The 11 yr of low activity of the magnetar XTE J1810-197

In 2003, the magnetar XTE J1810-197 started an outburst that lasted until early 2007. In the following 11 yr, the source stayed in a quiescent/low-activity phase. XTE J1810-197 is one of the closest magnetars, hence its X-ray properties can be studied in detail even in quiescence and an extended monitoring has been carried out to study its long-term timing and spectral evolution. Here, we report the results of new X-ray observations, taken between 2017 September and 2018 April, with XMM-Newton, Chandra, and NICER. We derived a phase-connected timing solution yielding a frequency derivative of -9.26(6) × 10^-14 Hz s^-1. This value is consistent with that measured between 2009 and 2011, indicating that the pulsar spin-down rate remained quite stable during the long quiescent period. A spectral analysis of all the X-ray observations taken between 2009 and 2018 does not reveal significant spectral and/or flux variability. The spectrum of XTE J1810-197 can be described by the sum of two thermal components with temperatures of 0.15 and 0.3 keV, plus a power-law component with photon index 0.6. We also found evidence for an absorption line at ∼1.2 keV and width of 0.1 keV. Due to the long exposure time of the summed XMM-Newton observations, we could also carry out a phase-resolved spectral analysis for this source in quiescence. This showed that the flux modulation can be mainly ascribed to the warmer of the two thermal components, whose flux varies by ∼45 per cent along the pulse phase.