A Study of the Outbursts in SS Cygni

Details A Study of the Outbursts in SS Cygni A Study of the Outbursts in SS Cygni

Sep 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998apj...505..344c&link_type=abstract

The Astrophysical Journal, Volume 505, Issue 1, pp. 344-351.

Astronomy and Astrophysics

Astronomy

17

Accretion, Accretion Disks, Stars: Novae, Cataclysmic Variables, Stars: Individual: Constellation Name: Ss Cygni

Scientific paper

We present a study of the characteristics of the rise and decay times in SS Cygni utilizing the daily means obtained by the American Association of Variable Star Observers. We concentrate on the most recent ~34 yr of data for which the most optimal daily means contain >~10 individual observations per daily mean. Our primary findings are the following: (1) the distribution of rise times shows a peak at ~= 0.5 days mag^-1 containing ~82% of all outbursts. The points within this peak form a distribution with deltat_rise/ ~= 0.25. The rise-time value of ~0.5 days mag^-1 may be strongly influenced by the ~1 day spacing in the daily means, and therefore represents an upper limit; (2) slow rise outbursts (historically referred to as ``anomalous'') form a broad distribution with no dominant peak; (3) the decay times show a peak at ~= 2.5 days mag^-1 containing ~86% of all outbursts. The points within this peak form a distribution with deltat_decay/ ~= 0.11; and (4) binned, composite light curves of the decay show that the functional form of the decay is close to exponential during the first half of the decay. During the second half, however, one sees a ~1-2 day interval, during which the locally defined decay time increases, sometimes substantially. Following this increase, the decay time returns to its previous level for ~1 day before entering quiescence. The two main implications for accretion disk instability theory are that (a) most outbursts must be triggered far from the inner disk to account for the preponderance of fast rises, and (b) the cooling wave that traverses the disk during the decay must travel slower for ~1 day near the end of the outburst to account for the temporary increase in the decay time. We also attempt to place our findings for SS Cygni in a broader context by considering the Bailey relation t_decay ~ P_orbital observed for dwarf novae, and its possible relevance for other types of interacting binaries.

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