Spectral analysis of the LMXB XTE J1810−189 with NICER data

Abstract

X-ray Timing Explorer 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 Neutron star Interior Composition Explorer, in the 1-10keV energy band. First, we fitted the spectra with a thermal Comptonization model. Our analysis revealed the lack of a significant direct emission from a blackbody-like component, therefore we calculated the optical depth of the Comptonizing 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 Comptonized 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 similar to 1.7 to similar to 2.2, and an average unabsorbed flux in the 1-10keV band of similar to 3.6 x 10(-10) ergcm(-2) s(-1). We searched for the presence of Fe K-shell emission lines in the range similar to 6.4-7keV, 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.