Return to Home Page
(1992 Apr. 13)
Subject: SW UMa
-----------------------------------------------------------------
Discovery of "super-QPOs" during a superoutburst of a dwarf nova
SW Urase Majoris
1. Introduction
Quasi-periodic oscillations (QPOs), oscillations of short coherence
length, are widely observed in systems with accretion disks, such as
cataclysmic binaries and X-ray binaries. Although several models
have been proposed to account for them, such as reporcessing of the
light by the oribiting bolobs, radial oscillation of the accretion disk,
it has not been evident what structure produces such variation and
period. This is partly because their amplitudes is usually small
and the observations are chiefly done for X-ray binaries, whose QPO
period is often too short to detect variations between inidividual QPO
profiles. QPOs are also widely observed in dwarf novae, in which
relativistic effect and the magnetic field of the compact objects can
be reasonably neglected, and they may play a place of simpler physics
for testing the theories.
SW UMa is an SU UMa type dwarf nova (Robinson et al. 1987) with an
orbital period of 0.0568 day, and the interval of outbursts is one
to several years. The star was detected in outburst on Mach 19, 1992,
and here we reports discovery of QPOs of great amplitude, which
we call super-QPOs, three days later.
2. Observation
Observations were done by CCD camera (Thomson 7882, 576x384 pixels)
attached to the Cassegrain focus of the 60cm reflector (focal length=4.8m)
at Ouda Station, Kyoto University (Ohtani et al. 1992).
The readout region of the CCD was reduced to 384x300 pixels,
which contained the variable
and two local standards. To reduce the readout dead time, on-chip
summation of 3x3 pixels to one pixel were adopted.
The Johnson V interference filter was used. The exposure time was
20 seconds and the dead time between them was 8 seconds. Bias frames
were taken in every 20 object frames.
Totally 309 object frames were obtained between March 22.526 and 22.629 UT,
1992.
The frames, after correcting by standard de-biasing and flat fielding,
were processed by microcomupter-based automatic aperture photometry
package developed by the author.
The aperture size was 18 arcseconds in diameter and the local sky levels
were determined from pixels located between 24 and 48 arcseconds from the
individual objects.
Comparison of the local standards confirms their constancy and gives
the expected standard error of differntial magnitudes for the variable
as 0.05 magnitude. Most of this error is due to the faintness of the
standard compared to the variable. Absolute photometry was not possible
because of the non-ideal sky condition.
3. Results
Figure 1 demonstrates the light curve. The differential magnitudes
of V(SW UMa) - C(comparison) were plotted. The comparison has the
coordinates of 08h 37m 10.8s, +53deg 30' 00" (2000.0) and the V magnitude
of 12.1 according to the Guide Star Catalog.
Superhumps with an amplitude of 0.2 magnitude were observed at 22.539
and 22.597 UT and the present outburst is confirmed to be a superoutburst.
The interval is consistent with the previously reported superhump period
(Robinson et al. 1987). Secondary humps were detected at 22.561 and
22.620 UT, which correspond to phase 0.40 of the superhump maxima.
Superimposed with the superhumps, there were prominent quasi-periodic
oscillations (QPOs) with the mean period of five minutes and the maximum
amplitude of 0.2 magnitude. The same period was also detected by
Robinson et al. (1987) but in smaller amplitude.
The present QPOs are peculiar in comparison with other QPOs in cataclysmic
variables and their characteristics are summarized as follows.
1) The amplitude is unusually large, typically ten times of usual ones.
2) The profiles are not sinusoidal, but more accurately described as
recurrent dips with a typical duration of one minute. The recurrence
of dips are not strictly periodic, confirming the quasi-periodic
nature of the oscillations.
3) Most of the dips have the same depth of 0.2 magnitude and have no
tendency of variation related to the superhump period or the orbital
period.
4) Some dips can be resolved to aggregates of shorter dips. The most
prominent ones can be seen around 22.600 UT, when two dips occurred
with an interval of one minute.
5) Outside the dips, there is still quasi-periodic variability with an
amplitude of 0.1 magnitude. This variablity has similar period with
the recurrence time of the dips. The brightness maxima outside dips
tend to occur just before the dips, but sometimes around midpoints of
adjacient dips.
Judging from the shortness and the sharpness of the inidividual dips,
and the constancy of the depth, the dips may be produced by the eclipse
of the central source of visual light which produces at least 20 percent
of the total accretion disk. Because the previously reported QPO period
is consistent with the recurrence time of the dips, and variation outside
dips has common character with other QPOs in outbursting dwarf novae,
it is natural to interprete the eclipsing body as the source of QPOs.
Although it is not evident whether the eclipsing body is the orbiting
blob in the accretion disk, or the vertical enlargement of the disk,
it must have the vertically extended structure because the binary system
is not eclipsing and the inclination angle must be smaller than 80 degrees.
This observation may be the first representation of QPOs as a body
of a measurable dimension. Because such dips have not been reported
in other cataclsmic variables and in another superoutburst of the same
object, it is possible that QPOs are greatly enhanced ("super-QPOs")
at the early phase of the superoutburst, when superhumps begin to appear.
This suggests the origin of super-QPOs may be connected with the tidal
instability, which is believed to prouce superoutbursts and superhumps,
and encourages further observations of early phases of superoutbursts.
References:
Robinson, E. L., Shafter, A. W., Hill, J. A., Wood, M. A. and
Mattei, J. A., 1987, Ap. J. 313, 772. \par
Return to the Powerful Daisaku Nogami
vsnet-adm@kusastro.kyoto-u.ac.jp
