The last 10 years of observations increases the number of X-ray data points in our database by a factor close to 2 (for the 0.5–2 keV and 200 keV light curves) or higher than 10 (5–100 keV, where RXTE data were collected). This significant improvement led naturally to concentrate our analysis on the X-ray emission and its connection to the other energy bands, as it has been shown in the previous sections. Furthermore, the update of the X-ray light curves required the collection and analysis of the X-ray spectra corresponding to all the performed observations. As a consequence, for the X-ray band it is possible to go a step further than a study of the flux variability and describe also the variations of the different parameters characterising 3C 273 X-ray spectrum.
As already pointed out in Sect. 8.1.2, in most of the cases the simple absorbed power law model chosen for the spectral analysis is a satisfactory representation of the X-ray emission of 3C 273, within the purposes of this work, i.e. the inclusion of fluxes and photon indices in the database. Nevertheless in a small percentage of the cases, an additional component or a different model gives a significantly better description of the data. If our “band by band” fitting procedure does not allow us to study the presence of a reflection component (Grandi & Palumbo 2004), it is possible to summarise general results on the variability of other features that describe the X-ray spectrum of this object: the spectral index, the soft-excess and the iron Kαemission line.
9.4.1 Photon index evolution
In order to study the evolution of the X-ray spectral shape of 3C 273, we choose the range where most of the data are available with a good sampling, i.e. the medium X-ray band. We exclude the soft-excess and the hard X-ray domains considering only spectra taken between 2–20 keV or in a
Figure 9.20:Structure function of the 2–20 keV photon index renormalised to the expected value of the upper plateau (continuous line). The dashed line is the expected value of the lower plateau. The power law fit is shown with a dotted line.
narrower band. We concentrate on the period 1996–2006 where a very good coverage is provided by RXTE, BeppoSAX, XMM and Swift. The photon indexΓmediumhas an average value of 1.71±0.07 and shows variations on both short and long time scales. The main feature of the photon index evolution with time (see Fig. 13 in Soldi et al. 2008) is an increase of∆Γmedium =0.2 within 2 years, followed by a slower decrease that seems to be still on going. The long term variations ofΓmediumare confirmed by the structure function analysis of this data set. The SF (Fig. 9.20) rises from the noise at about 18 hours and does not show a clear upper plateau, which translates into the presence of variability on time scales as short as∼1 day and longer than the sampled time period. The SF is well fitted by a power law with a slopeαSF =0.23±0.01.
In a number of Seyfert galaxies, a correlation has be found between the X-ray flux and the photon index (see Zdziarski et al. 2003 and references there), with the spectrum softening when the source brightens. This relation is interpreted as due to the interaction between a cold and a hot media. The X-rays would be produced by Compton upscattering in a hot plasma of the soft photons coming from the cold medium. As the flux variability of the soft photons is assumed to be stronger than that of the plasma, the larger the irradiating flux of seed photons, the softer and stronger the X-ray spectrum. We test the presence of such a relation in our data and find different results depending on the randomly selected time period. For example, no correlation is found for the complete data sample, a positive correlation (Rcorr = 0.55 and Prand = 3× 10−3) is found in 1999–2000 and an anti-correlation in 2001–2005 (Rcorr = −0.26 and Prand = 10−4; Fig. 9.21). This suggests that for 3C 273 there is not a clear trend in the flux−Γ relation and the found correlations are rather random occurrences. In particular, if the correlation of 1999–2000 was a signature of the hot corona component (Kataoka et al. 2002), it would be difficult to justify the anti-correlation in 2001–2005, period during which the minimum of the jet emission should imply the dominance at high energies of the thermal emission over the non-thermal one.
The X-ray spectral variability 143
Figure 9.21: Flux versus photon index in the 2–20 keV band. Open triangles indicate data taken in the period 1999–2000 where a correlation is found. Squares represent the 2001–2005 data set that shows an anti-correlation. The remaining points (dots) complete the 1996–2006 sample for which we do not obtain any significant correlation. A typical error bar is shown in the centre of the figure.
