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.

💡 Research Summary

The authors present a systematic study of very‑long‑term X‑ray variability in low‑mass X‑ray binaries (LMXBs) using the 13‑year light curves obtained with the All‑Sky Monitor (ASM) aboard the Rossi X‑ray Timing Explorer (RXTE). From the ASM archive they selected twelve persistently bright LMXBs that were continuously detected throughout the mission, comprising seven Atoll sources and five Z sources. After constructing daily averaged count‑rate histories and applying Lomb‑Scargle periodograms, they searched for periodicities on timescales far exceeding the well‑known orbital and super‑orbital periods (i.e., several to tens of years).

The analysis reveals quasi‑periodic modulations with characteristic timescales of roughly 5–20 yr in many Atoll systems, with fractional amplitude often exceeding 30 % and in some cases approaching 50 %. Z sources also show long‑term variations of comparable timescales, but their amplitudes are modest (5–10 %). The authors argue that this dichotomy reflects the different accretion regimes: Atoll sources operate well below the Eddington limit, so modest changes in the mass‑transfer rate (Ṁ) translate directly into large changes in X‑ray luminosity. Z sources, by contrast, are typically near the Eddington limit; any increase in Ṁ is largely radiatively capped, suppressing observable luminosity swings.

To explain the origin of the multi‑year modulation, the paper invokes a magnetic‑cycle driven mechanism originally proposed for cataclysmic variables (Richman, Applegate & Patterson 1994). In this “Applegate” scenario, cyclic magnetic field re‑configurations within the donor star alter its internal angular momentum distribution, causing small but systematic changes in the orbital period. The resulting shift in the Roche‑lobe geometry modifies the potential at the L1 point, leading to periodic variations in the mass‑transfer rate. Because the magnetic cycle period in late‑type donor stars is expected to be comparable to the solar 11‑year cycle, the observed X‑ray periods naturally fall in the several‑to‑tens‑of‑years range. The authors note that similar long‑term brightness variations have been documented in CVs, supporting the idea that donor magnetic activity can modulate accretion on very long timescales.

The paper discusses several caveats. ASM’s limited sensitivity and irregular sampling introduce uncertainties in the exact period and phase of the detected signals, and the quasi‑periodic nature (non‑sinusoidal, possibly drifting) complicates precise characterization. Moreover, the sample size is modest, and not all LMXBs exhibit clear long‑term cycles, suggesting that additional factors (e.g., donor spectral type, binary separation, irradiation effects) may influence the strength of the magnetic cycle.

Future work is outlined: continuous monitoring with newer all‑sky instruments such as NICER, MAXI, and eROSITA will extend the baseline beyond two decades, improving period determination. Simultaneous optical/infrared campaigns targeting donor‑star activity indicators (Hα emission, spot‑induced light‑curve modulations, Zeeman‑broadening) could directly test the magnetic‑cycle hypothesis. Detailed binary evolution models that incorporate magnetic braking and Applegate‑type angular‑momentum redistribution would also help quantify the expected Ṁ modulation amplitude.

In summary, the study provides compelling evidence that a subset of LMXBs exhibits very‑long‑term, quasi‑periodic X‑ray variability likely driven by solar‑cycle‑like magnetic activity in the donor star, which modulates the mass‑transfer rate. The larger response of Atoll sources compared with Z sources is naturally explained by their sub‑Eddington accretion regime. This work opens a new window on the interplay between stellar magnetic cycles and accretion physics in compact binaries.