We further discuss the Unipolar Inductor Model ( UIM ) coupled to GW emission ( Dall ’ Osso et al .
2005 ) 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 ( \sim 10 ^ { 30 } G cm ^ { 3 } ) 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 \dot { \omega } _ { o } \simeq 6 \times 10 ^ { -17 } rad s ^ { -2 } .
The latter is explained , in this framework , by the primary spin being slightly faster than the orbital motion ( \alpha \leq 1.1 ) .
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 .