**Publication date:** May 2010

**Abstract:**

Boutloukos et al. discovered twin-peak quasi-periodic oscillations

(QPOs) in 11 observations of the peculiar Z-source Circinus X-1. Among

several other conjunctions the authors briefly discussed the related

estimate of the compact object mass following from the geodesic

relativistic precession model for kHz QPOs. Neglecting the neutron star

rotation they reported the inferred mass M _{0} = 2.2 ± 0.3 M

_{sun}. We present a more detailed analysis of the estimate

which involves the frame-dragging effects associated with rotating

spacetimes. For a free mass we find acceptable fits of the model to data

for (any) small dimensionless compact object angular momentum j = cJ/GM

^{2}. Moreover, quality of the fit tends to increase very gently

with rising j. Good fits are reached when M ~ M _{0}[1 + 0.55(j

+ j ^{2})]. It is therefore impossible to estimate the mass

without independent knowledge of the angular momentum and vice versa.

Considering j up to 0.3 the range of the feasible values of mass extends

up to 3 M _{sun}. We suggest that similar increase of estimated

mass due to rotational effects can be relevant for several other

sources.

**Authors:**

Török, Gabriel; Bakala, Pavel; Šrámková, Eva; Stuchlík, Zdeněk; Urbanec, Martin;