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

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.

Radio and X-ray observations of giant pulses from XTE J1810-197

“…The temperature remains approximately constant for a year before starting to cool down gradually. We also note that the values of the warm and hot component are consistent with the analysis presented in Borghese et al (2021) with the same dataset and are also consistent with those estimated by Gotthelf & Halpern (2007) from XMM-Newton observations of the previous outburst. We also show the evolution of the two blackbody components during the time when GPs were observed.…”

“…The change in geometry is a likely scenario as many authors have suggested a shift in the magnetosphere of the neutron star due to a starquake if we assume that starquakes cause outbursts in magnetars (Beloborodov & Thompson 2007). While the phase lag between the radio and the X-ray profiles has not changed compared to the previous outburst (Gotthelf et al 2019a;Borghese et al 2021), the evolution of flux at radio wavelengths differs significantly compared to the previous outburst, suggesting an evolution in the location of the X-ray/radio emitting regions caused due to a change in the region traversed by the line of sight. This argument is also supported by the evolution of the pulsed fraction observed during the current outburst, which is significantly different from what was observed in the previous outburst (Figure 14).…”

“…Since we only use the NICER data, we decided to fit it with a two blackbody component model. We note that while we have used a similar dataset as Borghese et al (2021), we have analysed the data for a much larger baseline compared to what is presented in Borghese et al (2021) with an extension of almost another year. We discuss the implications of the results in the next sections.…”

The first 7 months of the 2020 X-ray outburst of the magnetar SGR J1935+2154

“…To gain some insight on the source geometry and on its evolution over the outburst decay, we introduce a simple model according to which thermal photons are produced by a circular cap on the star surface heated at the outburst onset. We assume that the cap is at uniform temperature, as suggested by the lack of multiple BB components in the observed spectrum (this is at variance with, e.g., the case of XTE J1810−197, Borghese et al 2021). The cap properties are fixed by the measured blackbody temperature (𝑘𝑇 c = 0.45 keV) and radius (𝑅 BB = 1.6 km which results in a semiaperture 𝜃 c ∼ 7 • for 𝑅 NS = 13 km); these values are representative of both the XMM-Newton observations of 2020 May 13 and October 1 since they do not change significantly between the two epochs.…”

Fast cooling and internal heating in hyperon stars

“…Additionally, inhomogeneous surface temperatures caused by magnetic fields are realistically more complicated (Igoshev et al 2021) than the limited number (usually a couple) of uniform-temperature regions of different sizes, commonly used in spectral modeling of observations (see e.g. Borghese et al (2021)). Moreover, in many cases the low photon counts and photoelectric absorption by the interstellar medium under ∼ 1 keV constitute further statistical and intrinsic uncertainties in 𝑇 s .…”