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

We present a comprehensive interstellar scintillation (ISS) study of the radio-loud magnetar XTE J1810-197, based on six years of multi-frequency monitoring (2018-2024) with the Shanghai Tian Ma Radio Telescope (TMRT) at 7.0, 8.6, and 14.0 GHz. The scintillation parameters—decorrelation bandwidth Δv_d, decorrelation time Δτ_d, and drift rate dt/dv—are fully characterized. Our measured Δτ_d implies Δτ_d < 4 s at 575-725 MHz under a Kolmogorov spectrum, which is shorter than the magnetar’s 5.54 s spin period. This result naturally explains the previously reported absence of pulse-to-pulse coherence at these frequencies. Kinematic modeling locates the dominant scattering screen at 1.6 ± 0.1 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 magnetar’s 2018 outburst suggested by earlier studies. A scintillation arc detected at 14.0 GHz represents the highest-frequency arc observed to date. The asymmetry of arcs is linearly correlated with a dispersion-measure gradient across the screen (r = 0.959, p < 10^-8). We also measure its refractive scintillation timescale, which is only 1.21 ± 0.19 d. Clear DISS 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 and provide a precise probe of the ionized interstellar medium in the Sagittarius Arm.