-expansion X-ray burst, but consistent with earlier distance estimates.

The structure of adsorbed bromide concurrent with the underpotential deposition (UPD) of Cu on Pt(111)

The adsorption of bromide anions concurrent with the underpotential deposition (UPD) of Cu on Pt(111) was examined utilizing ex-situ LEED and in-situ surface X-ray scattering for structure determination, in combination with coverage determination by rotating ring disk flux measurements with the Pt(111) single crystal as the disk electrode. The results show definitively that Cu UPD on Pt(111) in the presence of bromide is a two-step process, with the total amount of Cu deposited at underpotentials ∼0.95 ± 5% ML (1 ML = 1 adatom per Pt atom). The results also indicate that the surface coverage by adsorbed bromide undergoes only a small (<0.05 ML) change upon the deposition of Cu, even up to a nominal monolayer. We propose a model wherein the first stage of deposition occurs by displacement of the close-packed hexagonal layer of Br adatoms by Cu adatoms through a “turn-over” process, in which Cu is sandwiched between the Pt surface and the Br overlayer, leading to the formation of an ordered Pt(111)CuBr bilayer intermediate phase which closely resembles the (111) planes of the Cu(I)Br crystal. The coverage of both Cu and Br in this intermediate phase is ∼0.5 ML. The second stage is the filling-in of the Cu plane of the bilayer to form a pseudomorphic (1 × 1) Cu monolayer and a disordered Br adlayer with a coverage of ∼0.4 ML. The same mechanism is suggested for Cu UPD on Pt(111) in solutions containing chloride anions.

T Tauri variability in the context of the beat frequency model

We examine the implications of a beat frequency modulated model of T Tauri accretion. In particular we show that measurements of the variability of accretion generated lines can be used in conjunction with existing photometry to obtain a measurement of the underlying photospheric and disc flux. This provides an independent way of checking spectral energy distribution modelling. In addition, we show how spectroscopy of T Tauri stars can reveal the inclination angle between the magnetic axis and the plane of the disc.

The effects of redshifts and focusing on the spectrum of an accretion disk in the galactic center black hole candidate Sagittarius A[SUP]*[/SUP

There are firm indications that Sgr A(sup *), a compact, nonthermal radio source at the Galactic center, may be powered by the dissipation of gravitational energy as gas trapped from an ambient wind descends down the potential well, first through a quasi-spherical inflow (extending out to approximately 3 x 10(exp 16) cm) and then through a small accretion disk at less than or approximately = 5-10 Schwarzschild radii. Earlier three-dimensional Bondi-Hoyle numerical simulations have indicated that fluctuations in the accreted specific angular momentum can lead to a variability in the disk flux on a timescale of years. With greatly improved flux measurements at K and H, and the hint of a approximately 10 minute modulation in the IR luminosity, it is crucial to model the disk emission much more precisely than has been attempted thus far. In this Letter we take into account the effects of Doppler and gravitational redshifts, the light-travel time factor, and the light bending near the black hole to determine the measurable spectrum of Sgr A(sup *) in the increasingly important 10(exp 13) Hz less than or approximately = v less than or approximately = 10(exp 16) Hz frequency range. We find that the relativistic disk spectrum is much softer than its Newtonian counterpart, with a predicted UV flux roughly an order of magnitude smaller than had previously been anticipated. In addition, we find that when the physical conditions in the disk are taken to be consistent with the properties of the quasi-spherical infall (specifically, in terms of the accretion rate and disk size), only a slowly spinning or Schwarzschild black hole appears to fit the observations. Our calculations also reveal that the disk flux is much more weakly dependent on the observer's inclination angle than had been suspected on the basis of earlier Newtonian estimates.

HST eclipse mapping of dwarf nova OY Carinae in quiescence: an 'Fe II curtain' with Mach approx. = 6 velocity dispersion veils the white dwarf

