Detection of a spiral lens galaxy and optical variability in the gravitational lens system B1600+434

Abstract

The gravitationally lensed quasar B1600+434 (z=1.61, mV=21.6) has been observed at the 2.56m Nordic Optical Telescope (NOT). In this Letter we report the discovery of an edge-on late-type galaxy located between the two lensed components (separation 1\farcs4), close to the fainter image. The galaxy photometry indicates that its redshift is approximately 0.4. We detect a large colour difference between the two images due to significant obscuration of the faint image. The estimated amount of absorption as a function of colour indicates that the extinction may be due to dust in the lensing galaxy. We also present evidence of flux variability in B1600+434 with a detected change of 0.25mag in one year. The theoretically expected time delay is of the order of one month and so the system may be an interesting object for determining the Hubble constant.