Unipolar inductor model coupled to GW emission: energy budget and model application to RX J0806+15 and RX J1914+24

Abstract

We further discuss the Unipolar Inductor Model (UIM) coupled to GW emission (Dall'Osso et al. 2006, A&A, 447, 785) and compare it to observed properties of the two candidate ultrashort period binaries RX J0806+15 and RX J1914+24. We consider the measured orbital periods, period derivatives and inferred X-ray luminosities of these two sources and find constraints on system parameters in order for the model to account for them. We find that these properties point to the two sources being in different regimes of the UIM, with the requirement of low magnetic moment primaries (~1030 G cm3) for both. Given this weak magnetization, RX J0806+15 has a sufficiently low luminosity that it can be interpreted as having a primary spin almost synchronous to and just slightly slower than the orbital motion. Its measured orbital spin-up is only slightly affected by spin-orbit coupling and is mostly due to GW emission. RX J1914+24, on the other hand, is too bright in X-rays and has too slow an orbital spin-up for the same regime to apply. We suggest that this binary system may be emitting GWs at a significantly higher rate than implied by its measured rad s-2. The latter is explained, in this framework, by the primary spin being slightly faster than the orbital motion (). In this case, the associated spin-orbit coupling transfers to the orbit a significant amount of angular momentum, thus partially balancing that lost to GW emission. All expectations can be tested in the near future to confirm the viability of the model.