Implications for accretion flow dynamics from a spectral study of Swift J1357.2-0933

We report a detailed spectral study of the Swift J1357.2-0933 low-mass X-ray binary during its 2017 outburst using Swift and NuSTAR observations. We fit the data with a two-component advective flow (TCAF) model and a power-law model. We observe that the source is in a hard state during the outburst, where the size of the Compton cloud changes significantly with disc accretion rate. The typical disc accretion rate for this source is ∼ 1.5-2.0 per cent of the Eddington accretion rate (\dot{M}_Edd). The model-fitted intermediate shock compression ratio gives an indication of the presence of a jet, which is reported in the literature in different energy bands. We also split the NuSTAR data into three equal segments and fit them with the model. We check spectral stability using a colour-colour diagram and an accretion rate ratio (ARR) versus intensity diagram using different segments of the light curve, but do not find any significant variation in the hardness ratio or in the accretion rate ratio. To estimate the mass of the candidate, we use an important characteristic of TCAF that the the model normalization always remains a constant. We found that the mass comes out to be in the range of 4.0-6.8 M_⊙. From the model-fitted results, we study the disc geometry and different physical parameters of the flow in each observation. The count rate of the source appears to decay on a time-scale of {∼ } 45 d.