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[abridged] We present VLT/FORS1 spectroscopic follow-up observations andHST/WFPC2 imaging of the system COSMOS 5921+0638, which exhibits quadruplylensed images and a perfect Einstein ring. We investigate the nature of COSMOS5921+0638 by studying its photometric, spectroscopic and physical properties.We completed both an environmental analysis and detailed analytical andgrid-based mass modeling of the system. We measured the redshifts of thelensing galaxy in COSMOS 5921+0638 (z_l=0.551+/-0.001) and 9 additionalgalaxies in the field (5 of them at z~0.35). The redshift of the lensed sourcewas inferred by identifying a candidate Lya line at z_s=3.14+/-0.05. The massmodeling reveals the requirement of a small external shear (gamma=0.038), whichis slightly larger than the lensing contribution expected by galaxy groupsalong the line-of-sight (kappa_groups~0.01 and gamma_groups~0.005). Theestimated time-delays between the different images are of the order of hours tohalf a week and the total magnification of the background source is mu~150. Themeasured mass-to-light ratio of the lensing galaxy within the Einstein ring isM/L_B~8.5+/-1.6. Our analysis indicates that the ring and point-like structuresin COSMOS 5921+0638 consist of a lensed high redshift galaxy hosting a lowluminosity AGN (LLAGN). Flux ratio anomalies observed in the lensed AGN imagesare probably due to microlensing by stars in the lensing galaxy and/or acombination of static phenomena. Because of its short time-delays and thepossibility of microlensing, COSMOS 5921+0638 is a promising laboratory forfuture studies of LLAGNs.

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