COSMOS 5921+0638: characterization and analysis of a new strong gravitationally lensed AGN

Context. Strong lens candidates have been newly identified within the COSMOS field. We present VLT/FORS1 spectroscopic follow-up observations and HST/WFPC2 imaging of the system COSMOS 5921+0638, which exhibits quadruply lensed images and a perfect Einstein ring. Aims. We investigate the nature of COSMOS 5921+0638 by studying its photometric, spectroscopic and physical properties. Methods. By analyzing our VLT/FORS1 spectroscopy and Subaru/CFHT/HST imaging of COSMOS 5921+0638, we completed both an environmental analysis and detailed analytical and grid-based mass modeling to determine it properties. Results. We measured the redshifts of the lensing galaxy in COSMOS 5921+0638 (z(l) = 0.551 +/- 0.001) and 9 additional galaxies in the field (5 of them at z similar to 0.35). The redshift of the lensed source was inferred by identifying a candidate Ly alpha line at z(s) = 3.14 +/- 0.05. The mass modeling reveals the requirement of a small external shear (gamma = 0.038), which is slightly larger than the lensing contribution expected by galaxy groups along the line-of-sight obtained from the zCOSMOS optical group catalog (kappa(groups) similar to 0.01 and gamma(groups) similar to 0.005). The estimated time-delays between the different images are of the order of hours to half a week and the total magnification of the background source is mu approximate to 150. The measured mass-to-light ratio of the lensing galaxy within the Einstein ring is M/L-B approximate to 8.5 +/- 1.6. Anomalies are observed between the measured and expected flux ratios of the images of the background AGN. Conclusions. Our analysis indicates that the ring and point-like structures in COSMOS 5921+0638 consist of a lensed high redshift galaxy hosting a low luminosity AGN (LLAGN). The observed flux ratio anomalies are probably due to microlensing by stars in the lensing galaxy and/or a combination of static phenomena. Multi-epoch, multi-band space-based observations would allow us to differentiate between the possible causes of these anomalies, since static and/or dynamic variations could be identified. Because of its short time-delays and the possibility of microlensing, COSMOS 5921+0638 is a promising laboratory for future studies of LLAGNs.