On the soft excess in the x-ray spectrum of circinus X-1: Revisitation of the distance to circinus X-1

Abstract

We report on a 300 ks BeppoSAX(0.12-200 keV) observation of Circinus X-1 (CirX-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 × 1022 cm-2, adding absorption edges of 0 VII, 0 VIII, and Ne ix in the spectra extracted from the observation at phases 0.62-0.84 and adding absorption edges of 0 VII, 0 VIII, Mg XI, and Mg XII and absorption lines of 0 VII 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 with the ionized matter is -1013 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 × 1013 cm-3 at ∼1011 cm to ∼9 × 1010 cm-3 at ∼1013 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.