# NICER Observations of the 2018 Outburst of XTE J1810-197

## TL;DR

This paper reports early soft X-ray observations of the magnetar XTE J1810-197 after its 2018 outburst, analyzing flux decay, spectral models, and magnetospheric variations using NICER data.

## Contribution

First NICER observations of XTE J1810-197 shortly after outburst, providing insights into flux decay, spectral properties, and magnetospheric changes.

## Key findings

- X-ray flux decreased by 7% over 8 days
- Spectral fits favor blackbody plus power-law or two-blackbody models
- Pulsed fraction and phase are energy dependent up to 8 keV

## Abstract

We present the earliest available soft X-ray observations of XTE J1810-187, 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+/-0.007)x10^{-10} to (1.125+/-0.004)x10^{-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.