2024ApJ...975...67J

We present the results obtained from timing and spectral studies of 15 thermonuclear X-ray bursts from 4U 1820-30 observed with the Neutron Star Interior Composition Explorer (NICER) during its 5 yr of observations between 2017 and 2022. All bursts showed clear signs of photospheric radius expansion (PRE), where the neutron star (NS) photosphere expanded more than 50 km above the surface. One of the bursts produced a superexpansion with a blackbody emission radius of 902 km for the first time with NICER. We searched for burst oscillations in all 15 bursts and found evidence of a coherent oscillation at 716 Hz in a burst, with a 2.9σ detection level based on Monte Carlo simulations. If confirmed with future observations, 4U 1820-30 would become the fastest-spinning NS known in X-ray binary systems. The fractional rms amplitude of the candidate burst oscillation was found to be 5.8% in the energy range of 3-10 keV. Following the variable persistent model from burst time-resolved spectroscopy, an anticorrelation is seen between the maximum scaling factor value and the (preburst) persistent flux. We detected a low value of ionization at the peak of each burst based on reflection modeling of burst spectra. A partially interacting inner accretion disk or a weakly ionized outer disk may cause the observed ionization dip during the PRE phase.