## 3. Spectral analysisThe 3C 390.3 data were initially divided by the 3C 273 data
collected during the Because of the complexity of the 3C 390.3 spectrum, we decided to perform the spectral analysis in two steps, studying the continuum and the emission line separately. Special care was also taken in determining the absorbing column density along the line of sight. ## 3.1. Continuum: primary emission and reflection componentWe fitted the We first tested a single power law model (PL), with low energy absorption. The best fit parameters are reported in Table 1, with 90 confidence limits. This model is unacceptable as it gives a large value of , corresponding to a chance probability of about 0.004. A broken power law model (BKP) gives a better fit (see Table 1), but still not good enough in terms of the statistic (chance probability of 0.054).
Normalization factor at 1 keV ( 10^{b}^{-3} photons cm^{-2} sec^{-1} keV^{-1}). High-energy power law for model BKP. ^{c} Power-law break in model BKP, e-folding cut-off energy in model CPL+REF.^{d}We therefore tested a more complex spectral model including, in addition to a power law, a reflection component (PEXRAV in XSPEC; see Magdziarz & Zdziarski 1995). The PEXRAV model assumes that a cold reflector is irradiated by a primary isotropic X-ray source. The input X-ray spectrum can be modeled by a simple power law or by a power-law with a high energy exponential cut-off. The general functional form for this model is: where A is a global normalization factor at 1 keV
(photons cm The MECS and PDS data are better reproduced by the model including
reflection than by the single (PL) or broken power law (BKP) models.
We initially assumed a simple power law input spectrum (PL+REF). The
reduced decreases from 1.17 to the
well acceptable value of 1.09 (chance probability of 0.17). The
reflection component is unambiguously detected at
99 confidence level, as evident from
the contour plots shown in Fig. 1. We then included a high energy
cutoff (CPL+REF), allowing to vary.
As expected, when the input X-ray spectrum is a power law with cut-off
(E
In any case, the amount of reflection required by the
## 3.2. Column densityThe low energy absorption requires a column density larger than the
Galactic value, cm
When the historical column density is plotted as a function of the date of observation, several interesting variations are evident, clearly indicating that temporal modifications of the cold material along the line of sight occurred, with an estimated time scale of a few years (see Fig. 2). In particular, the value of seems to be steadily increasing since 1992, but was always found much lower than the first Einstein detection (Kruper et al. 1990).
We did not find any correlation between and the intensity at 1 keV (see Fig. 2 and Table 3). This fact suggests that changes in the geometry of the absorber, rather than variations of its ionization state (as expected in the case of a warm absorber responding to an ionizing continuum), are probably responsible for the observed long term variability of . Similar long-term absorption variability has been also observed in NGC4151 (Yaqoob et al. 1993).
## 3.3. Emission lineAn excess in emission with respect to the power law plus reflection
continuum is clearly present in the 5.5-7.0 keV energy interval (see
Fig. 3). The statistical significance of this excess, estimated by the
quadratic sum of the deviations in each bin, is at the level of
. The excess is most likely produced
by an Fe fluorescent line, for which we could estimate, using a
gaussian profile, a flux of 3.63 (-1.45,+0.78)
ph cm
In Fig. 5 the total 0.1-100 keV photon spectrum is shown with the residuals to a power-law-plus-reflection model when the gaussian line is added to the fit.
© European Southern Observatory (ESO) 1999 Online publication: March 1, 1999 |