Estimating the Black Hole Spin for the X-Ray Binary MAXI J1727-203 Based on Insight-HXMT

Abstract

We constrain the spin of the black hole (BH) candidate MAXI J1727-203 using Insight-HXMT data. Due to limited HXMT observations covering only part of the outburst, NICER data were used to analyze the full outburst's state transitions, we identified two of three HXMT observations in the high soft state and applied the continuum-fitting method to measure the spin. Based on previous estimates and continuum spectral fittings, we explored the parameter space and found that the best-fitting values were $(D, i, M) \approx (6\ \text{kpc}, 30^\circ, 12 M_{\odot})$. We also tested the variation of these parameters using Monte Carlo simulations, sampling over 3000 sets within the parameter ranges: $5.9 \text{kpc}< D<7 \text{kpc}$, $24^\circ<i< 35^\circ$, and $10 M_{\odot}<M<14 M_{\odot}$, yielding a spin measurement of $a=0.34_{-0.19}^{+0.15}$ (1$\sigma$). In addition, we analyzed NuSTAR data in low hard state and found a good fit with the {\tt tbabs*(diskbb+powerlaw)} model, with no significant iron line features observed in the residuals, then the previous reflection model results suggesting an extremely high spin would over-estimate the BH spin.