arXiv subjects

Christopher W. Mauche

Publications and source records attributed to Christopher W. Mauche.

At least 37 records · Page 2Linked to original sources

arXiv2002

Correlation of the Quasi-Periodic Oscillation Frequencies of White Dwarf, Neutron Star, and Black Hole Binaries

Using data obtained in 1994 June/July with the Extreme Ultraviolet Explorer deep survey photometer and in 2001 January with the Chandra X-ray Observatory Low Energy Transmission Grating Spectrograph, we investigate the extreme-ultraviolet (EUV) and soft X-ray oscillations of the dwarf nova SS Cyg in outburst. We find quasi-periodic oscillations (QPOs) at nu_0 ~ 0.012 Hz and nu_1 ~ 0.13 Hz in the EUV flux and at nu_0 ~ 0.0090 Hz, nu_1 ~ 0.11 Hz, and possibly nu_2 ~ nu_0 + nu_1 ~ 0.12 Hz in the soft X-ray flux. These data, combined with the optical data of Woudt & Warner for VW Hyi, extend the Psaltis, Belloni, & van der Klis nu_high-nu_low correlation for neutron star and black hole low-mass X-ray binaries (LMXBs) nearly two orders of magnitude in frequency, with nu_low ~ 0.08 nu_high. This correlation identifies the high-frequency quasi-coherent oscillations (so-called ``dwarf nova oscillations'') of cataclysmic variables (CVs) with the kilohertz QPOs of LMXBs, and the low-frequency QPOs of CVs with the horizontal branch oscillations (or the broad noise component identified as such) of LMXBs. Assuming that the same mechanisms produce the QPOs in white dwarf, neutron star, and black hole binaries, we find that the data exclude the relativistic precession model and the magnetospheric and sonic-point beat-frequency models (as well as any model requiring the presence or absence of a stellar surface or magnetic field); more promising are models that interpret QPOs as manifestations of disk accretion onto any low-magnetic field compact object.

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arXiv2002

EUVE Phase-Resolved Spectroscopy of V834 Centauri

(abridged) The Extreme Ultraviolet Explorer (EUVE) satellite was employed in 1999 February to acquire phase-resolved EUV photometric and spectroscopic observations of the AM Her-type cataclysmic variable V834 Centauri. Although we do not understand the EUV light curves in detail, they are explained qualitatively by a simple model of accretion from a ballistic stream along the field lines of a tilted magnetic dipole centered on the white dwarf. The 75-140 Angstrom EUVE spectra are well described by either a blackbody or a pure-H stellar atmosphere absorbed by a neutral hydrogen column density, but constraints on the size of the EUV emission region and its UV brightness favor the blackbody interpretation with temperature kT ~ 17.6 eV, hydrogen column density N_H ~ 7.4x10^{19} cm^{-2}, fractional emitting area f ~ 10^{-3}, and luminosity L_soft ~ 7.2x10^{32} (d/100 pc)^2 erg s^{-1}. The ratio of the EUV to X-ray luminosities is L_soft/L_hard ~ 40, signaling that some mechanism other than irradiation (e.g., blob heating) dominates energy input into the accretion spot.

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arXiv2002

The X-ray Spectra of VY Scl Stars Are Not Blackbodies

Using ASCA data, we find, contrary to other researchers using ROSAT data, that the X-ray spectra of the VY Scl stars TT Ari and KR Aur are poorly fit by an absorbed blackbody model but are well fit by an absorbed thermal plasma model. The different conclusions about the nature of the X-ray spectrum of KR Aur may be due to differences in the accretion rate, since this star was in a high optical state during the ROSAT observation, but in an intermediate optical state during the ASCA observation. TT Ari, on the other hand, was in a high optical state during both observations, so directly contradicts the hypothesis that the X-ray spectra of VY Scl stars in their high optical states are blackbodies. Instead, based on theoretical expectations and the ASCA, Chandra, and XMM spectra of other nonmagnetic cataclysmic variables, we believe that the X-ray spectra of VY Scl stars in their low and high optical states are due to hot thermal plasma in the boundary layer between the accretion disk and the surface of the white dwarf, and appeal to the acquisition of Chandra and XMM grating spectra to test this prediction.

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arXiv2001

Plasma Diagnostics in High-Resolution X-ray Spectra of Magnetic Cataclysmic Variables

Using the Chandra HETG spectrum of EX Hya as an example, we discuss some of the plasma diagnostics available in high-resolution X-ray spectra of magnetic cataclysmic variables. Specifically, for conditions appropriate to collisional ionization equilibrium plasmas, we discuss the temperature dependence of the H- to He-like line intensity ratios and the density and photoexcitation dependence of the He-like R line ratios and the Fe XVII I(17.10 A)/I(17.05 A) line ratio. We show that the plasma temperature in EX Hya spans the range from ~ 0.5 to ~ 10 keV and that the plasma density n > 2E14 cm^{-3}, orders of magnitude greater than that observed in the Sun or other late-type stars.

