A&A Supplement Series, Vol. 123, June II 1997, 569-574
Received February 6; accepted October 22, 1996
F.K. Liu
, B.F. Liu
, and G.Z. Xie
Send offprint request: B.F. Liu
International School for Advanced Studies,
Via Beirut 2-4, 34013
Trieste, Italy
e-mail: fkliu@sissa.it
Yunnan Observatory, Academia Sinica, P.O.Box 110,
Kunming 650011, China
Center for Astrophysics, CCAST (World Laboratory), Beijing,
China
All data available in B band for the BL Lac object Mkn 421 from 22
publications are used to construct a historical light curve, dating
back to 1900. It is found that the light curve is very complicated and
consists of a set of outbursts with very large duration. The
brightness of Mkn 421 varies from 11.6 magnitude to more than 16
magnitude. Analyses with Jurkevich method of computing periods of
cyclic phenomena reveal in the light curve two
kinds of behaviors. The first one is non-periodic with rapid, violent
variations in intensity on time scales of hours to days. The second one
is periodic with a possible period of 
years. Another possible
period of 
years is not very significant. We have tested
the robustness of the Jurkevich method. The period of about one year
found in the light curves of Mkn 421 and of other objects is a spurious
period due to the method and the observing window. We try to explain the period of 
years under the thermal instability of a slim
accretion disk around a massive black hole of mass 
.
keywords: accretion, accretion disk -- BL Lac objects: individual: Mkn 421