Christopher W. Mauche

Complete and simultaneous spectral observations of the black hole X-Ray nova XTE J1118+480

The X-ray nova XTE J1118+480 suffers minimal extinction (b = 62 degrees) and therefore represents an outstanding opportunity for multiwavelength studies. Hynes et al. (2000) conducted the first such study, which was centered on 2000 April 8 using UKIRT, EUVE, HST and RXTE. On 2000 April 18, the Chandra X-ray Observatory obtained data coincident with a second set of observations using all of these same observatories. A 30 ks grating observation using Chandra yielded a spectrum with high resolution and sensitivity covering the range 0.24-7 keV. Our near-simultaneous observations cover approximately 80% of the electromagnetic spectrum from the infrared to hard X-rays. The UV/X-ray spectrum of XTE J1118+480 consists of two principal components. The first of these is an approximately 24 eV thermal component which is due to an accretion disk with a large inner disk radius: > 35 Schwarzschild radii. The second is a quasi power-law component that was recorded with complete spectral coverage from 0.4-160 keV. A model for this two-component spectrum is presented in a companion paper by Esin et al. (2001).

The X-Ray Transient XTE J1118+480: Multiwavelength Observations of a Low-State Minioutburst

We present multiwavelength observations of the newly discovered X-ray transient XTE J1118+480 obtained in the rising phase of the 2000 April outburst. This source is located at unusually high Galactic latitude and in a very low absorption line of sight. This made the first Extreme Ultraviolet Explorer (EUVE) spectroscopy of an X-ray transient outburst possible. Together with our Hubble Space Telescope, Rossi X-Ray Timing Explorer, and United Kingdom Infrared Telescope data, this gives unprecedented spectral coverage. We find the source in the low hard state. The flat IR-UV continuum appears to be a combination of optically thick disk emission and possibly synchrotron, while at higher energies (including EUV), a typical low hard state power law is seen. EUVE observations reveal no periodic modulation, suggesting an inclination low enough that no obscuration by the disk rim occurs. We discuss the nature of the source and this outburst and conclude that it may be more akin to minioutbursts seen in GRO J0422+32 than to a normal X-ray transient outburst.

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

Using data obtained in 1994 June and 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 ν0 ≈ 0.012 and ν1 ≈ 0.13 Hz in the EUV flux and at ν0 ≈ 0.0090, ν1 ≈ 0.11, and possibly ν2 ≈ ν0 + ν1 ≈ 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 νhigh-νlow correlation for neutron star and black hole low-mass X-ray binaries (LMXBs) nearly 2 orders of magnitude in frequency, with νlow ≈ 0.08 νhigh. 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.

The X-ray and extreme-ultraviolet flux evolution of SS Cygni throughout outburst

We present the most complete multiwavelength coverage of any dwarf nova outburst: simultaneous optical, Extreme Ultraviolet Explorer and Rossi X-ray Timing Explorer observations of SS Cygni throughout a narrow asymmetric outburst. Our data show that the high-energy outburst begins in the X-ray waveband 0.9–1.4 d after the beginning of the optical rise and 0.6 d before the extreme-ultraviolet rise. The X-ray flux drops suddenly, immediately before the extreme-ultraviolet flux rise, supporting the view that both components arise in the boundary layer between the accretion disc and white dwarf surface. The early rise of the X-ray flux shows that the propagation time of the outburst heating wave may have been previously overestimated. The transitions between X-ray and extreme-ultraviolet dominated emission are accompanied by intense variability in the X-ray flux, with time-scales of minutes. As detailed by Mauche & Robinson, dwarf nova oscillations are detected throughout the extreme-ultraviolet outburst, but we find they are absent from the X-ray light curve. X-ray and extreme-ultraviolet luminosities imply accretion rates of 3 × 1015 g s−1 in quiescence, 1 × 1016 g s−1 when the boundary layer becomes optically thick, and ∼1018 g s−1 at the peak of the outburst. The quiescent accretion rate is two and a half orders of magnitude higher than predicted by the standard disc instability model, and we suggest this may be because the inner accretion disc in SS Cyg is in a permanent outburst state.

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

We show that the Fe XVII I(17.10 u/I(17.05 u 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 u/I(17.05 u line ratio can be explained if the plasma density ne > 3 × 10 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 Tbb > 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 × 10 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.

Observation of Quasi-Continuum Line Emission from Fe VII to Fe X in the Extreme-Ultraviolet Region below 140 Å

The line emission of Fe VII-Fe X ions in the extreme-ultraviolet region below 140 A was measured in controlled laboratory experiments under conditions representative of stellar coronae. The observations are compared with predictions from standard spectral models using the CHIANTI and MEKAL atomic databases. We find that the atomic databases miss most of the line flux in this region. While some of the missing lines form isolated features, most add up to form a quasi continuum in the 60-120 A region. This incompleteness can explain the poor fit when applying global-fitting techniques to spectra from cool stars measured by the Extreme-Ultraviolet Explorer satellite, the origin of which has been a source of controversy since the original observations were made.

Ultraviolet Emission-line Ratios of Cataclysmic Variables*

We present a statistical analysis of the ultraviolet emission lines of cataclysmic variables (CVs) based on ≈ 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 ~1 decade for log Si IV/C IV ≈ -0.5 and log He II/C IV ≈ -1.0 and ~1.5 decades for log N V/C IV ≈ -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 ~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 ~0.5 decades in the photoionization models.

IUE observations of the dwarf nova HL Canis Majoris and the winds of cataclysmic variables

An observational and theoretical study is conducted of the P Cygni profiles of cataclysmic variables, with attention to the profiles of the dwarf nova HL CMa, in light of the ionization structure of the wind for a given geometry. It is found that a spherically symmetric wind is capable of generating the observed profile shapes when the accretion disk is limb-darkened, provided that the acceleration of the wind is very low and that the wind represents a mass-loss rate of about 10 to the -11th/C IV ionization fraction of the solar mass per year; a wind of this magnitude, however, cannot be driven by radiation pressure.

Extreme Ultraviolet Explorer Observations of OY Carinae in Superoutburst

The Extreme Ultraviolet Explorer (EUVE) satellite was used for three days beginning on 1997 March 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 -1), 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 systems photoionized accretion disk wind. It is possible to trade off continuum luminosity for wind optical depth, but reasonable models have a boundary layer temperature Tbl ≈ 90-130 kK and a boundary layer and accretion disk luminosity Lbl = Ldisk 4 × 1034 ergs s-1 ≈ 10 L☉ corresponding to a mass accretion rate a 10-8 M☉ yr-1; an absorbing column density NH ≈ 1.6-3.5 × 1019 cm-2; and a wind mass-loss rate w 10-10 M☉ yr-1 ≈ 0.01a. 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.

A study of extreme-ultraviolet emission from cataclysmic variables

Voyager far- and extreme UV spectrophotometric observations of five cataclysmic variables (the dwarf novae SS Cyg and VW Hyi and the novalike variables V3885 Sgr, RW Sex, and IX Vel) are combined with neutral hydrogen column densities derived from the curve-of-growth analysis of interstellar absorption lines in high-resolution IUE spectra to place upper limits on the emitted flux in the 600-700 A EUV band. The Voyager observations of VW Hyi were obtained during both normal and superoutbursts. Detailed accretion disk model calculations show that most of the 600-700 A flux in these systems should originate in the inner accretion disk rather than in the boundary layer. For VW Hyi, the low neutral hydrogen column and excellent Voyager superoutburst data place the observed upper limit to the 600-700 A flux well below the expected EUV flux from the model calculations. 43 refs.