  Accretion disks and coronae around massive black holes have been studied
extensively, and they are known to be coupled. Over a period of 30 years,
however, the X-ray (coronal) flux of Mrk 817 increased by a factor of 40 while
its UV (disk) flux remained relatively steady. Recent high-cadence monitoring
finds that the X-ray and UV continua in Mrk 817 are also decoupled on time
scales of weeks and months. These findings could require mechanical beaming of
the innermost accretion flow, and/or an absorber that shields the disk and/or
broad line region (BLR) from the X-ray corona. Herein, we report on a 135 ks
observation of Mrk 817 obtained with NuSTAR, complemented by simultaneous X-ray
coverage via the Neil Gehrels Swift Observatory. The X-ray data strongly prefer
a standard relativistic disk reflection model over plausible alternatives.
Comparable fits with related models constrain the spin to lie in the range 0.5
< a < 1, and the viewing angle to lie between 10 deg. < theta < 22 deg.
(including 1-sigma statistical errors and small systematic errors related to
differences between the models). The spectra also reveal strong evidence of
moderately ionized absorption, similar to but likely less extreme than
obscuring events in NGC 5548 and NGC 3783. Archival Swift data suggest that the
absorption may be variable. Particularly if the column density of this absorber
is higher along the plane of the disk, it may intermittently mask or prevent
coupling between the central engine, disk, and BLR in Mrk 817.
