The long-term optical behavior of Mkn 421

¹ International School for Advanced Studies, Via Beirut 2-4, 34013 Trieste, Italy e-mail: fkliu@sissa.it
² Yunnan Observatory, Academia Sinica, P.O.Box 110, Kunming 650011, China
³ Center for Astrophysics, CCAST (World Laboratory), Beijing, China

Send offprint request to: B.F. Liu

Received: 6 February 1996
Accepted: 22 October 1996

Abstract

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 .