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arXiv2001

EUVE Observations of Nonmagnetic Cataclysmic Variables

We summarize EUVE's contribution to the study of the boundary layer emission of high accretion-rate nonmagnetic cataclysmic variables, especially the dwarf novae SS Cyg, U Gem, VW Hyi, and OY Car in outburst. We discuss the optical and EUV light curves of dwarf nova outbursts, the quasi-coherent oscillations of the EUV flux of SS Cyg, the EUV spectra of dwarf novae, and the future of EUV observations of cataclysmic variables.

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arXiv2001

First Simultaneous Optical and EUV Observations of the Quasi-Coherent Oscillations of SS Cygni

Using EUV photometry obtained with the Extreme Ultraviolet Explorer (EUVE) satellite and UBVR optical photometry obtained with the 2.7-m telescope at McDonald Observatory, we have detected quasi-coherent oscillations (so-called ``dwarf nova oscillations'') in the EUV and optical flux of the dwarf nova SS Cygni during its 1996 October outburst. There are two new results from these observations. First, we have for the first time observed ``frequency doubling:'' during the rising branch of the outburst, the period of the EUV oscillation was observed to jump from 6.59 s to 2.91 s. Second, we have for the first time observed quasi-coherent oscillations simultaneously in the optical and EUV. We find that the period and phase of the oscillations are the same in the two wavebands, finally confirming the long-held assumption that the periods of the optical and EUV/soft X-ray oscillations of dwarf novae are equal. The UBV oscillations can be simply the Rayleigh-Jeans tail of the EUV oscillations if the boundary layer temperature kT_bb <~ 15 eV and hence the luminosity L_bb >~ 1.2e34 (d/75 pc)^2 erg/s (comparable to that of the accretion disk). Otherwise, the lack of a phase delay between the EUV and optical oscillations requires that the optical reprocessing site lies within the inner third of the accretion disk. This is strikingly different from other cataclysmic variables, where much or all of the disk contributes to the optical oscillations.

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arXiv2001

First Application of the Fe XVII I(17.10 A)/I(17.05 A) Line Ratio to Constrain the Plasma Density of a Cosmic X-ray Source

We show that the Fe XVII I(17.10 A)/I(17.05 A) line ratio observed in the Chandra HETG spectrum of the intermediate polar EX Hydrae is significantly smaller than that observed in the Sun or other late-type stars. Using the Livermore X-ray Spectral Synthesizer, which calculates spectral models of highly charged ions based on HULLAC atomic data, we find that the observed I(17.10 A)/I(17.05 A) line ratio can be explained if the plasma density n_e > 3x10^{14} cm^{-3}. However, if photoexcitation is included in the level population kinetics, the line ratio can be explained for any density if the photoexcitation temperature T_bb > 55 kK. For photoexcitation to dominate the Fe XVII level population kinetics, the relative size of the hotspot on the white dwarf surface must be f < 2%. This constraint and the observed X-ray flux requires a density n > 2x10^{14} cm^{-3} for the post-shock flow. Either way, then, the Chandra HETG spectrum of EX Hya requires a plasma density which is orders of magnitude greater than that observed in the Sun or other late-type stars.

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arXiv2000

Optical and EUV Light Curves of Dwarf Nova Outbursts

We combine AAVSO and VSS/RASNZ optical and Extreme Ultraviolet Explorer EUV light curves of dwarf novae in outburst to place constraints on the nature of dwarf nova outbursts. From the observed optical-EUV time delays of 0.75-1.5 days, we show that the propagation velocity of the dwarf nova instability heating wave is ~ 3 km/s.

