The complex spectral behavior of the newly discovered neutron star X-ray binary Swift J1858.6-0814

We report on the NuSTAR observation of the newly discovered neutron star X-ray binary Swift J1858.6-0814 taken on 23rd March 2019. The light curve of the source exhibits several large flares during some time intervals of this observation. The source is softer in the high-intensity interval where the large flaring activity mainly occurs. We perform time-resolved spectroscopy on the source by extracting spectra for two different intensity intervals. The source was observed with a 3 - 79keV luminosity of ~9.68 × 10³⁶ ergs/s and ~4.78 × 10³⁶ ergs/s for high and low-intensity interval, respectively assuming a distance of 15 kpc. We find a large value of the absorbing column density ($\rm {N_{H}}\sim 1.1\times 10^{23}$ cm^-2), and it appears to be uncorrelated with the observed flux of the source. Each spectrum shows evidence of Fe Kα emission in the 5 - 7keV energy band, an absorption edge around ~7 - 8keV, and a broad Compton hump above 15keV, indicating the presence of a reflection spectrum. The observed features are well explained by the contribution of a relativistic reflection model and a partially covering absorption model. From the best-fit spectral model, we found an inner disc radius to be $4.87_{-0.96}^{+1.63}\,\,R_{ISCO}$ (for the high-intensity interval) and $5.68_{-2.78}^{+9.54}\,\,R_{ISCO}$ (for the low-intensity interval), indicating a significant disc truncation. The disk inclination is found to be relatively low, i < 33⁰. We further place an upper limit on this source's magnetic field strength considering the disc is truncated at the magnetospheric radius.