Astron. Astrophys. 339, 327-336 (1998)

4. Beppo-SAX data analysis

4.1. Timing analysis

In Fig. 2 the full energy band MECS light curve of the Beppo-SAX observation is shown. It displays a smooth decreasing trend of during the  s of elapsed time. The hardness ratio between the count rates in the 1.8-3.0 and 3.0-10.5 keV bands (see Sect. 4) shows a regular trend, increasing in the first  s and decreasing thereafter. However, the for constant hypothesis is 4 over 6 degrees of freedom only. We extracted spectra during the time intervals when the HR is higher or lower than the mean value. The difference in the spectral indices of a simple power-law model between the two states is , comparable with the statistical uncertainties. We will therefore focus in the following on the time-averaged spectral behavior to achieve the maximum S/N ratio.

Fig. 2. Broadband (i.e. 1.8-10.0 keV) MECS light curve (upper panel ) and ratio (lower panel ) between the 3-10.5 and 1.8-3.0 keV bands count rates. Binning time is  s, corresponding approximately to one Beppo-SAX orbit.

4.2. Spectral analysis

A simple power-law with photoelectric absorption by cold matter is a rather good representation of the MECS spectrum (see Fig. 3). If the absorbing column density is left free to vary,  cm^-2. has been therefore constrained to be not lower than the Galactic value along the 1H0419-577 line of sight ( cm^-2, Dickey & Lockman 1990) and is always consistent with its minimum allowed value. The is formally acceptable ( d.o.f.). The spectral index is rather flat () if compared with the mean value for the Seyfert 1 Galaxies (, Nandra et al. 1997a) observed by ASCA. The unabsorbed flux in the 2-10 keV band is  erg s^-1 cm^-2 ( mCrab), corresponding to a luminosity  erg s^-1, which ranks 1H0419-577 among the most luminous Seyfert 1s in X-rays. The normalization at 1 keV is  photons cm^-2 s^-1.

Fig. 3. Spectrum (upper panel ) and residuals in units of standard deviations (lower panel ) when a simple power-law model with photoelectric absorption by cold matter is applied to the MECS and PDS data simultaneously. Each of the PDS data points has a S/N , each of the MECS data points a S/N .

If we superimpose the PDS points to the power-law best fit, they lie well on the power-law extrapolation ( d.o.f.), provided the usual relative normalization constant between MECS and PDS flux at 1 keV is assumed (Cusumano et al. 1998). The results are not affected by the residual uncertainty on .

Although we have not found clear evidence of any deviation from the simple power-law behavior, we have searched for narrow line emission features and/or changes in the continuum curvature in the broadband spectrum. If a broken power-law is used instead of a simple power-law, the is only marginally better [F-test (F) equal to 2.6, significant at only 96.0%]. We have also tried a more physical double power-law model, which results in a (basically unconstrained) very steep soft component and a . Although the improvement in the quality of the fit is not statistically negligible (99.7% significance level), an inspection by eye of the residuals (see Fig. 3) reveals that such a solution tends to account for the behavior of the lowest energy channels in the MECS bandpass. The deviation expressed in data/model ratio is there , i.e. of the same order of the calibration uncertainties; moreover, the feature below 2 keV is not the widest regular one in the residuals spectrum. We consider therefore such evidence as scarcely conclusive and will search for more robust and calibration-independent tests for it. The attempts at modeling the slight concavity of the spectrum at low energy either with thermal or partial covering models were even less successful. Table 2 reports a summary of the fit results.

Table 2. Results of the simultaneous fits of MECS and PDS data. A photoelectric absorption from cold matter with  cm^-2 was added to all the quoted models. PO=power-law, BKNPO=broken power-law, GA=Gaussian line.

Broad fluorescent lines from neutral or low-ionized iron have been detected in almost all the Seyfert 1s observed by Ginga (Nandra & Pounds 1994) and ASCA (Nandra et al. 1997a) so far. Adding a narrow (i.e. ) Gaussian emission profile to the simple power-law model improves only marginally the fit (, significant at 84.6%). If the centroid energy of the line is frozen at 6.4 keV (neutral iron) or 6.7 keV (He-like iron), the equivalent widths (EW) are  eV and  eV respectively. If the widths of the lines are allowed to vary as free parameters, the decrease of the in comparison to the narrow-line case is always negligible (). When the widths of the lines are held fixed to the mean in Nandra et al. sample (430 eV, 1997a), the upper limits on the EW are  eV.

Flattened spectra can be produced if a Compton reflection component is superimposed on a steeper intrinsic continuum. This hypothesis is supported in Seyfert galaxies by the flattening of the continuum shape at observed by Ginga (Pounds et al. 1990, Piro et al. 1990) and by the detection of broad fluorescent iron lines (Tanaka et al. 1995; Nandra et al. 1997a), which are considered to be signatures of reprocessing of the nuclear X-rays by optically thick matter, possibly in the form of a rotating accretion disk around the central black hole. We have tested such an hypothesis with the model pexrav in XSPEC , leaving as free parameters only the intrinsic spectral index and the relative normalization R between the direct and the reflected component. The other parameters are basically unconstrained and we have therefore fixed the cut-off energy of the direct spectrum at  keV (Gondek et al. 1996) and the angle between the disk axis and the line-of-sight at (Nandra et al. 1997a). The improvement of the fit is significant at 98.4% (), and the intrinsic photon index turns out to be even steeper than typical values observed in Seyfert galaxies (). The nominal best-fit values correspond to an unplausibly high amount of reflection () but the spectral parameters are basically unconstrained. The upper limits on the EW of a narrow iron line added to such a continuum are  eV and  eV in the "neutral" and "ionized" cases, respectively. Leaving the widths of the lines free results in no (i.e. ) further improvement. If we fix the intrinsic spectral index to the average value found by Nandra et al. (1997a), , whereas the EW of a narrow (broad) neutral fluorescent line is (200) eV. It is therefore unlikely that the line originates in the same relativistic X-ray illuminated disc which could be advocated as the responsible for the huge continuum reflection component, unless the disc is substantially ionized. We consider hereafter a simple power-law with photon index the best modeling of the spectral shape in the whole 1.8-40 keV band.

© European Southern Observatory (ESO) 1998

Online publication: October 21, 1998
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