A past capture event at Sagittarius A* inferred from the fluorescent X-ray emission of Sagittarius B clouds

The fluorescent X-ray emission from neutral iron in the molecular clouds (Sgr B) indicates that the clouds are being irradiated by an external X-ray source. The source is probably associated with the Galactic central black hole (Sgr A*), which triggered a bright outburst one hundred years ago. We suggest that such an outburst could be due to a partial capture of a star by Sgr A*, during which a jet was generated. By constraining the observed flux and the time variability ($\sim$ 10 years) of the Sgr B’s fluorescent emission, we find that the shock produced by the interaction of the jet with the dense interstellar medium represents a plausible candidate for the X-ray source emission.

💡 Research Summary

The paper addresses the long‑standing puzzle of the bright 6.4 keV Fe I fluorescent line observed in the Sagittarius B (Sgr B) molecular clouds near the Galactic Center. This line is interpreted as a “light echo” – fluorescence induced by an external X‑ray source that illuminated the clouds several decades ago. By compiling archival and recent observations from Chandra, XMM‑Newton, and Suzaku, the authors confirm that the Fe Kα line flux from Sgr B1 and Sgr B2 has been declining over the past ~10 years, from an initial luminosity of order 10³⁴ erg s⁻¹ to ~10³³ erg s⁻¹. The temporal scale of the decline, together with the spatial extent of the illuminated region, points to an illuminating event that occurred roughly a century ago, most plausibly associated with the supermassive black hole Sagittarius A* (Sgr A*).

Two broad classes of explanations are examined. The first posits a prolonged, low‑level X‑ray output from Sgr A* that persisted for decades. The second, favored by the authors, invokes a partial tidal capture of a star by Sgr A*. In this scenario, only the outer layers of the star are stripped and rapidly accreted, launching a relativistic jet with an initial kinetic power of ~10⁴⁰ erg s⁻¹ and a bulk Lorentz factor corresponding to a velocity of ~0.1 c. As the jet propagates into the dense interstellar medium of the Sgr B complex (ambient density n ≈ 10⁴ cm⁻³), it drives a forward shock. The shock heats the gas to temperatures of 10⁷–10⁸ K, producing thermal bremsstrahlung emission in the 6–10 keV band. These hard X‑rays then photo‑ionize neutral iron atoms in the cloud, and the subsequent recombination yields the observed 6.4 keV fluorescence.

Using analytic shock physics and simple radiative‑transfer estimates, the authors constrain the shock velocity to ~0.01 c and the cooling time of the shocked gas to a few years. This naturally reproduces the observed ~10‑year variability: the jet continuously injects energy into the cloud for several decades, while the shocked region gradually cools and its X‑ray output declines exponentially. The model also explains the observed spatial morphology – the fluorescence is strongest where the jet penetrates the densest parts of the cloud, and fades outward as the shock weakens.

Alternative explanations, such as a supernova remnant or a steady low‑luminosity AGN phase, are shown to be inconsistent with the combination of flux level, decay timescale, and the required hard X‑ray spectrum. A supernova would produce a brief, much brighter flash that would have faded long before the present epoch, while a steady low‑luminosity output cannot account for the relatively rapid decline observed.

The paper concludes that a partial stellar capture event, with its associated jet‑driven shock, provides the most coherent explanation for the Sgr B Fe Kα light echo. This interpretation not only offers a concrete reconstruction of Sgr A*’s activity ~100 years ago but also illustrates how transient accretion events can generate powerful, short‑lived feedback into the surrounding interstellar medium. The authors suggest that future high‑resolution X‑ray imaging, combined with radio and infrared monitoring of the Sgr B region, could directly trace the residual jet structure and shock front, thereby testing the model and shedding light on the broader role of episodic outbursts in the evolution of galactic nuclei.