The variable spin-down rate of the transient magnetar XTE J1810--197

TL;DR

This study analyzes over 11 years of X-ray observations of the transient magnetar XTE J1810-197, revealing distinct spin-down regimes during outburst decay and quiescence, consistent with a magnetospheric twist model.

Contribution

It provides the first long-term phase-connected timing solution for XTE J1810-197 during quiescence, showing stable spin-down and a positive second derivative, supporting the magnetospheric twist hypothesis.

Findings

Variable spin-down rate during outburst decay.

Stable spin-down rate during quiescence.

Evidence for a magnetospheric twist driving the outburst.

Abstract

We have analyzed XMM-Newton and Chandra observations of the transient magnetar XTE J1810-197, spanning more than 11 years, from the initial phases of the 2003 outburst to the current quiescent level. We investigated the evolution of the pulsar spin period and we found evidence for two distinct regimes: during the outburst decay, the spin derivative (nu_dot) was highly variable in the range -(2-4.5)x10^-13 Hz/s, while during quiescence the spin-down rate was more stable at an average value of -1x10^{-13} Hz/s. Only during ~3000 days (from MJD 54165 to MJD 56908) in the quiescent stage, it was possible to find a phase-connected timing solution, with nu_dot = -4.9x10^{-14} Hz/s, and a positive second frequency derivative, nu_dotdot = 1.8x10^{-22} Hz/s. These results are in agreement with the behavior expected if the outburst of XTE J1810-197 was due to a strong magnetospheric twist.