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Astron. Astrophys. 346, 407-414 (1999)

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3. Timing analysis

NGC 4593 is well known to display rapid fluctuations in X-rays, with two-folding time [FORMULA] h (Barr et al. 1987). The count rates in the BeppoSAX instruments span indeed a dynamical range of about a factor of 2 on timescales as low as [FORMULA] s. We have therefore extensively searched for spectral variability associated with the flux changes, extracting light curves and Hardness Ratios (HR) in different energy bands and with various binning times. In Figs. 1 and 2 we show the light curves in the 0.7-3 keV, 3-10 keV and 13-200 keV with the corresponding contiguous HR. The bands have been chosen in order to sample energy intervals where different spectral components dominate, see Sect. 4. In particular, the softer band is affected by the warm absorber, which is basically transparent for photons of energy higher than 3 keV. No significant variability of the HR is detected. If a fit with a constant is performed on the 3-10 keV vs. 0.7-3 keV HR, the [FORMULA] is 29.3 for 38 degrees of freedom (dof). The same quantity for the 13-200 keV vs. 3-10 keV HR is 4.8/6 dof. A tiny perturbation of the intermediate X-ray flux in a [FORMULA] s interval around [FORMULA] s after the beginning of the observation is not accompanied by a similar phenomenology in the soft X-rays and yields therefore a decrease of the HR by a factor [FORMULA]. However, no variation in the spectral parameters above the statistical uncertainties has been detected in time-resolved spectroscopy (assuming the baseline model of Sect. 4.1). Similar results are obtained if the bands 0.1-3 and 3-10 keV are considered (see Fig. 3). We will therefore focus in the following on the time-averaged spectral behavior.

[FIGURE] Fig. 1. Light curves in the 0.7-3 keV (LECS, upper panel ) and 3-10 keV (MECS, central panel ) energy bands and the corresponding HR (lower panel ). The binning time is 2048 s. The light curves are not background-subtracted

[FIGURE] Fig. 2. Light curves in the 3-10 keV (MECS. upper panel ), 13-200 keV (PDS, central panel ) energy bands and the corresponding HR (lower panel ). The binning time is 25000 s. The MECS light curve is not background-subtracted

[FIGURE] Fig. 3. 0.1-3 vs. 3-10 keV HR versus the 3-10 keV count rate. Each data point corresponds to a 2048 s light curve bin

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© European Southern Observatory (ESO) 1999

Online publication: May 21, 1999
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