Spectral Analysis of the LMXB XTE J1810-189 with NICER Data

TL;DR

This study analyzes NICER X-ray data of the transient neutron star system XTE J1810-189 during its 2020 outburst, revealing spectral properties, corona characteristics, and burst behavior without entering a high soft state.

Contribution

It provides the first detailed spectral analysis of XTE J1810-189 during outburst, including corona properties and burst characteristics, using NICER data.

Findings

No significant black-body emission detected.

Optical depth of corona constrained to <4.5.

Detected broad Fe K emission in some observations.

Abstract

XTE J1810-189 is a Low-Mass X-ray Binary transient system hosting a neutron star, which underwent a three-month-long outburst in 2020. In order to study its spectral evolution during this outburst, we analysed all the available observations performed by NICER, in the 1-10 keV energy band. Firstly, we fitted the spectra with a thermal Comptonisation model. Our analysis revealed the lack of a significant direct emission from a black-body-like component, therefore we calculated the optical depth of the Comptonising region, deriving an upper limit of 4.5, which suggests the presence of a moderately thick corona. We also attempted to fit the spectrum with an alternative model, i.e. a cold Comptonised emission from a disc and a direct thermal component from the neutron star, finding a similarly good fit. The source did not enter a full high luminosity/soft state throughout the outburst, with a photon index ranging from 1.7 to 2.2, and an average unabsorbed flux in the 1-10 keV band of 3.6x10^(-10) erg cm^(-2) s^(-1). We searched for the presence of Fe K-shell emission lines in the range 6.4-7 keV, significantly detecting a broad component only in a couple of observations. Finally, we conducted a time-resolved spectral analysis of the detected type-I X-ray burst, observed during the outburst, finding no evidence of a photospheric radius expansion. The type-I burst duration suggests a mix of H/He fuel.