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Astron. Astrophys. 361, 153-158 (2000)

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Kilohertz QPOs, the marginally stable orbit, and the mass of the central sources - a maximum likelihood test

T. Bulik 1, W. Kluniak 1 and W. Zhang 2

1 Copernicus Astronomical Center, Bartycka 18, 00-716 Warszawa, Poland
2 Laboratory for High Energy Astrohysics NASA/GSFC, Greenbelt, MD 20771, USA

Received 10 January 2000 / Accepted 10 July 2000


Kilohertz quasi perodic oscillations (QPOs) have been detected in more than a dozen LMXBs. We show that the observations are consistent with the assumption that the maximum-frequency QPOs occur in the innermost (marginally) stable orbit allowed by general relativity. On this assumption, taking the masses of the compact objects to be randomly distributed in some interval, we use the maximum likelihood method to determine the minimum and maximum mass in the parent distribution of kHz QPO sources. At the 99% confidence level, we find the mass range of the accreting compact objects to be [FORMULA], in remarkable agreement with the expected mass range of neutron stars in LMXBs.

Key words: stars: binaries: general – stars: oscillations – X-rays: stars

Send offprint requests to: T. Bulik (bulik@camk.edu.pl)

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© European Southern Observatory (ESO) 2000

Online publication: September 5, 2000