Kavli Affiliate: Deepto Chakrabarty
| First 5 Authors: Gaurava K. Jaisawal, Z. Funda Bostancı, Tuğba Boztepe, Tolga Güver, Tod E. Strohmayer
| Summary:
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 five years of observations
between 2017-2022. All bursts showed clear signs of photospheric radius
expansion, where the neutron star (NS) photosphere expanded more than 50 km
above the surface. One of the bursts produced a super-expansion 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 2.9$sigma$ 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 anti-correlation is seen between the
maximum scaling factor value and the (pre-burst) 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
photospheric radius expansion phase.
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