The X-ray evolution and geometry of the 2018 outburst of XTE J1810-197

TL;DR

This study monitors the 2018 outburst of magnetar XTE J1810-197, revealing spectral and geometric evolution, including energy-dependent pulse phase shifts and changes in surface emission geometry, using multi-instrument X-ray observations.

Contribution

First detailed spectral and geometric analysis of the 2018 outburst, showing evolution from the 2003 event with unique energy-dependent pulse phase shifts.

Findings

Reproduced similar outburst behavior with a twofold change in spin-down rate.

Confirmed energy-dependent phase shift of the pulse profile.

Modeled surface emission with multiple blackbody components indicating evolving geometry.

Abstract

After 15 years, in late 2018, the magnetar XTE J1810-197 underwent a second recorded X-ray outburst event and reactivated as a radio pulsar. We initiated an X-ray monitoring campaign to follow the timing and spectral evolution of the magnetar as its flux decays using Swift, XMM-Newton, NuSTAR, and NICER observations. During the year-long campaign, the magnetar reproduced similar behaviour to that found for the first outburst, with a factor of two change in its spin-down rate from $\sim7.2\times10^{-12}$ s s$^{-1}$ to $\sim1.5\times10^{-11}$ s s$^{-1}$ after two months. Unique to this outburst, we confirm the peculiar energy-dependent phase shift of the pulse profile. Following the initial outburst, the spectrum of XTE J1810-197 is well-modelled by multiple blackbody components corresponding to a pair of non-concentric, hot thermal caps surrounded by a cooler one, superposed to the colder star surface. We model the energy-dependent pulse profile to constrain the viewing and surface emission geometry and find that the overall geometry of XTE J1810-197 has likely evolved relative to that found for the 2003 event.