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

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3. ASCA observation

3.1. Data reduction

PDS 456 was observed by the ASCA satellite (Tanaka et al. 1994) on 1998 March 7 during the AO6 phase. The focal plane instruments consist of two solid-state imaging spectrometers (SIS, Gendreau 1995) and two gas scintillation imaging spectrometers (GIS, Makishima et al. 1996), characterized by a good spectral resolution (about 2% and [FORMULA] 8% at 5.9 keV, respectively) and broad-band ([FORMULA] 0.6-10 keV) capabilities. The observations were performed in FAINT mode and then corrected for dark frame error and echo uncertainties as suggested by Otani & Dotani (1994). The data were screened with the version 1.4b of the XSELECT package with standard criteria. The final observing time is about 41 ks for both SIS and GIS detectors. Source counts were extracted from circles of 6´ radius for GIS and 3´.5 for SIS centered on the source, and background spectra were extracted from source-free regions from the same CCD chip for SIS and from the same field of view for GIS. The source count rates (after background-subtraction) are 6.90[FORMULA] [FORMULA] 10-2 and 6.13[FORMULA] [FORMULA] 10-2 counts per second for SIS and GIS, respectively.

3.2. ASCA spectral results

SIS and GIS data were fitted simultaneously allowing the relative normalizations to be free to vary, to account for residual discrepancies in the absolute flux calibration. The residuals of a single power law fit ([FORMULA], Fig. 5) are similar to those of BeppoSAX and fully consistent with the results reported by Reeves & Turner (2000) in their analysis of the same ASCA data. In particular, a soft excess and an edge appear prominently. No emission line has been detected. The 90% upper limits for the neutral, He-like and H-like lines are in the 45-70 eV range while, the 3[FORMULA] upper limits are of the order of 100-130 eV.

[FIGURE] Fig. 5. The residuals obtained by fitting the ASCA spectrum with a single power law. The soft excess at E [FORMULA] 1 keV and the counts deficit at E [FORMULA] 7 keV are similar to the BeppoSAX data.

The presence of an ionized edge is fully confirmed by ASCA data ([FORMULA]/[FORMULA]dof = 47/2 with respect to a single power law model). The edge spectral parameters are consistent with the BeppoSAX observation but less constrained due to the lower ASCA effective area at E [FORMULA] 7 keV. At low-energies, the spectrum clearly shows a soft component. The addition of a power law improves the fit.

Looking for consistency between the BeppoSAX and the ASCA data of PDS 456, we then fitted our chosen parameterization of the BeppoSAX data (model D) to the ASCA data. The edge is fully accounted for by this model. Both the ionization parameter and the warm absorbing column density are in agreement with BeppoSAX values. The ASCA data require a steeper spectral slope ([FORMULA] [FORMULA] 1.6-2.0) and a moderately lower neutral column density, but consistent, within the errors, with the BeppoSAX results.

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© European Southern Observatory (ESO) 2000

Online publication: October 30, 19100