Reflection modeling of the transient black hole X-ray binary MAXI J1803-298 with NuSTAR

The newly discovered transient X-ray binary MAXI J1803-298 went into outburst on May 1, 2021, and was first detected with MAXI/GSC. Follow-up observations in the X-ray band, alongside optical SALT spectroscopy, suggested that the source was an accreting black hole with a high inclination due to periodic absorption dips seen with both NICER and NuSTAR. MAXI J1803-298 brightened in the hard state before undergoing a hard-to-soft state transition beginning on May 12. Here we report on the results of a NuSTAR Target-of-Opportunity observation of the source that began on May 14, while MAXI J1803-298 was still in its intermediate state and near its peak outburst flux (~200 counts/s in the NuSTAR band). We observe clear reflection signatures in the source spectrum, and carry out detailed modeling of the full NuSTAR spectrum in order to measure properties of interest such as the black hole spin. After our observation, MAXI J1803-298 continued its transition into the soft state and then dropped in flux - we also report on the monitoring of the source in the months after its initial outburst, which saw a spectral transition back into the hard state at a low flux level, and another NuSTAR observation carried out on November 16, 2021.