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Astron. Astrophys. 362, 69-74 (2000)

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4. Discussion

ASCA and BeppoSAX observations of PDS 456 were taken about 5 months (observer frame) apart. The flux level of the source was lower during the ASCA observation ([FORMULA] [FORMULA] 3.6 [FORMULA] 10- 12 erg cm-2 s-1) than during the BeppoSAX one ([FORMULA] [FORMULA] 5.7 [FORMULA] 10- 12 erg cm-2 s-1). A 50% flux variation over 3 days has been detected by RXTE during a quasi-simultaneous observation with ASCA. The RXTE range of fluxes encompass the ASCA and BeppoSAX measurements. In addition, a strong flare with a doubling time of about 17 ksec is present in the RXTE observation (Reeves et al. 2000). The underlying continuum of the ASCA/RXTE spectrum ([FORMULA] 2.3-2.4) is steeper than the slope obtained from the present analysis of BeppoSAX and ASCA data and from the independent analysis of the ASCA spectrum discussed in Reeves & Turner (2000). The origin of such a discrepancy is unclear. Intercalibration errors between RXTE and ASCA and/or a significant steepening of the continuum spectrum above 10 keV may provide a plausible explanation.

The presence of a prominent ionized iron K edge, with similar values for the best-fit energy and optical depth in RXTE and BeppoSAX data, represents the most striking observed feature in PDS 456 and unambiguosly points to a highly ionized nuclear environment in this quasar. The edge parameters are equally well reproduced by either reflection off or transmission through a highly ionized gas. In the first case the ionization status inferred from the edge energy would imply a strong ([FORMULA] 300-500 eV; Matt et al. 1993) ionized (6.7-6.97 keV) iron line. If the edge originates in a high column density ([FORMULA] a few 1024 cm-2) warm gas, no strong iron line is expected (see below). The BeppoSAX and ASCA upper limits on the line intensity derived from the present analysis would then favour a transmission model. On the other hand, the tentative detection of a broad iron line in the RXTE data ([FORMULA] 1 keV, EW [FORMULA] 350 eV) would instead favour the reflection scenario (Reeves et al. 2000).

To further investigate the properties of the high column density, highly ionized absorber we have carried out more detailed calculations using the photoionization models described in Nicastro et al. (1999) and Nicastro et al. (2000a), which includes photoelectric and resonant absorption as well as gas emission. The best-fit value of the ionization parameter implies that the iron atoms are equally distributed among the 3 highest ionization states: FeXXV ([FORMULA] 30%), FeXXVI ([FORMULA] 40%) and FeXXVII (i.e. fully ionized, [FORMULA] 30%). In a spherical configuration, the net intensity of the corresponding emission lines depends (a) on the fraction of solid angle (as seen by the central source) covered by the absorber/emitter, and (b) the ratio between the outflowing ([FORMULA]) and the turbolence ([FORMULA]) gas velocities. We then ran our models for different values of the covering factor between 0.1 and 1. To maximize the net contribute of line emission, compared to absorption, we used [FORMULA]/[FORMULA] = 5, which guarantees a peak-to-peak separation of absorption and emission lines by the same transition greater than 3 times the width of these features (Nicastro et al. 2000b). The rather uncertain knowledge of the dynamical status of the gas prevents us from a more detailed treatment. The results indicate that the total equivalent width of the line blend (including recombination, fluorescent and intercombination lines) never exceeds [FORMULA]100 eV, a value lower than the upper limits derived from ASCA and BeppoSAX data, leading further support to the transmission model. Finally, the 0.1-2 keV continuum-plus-line fluxes predicted by these models allowed us to estimate the maximum contribution of gas emission to the soft component measured in both the BeppoSAX and ASCA data to be not larger than [FORMULA] 20%.

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© European Southern Observatory (ESO) 2000

Online publication: October 30, 19100