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

Abstract

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 $Δν_{\rm d}$, decorrelation time $Δτ_{\rm d}$, and drift rate $dt/dν$--are fully characterized. Our measured $Δτ_{\rm d}$ implies $Δτ_{\rm d} < 4$~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 $1.6\pm0.1$~kpc away from the Earth, within the Sagittarius Arm. The screen coincides with the HII 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 $1.21\pm0.19$~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.

Figures (5)

• Figure 1: Representative plots of ISS data analysis results for observations listed in Table \ref{['tab:ourresults']}. (a) 2D-ACFs: The normalized 2D-ACFs are shown with color-filled contours (scaled with the color-bar on the right), and the 2D-Exponential form fitting results are black-line contours. The major axis is marked with the dashed line. The bottom and left sub-panels are the 1D cuts of the 2D-ACF (solid curves) and the best-fit model (dashed curves) at zero time and frequency lag. (b) Dynamic spectra: The intensity of each pixel is linearly scaled with the color-bar on the right. (c) Secondary spectra: The white dashed curves and white dotted curves denote the boundaries and center of the best-fit arc regions, respectively.
• Figure 2: Observed $V_{\rm iss, 0}$ and the fitting curve. $V_{\rm iss, 0}$ at 7.0 and 8.6 GHz and their 1-$\sigma$ error ranges are marked with hollow green and solid blue points with error-bars. The blue curve with a shaded region is the fitting curve and its 1-$\sigma$ error ranges.
• Figure 3: The $\eta$ values changing with the observation time. $\eta$ at 7.00, 8.60, and 14.0 GHz and their 1-$\sigma$ error ranges are hollow brown squares, hollow deeppink diamonds, and solid purple points with asymmetric error-bars, respectively. The best-fit curves for isotropic (blue) and anisotropic (green) scattering scenarios are also shown on the plots.
• Figure 4: $\ln({S_{\rm r}}/{S_{\rm l}})$ and ${\partial DM}/{\partial t}$ of XTE J1810$-$197 at 8.60 GHz. The left panel displayed how ${\partial DM}/{\partial t}$ and $\ln({S_{\rm r}}/{S_{\rm l}})$ varied with time. $\ln({S_{\rm r}}/{S_{\rm l}})$, ${\partial DM}/{\partial t}$ and their 1-$\sigma$ error ranges are marked with red and blue points with error-bars, respectively. The right panel displays the linear relationship between $\ln({S_{\rm r}}/{S_{\rm l}})$ and ${\partial DM}/{\partial t}$. We use crossing error-bars to mark the 1-$\sigma$ error ranges
• Figure 5: $D(\tau)$ and the fitting results. $D(\tau)$ and their 1-$\sigma$ error ranges for unfiltered and filtered $F$ are plotted by green and blue dots with error-bars in the left and right panels, respectively. The red curve is the fitting result. Green and purple dashed lines are the $\tau_{\rm ch}$ and $\tau_{\rm r}$, respectively. Black and magenta dotted lines are $D_{\infty}$ and $2\sigma_{\rm r}^{2}$, respectively, where $\sigma_{\rm r}$ is the RISS modulation index. For a reliable fitting, $D_{\infty}\approx 2\sigma_{\rm r}^{2}$. The brown dotted line is $D_{0}$. When $D_{0}$ is close to zero, we would not plot it.