Very long-term X-ray variations in LMXBs: solar cycle-like variations in the donor?

Long-term monitoring of Low Mass X-ray Binaries (LMXBs) by the All Sky Monitor on board the Rossi X-ray Timing Explorer now covers ~13 yrs and shows that certain LMXB types display very long-term (~several to tens of years) quasi-periodic modulations. These timescales are much longer than any "super-orbital" periods reported hitherto and likely have a different origin. We suggest here that they are due to long-term variations in the mass-transfer rate from the donor, which are a consequence of solar-like magnetic cycles that lead to orbital period changes (as proposed by Richman, Applegate & Patterson 1994 for similar long-term variations in CVs). Atoll sources display much larger amplitude modulations than Z sources over these timescales, presumably because Z sources are Eddington limited and hence unable to respond as readily as Atoll sources to fluctuations in the mass-transfer rate from the donor.