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

After 15 yr, 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 2 change in its spin-down rate from ∼7.2 × 10−12 to ∼1.5 × 10−11 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.AB and FCZ are supported by Juan de la Cierva Fellowship. AB, NR, FCZ, DV, AI, and RS are supported by the ERC Consolidator Grant ‘MAGNESIA’ (nr. 817661), and acknowledge funding from grants SGR2017-1383 and PGC2018-095512-BI00. RT, MR, and SM acknowledge financial support from the Italian MIUR through grant ‘UNIAM’ (PRIN 2017LJ39LM). EVG and JAJA acknowledge support for this project from NASA grants 80NSSC20K0717, 80NSSC19K1462, 80NSSC20K0046, and 80NSSC21K0129. AP and MB gratefully acknowledge financial support by the research grant iPeska (P.I. Andrea Possenti) funded under the INAF national call Prin-SKA/CTA approved with the Presidential Decree 70/2016