ON THE SOFT EXCESS IN THE X-RAY SPECTRUM OF CIRCINUS X-1: REVISITATION OF THE DISTANCE TO CIRCINUS X-1

摘要

We report on a 300 ks BeppoSAX(0.12-200 keV) observation of Circinus X-1 (Cir X-1) at phases between 0.62 and 0.84 and on a 90 ks BeppoSAX observation of Cir X-1 at phases 0.11-0.16. Using the canonical model adopted until now to fit the energy spectrum of this source, large residuals appear below 1 keV. These are well fitted using an equivalent hydrogen column of 0.66 x 10~(22) cm~(-2), adding absorption edges of O VII, O VIII, and Ne IX in the spectra extracted from the observation at phases 0.62-0.84 and adding absorption edges of O VII, O VIII, Mg XI, and Mg XII and absorption lines of O vm and Mg xn in the spectra extracted from the observation at phases 0.11-0.16. During the observation at phases 0.62-0.84 the electron density associated with the ionized matter is ～10~(13) cm~(-3), remaining quite constant going away from the compact object. During the observation at phases 0.11-0.16, the electron density profile varies along the distance going from ～6 x 10~(13) cm~(-13) at ～10~(11) cm to ～9 x 10~(10) cm~(-3) at ～10~(13) cm. The equivalent hydrogen column toward Cir X-1 is 3 times lower than the value obtained from previous models. This low value would imply that Cir X-1 is at a distance of 4.1 kpc.