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Our aim is to examine the size, kinematics, and geometry of the broad-lineregion (BLR) in the double-lensed quasar Q 0957+561 by analyzing the impact ofmicrolensing on various rest-frame ultraviolet broad-emission lines (BELs). Weexplore the influence of intrinsic variability and microlensing on the C IV, CIII], and Mg II emission lines through multiple spectroscopic observationstaken between April 1999 and January 2017. By utilizing the line cores as areference for no microlensing and correcting for the long time delay betweenthe images, we estimate the sizes of the regions emitting the broad-line wingsusing a Bayesian approach. Our study of the microlensing amplitudes between thelensed images of the quasar Q 0957+561 reveals differing sizes of the regionsemitting the three prominent BELs C IV, C III], and Mg II. The strength of thedifferential microlensing indicates that the high-ionization line C IV arisesfrom a compact inner region of the BLR with a half-light radius of $R_{1/2}\gtrsim 16.0$ lt-days, which represents a lower limit on the overall size ofthe BLR and is comparable to the size of the region emitting the r-bandcontinuum in this system. A somewhat larger size of $R_{1/2}\gtrsim 44$ lt-daysis obtained for the semi-forbidden line C III]. Microlensing has a weak impacton the lower-ionization line Mg II, which is emitted from a region with ahalf-light radius of $R_{1/2} \gtrsim 50$ lt-days. These findings suggest thatthe BEL regions may have distinct geometries and kinematics, with the moreextended ones being spherically symmetric, and the most compact ones beingnonspherical, with motions likely confined to a plane.

Revealing the structure of the lensed quasar Q 0957+561 III. Constraints on the size of the broad-line region