The recent history of the X - ray absorber in NGC 3516

We present two BeppoSAX observations of the bright Seyfert 1 galaxy NGC3516, performed four months apart (late 1996/early 1997). The earlier spectrum is considerably weaker and harder in the whole 0.1–50 keV energy range. In addition, the warm absorber oxygen features, which were already observed with ROSAT (Mathur et al. 1997) and ASCA (Kriss et al. 1996), are much less pronounced. The most straightforward explanation is that in 1996 November NGC3516 was being seen through a substantial (NH ≃ 10 22 cm) column of cold material. This is the first confirmation with modern instrumentation that NGC3516 indeed undergoes phases of strong cold X-ray absorption. We speculate that these intervals may be connected to the onset of the remarkably variable UV absorption system, making NGC3516 the best known example of low-luminosity Broad Absorption Line (BAL) quasar. The absorbing matter could be provided by clouds ablated from the rim of the circumnuclear molecular torus, seen at a rather high inclination angle.