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Astron. Astrophys. 346, 407-414 (1999)

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6. Discussion

The broadband (0.1-200 keV) X-ray spectrum of the Seyfert 1 galaxy NGC 4593 shows most of the typical features of its class. The continuum is well described by the superposition of a power-law primary component with [FORMULA] and a Compton reflection component. The spectral index is broadly consistent both with Ginga (Nandra & Pounds 1994) and ASCA (Nandra et al. 1997) measurements, and close to the typical values observed in Seyfert 1s as a class (Nandra & Pounds 1994, Nandra et al. 1997). The best-fit nominal values on [FORMULA] is consistent with a slab geometry for the reflecting matter. The reflection component is therefore likely to originate in an X-ray illuminated relativistic accretion disk (AD), which subtends a [FORMULA] solid angle from the nuclear source. This picture is consistent with the properties of the iron emission line. Its centroid energy is well consistent with K[FORMULA] fluorescence from neutral or mildly ionized iron ([FORMULA]). The line is moderately broad ([FORMULA]). Fluorescence iron lines are expected to be produced, along with the Compton reflection continuum, in AD and broadened by the combination of gravitational and Doppler effects if the photons undergo the effects of the gravitational potential of a supermassive black hole. The [FORMULA] eV EW is around the expectation values if the AD matter has nearly solar abundances, given the measured value of [FORMULA] (Matt et al. 1992). NGC 4593 does not suffer the iron overabundance problem that affects the relativistic iron line measure in some bright objects (among which the best studied case so far: MCG-6-30-15, Tanaka et al. 1995; see also Nandra et al. 1997) or to require a contribution from a narrow line component, which may originate in the molecular torus surrounding the nuclear environment in the unification scenario (Ghisellini et al. 1994; Krolik et al. 1994), and observed in several cases (Guainazzi et al. 1996; Weaver et al. 1997; George et al. 1998; Guainazzi et al. 1998a). The line properties can in principle constrain the location of the emitting region and the geometry of the AD. Fitting the iron line profile with a relativistic model allows us to constrain the inclination of the system ([FORMULA]), under the assumptions that the bulk of the iron line emitting region is neutral.

The combination of intermediate X-ray continuum and iron emission line properties is hence perfectly consistent with the standard picture for the production of high-energy radiation in the nuclear region of radio-quiet nearby AGN. The primary continuum, with the "canonical" [FORMULA] power-law index, is isotropically produced in the neighborhood of the nuclear supermassive black hole and reprocessed via Compton down scattering and fluorescence by an optically thick and geometrically thin Shakura-Sunyaev disk (Shakura & Sunyaev 1973), whose extension is much larger than the typical size of the primary continuum production region. The reprocessing matter does not exhibit any substantial ionization or deviation from cosmic abundances.

EXOSAT (Ghosh & Soundararajaperumal 1993; Santos-Lleó et al. 1994) observed a strong and remarkably variable soft excess. Its intensity was between 0 and 45% of the extrapolated high-energy flux in seven EXOSAT observations, when soft and intermediate X-rays were measured simultaneously. In EXOSAT data, it seems to exist no obvious correlation between the soft excess and the intermediate X-ray flux or spectral index (Santos-Lleó et al. 1994). Santos-Lleó et al. (1994) discuss the soft excess in terms of thermal emission from an accretion disc and conclude that the maximum temperature [FORMULA] should be [FORMULA] eV. Such a disk blackbody component would be clearly detectable by the LECS (if not by the ASCA/SIS). If we add to the BeppoSAX "baseline" model the emission from a disc blackbody with temperature at the innermost disk radius [FORMULA] eV [FORMULA] (Pringle 1981), its luminosity is [FORMULA] erg s-1. A possible physical explanation for the disappearance of the soft excess may be a cooling of the disc temperature profile, which shifts [FORMULA] out of the BeppoSAX sensitive bandpass. Alternatively, the lack of soft excess detection could be simply due to the improved continuum determination, made possible by the broadband BeppoSAX spectral coverage. The 2-6 keV spectrum, against which Santos Lleó et al. (1994) measured the soft excess in EXOSAT data, was flatter than in our BeppoSAX baseline model ([FORMULA]). If one fits the LECS/MECS data between 2 and 6 keV, one gets indeed [FORMULA]. Extrapolating this power-law in the 0.1-2 keV BeppoSAX/LECS leaves a complex residual spectrum, with positive and negative wiggles, dominated by the OVII absorption feature. The interpretation of this residual spectrum in the broad EXOSAT/LE filters is not totally unambiguous. We conclude that the claimed soft excess might be simply an artifact, due to extrapolating the 2-6 keV spectrum into the low energy filter EXOSAT energy bandpass and/or to fitting a complex ionized absorber with the Galactic cold photoabsorption only. On the other hand, the lack of soft X-ray excess is in line with the lack of blue bump in this source (Santos-Lleó et al. 1994).

Another new result emerging from the BeppoSAX observation of NGC 4593 is the lack of any significant cutoff on the primary power-law, a lower limit on [FORMULA] being [FORMULA] keV. This outcome is consistent with the measures of cutoff energies on individual sources available so far: NGC 4151 (70-270 keV, Zdziarski et al. 1995; Piro et al. 1998), IC4329A (240-900 keV Madejski et al. 1995), MCG-6-30-15 (100-390 keV; Guainazzi et al. 1999), and NGC 5548 (110-200 keV; Nicastro et al. 1999).

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© European Southern Observatory (ESO) 1999

Online publication: May 21, 1999