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Astron. Astrophys. 320, 185-195 (1997)

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9. Hardness Ratios

The X-ray hardness ratios HR1 and HR2 are defined as


where [FORMULA], [FORMULA], [FORMULA], and [FORMULA] denote the count rates in the energy bands 0.1 to 0.4 keV, 0.5 to 2.1 keV, 0.5 to 0.9 keV and 0.9 to 2.1 keV, respectively.

As can be seen from Table 4, for our three samples of Lupus TTS (CTTS, 'on-cloud' WTTS, and 'off-cloud' WTTS) obviously the mean values of HR1 and HR2 decrease in the same order as the mean extinction for these sample, i.e. the CTTS have the hardest X-ray emission and the highest extinction, while the 'off-cloud' WTTS have the softest X-ray emission as well as the lowest extinction. In Fig. 9 we show HR1 (lower panel) as well as the error in HR1 (upper panel) as function of [FORMULA] for Lupus WTTS. It seems that most of the scatter in HR1 is caused by errors rather than by an intrinsic scatter in HR1, as both the errors in HR1 and the observed scatter are of comparable magnitude. For HR2 (not plotted) the situation is quite similar. These observations indicate that WTTS might have intrinsically soft X-ray spectra, and that harder spectra, i.e., higher values of the hardness ratio, are caused by absorption. We also presume, that the intrinsic spectral energy distribution of WTTS has a rather small scatter around some mean value, i.e. most of the observed spread in the hardness ratios is due to large errors for weak sources, as can be inferred from Fig. 9. This conclusion is not applicable to the CTTS in Lupus. These are generally strongly absorbed, weak X-ray sources with hard X-ray spectra, but large errors in HR. Thus, it is not clear whether their hard X-ray spectra are due solely to absorption, or whether they are intrinsically different to that of WTTS.

[FIGURE] Fig. 9. Plot of X-ray hardness ratio HR1 vs. [FORMULA] (lower panel) and error in HR1 vs. [FORMULA] (upper panel). 'On-cloud' WTTS are denoted by open circles and 'off-cloud' WTTS by starred symbols. In the upper panel the distinction between data from pointed observations and the RASS is clearly visible.
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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998