Radio Scrutiny of the X-ray-Weak Tail of Low-Mass Active Galactic Nuclei: A Novel Signature of High-Eddington Accretion?

The supermassive black holes (M ₁₇ 10^6-10^10~M_) that power luminous active galactic nuclei (AGNs), i. e. , quasars, generally show a correlation between thermal disk emission in the ultraviolet (UV) and coronal emission in hard X-rays. In contrast, some "massive" black holes (mBHs; M ₁₇ 10^5-10^6~M_) in low-mass galaxies present curious X-ray properties with coronal radiative output up to 100 weaker than expected. To examine this issue, we present new and archival 10 GHz Very Large Array observations of a sample of high-accretion-rate (Eddington ratios L ₁₎₋/L ₄₃₃ > 0. 1), mBH-powered AGNs with Chandra X-ray coverage. Empirical correlations previously revealed in samples of radio-quiet, high-Eddington AGNs indicate that the radio-X-ray luminosity ratio, L ₑ/L ₗ, is approximately constant. Through multiwavelength analysis, we instead find that the X-ray-weaker mBHs in our sample tend toward larger values of L ₑ/L ₗ even though they remain radio-quiet per their optical-UV properties. This trend results in a tentative but highly intriguing correlation between L ₑ/L ₗ and X-ray weakness, which we argue is consistent with a scenario in which X-rays may be preferentially obscured from our line of sight by a "slim" accretion disk. We compare this observation to weak emission-line quasars (AGNs with exceptionally weak broad-line emission and a significant X-ray-weak fraction) and conclude by suggesting that our results may offer a new observational signature for finding high-accretion-rate AGNs.