PMN J1838−3427: A New Gravitationally Lensed Quasar

We report the discovery of a new double-image quasar that was found during a search for gravitational lenses in the southern sky. Radio source PMN J1838-3427 is composed of two flat-spectrum components with separation 1.″0, flux density ratio 14 : 1, and matching spectral indices in VLA and VLBA images. Ground-based BRI images show the optical counterpart (total I = 18.6) is also double, with the same separation and position angle as the radio components. An HST/WFPC2 image reveals the lens galaxy. The optical flux ratio (27 : 1) is higher than the radio value, probably because of differential extinction of the components by the lens galaxy. An optical spectrum of the bright component contains quasar emission lines at z = 2.78 and several absorption features, including prominent Lyα absorption. The lens galaxy redshift could not be measured, but it is estimated to be z = 0.36 ± 0.08. The image configuration is consistent with the simplest plausible models for the lens potential. The flat radio spectrum and observed variability of PMN J1838-3427 suggest that the time delay between flux variations of the components is measurable and could thus provide an independent measurement of H0.