Assess the reality and physical origin of the northwestern spectral-index enhancement

Investigate whether the localized enhancement of the spectral index along the northwestern segment of M87*’s horizon-scale emission ring is intrinsic and determine its physical origin, including whether it reflects enhanced synchrotron self-absorption or altered plasma conditions near the jet–disk interface or the base of the emerging jet.

Background

The spatially resolved spectral-index map contains a localized region of comparatively high spectral index along the northwestern portion of M87*’s ring. This feature coincides spatially with the inferred foot point of the northern rim of the edge-brightened jet and could therefore provide information about the interaction between the inner accretion flow and the emerging jet.

The interpretation remains unresolved because the 86 and 230 GHz observations were not simultaneous and the 86 GHz data have limited (u,v) coverage. The authors identify future simultaneous, multi-frequency VLBI observations as the means to test whether the feature is real and to establish its physical origin.