Spin up in RX J0806+15: The shortest period binary

Abstract

RX J0806+15 has recently been identified as the binary system with the shortest known orbital period. We present a series of observations of RX J0806+15, including new optical observations taken one month apart. Using these observations and archival data, we find that the period of this system is decreasing over time. Our measurements imply that f = 6.11 × 10 -16 Hz s-1, which is in agreement with a rate expected from the gravitational radiation for two white dwarfs orbiting at a given period. However, a smaller value of f = 3.14 × 10-16 Hz s -1 cannot be ruled out. Our result supports the idea that the 321. 5-s period is the orbital period, that the system is the shortest period binary known so far and that it is one of the strongest sources of constant gravitational radiation in the sky. Furthermore, the decrease of the period strongly favours the unipolar inductor (or electric star) model rather than the accretion models.