2009A&A...507...35A

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. We investigate the nature of COSMOS 5921+0638 by studying its photometric, spectroscopic and physical properties. 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. 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∼0.35). The redshift of the lensed source was inferred by identifying a candidate Lyα line at z_s=3.14±0.05. The mass modeling reveals the requirement of a small external shear (γ=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 (κ_groups∼0.01 and γ_groups∼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 µ≃150. The measured mass-to-light ratio of the lensing galaxy within the Einstein ring is M/L_B≃8.5±1.6. Anomalies are observed between the measured and expected flux ratios of the images of the background AGN. 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.