63 research outputs found
The White Dwarf in EM Cygni: Beyond The Veil
We present a spectral analysis of the FUSE spectra of EM Cygni, a Z Cam DN
system. The FUSE spectrum, obtained in quiescence, consists of 4 individual
exposures (orbits): two exposures, at orbital phases phi ~ 0.65 and phi ~ 0.90,
have a lower flux; and two exposures, at orbital phases phi =0.15 and 0.45,
have a relatively higher flux. The change of flux level as a function of the
orbital phase is consistent with the stream material (flowing over and below
the disk from the hot spot region to smaller radii) partially masking the white
dwarf. We carry out a spectral analysis of the FUSE data, obtained at phase
0.45 (when the flux is maximual, using the codes TLUSTY and SYNSPEC. Using a
single white dwarf spectral component, we obtain a white dwarf temperature of
40,000K, rotating at 100km/s. The white dwarf, or conceivably, the material
overflowing the disk rim, shows suprasolar abundances of silicon, sulphur and
possibly nitrogen. Using a white dwarf+disk composite model, we obtain that the
white dwarf temperature could be even as high as 50,000K, contributing more
than 90% of the FUV flux, and the disk contributing less than 10% must have a
mass accretion rate reaching 1.E-10 Msun/yr.In both cases, however, we obtain
that the white dwarf temperature is much higher than previously estimated.Comment: accepted for publication in ApJ, 3 Tables, 12 Figures (including
color figures), 33 pages in present format (possibly 10 pages in ApJ format
Ultraviolet Emission Line Ratios of Cataclysmic Variables
We present a statistical analysis of the ultraviolet emission lines of
cataclysmic variables (CVs) based on 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 decade for log Si IV/C IV and log He II/C IV
and decades for log N V/C IV . 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 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 decades in the photoionization
models.Comment: LaTeX format, uses aaspp4.sty, 28 pages, 11 Postscript figures,
accepted for publication in The Astrophysical Journal 10/16/9
An Irradiation Effect in Nova DN Gem 1912 and the Significance of the Period Gap for Classical Novae
Continuous CCD photometry of the classical nova DN Gem during 52 nights in
the years 1992-98 reveals a modulation with a period 0.127844 d. The
semi-amplitude is about 0.03 mag. The stability of the variation suggests that
it is the orbital period of the binary system. This interpretation makes DN Gem
the fourth nova inside the cataclysmic variable (CV) period gap, as defined by
Diaz and Bruch (1997), and it bolsters the idea that there is no period gap for
classical novae. However, the number of known nova periods is still too small
to establish this idea statistically. We eliminate several possible mechanisms
for the variation, and propose that the modulation is driven by an irradiation
effect. We find that model light curves of an irradiated secondary star, fit
the data well. The inclination angle of the system is restricted by this model
to 10 deg < i < 65 deg. We also refine a previous estimate of the distance to
the binary system, and find d=1.6+/-0.6 kpc.Comment: 7 pages, Latex file, 2 .ps files and 3 .eps files. accepted for
publication in MNRAS. also available at:
ftp://ftp.astro.keele.ac.uk/pub/preprints/preprints.htm
From X-ray dips to eclipse: Witnessing disk reformation in the recurrent nova USco
The 10th recorded outburst of the recurrent eclipsing nova USco was observed
simultaneously in X-ray, UV, and optical by XMM-Newton on days 22.9 and 34.9
after outburst. Two full passages of the companion in front of the nova ejecta
were observed, witnessing the reformation of the accretion disk. On day 22.9,
we observed smooth eclipses in UV and optical but deep dips in the X-ray light
curve which disappeared by day 34.9, then yielding clean eclipses in all bands.
X-ray dips can be caused by clumpy absorbing material that intersects the line
of sight while moving along highly elliptical trajectories. Cold material from
the companion could explain the absence of dips in UV and optical light. The
disappearance of X-ray dips before day 34.9 implies significant progress in the
formation of the disk. The X-ray spectra contain photospheric continuum
emission plus strong emission lines, but no clear absorption lines. Both
continuum and emission lines in the X-ray spectra indicate a temperature
increase from day 22.9 to day 34.9. We find clear evidence in the spectra and
light curves for Thompson scattering of the photospheric emission from the
white dwarf. Photospheric absorption lines can be smeared out during scattering
in a plasma of fast electrons. We also find spectral signatures of resonant
line scattering that lead to the observation of the strong emission lines.
Their dominance could be a general phenomenon in high-inclination systems such
as Cal87.Comment: Submitted to ApJ. 16 pages, 16 figure
Secular changes in the quiescence of WZ Sge: the development of a cavity in the inner disk
We find a dimming during optical quiescence of the cataclysmic variable WZ
Sge by about half a magnitude between superoutbursts. We connect the dimming
with the development of a cavity in the inner part of the accretion disk. We
suggest that, when the cavity is big enough, accretion of material is governed
by the magnetic field of the white dwarf and pulsations from the weakly
magnetic white dwarf appear. The time scale of forming the cavity is about a
decade, and it persists throughout the whole quiescent phase. Such a cavity can
be accommodated well by the proposed magnetic propeller model for WZ Sge, where
during quiescence mass is being expelled by the magnetic white dwarf from the
inner regions of the accretion disk to larger radii.Comment: 10 pages, 4 figures, accepted for publication in Astronomy and
Astrophysics; following referee report, many textual changes, figures
improved, more historic data added, conclusions unchange
Does glucagon-like peptide-1 (GLP-1) receptor agonist stimulation reduce alcohol intake in patients with alcohol dependence:Study protocol of a randomised, double-blinded, placebo-controlled clinical trial
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).Comment: 19 pages, 4 figures, resubmitted to ApJ on March 2, 200
Homologues of potato chromosome 5 show variable collinearity in the euchromatin, but dramatic absence of sequence similarity in the pericentromeric heterochromatin
- …