On the Soft Excess in the X-ray Spectrum of Cir X-1: Revisitation of the Distance to Cir X-1

Abstract

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 $0.66 \times 10^{22}$ 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 VIII and Mg XII 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 to the ionized matter is $\sim 10^{13}$ cm$^{-3}$ remaining quit 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 $\sim 6 \times 10^{13}$ cm$^{-3}$ at $\sim 10^{11}$ cm to $\sim 9 \times 10^{10}$ cm$^{-3}$ at $\sim 10^{13}$ cm. The equivalent hydrogen column towards Cir X-1 is thre 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.