Abstract. We report on the detection of a rapidly variable narrow Fe Kα line in Mkn 841. The source has been observed two times by XMM-Newton and simultaneously with BeppoSAX. The two observations, of about 10 ks long each, were separated by ∼15 hours. The line flux reaches a maximum during the first observation and is significantly reduced in the second one. The continuum shape and flux, instead, keep roughly constant between the two pointings. Such rapid variability of a narrow (unresolved by the XMM-pn instrument) line has never been reported in the past. These results are not easily explained in the standard cold reflection model where the narrow line component is supposed to be produced far from the primary X-ray source (e.g. from the torus) and is thus not expected to vary rapidly. Different interpretations are discussed.

The radio galaxy Centaurus A was observed by the BeppoSAX satellite five times from 1997 to 2000. From 1999 July 6 to 1999 August 17, the source was also simultaneously observed by COMPTEL on board the Compton Gamma Ray Observatory. Centaurus A has a complex spectrum with multiple extended components and a strongly absorbed (N H $ 10 23 cm À2 ) nucleus well fitted by a power law (À $ 1:8) that bends at high energies. When the BeppoSAX and COMPTEL observations are combined together, an exponential cutoff with an e-folding energy of $600 keV gives an adequate description of the spectral steepening. A complex feature in emission at 6-7 keV is resolved into two Fe K components, one narrow cold line and an ionized line centered at 6.8 keV. Significant variations have been observed in the iron feature, with the less prominent ionized line seemingly being the only one responsible for them: its variations do not appear to correlate with the strength of the continuum. The high-energy cutoff and the Fe feature suggest the presence of an accretion flow in the Centaurus A nucleus. However, the absence of a significant reflection and the narrowness of the cold line, as well as the lack of correlation between the continuum and 6.8 keV line variations, disfavor a standard cold/ionized thin disk (at least in the inner regions). A more plausible configuration might be a hot, thick, optically thin accretion flow surrounded by material with different opacities. Finally, we note that the high-energy break observed by BeppoSAX and COMPTEL could be also reasonably explained by inverse Compton radiation from a jet. If this is the case, a structured jet with outer slow layers surrounding a beamed inner region is necessary to explain the strong Fe feature observed by BeppoSAX.

Recently, BeppoSAX and ASCA have observed an unusual resurgence of soft X‐ray emission during the afterglows of GRB 970508 and 970828, together with marginal evidence for the existence of Fe lines in both objects. We consider the implications of the existence of a torus of iron‐rich material surrounding the sites of gamma‐ray bursts, as would be expected in the supra‐nova model; in particular, we show that the fireball will quickly hit this torus, and bring it to a temperature of ≈3×107 K. Bremsstrahlung emission from the heated‐up torus will cause a resurgence of the soft X‐ray emission with all expected characteristics (flux level, duration and spectral hardening with time) identical to those observed during the re‐burst. Also, thermal emission from the torus will account for the observed iron line flux. These events are also observable, for instance by new missions such as SWIFT, when beaming away from our line of sight makes us miss the main burst, as fast (soft) X‐ray transients, with durations of ≈103 s and fluences of ≈10−7–10−4 erg cm−2. This model provides evidence in favour of the supra‐nova model for gamma‐ray bursts.

The analysis of Perseus data collected with the Medium Energy Concentrator Spectrometer (MECS) on board Beppo-SAX shows that the ratio of the flux of the 8 keV line complex (dominated by Fe K β emission) over the 6.8 keV line complex (dominated by Fe K α emission) is significantly larger than predicted by standard thermal emission codes. Moreover the analysis of spatially resolved spectra shows that the above ratio decreases with increasing cluster radius.We find that, amongst the various explanations we consider, the most likely requires the plasma to be optically thick for resonant scattering at the energy of the Fe K α line. We argue that if this is the case, then measures of the iron abundance made using standard thermal emission codes, that assume optically thin emission, can significantly underestimate the true iron abundance. In the case of the core of Perseus we estimate the true abundance to be ∼ 0.9 solar in a circular region with radius of ∼ 60 kpc and centered on NGC 1275. Finally we speculate that similar results may hold for the core of other rich clusters.

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