ON THE DYNAMICS OF ULTRA COMPACT X-RAY BINARIES: 4U 1850-087, 4U 0513-40, AND M15 X-2

Abstract

In this work we extend our dynamical study of Ultra Compact X-ray Binaries\n(UCXB) 4U 1820-30 from Prodan and Murray 2012 to three more UCXBs in globular\nclusters: 4U 1850-087, 4U 0513-40 and M15 X-2. These three UCXBs have orbital\nperiods < 20 mins. Two of them, 4U 1850-087 and 4U 0513-40, have suspected\nluminosity variations of order of ~ 1yr. There is insufficient observational\ndata to make any statements regarding the long periodicity in the light curve\nof M15 X-2 at this point. The properties of these three systems are quite\nsimilar to 4U 1820-30, which prompt us to model their dynamics in the same\nmanner. As in the case of 4U 1820-30, we interpret the suspected long periods\nas the period of small oscillations around a stable fixed point in the Kozai\nresonance. We provide a lower limit on the tidal dissipation factor Q which is\nin agreement with results obtained for the case of 4U 1820-30.\n