59 research outputs found
A multichannel fiber optic photometer present performance and future developments
A three channel photometer for simultaneous multicolor observations was designed with the aim of making possible highly efficient photometry of fast variable objects like cataclysmic variables. Experiences with this instrument over a period of three years are presented. Aspects of the special techniques applied are discussed with respect to high precision photometry. In particular, the use of fiber optics is critically analyzed. Finally, the development of a new photometer concept is discussed
Time-resolved, multi-color photometry and spectroscopy of Virgo 4 (OU Vir): a high orbital inclination, short orbital period dwarf nova
We present multi-color photometry and time resolved spectroscopy of OU Vir.
The analysis of the quiescent light curve shows that OU Vir is characterized by
i) strong cycle-to-cycle brightness variations, and ii) hot spot modulated
light curve with grazing eclipse of the impact region. Colors are derived both
in- and out- of eclipse. The time-resolved spectroscopy allows us to produce
the radial velocity curve from the H accretion disk emission line which
possibly reveals only weak evidence for hot spot line emission. The hot spot is
believed to be a turbulent optically thick region, producing mostly continuum
emission.Comment: 8 pages (including figures), 7 figures. To Be published in A&
Advection-Dominated Accretion and Black Hole Event Horizons
The defining characteristic of a black hole is that it possesses an event
horizon through which matter and energy can fall in but from which nothing
escapes. Soft X-ray transients (SXTs), a class of X-ray binaries, appear to
confirm this fundamental property of black holes. SXTs that are thought to
contain accreting black holes display a large variation of luminosity between
their bright and faint states, while SXTs with accreting neutron stars have a
smaller variation. This difference is predicted if the former stars have
horizons and the latter have normal surfaces.Comment: 11 pages, including 2 tables and 2 figures. To appear in The
Astrophysical Journal Letter
Hubble Space Telescope Far Ultraviolet Spectroscopy of the Dwarf Nova VW Hyi in Superoutburst
We obtained three consecutive HST spectroscopic observations of a single
superoutburst of the dwarf nova VW Hydri. The spectra cover the beginning,
middle, and end of the superoutburst. All of the spectra are dominated by
strong absorption lines due to CIII (1175 \AA), Lyman alpha (1216 \AA), NIV
(1238 \AA, 1242 \AA), SII (1260-65 \AA), SIII (1300 \AA), CII (1335 \AA), SIV
(1394 \AA, 1402 \AA) and CIV (1548 \AA, 1550 \AA). We discuss the evolution of
the far UV energy distribution and line structure during the superoutburst. We
note the absence of any P Cygni line structure in the STIS spectra. Using state
of the art accretion disk models by Wade and Hubeny, we have determined
accretion rates for all three spectra, for two white dwarf masses, 0.55
M_{\sun} and 0.8 M-{\sun}. For both white dwarf masses the accretion rate
during superoutburst decreased by a factor of two from early to late in the
superoutburst. The average accretion rate during superoutburst is M_{\sun}/yr depending on the white dwarf mass.Comment: 2007, PASP, in pres
The boundary layer of VW Hyi in quiescence
In this letter, we suggest that the missing boundary layer luminosity of
dwarf novae in quiescence is released mainly in the ultraviolet (UV) as the
second component commonly identified in the far ultraviolet (FUV) as the
"accretion belt".
We present the well-studied SU UMa-type system VW Hyi in detail as a
prototype for such a scenario. We consider detailed multiwavelength
observations and in particular the recent FUSE observations of VW Hyi which
confirm the presence of a second component (the "accretion belt") in the FUV
spectrum of VW Hyi in quiescence. The temperature (50,000K) and rotational
velocity (> 3,000km/s) of this second FUV component are entirely consistent
with the optically thick region (tau = 1) located just at the outer edge of
optically thin boundary layer in the simulations of Popham (1999).
