2008A&A...491..739M

A 12 year-long monitoring of the absorption caused by a z=0.89 spiral galaxy on the line of sight to the radio-loud gravitationally lensed quasar PKS 1830-211 reveals spectacular changes in the HCO⁺ and HCN (2-1) line profiles. The depth of the absorption toward the quasar NE image increased by a factor of ∼3 in 1998-1999 and subsequently decreased by a factor ≥6 between 2003 and 2006. These changes were echoed by similar variations in the absorption line wings toward the SW image. Most likely, these variations result from a motion of the quasar images with respect to the foreground galaxy, which could be due to a sporadic ejection of bright plasmons by the background quasar. VLBA observations have shown that the separation between the NE and SW images changed in 1997 by as much as 0.2mas within a few months. Assuming that motions of similar amplitude occurred in 1999 and 2003, we argue that the clouds responsible for the NE absorption and the broad wings of the SW absorption should be sparse and have characteristic sizes of 0.5-1pc.