EUVE photometry of SS Cygni: Dwarf nova outbursts and oscillations Page: 7 of 13
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C. W. Mauche: EUVE Photometry of SS Cygni
the material would reach the inner disk and boundary layer and there radiate at EUV
wavelengths. The transition to the high state was relatively slow, because the heating
wave had to "swim upstream" against the surface density profile of the disk and the
inward-spiraling gas. The 1994 outburst, on the other hand, was of the outside-in variety.
As the heating wave swept inward through the disk, it transformed successively smaller
annuli to the high state, and in the process set this material in motion toward the white
dwarf. News of the outburst arrived at the inner disk and boundary layer with the arrival
of the heating wave, just ahead of the flow of material which would power the boundary
layer. The transition to the high state was relatively fast, because the heating wave "ran
down" the surface density profile of the disk, in the same direction as the inward-spiraling
gas. The delay of a 1 day between the rise of the optical and EUV light curves is the
length of time required for the heating wave to travel the length of the accretion disk.
This delay of - 1 day between the EUV and optical light curves is longer than the delay
of ~ 0.5-0.75 day between the FUV (950-1150 1) and optical light curves measured by
Cannizzo, Wheeler, & Polidan (1986) for the 1980 May outside-in outburst of SS Cyg.
The length of the FUV delay is shorter than the EUV delay because the disk itself
produces the FUV flux and the heating wave travels a shorter distance before heating
an annulus of the disk to the temperature (- 30,000 K) required for it to radiate in the
FUV. This difference between the FUV and EUV light curves of SS Cyg highlights the
importance of the above EUV light curves: the optical through FUV light curves measure
the response of the disk to the outburst; the EUV light curves measure the rate at which
material arrives at the boundary layer. The combination of the EUV and optical, UV,
and/or FUV light curves makes for a powerful diagnostic of the mechanisms responsible
for dwarf nova outbursts. To follow up these results, we plan to observe VW Hyi this
summer simultaneously with Voyager and EUVE.
4. Dwarf Nova Oscillations
Superposed on the long-term photometric variations associated with its dwarf nova
outbursts, SS Cyg exhibits quasi-coherent photometric oscillations ("dwarf nova oscil-
lations," see, e.g., Patterson 1981; Warner 1995) on a time scale measured in seconds.
Optical oscillations were detected by Patterson, Robinson, & Kiplinger (1978) with a
period of 9.74 s, by Horne & Gomer (1980) with periods of 8.23 and 8.50 s, and by
Patterson (1981) with periods of 10.72, 10.90, and 8.9 s. Hildebrand, Spillar, & Stiening
(1981) tracked the oscillation over an interval of 6 days and observed its period fall from
7.53 s to 7.29 s and then rise again to 8.54 s. At soft X-ray energies, C6rdova et al.
(1980) and C6rdova et al. (1984) detected oscillations in HEA 0 1 A-2 data at a period
of 9 s and 11 s, respectively. Jones & Watson (1992) detected oscillations in EXOSAT
LE data at periods between 7.4 and 10.4 s.
To search for oscillations in the EUV flux measured by the DS instrument, we proceed
as follows. First, for each of the i satellite orbits, we construct DS count rate light
curves with 1 s time resolution. Next, we phase-fold these light curves using a range of
trial periods P = Pi + [j(P2 - Pi)/imax] for j = 0,1,... , jmax. Finally, we test for an
oscillation with the given period during the given orbit by means of the X2 statistic: xi} =
I (rk -- (r))2/ou, where rk is the count rate in phase bin.k, (r) = Ij" rk/kmax,
and rk = ri. For an oscillation to be detected by this means, it must have a period
within the range of trial periods, it must have a significant amplitude, and it must be
coherent in phase and period over the S 2000 s interval during each orbit when the source
is visible to the satellite.4
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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/m1/7/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.