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

TL;DR

This study identifies very long-term quasi-periodic X-ray variations in Low Mass X-ray Binaries, likely caused by solar cycle-like magnetic activity in the donor stars affecting mass transfer rates over decades.

Contribution

It proposes a new explanation for long-term X-ray modulations in LMXBs as magnetic cycle-induced mass transfer variations, extending understanding beyond previously known super-orbital periods.

Findings

Long-term (~decades) quasi-periodic X-ray modulations observed in LMXBs.

Atoll sources show larger amplitude variations than Z sources.

Magnetic cycles in donor stars likely cause these long-term variations.

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 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.