Hubble Space Telescope (HST) observations of the eclipsing dwarf nova OY Car in its quiescent state are used to isolate the ultraviolet spectrum (1150-2500 A at 9.2 A Full Width at Half Maximum (FWHM) resolution) of the white dwarf, the accretion disk, and the bright spot. The white dwarf spectrum has a Stark-broadened photospheric L(alpha) absorption, but is veiled by a forest of blended Fe II features that we attribute to absorption by intervening disk material. A fit gives T(sub w) approx. = 16.5 x 10(exp 3) K for the white dwarf with a solar-abundance, log g = 8 model atmosphere, and T approx. = 10(exp 4) K, n(sub e) approx. = 10(exp 13)/cu cm, N(sub H) approx. = 10(exp 22) sq cm, and velocity dispersion delta V approx. = 60 km/s for the veil of homogeneous solar-abundance local thermodynamic equilibrium (LTE) gas. The veil parameters probably measure characteristic physical conditions in the quiescent accretion disk or its chromosphere. The large velocity dispersion is essential for a good fit; it lowers (chi square)/778 from 22 to 4. Keplerian shear can produce the velocity dispersion if the veiling gas is located at R approx. = 5 R(sub W) with (delta R)/R approx. = 0.3, but this model leaves an unobscured view to the upper hemisphere of the white dwarf, incompatible with absorptions that are up to 80% deep. The veiling gas may be in the upper atmosphere of the disk near its outer rim, but we then require supersonic (Mach approx. = 6) but sub-Keplerian (delta V/V(sub Kep) approx. = 0.07) velocity disturbances in this region to produce both the observed radial velocity dispersion and vertical motions sufficient to elevate the gas to z/R = cos i = 0.12. Such motions might be driven by the gas stream, since it may take several Kepler periods to reestablish the disk's vertical hydrostatic equilibrium. The temperature and column density of the gas we see as Fe II absorption in the ultraviolet are similar to what is required to produce the strong Balmer jump and line emissions seen in optical spectra of OY Car and similar quiescent dwarf novae. The outer accretion disk is detected at mid-eclipse with a spectrum that rises from 0.05 to 0.3 mJy between 2000 and 2500 A, consistent with combinations of cool blackbodies, blended Fe II emission lines, and Balmer continuum emission. The total disk flux density is 0.5 mJy at 2500 A, and this shallow disk eclipse implies a roughly flat surface brightness distribution. The bright spot, somewhat bluer than the disk, has a flux density rising from 0.05 to 0.15 mJy between 1600 and 2500 A. The C IV emission line has a broad shallow eclipse, but the radial velocity variations observed during the eclipse do not clearly distinguish between a disk or wind origin. The only possible indications of boundary layer emission are fast UV flares that appear to arise from near the central object -- not from the bright spot.

Slowly-Varying Microwave Emissions from the Solar Corona

Measurements of the total microwavef emission from the solar disc at single and multiple wavelengths provide a useful indicator of solar activity, and correlate well with other indices of solar activity and with solar luminosity.Since activity in the solar atmosphere is driven by the emergence, redistribution and disappearance of magnetic flux, the close and linear correspondence between the microwave flux densities, as observed at 10.7 and 21 cm wavelengths, provide a convenient and objective indicator of solar magnetic activity.Microwave emissions from the “non-flaring” solar corona originate in a wide range of sources. These generally lie in active regions, and comprise two main types: bright, compact sources a few arc-seconds across and brightness temperatures of the order of 106K, and weaker, diffuse sources arc-minutes in size and having brightness temperatures of about 104K. There is also evidence of a weaker, widespread emission covering large areas of the solar disc. The compact sources are probably due to gyroresonance, although some could be non-thermal. The diffuse widespread emission are almost certainly free-free thermal emission from plasma concentrations in the corona.The strong correlation between the 10.7 cm flux and other activity indices, even when they are produced by disimilar processes and at different heights in the solar atmosphere, may at least partially be due to the overiding role played by magnetic activity in dictating the structure of the chromosphere and corona. However, the consistent correspondences over wide ranges of activity suggest dependences upon relatively few quantities and that the microwave emission from the non-flaring Sun is dominated by a single process, which is closely and simply-related to density and temperature, such as free-free (bremsstrahlung) emission from plasma concentrations trapped in coronal and chromospheric magnetic structures. Attempts to “budget” the total microwave emission at 2.8, 10.7 and 21 cm wavelengths suggest that this is the case.Where ambient magnetic fields exceed 3000/λ Gauss, gyroresonance can produce much higher optical depths than free-free emission, and accounts for the bright emission observed in the vicinity of sunspots. Although not the dominant contributor to the total emission from the solar disc, these sources stand out brightly in high-resolution maps of active regions.To examine the assumptions of dominance of free-free thermal emission, and simple dependences upon few parameters, we compute the spectrum for a range of coronal density models, and compare the results with observations. The agreement is good, suggesting that the total disc flux is dominated by thermal, free-free emission, and that the intensity is controlled mainly by the density at the base of the corona, but is otherwise fairly tolerant of the density model used.

