  We present the direct imaging discovery of a low-mass companion to the nearby
accelerating F star, HIP 5319, using SCExAO coupled with the CHARIS, VAMPIRES,
and MEC instruments in addition to Keck/NIRC2 imaging. CHARIS $JHK$ (1.1-2.4
$\mu$m) spectroscopic data combined with VAMPIRES 750 nm, MEC $Y$, and NIRC2
$L_{\rm p}$ photometry is best matched by an M3--M7 object with an effective
temperature of T=3200 K and surface gravity log($g$)=5.5. Using the relative
astrometry for HIP 5319 B from CHARIS and NIRC2 and absolute astrometry for the
primary from $Gaia$ and $Hipparcos$ and adopting a log-normal prior assumption
for the companion mass, we measure a dynamical mass for HIP 5319 B of
$31^{+35}_{-11}M_{\rm J}$, a semimajor axis of $18.6^{+10}_{-4.1}$ au, an
inclination of $69.4^{+5.6}_{-15}$ degrees, and an eccentricity of
$0.42^{+0.39}_{-0.29}$. However, using an alternate prior for our dynamical
model yields a much higher mass of 128$^{+127}_{-88}M_{\rm J}$. Using data
taken with the LCOGT NRES instrument we also show that the primary HIP 5319 A
is a single star in contrast to previous characterizations of the system as a
spectroscopic binary. This work underscores the importance of assumed priors in
dynamical models for companions detected with imaging and astrometry and the
need to have an updated inventory of system measurements.