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arXiv2000

EUVE Observations of OY Carinae in Superoutburst

The Extreme Ultraviolet Explorer (EUVE) satellite was used for three days beginning on 1997 Mar 26.96 UT to obtain photometric and spectroscopic observations of the eclipsing SU UMa-type dwarf nova OY Carinae in superoutburst. Because of the longer time on source (143 ks), the larger number of eclipses observed (17), and the higher count rate in the detector (0.5-2.2 counts/s), we are able to significantly strengthen previous reports that there is little or no eclipse by the secondary of the EUV emission region of OY Car in superoutburst. The mean EUVE spectrum extends from 70 to 190 A and contains broad (FWHM~1 A) emission lines of N V, O V-O VI, Ne V-Ne VII, Mg IV-Mg VI, Fe VI-Fe VIII, and possibly Fe XXIII. Good fits of the observed spectrum are obtained with a model (similar to that of Seyfert 2 galaxies) wherein radiation from the boundary layer and accretion disk is scattered into the line of sight by the system's photoionized accretion disk wind. It is possible to trade off continuum luminosity for wind optical depth, but reasonable models have a boundary layer temperature T_{bl}=90-130 kK and a boundary layer and accretion disk luminosity L_{bl}=L_{disk} ~4x10^{34} erg/s ~ 10 Lsun, corresponding to a mass-accretion rate Mdota~10^{-8} Msun/yr; an absorbing column density N_H=1.6-3.5x10^{19} cm^{-2}; and a wind mass-loss rate Mdotw~10^{-10} Msun/yr ~ 0.01 Mdota. Because radiation pressure alone falls an order of magnitude short of driving such a wind, magnetic forces must also play a role in driving the wind of OY Car in superoutburst.

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arXiv1999

ORFEUS II and IUE Spectroscopy of EX Hydrae

Using ORFEUS-SPAS II FUV spectra, IUE UV spectra, and archival EUVE deep survey photometry, we present a detailed picture of the behavior of the magnetic cataclysmic variable EX Hydrae. Like HUT spectra of this source, the FUV and UV spectra reveal broad emission lines of He II, C II-IV, N III and V, O VI, Si III-IV, and Al III superposed on a continuum which is blue in the UV and nearly flat in the FUV. Like ORFEUS spectra of AM Her, the O VI doublet is resolved into broad and narrow emission components. Consistent with its behavior in the optical, the FUV and UV continuum flux densities, the FUV and UV broad emission line fluxes, and the radial velocity of the O VI broad emission component all vary on the spin phase of the white dwarf, with the maximum of the FUV and UV continuum and broad emission line flux light curves coincident with maximum blueshift of the broad O VI emission component. On the binary phase, the broad dip in the EUV light curve is accompanied by strong eclipses of the UV emission lines and by variations in both the flux and radial velocity of the O VI narrow emission component. The available data are consistent with the accretion funnel being the source of the FUV and UV continuum and the O VI broad emission component, and the white dwarf being the source of the O VI narrow emission component.

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arXiv1998

EUVE Spectroscopy of Polars

An admittedly pedantic but hopefully useful and informative analysis is presented of the EUVE 70--180 Angstrom spectra of nine polars. These spectra are fit with three different models---a blackbody, a pure-H stellar atmosphere, and a solar abundance stellar atmosphere---to reveal the presence of spectral features such as absorption lines and edges, and to investigate the sensitivity of the derived (kT, N_H, solid angle) and inferred (fractional emitting area, bolometric luminosity) parameters to the model assumptions. Among the models tested, the blackbody model best describes the observed spectra, although the untested irradiated solar abundance stellar atmosphere model is likely a better overall description of the EUV/soft X-ray spectra of polars.

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arXiv1998

ORFEUS II Far-UV Spectroscopy of AM Herculis

Six high-resolution (λ/Δλ~ 3000) far-UV (λλ= 910-1210 Å) spectra of the magnetic cataclysmic variable AM Herculis were acquired in 1996 November during the flight of the ORFEUS-SPAS II mission. AM Her was in a high optical state at the time of the observations, and the spectra reveal emission lines of O VI λλ1032, 1038, C III λ977, λ1176, and He II λ1085 superposed on a nearly flat continuum. Continuum flux variations can be described as per Gansicke et al. by a ~ 20 kK white dwarf with a ~ 37 kK hot spot covering a fraction f~0.15 of the surface of the white dwarf, but we caution that the expected Lyman absorption lines are not detected. The O VI emission lines have narrow and broad component structure similar to that of the optical emission lines, with radial velocities consistent with an origin in the irradiated face of the secondary and the accretion funnel, respectively. The density of the narrow- and broad-line regions is n_{nlr} ~ 3\times 10^{10} cm^{-3} and n_{blr} ~ 1\times 10^{12} cm^{-3}, respectively, yet the narrow-line region is optically thick in the O VI line and the broad-line region is optically thin; apparently, the velocity shear in the broad-line region allows the O VI photons to escape, rendering the gas effectively optically thin. Unexplained are the orbital phase variations of the emission-line fluxes.

