An Einstein ring fingerprint around SMBHs illuminated by BLR spectral lines

Abstract

The continuum emission from the hot and ionized inner regions of a supermassive black hole (SMBH) accretion disk that is strongly lensed by the light-ring (i.e., the unstable photon orbit), is always superimposed on that of the locally emitting plasma near the innermost stable circular orbit (ISCO), masking strong-gravity effects and making their study difficult. A cleaner image of the light-ring against a non-luminous background, not affected by the emission and all the dynamical effects taking place near the ISCO, would thus be preferable. A more distant SMBH illumination source that could accommodate this can be provided by the unique spectral lines emitted by the cooler parts of the accretion disk, the so-called Broad Line Region (BLR). Spectral line emission from the transitional region between the inner disk and the outer BLR may be ideal for obtaining a cleaner image of the light-ring, and better suited for strong gravity tests. A crucial first order effect of a BLR spectral line illumination of the SMBHs in galactic centers, and a possible smoking gun signal of strong SMBH lensing, will be an Einstein ring, whose presence could be evident even in unresolved sources where only the total line profile can be acquired. In resolved sources, the combination of information from the image and the spectrum can even facilitate the measurement of the mass of the SMBH.