We present the earliest available soft X-ray observations of XTE J1810 - 197 , the prototypical transient magnetar , obtained 75–84 days after its 2018 outburst with the Neutron Star Interior Composition Explorer ( NICER ) .
Using a series of observations covering eight days we find that its decreasing X-ray flux is well-described by either a blackbody plus power-law or a two-blackbody spectral model .
The 2–10 keV flux of the source varied from ( 1.206 \pm 0.007 ) \times 10 ^ { -10 } to ( 1.125 \pm 0.004 ) \times 10 ^ { -10 } ~ { } { erg } ~ { } { s } ^ { -1 } ~ { } { cm } ^ { -2 } , a decrease of about 7 % within our observations and 44 % from that measured 7–14 days after the outburst with NuSTAR .
We confirm that the pulsed fraction and spin pulse phase of the neutron star are energy dependent up to at least 8 keV .
Phase resolved spectroscopy of the pulsar suggests magnetospheric variations relative to the line of sight .