Interstellar Scintillation of the Radio-Loud Magnetar XTE J1810-197

In this Letter, we present a comprehensive interstellar scintillation (ISS) study of the radio-loud magnetar XTE J1810-197 based on six years (2018-2024) of multifrequency monitoring (7.0, 8.6, and 14.0 GHz) with the Shanghai Tian Ma Radio Telescope. Scintillation parameters such as decorrelation bandwidth \Delta\nu_\rm d, decorrelation time \Delta\tau_\rm d, and drift rate \rm dt/\rm d\nu, are fully characterized by the two-dimensional auto-correlation analysis. The measured \Delta\tau_\rm d is less than 4 s at 575-725 MHz under a Kolmogorov spectrum, which is shorter than the 5.54 s spin period of the magnetar, thereby explaining the previously reported absence of pulse-to-pulse coherence at these frequencies. Kinematic modeling is utilized to locate the dominant scattering screen 1.6\pm0.1 kpc away from the Earth, within the Sagittarius Arm. The screen coincides with the H_Ⅱ region JCMTSE J180921.2-201932 and is unrelated to the 2018 outburst of the magnetar, as suggested by earlier studies. The scintillation arc detected at 14.0 GHz is the highest-frequency arc observed to date. The arc asymmetry is linearly correlated with the dispersion-measure gradient across the screen (r = 0.959, p < 10^-8). Further, we measure its refractive scintillation timescale, which is only 1.21\pm0.19 d. Clear diffractive interstellar scintillation at 14 GHz effectively resolves the debate over a possible strong-to-weak scattering transition at this frequency. These results extend the ISS characterization of magnetars to previously unexplored frequencies, providing a precise probe for ionized interstellar media in the Sagittarius Arm.