Reflection and two QPOs in a NuSTAR observation of MAXI J1803-298 compared to simulated HEX-P observations

The transient black hole X-ray binary MAXI J1803-298 was discovered in May 2021 when it was observed in outburst for the first time. As the source rose in flux in the hard state, it showed periodic absorption dips, which stopped when it transitioned into an intermediate state near the peak of its outburst. Here we report on the spectral and timing analysis of a NuSTAR target-of-opportunity observation near the outburst peak. We detect two intermittent quasi-periodic oscillation (QPO) signals, which are not harmonically related at 5.4 ± 0.2 Hz, and 9.4 ± 0.3 Hz, the former of which may be the first Type-B QPO detected with NuSTAR. Curiously, the QPOs are only apparent during periods of lower flux in our observation. In the source spectrum, clear reflection features are evident. The reflection spectrum is best fit with high-density models, and we are able to constrain the inclination of the inner accretion disk to be roughly 75 degrees. An estimate of the black hole spin, however, depends on the reflection model used, since two separate accretion geometries can fit the NuSTAR spectra equally well. We therefore simulate a High Energy X-ray Probe (HEX-P) observation based on our results with these distinct geometries: a lamppost model, and a broken-power law disk emissivity model. We then re-fit the HEX-P simulated spectra to test whether the improved data will allow us to distinguish between these two accretion geometries. We also consider HEX-P’s ability to further investigate the two QPOs detected.