Basic parameters of the helium accreting X-ray bursting neutron star in 4U 1820-30

Abstract

The ultracompact low-mass X-ray binary 4U 1820-30 situated in the globular cluster NGC 6624 has an orbital period of only $\approx$11.4 min which likely implies a white dwarf companion. The observed X-ray bursts demonstrate a photospheric radius expansion phase and therefore are believed to reach the Eddington luminosity allowing us to estimate the mass and the radius of the neutron star (NS) in this binary. Here we re-analyse all Rossi X-ray Timing Explorer observations of the system and confirm that almost all the bursts took place during the hard persistent state of the system. This allows us to use the recently developed direct cooling tail method to estimate the NS mass and radius. However, because of the very short, about a second, duration of the cooling tail phases that can be described by the theoretical atmosphere models, the obtained constraints on the NS radius are not very strict. Assuming a pure helium NS atmosphere we found that the NS radius is in the range 10-12 km, if the NS mass is below 1.7 $M_{\odot }$, and in a wider range of 8-12 km for a higher 1.7-2.0 $M_{\odot }$ NS mass. The method also constrains the distance to the system to be 6.5$\pm$0.5 kpc, which is consistent with the distance to the cluster. For the solar composition atmosphere, the NS parameters are in strong contradiction with the generally accepted range of possible NS masses and radii.

9 pages, 9 figures, 2 tables, accepted for publication in MNRAS

References in corpus (14)

• Neutron Star Observations: Prognosis for Equation of State Constraints
• Thermonuclear (type-I) X-ray bursts observed by the Rossi X-ray Timing Explorer
• The Mass and Radius of the Neutron Star in 4U 1820-30
• X-ray bursting neutron star atmosphere models using an exact relativistic kinetic equation for Compton scattering
• The effect of accretion on the measurement of neutron star mass and radius in the low-mass X-ray binary 4U 1608-52
• Spread of Matter over a Neutron-Star Surface During Disk Accretion: Deceleration of Rapid Rotation
• Evidence for enhanced persistent emission during sub-Eddington thermonuclear bursts
• The influence of accretion geometry on the spectral evolution during thermonuclear (type-I) X-ray bursts
• The superorbital variability and triple nature of the X-ray source 4U 1820-303
• The direct cooling tail method for X-ray burst analysis to constrain neutron star masses and radii
• Absorption features in the spectra of X-ray bursting neutron stars
• Detection of burning ashes from thermonuclear X-ray bursts
• A state-dependent influence of the type-I bursts on the accretion in 4U 1608--52?
• 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

Cited by in corpus (10)

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• Burst-induced coronal cooling in GS 1826-24 The clock wagging its tail
• NICER observations of the evidence of Poynting-Robertson drag and disk reflection during type I X-ray bursts from 4U 1636$-$536
• NICER views moderate, strong, and extreme photospheric expansion bursts from the ultracompact X-ray binary 4U 1820$-$30
• A Comprehensive Study of Thermonuclear X-ray Bursts from 4U 1820-30 with NICER: Accretion Disk Interactions and a Candidate Burst Oscillation
• SRG/ART-XC discovery of SRGAJ144459.2-604207: a well-tempered bursting accreting millisecond X-ray pulsar
• Anti-de Sitter neutron stars in the theory of gravity with nonminimal derivative coupling
• Neutron star atmospheres composed of fusion ashes
• The mass of the neutron star in 4U 1820-30 revisited