Abstract
Astroph. J. 440, L61 (1995) The soft gamma-ray repeater (SGR) 1806$-$20 is associated with the
center-brightened non-thermal nebula G~10.0$-$0.3, thought to be a plerion. As
in other plerions, a steady \Xray\ source, AX~1805.7$-$2025, has been detected
coincident with the peak of the nebular radio emission. Vasisht et al.\ have
shown that the radio peak has a core-jet appearance, and argue that the core
marks the true position of the SGR. At optical wavelengths, we detect three
objects in the vicinity of the radio core. Only for the star closest to the
core, barely visible in the optical but bright in the infrared ($K=8.4\,$mag.),
the reddening is consistent with the high extinction ($A_V\simeq30\,$mag.) that
has been inferred for AX~1805.7$-$2025. From the absence of CO band absorption,
we infer that the spectral type of this star is earlier than late~G/early~K.
The large extinction probably arises in a molecular cloud located at a distance
of 6$\,$kpc, which means that the star, just like AX~1805.7$-$2025, is in or
behind this cloud. This implies that the star is a supergiant. Since
supergiants are rare, a chance coincidence with the compact radio core is very
unlikely. To our knowledge, there are only three other examples of luminous
stars embedded in non-thermal radio nebulae, SS~433, \mbox{Cir X-1} and
G~70.7+1.2. Given this and the low coincidence probability, we suggest that the
bright star is physically associated with SGR~1806$-$20, making it the first
stellar identification of a high-energy transient.