Date Presented:
2023/01/1
Abstract:
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 15-50keV flux of ~12Crab. 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 it rapidly spins-up as the outburst progresses. The NICER and NuSTAR observations, which monitor the evolution of the spectral shape, show the 1-79 keV luminosity peaks at (1.354 ± 0.005)×1038 erg s-1, which is above the inferred critical luminosity of ~6.3×10
37 erg s-1 in the accretion column. 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 suggests that the size of the Fe fluorescent emission region is larger than the Alfv ́en radius. Even when the measured 1-79 keV luminosity is above the critical value, we find that the spectral shape hardens with increasing luminosity. The pulse-phase-averaged NuSTAR data show that the energy of the CRSF around ~45keV remains unchanged with respect to X-ray flux, but the line depth decreases with increasing X-ray flux. This may be a result of photon spawning.
Website
