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 X-ray 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 } \simeq 30 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 , 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 .