The X-ray evolution and geometry of the 2018 outburst of XTE J1810-197
Tiengo, A.; Götz, D.; Mereghetti, S.; Rea, N.; Esposito, P.; Gotthelf, E. V.; Possenti, A.; Burgay, M.; Israel, G. L.; Zane, S.; Turolla, R.; Sathyaprakash, R.; Coti Zelati, F.; Perna, R.; Rigoselli, M.; Borghese, A.; Alford, J. A. J.; Viganò, D.; Ibrahim, A.; Pons, J.
Spain, Italy, United Kingdom, United States, France
Abstract
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.