Presentation #406.01 in the session “Stellar/Compact II (Oral)”.
We report on Neutron Star Interior Composition Explorer (NICER), Nuclear Spectroscopic Telescope Array (NuSTAR) and Fermi Gamma-ray Burst Monitor (GBM) observations of the Be-X-ray binary pulsar 1A 0535+262 performed during its giant Type II outburst in 2020, which peaked at a 15-50 keV flux of ~12 Crab. From the Fermi GBM, NICER and NuSTAR measurements, we find the neutron star rotation period decreases from 103.622±0.004 s to 103.24±0.02 s, suggesting that the neutron star rapidly spins-up as the outburst progressed. The NICER and NuSTAR observations, which monitor the evolution of the spectral shape, show the 1-79 keV luminosity peaks at ~1.4 × 1038 erg s-1. This is clearly above ~6.3 × 1037 erg s-1, which is expected around the critical luminosity. We find that the pulse profiles in each observation show a strong energy dependence as well as a strong dependence on luminosity. Emission lines from Fe Kα, Fe XXV Kα and Fe XXVI Kα are observed in the joint NICER and NuSTAR spectra. The line strengths are correlated with the 7.2-10 keV unabsorbed continuum flux. The Fe fluorescent emission lines show no evidence of pulsations, which may suggest that the size of the Fe fluorescent emission region is larger than the Alfvén radius. Although the measured 1-79 keV luminosities are clearly in the supercritical accretion regime, we find that the spectral shape hardens with increasing luminosity. The pulse-phase-averaged NuSTAR data show that the energy of the CRSF around ~45 keV remains virtually unchanged, but the line depth decreases with increasing X-ray flux. This may be a result of photon spawning.