Ring–disc electrodes. Part 22.—Theory of the measurement of proton fluxes at the disc

Proton fluxes at the disc of a ring–disc electrode can be measured by potentiometric measurement with a bismuth oxide ring electrode. When the bulk pH is close to 7, fluxes as low as 10–12 mol s–1 cm–2 can be measured. Theory is developed to describe the relation between the disc flux and the ring pH, allowing for the fact that the diffusion layer may be divided into an H+ region and an OH– region with a titration reaction taking place on the interface between the regions. Analytical expressions, allowing for the different diffusion coefficients of H+ and OH–, are derived for four separate cases. A case diagram is constructed to show the interrelation between the different cases and to show the relation between the disc flux, the bulk pH and the pH at the ring.

The Temperatures of White Dwarfs in Accreting Binaries

Through the use of accreting binary systems, it is possible to study the effects of the deposition of matter and energy on the surface of a white dwarf. The observed atmospheric properties of composition and temperature obtained from direct observation of the spectral lines and the continuum flux can be used to compare with those of single white dwarfs to understand the consequences of mass accretion on binary evolution.Cataclysmic variables provide one of the best targets for this type of study because a) the primaries are all white dwarfs b) the level and the timescale of the accretion cover a large range from the high rate, relatively steady novalike accretors to the dwarf novae systems which are modulated on short timescales in a quasi-periodic manner. Unfortunately, due to the mass transfer process, an accretion disk builds up to the point where its radiation overwhelms the white dwarf light in most cases. Thus, to study the effects on the stellar primary, systems must be found which have low mass transfer rates (generally the short orbital period systems (Patterson 1984)) and/or high inclinations (since most of the disk flux emerges perpendicular to the plane of the disk). The best identification of the white dwarf emerges from IUE spectra which show a broad Lyman α absorption profile (in contrast to the normal emission lines from a disk at quiescence). The shape of this profile provides a sensitive indicator of the temperature and gravity. In some cases, broad absorption lines are also evident in the optical Balmer lines, although the broad emission lines from the disk usually make these difficult to detect. The steeply falling flux distribution of a white dwarf throughout the optical region, combined with a flat disk distribution usually means that the white dwarf contributes a minor amount to the optical flux. However, in the ultraviolet, the rising energy distribution of the white dwarf easily dominates the falling energy distribution of a low accretion rate disk (Mateo and Szkody 1984). White dwarfs are generally acknowledged to be prominent in the dwarf novae U Gem (Panek and Holm 1984), VW Hyi (Mateo and Szkody 1984) and Z Cha (Marsh, Horne and Shipman 1987) and suggested in EK TrA and WZ Sge (Verbunt 1987). In addition, the white dwarf has been seen in some novalike systems which sporadically turn off their mass transfer, (resulting in the disappearance of most of the disk and the resulting appearance of the white dwarf). This has been the case in TT Ari (Shafter et al. 1985) and some limits have been determined for MV Lyr (Szkody and Downes 1982) and V794 Aql (Szkody, Downes and Mateo 1988). Several magnetic white dwarfs have also been seen when the mass transfer ceases in the AM Her systems (summarized in Szkody, Downes and Mateo 1988).

The behavior of microdisk and microring electrodes

An exact analysis of diffusion to microdisk and microring electrodes is presented for steady-state conditions and for the assumptions of constant concentration over the surface of the disk and constant flux over the surface of the ring electrodes. Mass transfer to ring electrodes is progressively enhanced by reducing their thickness, which also reduces changes in concentration with position over the surface of the ring as well as the differences between the mass transfer coefficient for the constant concentration and constant flux conditions. Thin rings are therefore recommended for investigations of electrode kinetics.

A three‐component model of the variability of the solar ultraviolet flux: 145–200 nM

A three‐component model has been developed to examine the variation with solar activity of the far ultraviolet irradiance between 145 and 200 nm. This model is based on spatially resolved observations of the Call K chromosphere and includes the contributions to the full disk flux from both plage and active network emission. The 27‐day modulation of the ultraviolet flux is explained by the evolution and rotation of the plage regions on the solar disc. Over the longer time scale of the eleven‐year cycle it is essential that changes in the active network arising from the decay of plage regions also be considered as a source of ultraviolet flux variability. Only by using a three‐component model of the solar flux is it possible to simultaneously reproduce the 27‐day variability observed by the solar backscatter ultraviolet experiment on the Nimbus 7 satellite and the changes from the minimum to the maximum of the solar activity cycle observed by the rocket experiments of the Laboratory for Atmospheric and Space Physics and by the extreme ultraviolet spectrometer on the Atmospheric Explorer E satellite. It is shown that the AE‐E experiment measured a smaller solar cycle variability for the ultraviolet irradiances than is predicted by the model calculations because of the spatially restricted field of view of this instrument.

