NEUTRON-STAR MASSES AND RADII AS INFERRED FROM KILOHERTZ QUASI-PERIODIC OSCILLATIONS

Kilohertz quasi-periodic oscillations (kHz QPOs) have been discovered in the X-ray fluxes of eight low-mass X-ray binaries (LMXBs) with the Rossi X-Ray Timing Explorer (RXTE). The characteristics of these QPOs are remarkably similar from one source to another. In particular, the highest observed QPO frequencies for sis of the eight sources fall in a very narrow range: from 1066 to 1171 Hz. This is all the more remark able when one considers that these sources are thought to have very di fferent luminosities and magnetic fields and produce very different co unt rates in the RXTE detectors. Therefore, it is highly unlikely that this near-constancy of the highest observed frequencies is due to som e unknown selection effect or instrumental bias. In this Letter we pro pose that the highest observed QPO frequency can be taken as the orbit al frequency of the marginally stable orbit. This leads to the conclus ions that the neutron stars in these LMXBs are inside their marginally stable orbits and have masses in the vicinity of 2.0 M.. This mass is consistent with the hypothesis that these neutron stars were born wit h about 1.4 M. and have been accreting matter at a fraction of the Edd ington limit for 10(8) yr.