  We report on the discovery of a luminous blue variable (LBV) lying ~7 pc in
projection from the Quintuplet cluster. This source, which we call LBV
G0.120-0.048, was selected for spectroscopy owing to its detection as a strong
source of Paschen-alpha excess in a recent narrowband imaging survey of the
Galactic center region with HST/NICMOS. The K-band spectrum is similar to that
of the Pistol Star and other known LBVs. The new LBV was previously cataloged
as a photometric variable star, exhibiting brightness fluctuations of up to ~1
mag between 1994 and 1997, with significant variability also occurring on
month-to-month time scales. The luminosity of LBV G0.120-0.048, as derived from
2MASS photometry, is approximately equivalent to that of the Pistol Star.
However, the time-averaged brightness of LBV G0.120-0.048 between 1994 and 1997
exceeded that of the Pistol Star; LBV G0.120-0.048 also suffers more
extinction, which suggests that it was intrinsically more luminous in the
infrared than the Pistol Star between 1994 and 1997. Paschen-alpha images
reveal a thin circular nebula centered on LBV G0.120-0.048 with a physical
radius of ~0.8 pc. We suggest that this nebula is a shell of ejected material
launched from a discrete eruption that occurred between 5000 and 10,000 years
ago. Because of the very short amount of time that evolved massive stars spend
in the LBV phase, and the close proximity of LBV G0.120-0.048 to the Quintuplet
cluster, we suggest that this object might be coeval with the cluster and may
have once resided within it.
