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Astron. Astrophys. 350, L31-L34 (1999)

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1. Introduction

One of the results of the X-ray satellite ROSAT (Trümper 1983) has been the identification of a large number of bright soft-X-ray selected AGN (e.g Thomas et al. 1998, Beuermann et al. 1999, Grupe et al. 1998a, 1999a) during its All-Sky Survey (RASS, Voges et al. 1996). Several of these sources have turned out to be transient in the ROSAT Position Sensitive Proportional Counter 0.1-2.4 keV energy window (PSPC, Pfeffermann et al. 1986). These AGN were bright only in their `high' state phase during the RASS, and they had become dramatically fainter or even disappeared by the time of their pointed observations years later. Some prominent examples are IC 3599 (Brandt et al. 1995, Grupe et al. 1995a), WPVS007 (Grupe et al. 1995b), or RX J0134.2-4258 (Grupe et al. 1999b, Komossa 1999). These three examples can be considered to be representative of three different kinds of transience. In IC 3599, an X-ray outburst (similar to that in NGC 5905, Bade et al. 1996) was observed during the RASS. An associated response was observed in the optical emission lines, as the high ionization "coronal" iron lines became much fainter when the X-ray flux decreased. WPVS007 had practically turned off by the time of its pointed observation about three years after the RASS, and RX J0134.9-4258 showed a dramatic change in its X-ray spectral shape between the RASS and the pointed observation two years later.

In this paper we discuss the case of RX J1624.9+7554. While many of the other transient soft X-ray selected galaxies show nuclear emission in their `high' and `low' states, e.g. WPVS007, and are optically identified as AGN, RX J1624.9+7554 shows the spectrum of a normal non-active galaxy in its low state.

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© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999