SS Cygni - The accretion disk in eruption and at minimum light

view Abstract Citations (84) References (43) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS SS CYG : the accretion disk in eruption and at minimum light. Kiplinger, A. L. Abstract Absolute spectrophotometric observations of the dwarf nova SS Cygni have been obtained at maximum light, during the subsequent decline, and at minimum light. In order to provide a critical test of accretion disk theory, a model for a steady-state α-model accretion disk has been constructed which utilizes a grid of stellar energy distributions to synthesize the disk flux. Physical parameters for the accretion disk at maximum light are set by estimates of the intrinsic luminosity of the system that result from a desynthesis of a composite minimum light energy distribution. At maximum light, agreements between observational and theoretical continuum slopes and the Balmer jump are remarkably good. The model fails, however, during the eruption decline and at minimum light. It appears that the physical character of an accretion disk at minimum light must radically differ from the disk observed at maximum light. Publication: The Astrophysical Journal Pub Date: December 1979 DOI: 10.1086/157584 Bibcode: 1979ApJ...234..997K Keywords: Astronomical Spectroscopy; Binary Stars; Dwarf Stars; Stellar Luminosity; Stellar Mass Accretion; Balmer Series; Disks (Shapes); Spectral Energy Distribution; Spectrum Analysis; Stellar Models; Stellar Rotation; Astrophysics full text sources ADS | data products SIMBAD (2)

Z Cam at Standstill and in Eruption

Absolute spectrophotometric observations of the prototype dwarf nova Z Cam have been obtained at standstill and during the course of an eruption. Optical continuum slopes during the final stages of the rise to maximum appear steeper (bluer) than those observed during decline and at standstill. The Balmer jump is observed to be in shallow absorption during decline and in deeper absorption during the rise to maximum and at standstill. A composite energy distribution constructed for Z Cam at standstill is compared with fluxes derived from steady state (a-raodel) accretion disk models which utilize a grid of stellar energy distributions to synthesize the disk fluxes. The most reasonable accretion disk fit to the observations at standstill suggests a mass transfer rate of ~3 × 1017 g sec-1. The steady state model does not fit the observations of Z Cam during decline, nor does any arbitrary linear combination of stellar energy distributions. The results suggest that the accretion disk of decline is time dependent and exhibits an emission character which differs from that seen at standstill.

Angular momentum of a black hole in a dense stellar system

view Abstract Citations (15) References (14) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Angular momentum of a black hole in a dense stellar system. Young, P. J. Abstract The capture of stars from plunge orbits by a massive black hole in a dense stellar system provides a mechanism to reduce the angular momentum of the hole to what is effectively a Schwarzschild state as the mass increases from M = l0 M0 (spin a* = aIM = 0.998, efficiency of energy conversion in accretion disk = 0.32) to 108 M (a* - 0.05, = 0.06), cutting the luminosity from the disk by a factor of 5 compared with that expected from a Kerr hole (a* = 0.998). This spin-down is due to the greater cross section of a Kerr hole for orbits with negative angular momentum with respect to the spin of the hole. A small black hole (M = l0 M0) in a globular cluster is also likely to be Schwarzschild in nature provided sufficient time has elapsed for significant growth of the hole. A growth in mass by a factor of 2 will reduce a* = 0.998 ( = 0.32) to a* = 0.10 ( = 0.06). The presumption is made that an accretion disk supplied by the tidal disruption of stars attempts to increase the angular momentum of the hole. The disk competes with the plungeorbit flux which is successful in reducing the angular momentum of the hole to a* < 0.6 ( < 0.1) if it is supplied with mass at a rate > 50% of the disk flux. The conclusions may be destroyed if a strong source of flux to the disk is provided which is independent of the relaxation of the nucleus (which controls both the plunge orbit and the tidal disruption fluxes). Subject headings: black holes - clusters: globular - stars: accretion - stars: stellar dynamics Publication: The Astrophysical Journal Pub Date: February 1977 DOI: 10.1086/155041 Bibcode: 1977ApJ...212..227Y Keywords: Angular Momentum; Astrodynamics; Black Holes (Astronomy); Globular Clusters; Spin Dynamics; Stellar Systems; Cosmology; Energy Conversion Efficiency; Space Density; Star Distribution; Stellar Luminosity; Astrophysics full text sources ADS |