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;