EUVE photometry of SS Cygni: Dwarf nova outbursts and oscillations Page: 4 of 13
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1995, Proceedings of IA U Colloquium No. 152--Astrophysics in the Extreme Ultraviolet,
ed. S. Bowyer & B. Haisch (Cambridge: Cambridge University Press), in press
EUVE Photometry of SS Cygni:
Dwarf Nova Outbursts and Oscillations
By CHRISTOPHER W. MAUCHE
Lawrence Livermore National Laboratory, L-41, P.O. Box 808, Livermore, CA 94550, USA
We present EUVE Deep Survey photometry and AAVSO optical measurements of the 1993
August and 1994 June/July outbursts of the dwarf nova SS Cygni. The EUV and optical light
curves are used to illustrate the different response of the accretion disk to outbursts which
begin at the inner edge and propagate outward, and those which begin at the outer edge and
propagate inward. Furthermore, we describe the properties of the quasi-coherent 7-9 s sinusoidal
oscillations in the EUV flux detected during the rise and plateau stages of these outbursts.
1. Introduction
Simple theory predicts that nonmagnetic cataclysmic variables (CVs; nova-like vari-
ables and dwarf novae) should be bright EUV sources. As material sinks through the
accretion disk of a CV, half of its specific gravitational potential energy is converted
into heat, which is radiated away locally, and half is converted into orbital kinetic en-
ergy via the balance of gravitational and centrifugal forces. Consequently, the material
in the disk orbiting just above the surface of the white dwarf has a specific kinetic en-
ergy equal to GMwd/2Rwd, where Mwd and Rwd are the mass and radius of the white
dwarf. Unless the white dwarf is rotating near breakup, the material in the disk must
dissipate this amount of energy in the boundary layer between the disk and the sur-
face of the white dwarf before accreting onto the star. The luminosity of the boundary
layer will be GMwdM/2Rwd . 20 Lp for a 1 MC white dwarf accreting at a rate of
M = 10-8 MD yr-1. For the expected size of the boundary layer, this luminosity will be
radiated at EUV to soft X-ray energies.
Although the luminosities of the boundary layers in VW Hyi (Mauche et al. 1991)
and SS Cyg (Mauche, Raymond, & Mattei 1995) appear to be lower than predicted by
theory by roughly an order of magnitude, these systems are still sufficiently luminous,
sufficiently close (d 100 pc), and have sufficiently low interstellar column densities
(NH S 3 x 1019 cm-2) to be observed as bright EUV sources. Indeed, dwarf novae have
proven to be particularly rewarding targets for EUVE: their EUV flux varies on time
scales ranging from seconds to days, they produce high signal-to-noise ratio spectra, and
their spectra are bewilderingly complex. On the down side, dwarf novae can only be
observed as targets of opportunity, and so require significant and often urgent effort to
observe. The success of the existing EUVE observations of dwarf novae is due to the
efforts of the members, staff, and director of the American Association of Variable Star
Observers (AAVSO), EUVE Deputy Project Scientist Ron Oliversen, the staffs of the
Center for EUV Astrophysics (CEA), the EUVE Science Operations Center at CEA,
and the Flight Operations Team at Goddard Space Flight Center.
To date, EUVE has observed the nova-like variable IX Vel (van Teeseling et al. 1995)
and the dwarf novae SS Cyg (twice), U Gem (Long et al. 1995), and VW Hyi. Having
discussed at length the spectroscopic results from the first EUVE observation of SS Cyg
(Mauche, Raymond, & Mattei 1995), I present here a fuller, though preliminary and
largely descriptive, discussion of the photometric results from the Deep Survey (DS)
instrument from both EUVE observations of SS Cyg.
1
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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/m1/4/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.