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;