Orbital Evolution of an Accreting Millisecond Pulsar: Witnessing the Banquet of a Hidden Black Widow?

Abstract

We have performed a timing analysis of all the four X-ray outbursts from the accreting millisecond pulsar SAX J1808.4−3658 observed so far by the PCA on board RXTE. For each of the outbursts we derived the best-fit value of the time of ascending node passage. We find that these times follow a parabolic trend, which gives an orbital period derivative ˙ Porb = (3.40 ± 0.18) × 10 −12 s/s, and a refined estimate of the orbital period, Porb = 7249.156499 ± 1.8 × 10 −5 s (reference epoch T0 = 50914.8099 MJD). This derivative is positive, suggesting a degenerate or fully convective companion star, but is more than one order of magnitude higher than what is expected from secular evolution driven by angular momentum losses caused by gravitational radiation under the hypothesis of conservative mass transfer. Using simple considerations on the angular momentum of the system, we propose an explanation of this puzzling result assuming that during X-ray quiescence the source is ejecting matter (and angular momentum) from the inner Lagrangian point. We have