The BeppoSAX 0.1 - 18 keV Spectrum of the Bright Atoll Source GX 9+1: an Indication of the Source Distance

Abstract

We report the results of a long, 350 ks, BeppoSAX observation of the bright atoll source GX 9+1 in the 0.12 - 18 keV energy range. During this observation GX 9+1 showed a large count rate variability in its lightcurve. From its color - color diagram we selected six zones and extracted the source energy spectrum from each zone. We find that the model, composed of a blackbody plus a Comptonized component absorbed by an equivalent hydrogen column of similar to 1.4 x 10(22) cm(-2), fits the spectra in the energy range 1 - 18 keV well; however, below 1 keV a soft excess is present. We find that the spectrum of GX 9+1, in the 0.12 - 18 keV energy range, is well fitted by the model above, if we use an equivalent hydrogen column of similar to 0.8 x 10(22) cm(-2), together with several absorption edges from ionized matter ( O VII, O VIII, Ne IX, Ar XVII, and Ca XX) and an absorption line from Si XIV. From the study of these features we deduce that the electron density of the plasma and the equivalent hydrogen column density associated with the ionized matter is similar to 10(12) cm(-3) and similar to 10(23) cm(-2), respectively, at a distance from the central object of r similar to 10(11) cm, while at larger distances the equivalent hydrogen column density associated to the ionized matter decreases down to similar to 10(22) cm(-2).