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

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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 [FORMULA] during the [FORMULA] 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 [FORMULA] s and decreasing thereafter. However, the [FORMULA] 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 [FORMULA], 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.

[FIGURE] 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 [FORMULA] 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, [FORMULA] cm-2. [FORMULA] has been therefore constrained to be not lower than the Galactic value along the 1H0419-577 line of sight ([FORMULA] cm-2, Dickey & Lockman 1990) and is always consistent with its minimum allowed value. The [FORMULA] is formally acceptable ([FORMULA] d.o.f.). The spectral index is rather flat ([FORMULA]) if compared with the mean value for the Seyfert 1 Galaxies ([FORMULA], Nandra et al. 1997a) observed by ASCA. The unabsorbed flux in the 2-10 keV band is [FORMULA] erg s-1 cm-2 ([FORMULA] mCrab), corresponding to a luminosity [FORMULA] erg s-1, which ranks 1H0419-577 among the most luminous Seyfert 1s in X-rays. The normalization at 1 keV is [FORMULA] photons cm-2 s-1.

[FIGURE] 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 [FORMULA], each of the MECS data points a S/N [FORMULA].

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

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 [FORMULA] 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 [FORMULA]. 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 [FORMULA], 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 [FORMULA] cm-2 was added to all the quoted models. PO=power-law, BKNPO=broken power-law, GA=Gaussian line.

Broad [FORMULA] 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. [FORMULA]) Gaussian emission profile to the simple power-law model improves only marginally the fit ([FORMULA], 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 [FORMULA] eV and [FORMULA] eV respectively. If the widths of the lines are allowed to vary as free parameters, the decrease of the [FORMULA] in comparison to the narrow-line case is always negligible ([FORMULA]). 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 [FORMULA] 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 [FORMULA] 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 [FORMULA] keV (Gondek et al. 1996) and the angle between the disk axis and the line-of-sight at [FORMULA] (Nandra et al. 1997a). The improvement of the fit is significant at 98.4% ([FORMULA]), and the intrinsic photon index turns out to be even steeper than typical values observed in Seyfert galaxies ([FORMULA]). The nominal best-fit values correspond to an unplausibly high amount of reflection ([FORMULA]) but the spectral parameters are basically unconstrained. The upper limits on the EW of a narrow iron line added to such a continuum are [FORMULA] eV and [FORMULA] eV in the "neutral" and "ionized" cases, respectively. Leaving the widths of the lines free results in no (i.e. [FORMULA]) further improvement. If we fix the intrinsic spectral index to the average value found by Nandra et al. (1997a), [FORMULA], whereas the EW of a narrow (broad) neutral fluorescent line is [FORMULA] (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 [FORMULA] the best modeling of the spectral shape in the whole 1.8-40 keV band.

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© European Southern Observatory (ESO) 1998

Online publication: October 21, 1998