Kavli Affiliate: Luis C. Ho
| First 5 Authors: Jeremiah D. Paul, Richard M. Plotkin, W. N. Brandt, Christopher H. Ellis, Elena Gallo
| Summary:
The supermassive black holes ($M_{rm 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_{rm 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 $L_{rm bol}/L_{rm Edd} > 0.1$), mBH-powered AGNs ($M_{rm
BH} sim 10^{6}~M_odot$) 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, $L_{rm R}/L_{rm X}$, is
approximately constant. Through multiwavelength analysis, we instead find that
the X-ray-weaker mBHs in our sample tend toward larger values of $L_{rm
R}/L_{rm X}$ even though they remain radio-quiet per their optical$-$UV
properties. This trend results in a tentative but highly intriguing correlation
between $L_{rm R}/L_{rm X}$ 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.
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