  We report the discovery and characterization of a new symbiotic star of the
accreting-only variety, which we observed in the optical/near-infrared (NIR)
with VLT/X-Shooter and in the X-rays/ultraviolet with Swift/UVOT+XRT. The new
symbiotic star, THA 15-31, was previously described as a pre-main sequence star
belonging to the Lupus~3 association. Our observations, ancillary data, and
Gaia EDR3 parallax indicate that THA 15-31 is a symbiotic star composed of an
M6III red giant and an accreting companion, is subject to E(B-V)=0.38
reddening, and is located at a distance of ~12 kpc and at 1.8 kpc above the
Galactic plane in the outskirts of the Bulge. The luminosity of the accreting
companion is ~100 Lsun, placing THA 15-31 among the symbiotic stars accreting
at a high rate (2.5e-08 Msun/yr if the accretion is occurring on a white dwarf
of 1 Msun). The observed emission lines originate primarily from HI, HeI, and
FeII, with no HeII or other high-excitation lines observed; a sharp central
absorption superimposed on the Balmer emission lines is observed, while all
other lines have a simple Gaussian-like profile. The emission from the
companion dominates over the M6III red giant at $U$ and $B$-band wavelengths,
and is consistent with an origin primarily in an optically thick accretion
disk. No significant photometric variability is observed at optical or NIR
wavelengths, suggesting either a face-on orbital orientation and/or that the
red giant is far from Roche-lobe filling conditions. The profile of emission
lines supports a low orbital inclination if they form primarily in the
accretion disk. An excess emission is present in AllWISE W3 (12 micron) and W4
(22 micron) data, radiating a luminosity ~35 Lsun, consistent with thermal
emission from optically thin circumstellar dust.