9.4.2 The soft-excess
Using data from the satellites that cover simultaneously the band from 0.5 keV to 8–10 keV and for which a separated fit is performed for the energy ranges below and above 2 keV (i.e. BeppoSAX, XMM and Swift), we study the evolution over the last 10 years of the flux of the soft-excess. In order to estimate the importance of the soft-excess we compute the ratio Fmeas/Fextrbetween the 1 keV flux measured through the 0.5–2 keV fit and the 1 keV flux extrapolated from the best fit to the 2–10 keV data. Whenever this ratio is larger than 1, an excess emission at soft energies is detected. BeppoSAX fluxes are rescaled following the cross-calibration results shown in Table 2 of Soldi et al. (2008) in order to match what was obtained with the XMM quasi-simultaneous observations. In Fig. 9.22 the evolution of the Fmeas/Fextrratio with time shows how the soft-excess was always present in the last 10 years of observations (see also Grandi & Palumbo 2004, Page et al. 2004). The only exception could be found in the Swift data set of December 17, 2005, but the error bars are too large to draw a firm conclusion. The soft-excess was already found to vary (e.g. Turner et al. 1990, Page et al. 2004) and this trend is confirmed by our data, with large variability on both short and long time scales and flux changes up to about 50%.
9.4.3 The broad line at 6.4 keV
In spite of the fact that most of the X-ray spectra presented here are well fitted (χ2red <1) by a simple absorbed power law in the 2–10 keV range, adding a redshifted Gaussian line significantly improves the fit in a number of cases. Due to the evidence found by Yaqoob & Serlemitsos (2000) and Kataoka et al. (2002) of a broad iron Kα emission line (although a narrow line was also detected, see for example Grandi & Palumbo 2004), and due to the limited sensitivity of RXTE/PCA, we choose to fix the energy and width of the line at 6.4 keV (in the source rest frame) and 0.8 keV, respectively
Figure 9.22: Evolution with time of the ratio between the flux at 1 keV obtained from the 0.5–2 keV fit and that extrapolated from the fit in the 2–10 keV range. When this ratio is larger than one, a soft-excess is present.
BeppoSAXdata are represented by triangles,XMMby squares andSwiftby stars.
(Kataoka et al. 2002) and apply the same fitting procedure to XMM, BeppoSAX and Swift spectra. To test the significance in the improvement of the fit, we apply a F-test and consider as significant the cases for which the probability associated was lower than 1%. We are aware that the use of the F-test in case of an emission line might lead to understate the presence of the line, and then to possibly miss the detection of weak lines (Protassov et al. 2002). Nevertheless, we use the F-test just to present a qualitative study of the iron line, as a deep analysis of this feature is beyond the scope of this work. In 85 cases the F-test indicates that a line is significantly detected (see an example in Fig. 9.23). Among them, there are 81 RXTE detections (see also Yaqoob & Serlemitsos 2000 and Kataoka et al. 2002), 4 XMM (Chernyakova et al. 2007) and 3 BeppoSAX ones (Grandi et al. 1997; Grandi & Palumbo 2004).
10 5
−0.2−0.100.10.2
counts/sec/keV
Energy [keV]
10 5
−0.2−0.100.10.2
counts/sec/keV
Energy [keV]
Figure 9.23: Residuals obtained when fitting theRXTE/PCA data collected in February 1, 2002, with a simple absorbed power law (left panel) and with an absorbed power law plus a redshifted Gaussian line (right panel).
The addition of the line results in a significant improvement of the χ2red corresponding to a probability of 10−5. The energy and width of the line are fixed (see text) and the line flux resulting from the fit is5.6× 10−4ph cm−2s−1.
The global picture 145 The intensity of the line varies in these spectra between 5.3×10−5and 6.7×10−4ph cm−2s−1, with an average value of 3.1×10−4ph cm−2s−1. No correlation is found between the parameters of the line and the slope and intensity of the underlying continuum. We therefore detect the presence of a broad line in less than 10% of the spectra analysed and confirm the variability of this feature, whose detection supports the presence of a Seyfert-like component, whereas its non-detection in most of the cases testifies the dominance of the non-thermal jet component in the X-ray emission of 3C 273.