EUVE photometry of SS Cygni: Dwarf nova outbursts and oscillations Page: 9 of 13
10 p.View a full description of this report.
Extracted Text
The following text was automatically extracted from the image on this page using optical character recognition software:
C. W. Mauche: EUVE PhoLometry of SS Cygni
FIGURE 3. Power spectrum of the DS count rate from the half-hour interval centered on MJD
9222.06. The fundamental lies at 7.50 s and the positions of the first and second harmonics and
the first subharmonic are indicated.
order equal to the period on the decline. This is true despite the fact that the period
derivative P is significantly higher on the rise compared to that on decline. Evidently,
the period of the oscillation does not depend to any significant degree on P, but only
on the DS count rate, and, by inference, only on the mass-accretion rate through the
boundary layer. Second, the amplitude of the oscillation correlates with the count rate,
being high (. 100%) when the count rate is low, and moderate (20% 10%) when the
count rate is high. Furthermore, the amplitude of the oscillation is equal at a given
count rate between the 1993 and 1994 outbursts. This fact is particularly striking in the
interval log DS count rate = -1.5 to 0, which is traversed slowly on the rise of the 1993
outburst, and very rapidly on the rise of the 1994 outburst. Finally, based on the shape
of the phase-folded light curves and on the absence of power at any of the harmonics in
power spectra of the light curves (see Fig. 3), the oscillations are purely sinusoidal.
What is the physical mechanism responsible for these oscillations? For a review of this
subject, refer to Warner (1995). The low period stability of the oscillations rules out the
rotating white dwarf (the DQ Her mechanism) as well as non-radial pulsations of the
white dwarf as the cause of the oscillations; pulsations are observed in high inclination
systems, ruling out the eclipse by the white dwarf of luminous blobs of material in the
inner disk and boundary layer; r-modes and trapped g-modes fail because more than one
mode would be excited; oscillations of the accretion disk fail because they are not confined
to a particular annulus and hence to a particular period. Viable mechanisms are more
difficult to construct. King (1985) describes a mechanism employing transient magnetic
fields which channel electrons from a hot corona down to the surface of the white dwarf.
Molteni, Sponholz, & Chakrabarti (1995) argue that shocks are an inevitable consequence
of gas flow near the inner edge of an accretion disk and that shock oscillations lead to a
cycling of the accretion luminosity when the cooling time of the shocked gas is comparable
to the radial infall time. Warner & Livio (1995) propose that the oscillations are due to
the combined action of the differentially rotating surface layers of the white dwarf and
magnetically controlled accretion.
The EUVE observations of the 1994 outburst of SS Cyg severely constrain the Warner
& Livio model. The time derivative of the kinetic energy of the rotating surface layers
of the white dwarf is Ex = 4r2IP/P3 where I = MR2 is the moment of inertia of6
Upcoming Pages
Here’s what’s next.
Search Inside
This report can be searched. Note: Results may vary based on the legibility of text within the document.
Tools / Downloads
Get a copy of this page or view the extracted text.
Citing and Sharing
Basic information for referencing this web page. We also provide extended guidance on usage rights, references, copying or embedding.
Reference the current page of this Report.
Mauche, C.W. EUVE photometry of SS Cygni: Dwarf nova outbursts and oscillations, report, May 15, 1995; California. (https://digital.library.unt.edu/ark:/67531/metadc793210/m1/9/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.