Einstein ring fingerprint around supermassive black holes illuminated by broad line region spectral lines

Abstract

The continuum emission from the hot and ionized inner regions of accretion disks around supermassive black holes (SMBHs), which is strongly lensed by the light ring (i.e., the unstable photon orbit) is always superimposed on that from the locally emitting plasma near the innermost stable circular orbit, masking strong-gravity effects and making their study difficult. Cleaner images of (strong-lensing)-generated brightness distributions set against a nonluminous background would thus be preferable. An SMBH illumination source that allows for this could be the more distant, cooler, neutral gas reservoir, underlying the so-called broad line region. Such a far-field illumination is ideal for producing the aforementioned images, and thus better suited for strong gravity tests. In such an illumination scenario, however, two prominent first-order effects stand out, which will be the focus of this Letter: an hourglass-like brightness distribution around the SMBH, and an associated double-peaked inner feature imprinted on the disk's global double-horned line profile, for lines of sight near the accretion disk plane. The latter effect can be discerned even in unresolved sources where only the total line profile is accessible. Interferometers operating in spectral line mode can reveal this hourglass image {\it and} the inner double peak spectral feature it imprints on the global line profile, and yield SMBH mass estimates for edge-on systems.

7 pages, 4 figures