The 2001 Superoutburst of WZ Sagittae

Joseph Patterson, Gianluca Masi, Michael Richmond, Brian Martin, Edward Beshore, David Skillman, Jonathan Kemp, Tonny Vanmunster, Robert Rea, William Allen, Stacey Davis, Tracy Davis, Arne Henden, Donn Starkey, Jerry Foote, Arto Oksanen, Lewis Cook, Robert Fried, Dieter Husar, Rudolf Novák, Tut Campbell, Jeff Robertson, Thomas Krajci, Elena Pavlenko, Panos Niarchos, Orville Brettman, and Stan Walker

Publications of the Astronomical Society of the Pacific

July 2002, Volume 114, Page 721

We report the results of a worldwide campaign to observe WZ Sagittae during its 2001 superoutburst. After a 23-year slumber at V=15.5, the star rose within 2 days to a peak brightness of 8.2, and showed a main eruption lasting 25 days. The return to quiescence was punctuated by 12 small eruptions, of ~1 mag amplitude and 2 day recurrence time; these "echo outbursts" are of uncertain origin, but somewhat resemble the normal outbursts of dwarf novae. After 52 days, the star began a slow decline to quiescence.

Periodic waves in the light curve closely followed the pattern seen in the 1978 superoutburst: a strong orbital signal dominated the first 12 days, followed by a powerful common superhump at 0.05721(5) d, 0.92(8)% longer than P_orb. The latter endured for at least 90 days, although probably mutating into a "late" superhump with a slightly longer mean period [0.05736(5) d]. The superhump appeared to follow familiar rules for such phenomena in dwarf novae, with components given by linear combinations of two basic frequencies: the orbital frequency w_o and an unseen low frequency W, believed to represent the accretion disk's apsidal precession. Long time series reveal an intricate fine structure, with ~20 incommensurate frequencies. Essentially all components occurred at a frequency nw_o-mW, with m = 1, ..., n. But during its first week, the common superhump showed primary components at nw_o-W, for n = 1, 2, 3, 4, 5, 6, 7, 8, 9 (i.e., m=1 consistently); a month later, the dominant power shifted to components with m=n-1. This may arise from a shift in the disk's spiral-arm pattern, likely to be the underlying cause of superhumps.

The great majority of frequency components are red-shifted from the harmonics of w_o, consistent with the hypothesis of apsidal advance (prograde precession). But a component at 35.42 c/day suggests the possibility of a retrograde precession at a different rate, probably N=0.13±0.02 c/day.

The eclipses permit measuring the location and brightness of the mass-transfer hot spot. The disk must be very eccentric and nearly as large as the white dwarf's Roche lobe. The hot-spot luminosity exceeds its quiescent value by a factor of up to 60. This indicates that enhanced mass transfer from the secondary plays a major role in the eruption.

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Copyright © 2002 Astronomical Society of the Pacific.