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arXiv · 2504.07328

A Comprehensive Study of Thermonuclear X-ray Bursts from 4U 1820-30 with NICER: Accretion Disk Interactions and a Candidate Burst Oscillation

Gaurava K. Jaisawal·Z. Funda Bostancı·Tuğba Boztepe·Tolga Güver·Tod E. Strohmayer·David R. Ballantyne·Jens H. Beck·Ersin Göğüş·Diego Altamirano·Zaven Arzoumanian·Deepto Chakrabarty·Keith C. Gendreau·Sebastien Guillot·Renee M. Ludlam·Mason Ng·Andrea Sanna·Jérôme Chenevez
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Abstract

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$σ$ 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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Gaurava K. Jaisawal, Z. Funda Bostancı, Tuğba Boztepe, Tolga Güver, Tod E. Strohmayer, David R. Ballantyne, Jens H. Beck, Ersin Göğüş, Diego Altamirano, Zaven Arzoumanian, Deepto Chakrabarty, Keith C. Gendreau, Sebastien Guillot, Renee M. Ludlam, Mason Ng, Andrea Sanna, Jérôme Chenevez. 2025-04-09. A Comprehensive Study of Thermonuclear X-ray Bursts from 4U 1820-30 with NICER: Accretion Disk Interactions and a Candidate Burst Oscillation. https://doi.org/10.3847/1538-4357%2Fad794e

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