Identify the acceleration mechanism for TeV electrons in the nuclear bubbles

Determine which local particle-acceleration mechanism—magnetic reconnection, direct electric-field acceleration, intermittent internal shocks, or second-order Fermi acceleration—produces the hard X-ray-emitting TeV electrons in the interiors of the NGC 4438 nuclear bubbles, distinguishing it from the shock acceleration responsible for the shell-dominated radio emission.

Background

The joint radio–X-ray analysis indicates that the nuclear-bubble emission cannot be explained by a single electron population. The radio-emitting GeV electrons are most naturally associated with shock acceleration at the expanding bubble shell, whereas the hard X-ray emission requires a separate population of rapidly cooling TeV electrons that must be continuously accelerated within the bubble interior.

The paper discusses several candidate mechanisms for this second acceleration channel, including magnetic reconnection, direct electric-field acceleration near the black hole, intermittent internal shocks, and second-order Fermi acceleration driven by turbulence. The observations do not yet discriminate among these possibilities, leaving the physical origin of the TeV electrons unresolved.

References

Although the current data do not allow us to distinguish uniquely among these scenarios, they strongly suggest that the hard X-ray synchrotron component requires ongoing local particle acceleration distinct from the process that produces the shell-dominated radio synchrotron emission.

— Energy Partition in AGN-driven Bubbles of NGC 4438: From Nuclear Bubbles to a Galaxy-scale Outflow  (2608.25552 - Luan et al., 26 Aug 2026) in Section 4.1, “Origin of the Non-thermal Emission in the Nuclear Bubbles”