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ABSTRACT Irradiation of the accretion disc causes reflection signatures in the observed X-ray spectrum, encoding important information about the disc structure and density. A Type I X-ray burst will strongly irradiate the accretion disc and alter its properties. Previous numerical simulations predicted the evolution of the accretion disc due to an X-ray burst. Here, we process time-averaged simulation data of six time intervals to track changes in the reflection spectrum from the burst onset to just past its peak. We divide the reflecting region of the disc within r ≲ 50 km into six to seven radial zones for every time interval and compute the reflection spectra for each zone. We integrate these reflection spectra to obtain a total reflection spectrum per time interval. The burst ionizes and heats the disc, which gradually weakens all emission lines. Compton scattering and bremsstrahlung rates increase in the disc during the burst rise, and the soft excess at <3 keV rises from ≈4 to ≈38 per cent of the total emission at the burst peak. A soft excess is expected to be ubiquitous in the reflection spectra of X-ray bursts. Structural disc changes such as inflation because of heating or drainage of the inner disc due to Poynting–Robertson drag affect the strength of the soft excess. Further studies on the dependence of the reflection spectrum characteristics to changes in the accretion disc during an X-ray burst may lead to probes of the disc geometry.
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This content will become publicly available on June 3, 2026
On the Origin of Spectral Features Observed during Thermonuclear X-Ray Bursts and in the Aftermath Emission of a Long Burst from 4U 1820–30
Abstract We study 15 thermonuclear X-ray bursts from 4U 1820–30 observed with the Neutron Star Interior Composition Explorer (NICER). We find evidence of a narrow emission line at 1.0 keV and three absorption lines at 1.7, 3.0, and 3.75 keV, primarily around the photospheric radius expansion phase of most bursts. The 1.0 keV emission line remains constant, while the absorption features, attributed to wind-ejected species, are stable but show slight energy shifts, likely due to combined effects of Doppler and gravitational redshifts. We also examine with NICER the “aftermath” of a long X-ray burst (a candidate superburst observed by MAXI) on 2021 August 23 and 24. The aftermath emission recovers within half a day from a flux depression. During this recovery phase, we detect two emission lines at 0.7 and 1 keV, along with three absorption lines whose energies decrease to 1.57, 2.64, and 3.64 keV. Given the nature of the helium white dwarf companion, these absorption lines during the aftermath may originate from an accretion flow, but only if the accretion environment is significantly contaminated by nuclear ashes from the superburst. This provides evidence of temporary metal enhancement in the accreted material due to strong wind loss. Moreover, we suggest that the absorption features observed during the short X-ray bursts and in the superburst aftermath share a common origin in heavy nuclear ashes enriched with elements like Si, Ar, Ca, or Ti, either from the burst wind or from an accretion flow contaminated by the burst wind.
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- PAR ID:
- 10635613
- Publisher / Repository:
- IOP
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 986
- Issue:
- 1
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 16
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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