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arXiv1997

X-ray and EUV Spectroscopy of the Boundary Layer Emission of Nonmagnetic Cataclysmic Variables

EUVE, ROSAT, and ASCA observations of the boundary layer emission of nonmagnetic cataclysmic variables (CVs) are reviewed. EUVE spectra reveal that the effective temperature of the soft component of high-Mdot nonmagnetic CVs is kT ~ 10-20 eV and that its luminosity is ~ 0.1-0.5 times the accretion disk luminosity. Although the EUV spectra are very complex and belie simple interpretation, the physical conditions of the boundary layer gas are constrained by emission lines of highly ionized Ne, Mg, Si, and Fe. ROSAT and ASCA spectra of the hard component of nonmagnetic CVs are satisfactorily but only phenomenologically described by multi-temperature thermal plasmas, and the constraints imposed on the physical conditions of this gas are limited by the relatively weak and blended lines. It is argued that significant progress in our understanding of the X-ray spectra of nonmagnetic CVs will come with future observations with XMM, AXAF, and Astro-E.

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arXiv1997

The EUV and X-ray Emission of Nonmagnetic Cataclysmic Variables

Recent results are presented and discussed regarding the EUV and X-ray emission of nonmagnetic cataclysmic variables. Emphasis is given to high accretion rate systems (novalike variables and dwarf novae in outburst), and to a number of apparent discrepancies between observations and the theory of the boundary layer between the accretion disk and the surface of the white dwarf. Discussed are EUV and X-ray light curves, dwarf nova oscillations, and spectra, with new and previously unpublished results on SS Cyg and OY Car.

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arXiv1997

The Winds of Cataclysmic Variables

We present an observational and theoretical review of the winds of cataclysmic variables (CVs). Specifically, we consider the related problems of the geometry, ionization state, and mass-loss rate of the winds of CVs. We present evidence of wind variability and discuss the results of studies of eclipsing CVs. Finally, we consider the properties of accretion disk wind and magnetic wind models. Some of these models predict substantial angular momentum loss, which could affect both disk structure and binary evolution.

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arXiv1996

Ultraviolet Emission Line Ratios of Cataclysmic Variables

We present a statistical analysis of the ultraviolet emission lines of cataclysmic variables (CVs) based on $\approx 430$ ultraviolet spectra of 20 sources extracted from the International Ultraviolet Explorer Uniform Low Dispersion Archive. These spectra are used to measure the emission line fluxes of N V, Si IV, C IV, and He II and to construct diagnostic flux ratio diagrams. We investigate the flux ratio parameter space populated by individual CVs and by various CV subclasses (e.g., AM Her stars, DQ Her stars, dwarf novae, nova-like variables). For most systems, these ratios are clustered within a range of $\sim 1$ decade for log Si IV/C IV $\approx -0.5$ and log He II/C IV $\approx -1.0$ and $\sim 1.5$ decades for log N V/C IV $\approx -0.25$. These ratios are compared to photoionization and collisional ionization models to constrain the excitation mechanism and the physical conditions of the line-emitting gas. We find that the collisional models do the poorest job of reproducing the data. The photoionization models reproduce the Si IV/C IV line ratios for some shapes of the ionizing spectrum, but the predicted N V/C IV line ratios are simultaneously too low by typically $\sim 0.5$ decades. Worse, for no parameters are any of the models able to reproduce the observed He II/C IV line ratios; this ratio is far too small in the collisional and scattering models and too large by typically $\sim 0.5$ decades in the photoionization models.

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arXiv1996

The Quasi-Coherent Oscillations of SS Cygni

Properties of the quasi-coherent oscillations in the extreme ultraviolet (EUV) flux of the dwarf nova SS Cygni are described, with emphasis on (1) the scaling between the period of the oscillation and the EUV flux and (2) the mean power spectra and waveforms. In addition, a quasi-coherent oscillation at ν= 0.012 +/- 0.002 Hz (FWZI) is discovered and proposed to be the spin frequency of the white dwarf.

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arXiv1996

The EUV Spectrum of the Quasi-Coherent Oscillations of the Dwarf Nova SS Cygni

Data obtained by the Extreme Ultraviolet Explorer satellite are used to determine the EUV spectrum of the quasi-coherent oscillations of the dwarf nova SS Cygni. It is found that the spectrum of the oscillations is neither blue nor red nor grey relative to the net (oscillation-phase integrated) spectrum, and hence that the oscillations cannot be explained by variations in the effective temperature, absorbing column density, or effective area, respectively. Instead, it is found that the amplitude of the oscillations is high at the relative maxima of the net spectrum, and low to zero at the relative minima of the net spectrum. This behavior can be explained by either variations in the emission line flux atop a constant underlying continuum, or variations in the optical depth of a haze of overlapping absorption lines, in which case the optical depths must be $τ<1$ at the relative maxima of the net spectrum, and $τ\gg 1$ at the relative minima.

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