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

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4. Discussion

4.1. The nature of RX J1624.9+7554

RX J1624.9+7554 is one of a few X-ray transient sources for which the X-ray emission vanished between the RASS and pointed observations. We found an intrinsic 0.2-2.0 keV luminosity in the rest frame of log [FORMULA] [W] [FORMULA] using the parameters from the best-fitting absorbed power law model. This high luminosity excludes the possibility of emission from a highly luminous X-ray binary in the host galaxy. It also excludes an extremely bright supernova. First, in the last 100 years no supernova has been reported around the position of RX J1624.9+7554 and second, the brightest supernovae have luminosities in the order of log L [W] = 34. Gamma Ray Burst (GRB) afterglows may be excluded because of the long relatively constant X-ray light curve of RX J1624.9+7554 over a period of more than a week during the RASS. There are several arguments against RX J1624.9+7554 being a BL Lac. First, we do not find radio emission for this source. Second, it is essentially unpolarized, third it is a spiral galaxy and so far BL Lacs have almost always been reported to be hosted by elliptical galaxies (e.g. Urry et al. 1999), and fourth the X-ray spectrum would be at the very steep end if RX J1624.9+7554 is a BL Lac (see e.g. Greiner et al. 1996, Perlman et al. 1996, Urry et al. 1996, Lamer et al. 1996 for comparison). Each argument for itself does not exclude the possibility that it is a BL Lac. However, considered all together it becomes very unlikely that RX J1624.9+7554 is a BL Lac. Another, rather simple, explanation for the vanishing of the X-ray source RX J1624.9+7554 could be a big cloud of cold absorbing gas in the line of sight. However, in this case we would expect signs of activity in the galaxy, which we do not see

Why do we consider this source to be an AGN even though it does not show any signs of activity in its optical spectrum? The source has shown a dramatic turn-off in X-rays on a timescale of less than two years. Variability on such a short timescale would be impossible in an ordinary galaxy simply because of the large extent. The X-ray event must have happened in a very small region that can produce both a high X-ray luminosity. The only machine that fulfills those constraints would be an AGN engine. On the first view, the vanishing of RX J1624.9+7554 in X-rays appears similar to the case of WPVS007 (Grupe et al. 1995b); we interpreted that to be due to a shift of the soft X-ray spectrum out of the ROSAT PSPC energy window. However, WPVS007 is an active Narrow-Line Seyfert 1 galaxy and RX J1624.9+7554 does not show any signs of nuclear activity, at least not in its optical spectra from 1998 and 1999. A possible explanation of the dramatic X-ray event can be the tidal disruption of a star by the central black hole.

4.2. Tidal disruption of a star

The X-ray results of RX J1624.9+7554 can be caused by an X-ray outburst similar to that seen in IC 3599 (Brandt et al. 1995, Grupe et al. 1995a) or NGC 5905 (Komossa & Bade 1999). In both cases, a tidal disruption of a star by the central black hole is considered to be a likely cause of the outburst (see Komossa & Bade and references therein). Similar to RX J1624.9+7554, the optical spectrum from NGC 5905 does not show nuclear activity. A tidal disruption of a star can occur if a star orbiting a supermassive black hole is disrupted by the gravitational field of the black hole. Part of the debris will orbit and part will fall into in black hole. This will produce an X-ray outburst such as seen for example in IC 3599. Rees (1990) estimated that statistically every 10000 years such a tidal disruption event can happen around a massive black hole in a galaxy. The estimated duration for a tidal disruption of a star is on the order of one year for a star `eaten' by a [FORMULA] black hole. However, in this case we would expect signs of activity in the galaxy, which we do not see. Tidal disruption of a star also explains why an X-ray outburst can be seen in a non-active galaxy. Outbursts can also potentially come from instabilities in an accretion disk; however, again in this case we would expect to see signs of activity in the optical spectrum. Meanwhile, another X-ray outburst has been discovered in a non-active galaxy, RX J1242.6-1119 (Komossa & Greiner 1999).

Our study of RX J1624.9+7554 is lacking one aspect: we do not have simultaneous optical and X-ray data. Therefore, unfortunately we do not know what the optical spectrum looked like during the X-ray outburst. In the case of IC 3599, we were lucky that optical observations were made about half a year after the RASS (Brandt et al. 1995). It is important for future missions to perform repeated surveys, such as it was planned for ABRIXAS. In this way we would be able to detect activity and react much faster than we were able to in the case of RX J1624.9+7554.

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© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999