Kavli Affiliate: Claudio Ricci
| First 5 Authors: Peter G. Boorman, Daniel Stern, Roberto J. Assef, Abhijeet Borkar, Murray Brightman
| Summary:
Recent models suggest approximately half of all accreting supermassive black
holes (SMBHs; $M_{rm BH}$ $gtrsim$ 10$^{5}$ M$_{odot}$) are expected to
undergo intense growth phases behind Compton-thick ($N_{rm H}$ $>$ 1.5
$times$ 10$^{24}$ cm$^{-2}$) veils of obscuring gas. However, despite being a
viable source for the seeding of SMBHs, there are currently no examples known
of a Compton-thick accreting intermediate mass black hole (IMBH; $M_{rm BH}$
$sim$ 10$^{2}$ $-$ 10$^{5}$ M$_{odot}$). We present a detailed X-ray spectral
analysis of IC 750 $-$ the only AGN to-date with a precise megamaser-based
intermediate mass $<$ 10$^{5}$ M$_{odot}$. We find the equivalent width of
neutral 6.4 keV Fe K$alpha$ to be 1.9$^{+2.2}_{-1.0}$ keV via phenomenological
modelling of the co-added 177 ks Chandra spectrum. Such large equivalent widths
are seldom produced by processes other than fluorescence from dense
obscuration. We fit three physically-motivated X-ray spectral models to infer a
range of possible intrinsic 2$-$10 keV luminosity posteriors that encompass the
systematic uncertainties associated with a choice of model. Despite a wide
range of predicted intrinsic 2$-$10 keV luminosities between $sim$ 10$^{41}$
and 10$^{43}$ erg s$^{-1}$, all three models agree that IC 750 has a
Compton-thick line-of-sight column density to $>$ 99% confidence.
Compton-thick obscuration is well-documented to impinge substantial bias on the
pursuit of SMBH AGN. Our results thus provide the first indication that
Compton-thick obscuration should also be properly considered to uncover and
understand the IMBH population in an unbiased manner.
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