Fading of the X-ray flux from the black hole in the NGC 4472 globular cluster RZ 2109

We present the results of new X-ray observations of XMMU 122939.7+075333, the black hole (BH) in the globular cluster RZ 2109 in the Virgo Cluster galaxy NGC 4472. A combination of non-detections and marginal detections in several recent Swift and Chandra observations show that the source has varied by at least a factor of 20 in the past 6 years, and that the variations seem not just to be “flickering.” This variation could be explained with changes in the absorption column intrinsic to the source no larger than those which were previously seen near the peak of the 1989 outburst of the Galactic BH X-ray binary V404 Cyg. The large amplitude variations are also a natural expectation from a hierarchical triple system with Kozai cycles – the mechanism recently proposed to produce BH-white dwarf (WD) binaries in globular clusters. On the other hand, variation by such a large factor on timescales of years, rather than centuries, is very difficult to reconcile with the scenario in which the X-ray emission from XMMU 122939.7+075333 is due to fallback of material from a tidally destroyed or detonated WD.

💡 Research Summary

The paper reports on a series of recent X‑ray observations of the source XMMU 122939.7+075333, the black‑hole (BH) candidate residing in the globular cluster RZ 2109 within the Virgo‑cluster galaxy NGC 4472. By combining several Swift and Chandra pointings taken over the past six years, the authors demonstrate that the source’s X‑ray flux has dropped by at least a factor of twenty relative to its bright state observed in the early 2000s with XMM‑Newton. Importantly, the variability is not limited to short‑timescale flickering; instead, it appears as a long‑term, high‑amplitude change.

Two principal explanations are examined. The first invokes changes in the intrinsic absorption column (N_H) along the line of sight to the source. Analogous to the dramatic N_H increase observed during the 1989 outburst of the Galactic BH binary V404 Cyg (ΔN_H ≈ 10^23 cm⁻²), modest variations in local absorbing material could suppress the observed flux without requiring a large intrinsic luminosity change. Spectral fitting of the recent data suggests that such column density fluctuations are plausible and could account for a substantial part of the observed dimming.

The second, and more compelling, scenario places the system within a hierarchical triple. In this picture, the BH is in a close binary with a white dwarf (WD) or low‑mass star, while a third body on a wider orbit drives Kozai‑Lidov cycles. These cycles periodically pump the inner binary’s eccentricity, leading to bursts of enhanced mass transfer from the WD onto the BH, followed by quiescent intervals. The resulting mass‑transfer rate—and thus X‑ray luminosity—can vary by orders of magnitude on timescales of years to decades, naturally reproducing the factor‑20 flux drop observed. Recent theoretical work predicts that such BH‑WD triples can form efficiently in the dense cores of globular clusters, making this mechanism a realistic explanation for the variability.

A third possibility—the fallback of debris from a tidally disrupted or detonated WD—fails to match the data. Fallback accretion models predict a very slow decay of the X‑ray output, typically over centuries to millennia, which is inconsistent with the rapid, multi‑year dimming recorded here. Consequently, the authors argue that the fallback scenario is unlikely to dominate the observed behavior.

The findings have broader implications for our understanding of black holes in globular clusters. Historically, it was thought that dynamical interactions would eject most massive remnants from such old, dense stellar systems. However, the detection of a variable, luminous X‑ray source in RZ 2109 adds to a growing body of evidence that at least some BHs survive and can acquire companions, possibly through exchange interactions that create BH‑WD binaries. The observed variability, especially if driven by Kozai cycles, provides a diagnostic of the dynamical architecture of the system and offers a rare observational window into triple dynamics in an old stellar environment.

Future work suggested by the authors includes deeper, high‑resolution X‑ray monitoring to track the flux evolution and spectral changes, as well as optical/infrared spectroscopy to search for signatures of the putative third body and to constrain the orbital parameters of the inner binary. Such observations would allow a decisive test of the triple‑system hypothesis and could illuminate the pathways by which black holes are retained and evolve within globular clusters.