This second component contributes 20% of the FUV flux, therefore implying a
boundary layer luminosity: , while the theory (Klu\'zniak 1987) predicts, for the
rotation rate of VW Hyi's WD, . The remaining
accretion energy () is apparently advected into the star as
expected for optically thin advection dominated boundary layers. This scenario
is consistent with the recent simultaneous X-ray and UV observations of VW Hyi
by (Pandel, C\'ordova & Howell 2003), from which we deduced here that the alpha
viscosity parameter in the boundary layer region must be as small as .Comment: 4 page
Far Ultraviolet Observations of the Dwarf Nova VW Hyi in Quiescence
We present a 904-1183 A spectrum of the dwarf nova VW Hydri taken with the
Far Ultraviolet Spectroscopic Explorer during quiescence, eleven days after a
normal outburst, when the underlying white dwarf accreter is clearly exposed in
the far ultraviolet. However, model fitting show that a uniform temperature
white dwarf does not reproduce the overall spectrum, especially at the shortest
wavelengths. A better approximation to the spectrum is obtained with a model
consisting of a white dwarf and a rapidly rotating ``accretion belt''. The
white dwarf component accounts for 83% of the total flux, has a temperature of
23,000K, a v sin i = 400 km/s, and a low carbon abundance. The best-fit
accretion belt component accounts for 17% of the total flux, has a temperature
of about 48,000-50,000K, and a rotation rate Vrot sin i around 3,000-4,000
km/s. The requirement of two components in the modeling of the spectrum of VW
Hyi in quiescence helps to resolve some of the differences in interpretation of
ultraviolet spectra of VW Hyi in quiescence. However, the physical existence of
a second component (and its exact nature) in VW Hyi itself is still relatively
uncertain, given the lack of better models for spectra of the inner disk in a
quiescent dwarf nova.Comment: 6 figures, 10 printed page in the journal, to appear in APJ, 1 Sept.
2004 issue, vol. 61
Dwarf nova oscillations and quasi-periodic oscillations in cataclysmic variables - VII. OY Carinae and oscillations in dwarf novae in quiescence
We have observed dwarf nova oscillations (DNOs) in OY Car during outburst,
down through decline and beyond; its behaviour is similar to what we have
previously seen in VW Hyi, making it only the second dwarf nova to have DNOs
late in outburst that continue well into quiescence. There are also occasional
examples of DNOs in deep quiescence, well away from outburst - they have
properties similar to those during outburst, indicating similar physical causes
and structures. We discuss the occurrence of DNOs in other dwarf novae and
conclude that DNOs during quiescence are more common than often supposed and
exhibit properties similar to those seen in outburst.Comment: Accepted for publication in MNRAS; paper contains 11 figures and 2
tables. This paper has become paper VII in the serie
QPOs in Cataclysmic Variables and in X-ray Binaries
Recent observations, reported by Warner and Woudt, of Dwarf Nova Oscillations
(DNOs) exhibiting frequency drift, period doubling, and 1:2:3 harmonic
structure, can be understood as disc oscillations that are excited by
perturbations at the spin frequency of the white dwarf or of its equatorial
layers. Similar quasi-periodic disc oscillations in black hole low-mass X-ray
binary (LMXB) transients in a 2:3 frequency ratio show no evidence of frequency
drift and correspond to two separate modes of disc oscillation excited by an
internal resonance. Just as no effects of general relativity play a role in
white dwarf DNOs, no stellar surface or magnetic field effects need be invoked
to explain the black hole QPOs.Comment: Revised version. Astronomy & Astrophysics (Letters), in pres
Chandra Observations of the Dwarf Nova WX Hyi in Quiescence
We report Chandra observations of the dwarf nova WX Hyi in quiescence. The
X-ray spectrum displays strong and narrow emission lines of N, O, Mg, Ne, Si, S
and Fe. The various ionization states implied by the lines suggest that the
emission is produced within a flow spanning a wide temperature range, from T ~
10^6 K to T >~ 10^8 K. Line diagnostics indicate that most of the radiation
originates from a very dense region, with n ~ 10^{13}-10^{14} cm^{-3}. The
Chandra data allow the first tests of specific models proposed in the
literature for the X-ray emission in quiescent dwarf novae. We have computed
the spectra for a set of models ranging from hot boundary layers, to hot
settling flows solutions, to X-ray emitting coronae. WX Hyi differs from other
dwarf novae observed at minimum in having much stronger low temperature lines,
which prove difficult to fit with existing models, and possibly a very strong,
broad O VII line, perhaps produced in a wind moving at a few x 10^3 km/s. The
accretion rate inferred from the X-rays is lower than the value inferred from
the UV. The presence of high-velocity mass ejection could account for this
discrepancy while at the same time explaining the presence of the broad O VII
line. If this interpretation is correct, it would provide the first detection
of a wind from a dwarf nova in quiescence.Comment: accepted to ApJ; 19 pages, 3 figures, 1 tabl
High Speed Photometry of SDSS J013701.06-091234.9
We present high speed photometry of the Sloan Digital Sky Survey cataclysmic
variable SDSS J013701.06-091234.9 in quiescence and during its 2003 December
superoutburst. The orbital modulation at 79.71\pm0.01 min is double humped; the
superhump period is 81.702\pm0.007 min. Towards the end of the outburst late
superhumps with a period of 81.29\pm0.01 min were observed. We argue that this
is a system of very low mass transfer rate, and that it probably has a long
outburst interval.Comment: 5 pages, 8 figures. Accepted for publication in MNRA
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