It is widely accepted that the huge energy output of Active Galactic Nuclei (AGN) is due to the release of gravitational energy by matter falling onto a supermassive (-) black hole. It has been known since the beginning of the X-ray astronomy that the spectrum of Seyfert 1 galaxies above a few keV and up to a few tens of keV can be described at the 0-th order by a simple power-law, with typical photon index (Mushotzky 1984). Later on, an emission line from neutral or mildly ionized iron (Holt et al. 1980; Perola et al. 1986; Pounds et al. 1990) and a flattening of the spectrum above keV (Pounds et al. 1990; Piro et al. 1990) were discovered to be common spectral features in the class as well. Both these features have been interpreted as the effect of reprocessing of the primary radiation by optically thick matter surrounding the nucleus (George & Fabian 1991; Matt et al. 1991). The detection of the double-horned and redshifted profile of the iron line in a long-look ASCA observation of the archetypical Seyfert 1 MCG-6-30-15 (Tanaka et al. 1995; Iwasawa et al. 1996), together with the general evidence that the iron lines are on the average broad in this class (Nandra et al. 1997), gave the first direct evidence that the reprocessing matter is located close to the black hole, probably in a Keplerian accretion disk.
In soft X-rays, an excess above the extrapolation of the high-energy power-law was measured in 50% of the Seyferts observed by EXOSAT (Turner & Pounds 1989) and about 90% of those observed by ROSAT (Turner et al. 1993; Walter & Fink 1993). The good correlation with Optical/UV (Walter & Fink 1993; Puchnarewicz et al. 1996; Laor et al. 1997) has traditionally supported the idea that this excess represents the hard tail of thermal emission from the accretion disk (Czerny & Elvis 1987), although in at least a few cases reprocessing by ionized matter seems a more viable explanation (Piro et al. 1997; Guainazzi et al. 1998b). However, caution must be employed when evaluating the soft excess in band-limited detectors and/or from multi-instrumental fits of not simultaneous observations. The extrapolation of 1-10 keV power-law fits can yield "faked" soft excesses, because the so determined spectral indices can be harder than the true one, due to the flattening contribution of the reflection component. Eventually, the discovery of absorption edges from highly ionized species of oxygen in almost 50% of the Seyfert 1s observed by ASCA so far (Reynolds 1997; George et al. 1998) has suggested that in some cases the apparent soft excess could be simply due to a mis-fit of a complex and ionized absorber.
Given the spectral complexity outlined above a detailed and self-consistent description of the X-ray spectra of Seyfert 1s requires broadband spectral observations. Moreover, the various spectral components are expected to be produced in different physical regions (see Mushotzky et al. 1993 for a review of the pre-ASCA interpretative scenario, which is still largely valid) and are therefore expected to exhibit different variability patterns and/or a delayed response to the changes of the nuclear radiation flux. The simultaneous measure of all the spectral components is a crucial requirement for any X-ray observation of Seyfert galaxies (see the discussion in Cappi et al. 1996 about the dependence of the iron line measurements on the underlying continuum determination). The Italian-Dutch satellite BeppoSAX (Boella et al. 1997a) carries a scientific payload which covers the unprecedented wide energy band between 0.1 and 200 keV, with imaging capabilities and good energy resolution in the 0.1-10 keV band. A program of spectral survey of a sizeable sample of Seyfert 1 galaxies is ongoing, and in this paper we report the results of the observation of NGC 4593.
NGC 4593 (, ) is a nearby () barred spiral galaxy of Hubble type SBb, which hosts a Seyfert 1 nucleus. The Spectral Energy Distribution (SED) is characterized by a very weak or missing "blue bump" (Santos-Lleó et al. 1994). In X-rays, the source is variable, on timescales of weeks-months, by a factor of 3 in the intermediate (i.e. : 2-10 keV) and 4-5 in the soft X-rays (Ghosh & Soundararajaperumal 1993). In NGC 4593 a soft excess above the extrapolation of the intermediate X-ray power-law was observed both by EXOSAT (Ghosh & Soundararajaperumal 1993; Santos-Lleó at al. 1995) and ROSAT (Walter & Fink 1993). It was claimed to be variable, ranging from 0 to of the intermediate X-ray extrapolated flux. However, this picture is further complicated by the ASCA discovery of a warm absorber (Reynolds 1997), with optical depths of the OVII and OVIII photoionization edges equal to and 0.1, respectively. There was no significant evidence of broadening of a weak (Equivalent Width eV) fluorescent line from neutral iron (centroid energy keV) in the ASCA data (Nandra et al. 1997). In the hard X-ray regime, BATSE measured a 20-100 keV flux of erg cm-2 s-1 (Malizia et al. 1997), whereas the 50-150 keV OSSE 2- upper limit was erg cm-2 s-1 (Johnson et al. 1993).
This paper is organized as follows. In Sect. 2 we describe the data reduction and preparation. Sect. 3 and 4 deal with the timing and spectral analysis of BeppoSAX data, respectively. In Sect. 5, we compare our findings with a reanalysis of archival ASCA data of the same object. The results are discussed in Sect. 6.
© European Southern Observatory (ESO) 1999
Online publication: May 21, 1999