VLBI aperture synthesis observations of 1720-MHz OH in NGC 7538 (IRS1)

Abstract

The 1720-MHz OH source in NGC 7538 (IRS1) has been observed with four telescopes in the European VLBI network. Aperture synthesis mapping with a beam of 0.03 arcsec shows six discrete maser positions concentrated in an elongated region of size 0.3 × 0.15 arcsec2. The maser emission arises from two groups of unresolved (< 0.02 arcsec) spots, with apparent brightness temperatures |$\gt\,{10}^{10}\text K$|⁠. The two separate spatial groups correspond to two distinct velocity groups.