NICER views moderate, strong, and extreme photospheric expansion bursts from the ultracompact X-ray binary 4U 1820$-$30

Abstract

Type I X-ray bursts in the ultracompact X-ray binary 4U 1820$-$30 are powered by the unstable thermonuclear burning of hydrogen-deficient material. We report the detection of 15 type I X-ray bursts from 4U 1820$-$30 observed by NICER in between 2017 and 2023. All these bursts occurred in the low state for the persistent flux in the range of $2.5-8\times 10^{-9}\mathrm{erg}{\mathrm{s}}^{-1}\mathrm{c}{\mathrm{m}}^{-2}$ in 0.1$-$250 keV. The burst spectra during the tail can be well explained by blackbody model. However, for the first $\sim$5 s after the burst onset, the time-resolved spectra showed strong deviations from the blackbody model. The significant improvement of the fit can be obtained by taking into account of the enhanced persistent emission due to the Poynting-Robterson drag, the extra emission modelled by another blackbody component or by the reflection from the surrounding accretion disk. The reflection model provides a self-consistent and physically motivated explanation. We find that the accretion disk density changed with 0.5 s delay as response to the burst radiation, which indicates the distortion of the accretion disk during X-ray bursts. From the time-resolved spectroscopy, all bursts showed the characteristic of photospheric radius expansion (PRE). We find one superexpansion burst with the extreme photospheric radius $r_{\mathrm{ph}}>10^3$ km and blackbody temperature of $\sim 0.2$ keV, thirteen strong PRE bursts for $r_{\mathrm{ph}}>10^2$ km, and one moderate PRE burst for $r_{\mathrm{ph}}\sim 55$ km.

16 pages, 13 figures, 4 tables. It matches the published version in Astronomy & Astrophysics

References in corpus (21)

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• The Mass and Radius of the Neutron Star in 4U 1820-30
• An Empirical Background Model for the NICER X-ray Timing Instrument
• Accretion disks and coronae in the X-ray flashlight
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• The cooling phase of Type I X-ray bursts observed with RXTE in 4U 1820-30 does not follow the canonical F - T^4 relation
• Variable spreading layer in 4U 1608-52 during thermonuclear X-ray bursts in the soft state
• Maxi observations of long X-ray bursts
• Type I X-ray bursts' spectra and fuel composition from the atoll and transient source 4U 1730$-$22
• Evolution of accretion disc reflection spectra due to a Type I X-ray burst
• Discovery of transition from marginally stable burning to unstable burning after a superburst in Aql X-1
• Discovery of a 584.65 Hz Burst Oscillation in the Low Mass X-Ray Binary 4U 1730--22
• Expanded atmospheres and winds in Type I X-ray bursts from accreting neutron stars
• Radiation Driven Warping of Accretion Discs Due to X-ray Bursts

Cited by in corpus (9)

• A Comprehensive Study of Thermonuclear X-ray Bursts from 4U 1820-30 with NICER: Accretion Disk Interactions and a Candidate Burst Oscillation
• Quenching and recovery of persistent X-ray emission during a superburst in 4U 1820$-$30
• Discovery of a long thermonuclear X-ray burst from the ultra-compact binary 4U 1850$-$087
• A comprehensive study of type I (thermonuclear) bursts in the new transient SRGA J144459.2$-$604207
• Emission and Absorption Lines in Photospheric Radius Expansion Bursts of 4U 1820$-$30
• NICER observations of type-I X-ray bursts from the ultra-compact X-ray binary M15 X-2
• A transition from mixed-fuel to pure-helium thermonuclear bursts in Terzan 5 X-3/Swift J174805.3-244637
• Mass-Radius Constraints for 2S 0918-549 from an RXTE Superexpansion Burst: A Direct Cooling-Tail Analysis
• Long thermonuclear burst driven thermal-viscous instability of accretion disk: triggering an outburst-like X-ray flare