XTE J1810−197 (XTE J1810) unveiled its magnetar nature in 2003 when the Rossi X-ray Timing Explorer ( RXTE) detected an increase in its X-ray flux by a factor of ∼ 100 with respect to the quiescent level measured by ROSAT in 1993, ∼5 × 10 −13 erg s −1 cm −2 (0.5–10 keV Gotthelf et al.
2016) and the source at the centre of the supernova remnant RCW 103 (D’Aì et al. Up to now, about 30 sources are listed as magnetars, although magnetar-like activity has been recorded from other classes of isolated neutron stars too, such as high- B radio pulsars (e.g. Then, it usually relaxes back to the pre-outburst level on time-scales spanning from weeks to months/years. During an outburst, the persistent X-ray flux increases by up to three orders of magnitude above the quiescent level. These flaring events often indicate that the source has entered an active phase, commonly referred to as outburst (see the Magnetar Outburst Online Catalog Coti Zelati et al.
The main feature of these isolated neutron stars is the unpredictable and variable bursting activity in the X-/gamma-ray bands on different time-scales. With spin periods in the 0.3–12 s range and relatively large spin-down rates, these objects have a persistent X-ray luminosity of L X ∼ 10 31–10 36 erg s −1, generally larger than their rotational energy loss rate. Kaspi & Beloborodov 2017 Esposito, Rea & Israel 2021, for reviews). Magnetars are isolated pulsars whose emission is thought to be powered by the decay and instabilities of their extreme magnetic fields, typically B ∼ 10 14–10 15 G (see e.g. Stars: individual: XTE J1810−197, stars: magnetars, stars: neutron, X-rays: bursts 1 INTRODUCTION 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. 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. Unique to this outburst, we confirm the peculiar energy-dependent phase shift of the pulse profile. 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. 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. 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.
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