1999/04/09 by Dong Lai, Lai, Dong, R. V. E. Lovelace +4
Decision Sciences · Earth and Planetary Sciences · Physics and Astronomy · #Astrophysical Phenomena and Observations #Astrophysics (astro-ph) #FOS: Physical sciences #High-pressure geophysics and materials #Scientific Measurement and Uncertainty Evaluation #astro-ph
paper · pdf · doi:10.48550/arxiv.astro-ph/9904111
AASTeX, 19 pages including 2 ps figure; submitted to ApJ 3/11/99
arxiv created 1999/04/09 · openalex publication_date 1999/04/09 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
It has recently been suggested that the maximum observed quasi-periodic oscillation (QPO) frequencies, νmax, for several low-mass X-ray binaries, particularly 4U 1820-30, correspond to the orbital frequency at the inner-most stable orbit of the accretion disk. This would imply that the neutron stars in these systems have masses \go 2~M_\odot, considerably larger than any well-measured neutron star mass. We suggest that the levelling off of νQPO may be also understood in terms of a steepening magnetic field which, although possibly dipolar at the stellar surface, is altered substantially by disk accretion, and presents a ``wall'' to the accretion flow that may be outside the innermost stable orbit. General relativistic effects add to the flattening of the νQPO-\mdot relation at frequencies below the Kepler frequency at the innermost stable orbit. We offer two other possible ways to reconcile the low value of νmax (≈ 1060 Hz for 4U 1820-30) with a moderate neutron star mass, ≈ 1.4\msun: at sufficiently large \mdot, either (i) the disk terminates in a very thin boundary layer near the neutron star surface, or (ii) νQPO is not the orbital frequency right at the inner edge of the disk, but rather at a somewhat larger radius, where the emissivity of the disk peaks.