2024ApJ...967...35L

Iron line spectroscopy has been one of the leading methods not only for measuring the spins of accreting black holes but also for testing fundamental physics. Basing on such a method, we present an analysis of a data set observed simultaneously by NuSTAR and NICER for the black hole binary candidate MAXI J1803-298, which shows prominent relativistic reflection features. Various relxill_nk flavors are utilized to test the Kerr black hole hypothesis. The results obtained from our analysis provide stringent constraints on Johannsen deformation parameter α₁₃ with the highest precise to date, namely ${\alpha }_{13}={0.023}_{-0.038}^{+0.071}$ from relxillD_nk and ${\alpha }_{13}={0.006}_{-0.022}^{+0.045}$ from relxillion_nk, respectively, in 3σ credible lever, where the Johannsen metric reduces to the Kerr metric when α₁₃ vanishes. Furthermore, we investigate the best model fit results using Akaike information criterion and assess its systematic uncertainties.