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

Abstract

The supermassive black holes ($M_{\mathrm{BH}}\sim 10^6$$-$$10^{10}{M}_{\odot }$) 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_{\mathrm{BH}}\sim 10^5$$-$$10^6{M}_{\odot }$) in low-mass galaxies present curious X-ray properties with coronal radiative output up to 100$\times$ weaker than expected. To examine this issue, we present a pilot study incorporating Very Large Array radio observations of a sample of 18 high-accretion-rate (Eddington ratios $L_{\mathrm{bol}}/L_{\mathrm{Edd}}>0.1$), mBH-powered AGNs ($M_{\mathrm{BH}}\sim 10^6{M}_{\odot }$) 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, $L_{\mathrm{R}}/L_{\mathrm{X}}$, is approximately constant. Through multiwavelength analysis, we instead find that the X-ray-weaker mBHs in our sample tend toward larger values of $L_{\mathrm{R}}/L_{\mathrm{X}}$ even though they remain radio-quiet per their optical$-$UV properties. This trend results in a tentative but highly intriguing correlation between $L_{\mathrm{R}}/L_{\mathrm{X}}$ 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.

Accepted for publication in ApJ. 26 pages, 8 figures, 6 tables

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