NuSTAR monitoring of MAXI J1348-630. Evidence of high density disc reflection

We present the broad-band spectral analysis of all the six hard, intermediate, and soft state NuSTAR observations of the recently discovered transient black hole X-ray binary MAXI J1348-630 during its first outburst in 2019. We first model the data with a combination of a multicolour disc and a relativistic blurred reflection, and, whenever needed, a distant reflection. We find that this simple model scheme is inadequate in explaining the spectra, resulting in a very high iron abundance. We therefore explore the possibility of reflection from a high-density disc. We use two different sets of models to describe the high-density disc reflection: RELXILL-based reflection models, and REFLIONX-based ones. The REFLIONX-based high-density disc reflection models bring down the iron abundance to around the solar value, while the density is found to be 1020.3-21.4 cm-3. We also find evidence of a high-velocity outflow in the form of ∼7.3 keV absorption lines. The consistency between the best-fitting parameters for different epochs and the statistical significance of the corresponding model indicates the existence of high-density disc reflection in MAXI J1348-630.