Exploring Gravitationally-Lensed $z\gtrsim6$ X-ray AGN Behind the RELICS
  clusters
  Although observations of high-redshift quasars demonstrate that many
supermassive black holes (BHs) reached large masses within one billion years
after the Big Bang, the origin of the first BHs is still a mystery. A promising
way to constrain the origin of the first BHs is to explore the average
properties of $z\gtrsim6$ BHs. However, typical BHs remain hidden from X-ray
surveys, which is due to their relatively faint nature and the limited
sensitivity of X-ray telescopes. Gravitational lensing provides an attractive
way to study this unique galaxy population as it magnifies the faint light from
these high-redshift galaxies. Here, we study the X-ray emission originating
from 155 gravitationally-lensed $z\gtrsim6$ galaxies that were detected in the
RELICS survey. We utilize Chandra X-ray observations to search for AGN in the
individual galaxies and in the stacked galaxy samples. We did not identify an
individual X-ray source that was undoubtedly associated with a high-redshift
galaxy. We stack the signal from all galaxies and do not find a statistically
significant detection. We split our sample based on stellar mass,
star-formation rate, and lensing magnification and stack these sub-samples. We
obtain a $2.2\sigma$ detection for massive galaxies with an X-ray luminosity of
$(3.7\pm1.6)\times10^{42} \ \rm{erg \ s^{-1}}$, which corresponds to a
$(3.0\pm1.3)\times10^5 \ \rm{M_{\odot}}$ BH accreting at its Eddington rate.
Other stacks remain undetected and we place upper limits on the AGN emission.
These limits imply that the bulk of BHs at $z\gtrsim6$ either accrete at a few
percent of their Eddington rate and/or are $1-2$ orders of magnitude less
massive than expected based on the stellar mass of their host galaxy.
