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

Abstract

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 yr and shows that certain LMXB types display very long-term (approximately several to tens of years) quasi-periodic modulations. These time-scales 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 Porb changes (as proposed by Richman, Applegate & Patterson for similar long-term variations in cataclysmic variables). Atoll sources display much larger amplitude modulations than Z sources over these time-scales, 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. © 2009 The Authors. Journal compilation © 2009 RAS.