On mass-constraints implied by the relativistic precession model of twin-peak quasi-periodic oscillations in Circinus X-1

TL;DR

This paper refines the mass estimates of the neutron star in Circinus X-1 by incorporating relativistic frame-dragging effects, showing that rotation significantly affects the inferred mass from QPO observations.

Contribution

It provides a detailed analysis of how neutron star rotation influences mass estimates derived from the relativistic precession model for twin-peak QPOs.

Findings

Mass estimates increase with angular momentum j.

Acceptable fits occur for small j values.

Mass can extend up to 3 solar masses when considering rotation.

Abstract

Boutloukos et al. (2006) 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 the 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 3M_\sun. We suggest that similar increase of estimated mass due to rotational effects can be relevant for several other sources.