A Constraint on Dark Matter Self-Interaction from Combined Strong Lensing and Stellar Kinematics in MACS J0138-2155

Abstract: Self-Interacting Dark Matter (SIDM) represents a compelling alternative to collisionless dark matter, with diverse phenomenological signals from dwarf galaxy to galaxy cluster scales. We present new constraints on the SIDM cross section from the galaxy cluster MACS J0138-2155, host to the strongly lensed supernovae Requiem and Encore. Our analysis combines strong gravitational lensing with spatially resolved stellar kinematics of the central galaxy, employing several methodological advances over previous cluster-scale SIDM studies. The result is a self-consistent measurement of the density profile of MACS J0138-2155 across two orders of magnitude in radius. Our lensing and kinematics analyses individually yield highly consistent results, and from their combination we report a 95% confidence upper limit on the SIDM cross section of $\sigma/m < 0.613$ cm$^2$/g, at an interaction velocity of $\langle v_\text{pair}\rangle < 2090$ km/s. This constraint, derived from the most detailed single-system analysis to date, is competitive with previous cluster-scale limits while demonstrating the power of combining complementary gravitational probes. The methodology developed here advances precision cluster lens modeling and will inform future studies of dark matter physics, as well as time-delay cosmography in this unique strong lensing system. Additionally, our results imply SN Requiem will reappear sooner than previously reported, with a 1$\sigma$ CL between January 2027 and November 2028 at H$_0 = 67.7$ km s$^{-1}$ Mpc$^{-1}$.