Does the radio-active phase of XTE~J1810$-$197 recur following the same evolutionary pattern?

TL;DR

This paper investigates whether the radio-active phase of XTE J1810$-$197 recurs with the same evolutionary pattern as its 2003 outburst. Using continuous dual-frequency observations at 2.25 and 8.60 GHz with the TMRT, it traces the full radio-active evolution, revealing four distinct phases in the spin-down rate $\dot{ν}$, long-term profile evolution, and largely flat spectra with episodic steepening. A twisted-magnetosphere/untwisting model quantitatively accounts for the torque evolution, profile narrowing, and abrupt radio cessation, and the study finds striking similarities between the 2003 and 2018 outbursts in timing, flux, and magnetospheric geometry, arguing for a repeatable underlying mechanism. By placing XTE J1810$-$197 in the context of other radio-loud magnetars and high-$B$ pulsars, the work suggests a universal magnetospheric process governing radio activity and offers avenues to predict future outbursts and connect magnetar radio emission to intermittent pulsars.

Abstract

Magnetars are the most strongly magnetized compact objects known in the Universe and are regarded as one of the primary engines powering a variety of enigmatic, high-energy transients. However, our understanding of magnetars remains highly limited, constrained by observational sample size and radiative variability. XTE~J1810$-$197, which re-entered a radio-active phase in 2018, is one of only six known radio-pulsating magnetars. Leveraging the distinctive capability for simultaneous dual-frequency observations, we utilized the Shanghai Tianma Radio Telescope (TMRT) to monitor this magnetar continuously at both 2.25 and 8.60~GHz, capturing its entire evolution from radio activation to quenching. This enabled precise characterization of the evolution in its integrated profile, spin frequency, flux density, and spectral index ($α$, defined by $S \propto f^α$). The first time derivative of its spin frequency $\dotν$ passed through four distinct phases -- rapid decrease, violent oscillation, steady decline, and stable recovery -- before returning to its pre-outburst value concomitant with the cessation of radio emission. Remarkably, both the amplitudes and the characteristic time-scales of these $\dotν$ variations match those observed during the previous outburst that began in 2003, providing the first demonstration that post-outburst rotational evolution and radiative behavior in a magnetar are repeatable. A twisted-magnetosphere model can qualitatively account for this repeatability as well as for the progressive narrowing and abrupt disappearance of the radio pulse radiation, thereby receiving strong observational support.

Figures (5)

• Figure 1: Representative integrated profiles of XTE J1810$-$197 at 2.25/8.60 GHz, showcasing four distinct morphological types. For each type, a representative pair of integrated profiles is displayed in one row: the red profile on the left at 2.25 GHz and the blue profile on the right at 8.60 GHz, both obtained from the same observation. Type labels ($Type~1$–$4$) and durations are indicated to the left and right of the integrated profiles, respectively. Prominent components are labeled P1-P5 and marked above the profiles.
• Figure 2: Temporal evolution of observational and derived parameters of XTE J1810$-$197. (a) Red and blue vertical bars show $T_{obs}$ at 2.25/8.60 GHz, respectively, for epochs used in timing analysis. (b) $\nu$ evolution, with dual-frequency timing results shown as black downward triangles and 8.60 GHz-only results as blue open circles. The black solid line indicates a linear fit across the active phase. (c) $\dot{\nu}$ evolution. (d) Flux density measurements at both frequencies, with red open circles and blue open triangles indicating 2.25/8.60 GHz detections, respectively. For the final two observations, flux upper limits are shown as downward arrows. (e) Spectral index ($\alpha$) derived from dual-frequency power-law fits. (f) Evolution of the phase-resolved modulation index ($\bar{m}$) for the central component ($P1/2$). (g) Temporal evolution of $W_{10}$ for the $P1/2$ region, determined through Gaussian fitting. The vertical green dashed lines separate results from our previous work and those from this study hys2023.
• Figure 3: Comparison of the $\dot{\nu}$ evolution of XTE J1810$-$197 during its 2003 and 2018 outbursts. The $\dot{\nu}$ values from the 2003 outburst (orange) and 2018 outburst (light blue) are aligned such that the epochs of radio disappearance coincide. Orange diamonds are from Ibrahim et al. (2004) ims2004, orange filled circles are from Camilo et al. (2016) crh2016, light blue open triangles are from Levin et al. (2019) lld2019, light blue triangles are from Caleb et al. (2022) crd2022, light blue open squares are our previous work hys2023, and light blue open diamonds represent results from this work.
• Figure 4: Evolution of $B_{\text{lc}}$ (orange solid line with circles) and spin-down luminosity $\dot{E}$ (light blue solid line with open diamonds) of XTE J1810$-$197. The orange and light blue dashed lines mark the corresponding values of $B_{\text{lc}}$ and $\dot{E}$ during the magnetar's quiescent state, respectively.
• Figure 5: Spin-down rate ratio $\dot{\nu}$ (on/off) for intermittent pulsars (green triangles), other radio magnetars (light blue diamonds), and XTE J1810$-$197 (coral circles). The red dashed line indicates the spin-down rate ratio of XTE J1810$-$197, derived from the average $\dot{\nu}$ across its entire radio-active period compared to its quiescent state scs2017hys2021rcv2020lys2023.