NICER Detection of Strong Photospheric Expansion during a Thermonuclear X-Ray Burst from 4U 1820-30

Abstract

The Neutron Star Interior Composition Explorer (NICER) on the International Space Station (ISS) observed strong photospheric expansion of the neutron star in 4U 1820-30 during a Type I X-ray burst. A thermonuclear helium flash in the star's envelope powered a burst that reached the Eddington limit. Radiation pressure pushed the photosphere out to ~200 km, while the blackbody temperature dropped to 0.45 keV. Previous observations of similar bursts were performed with instruments that are sensitive only above 3 keV, and the burst signal was weak at low temperatures. NICER's 0.2-12 keV passband enables the first complete detailed observation of strong expansion bursts. The strong expansion lasted only 0.6 s, and was followed by moderate expansion with a 20 km apparent radius, before the photosphere finally settled back down at 3 s after the burst onset. In addition to thermal emission from the neutron star, the NICER spectra reveal a second component that is well fit by optically thick Comptonization. During the strong expansion, this component is six times brighter than prior to the burst, and it accounts for 71% of the flux. In the moderate expansion phase, the Comptonization flux drops, while the thermal component brightens, and the total flux remains constant at the Eddington limit. We speculate that the thermal emission is reprocessed in the accretion environment to form the Comptonization component, and that changes in the covering fraction of the star explain the evolution of the relative contributions to the total flux.

arXiv comment6 pages, 4 figures, 1 table, published by ApJ Letters

References in corpus (25)

• On the Absorption of X-rays in the Interstellar Medium
• Thermonuclear (type-I) X-ray bursts observed by the Rossi X-ray Timing Explorer
• Calibration of the Rossi X-ray Timing Explorer Proportional Counter Array
• Photospheric radius expansion X-ray bursts as standard candles
• The Mass and Radius of the Neutron Star in 4U 1820-30
• Exposing the Nuclear Burning Ashes of Radius Expansion Type I X-ray Bursts
• Evidence for accretion rate change during type I X-ray bursts
• X-ray bursting neutron star atmosphere models using an exact relativistic kinetic equation for Compton scattering
• Models of Type I X-Ray Bursts in 4U 1820-30
• The Red Giant Branch in the Near-Infrared Colour-Magnitude Diagrams. I: The calibration of photometric indices
• Spectra of the spreading layers on the neutron star surface and constraints on the neutron star equation of state
• The Evolution of the Accretion Disk around 4U 1820-30 During a Superburst
• XMM-Newton observation of 4U 1820-30: Broad band spectrum and the contribution of the cold interstellar medium
• Evidence of heavy-element ashes in thermonuclear X-ray bursts with photospheric superexpansion
• A bright thermonuclear X-ray burst simultaneously observed with Chandra and RXTE
• Anisotropy of X-ray bursts from neutron stars with concave accretion disks
• On the Dynamics of Suddenly Heated Accretion Disks around Neutron Stars
• NICER Observes the Effects of an X-Ray Burst on the Accretion Environment in Aql X-1
• On the spreading layer emission in luminous accreting neutron stars
• Winds from Neutron Stars and Strong Type I X--Ray Bursts
• Superexpansion as a possible probe of accretion in 4U 1820-30
• The cooling rate of neutron stars after thermonuclear shell flashes
• Neutron star cooling and the rp process in thermonuclear X-ray bursts
• Coherent oscillations and the evolution of the emission area in the decaying phase of radius-expansion bursts from 4U 1636-53
• Variable spreading layer in 4U 1608-52 during thermonuclear X-ray bursts in the soft state

Cited by in corpus (21)

• A NICER thermonuclear burst from the millisecond X-ray pulsar SAX J1808.4-3658
• Phase transitions in neutron stars and their links to gravitational waves
• Horizons: Nuclear Astrophysics in the 2020s and Beyond
• A uniform search for thermonuclear burst oscillations in the RXTE legacy dataset
• GRMHD Simulations of Accreting Neutron Stars with Non-Dipole Fields
• NICER observes a secondary peak in the decay of a thermonuclear burst from 4U 1608-52
• Discovery of thermonuclear (Type I) X-ray bursts in the X-ray binary Swift J1858.6-0814 observed with NICER and NuSTAR
• A NICER look at thermonuclear X-ray bursts from Aql X-1
• NICER Discovers Spectral Lines During Photospheric Radius Expansion Bursts from 4U 1820-30: Evidence for Burst-driven Winds
• Super-Eddington winds from Type I X-ray bursts
• Burst-Disk Interaction in 4U 1636-536 as observed by NICER
• NICER views moderate, strong, and extreme photospheric expansion bursts from the ultracompact X-ray binary 4U 1820$-$30
• A Comprehensive Study of Thermonuclear X-ray Bursts from 4U 1820-30 with NICER: Accretion Disk Interactions and a Candidate Burst Oscillation
• Expanded atmospheres and winds in Type I X-ray bursts from accreting neutron stars
• Insight-HXMT observation on 4U~1608--52: evidence of interplay between thermonuclear burst and accretion environments
• Modelling spin-up episodes in accreting millisecond X-ray pulsars
• Broadband time-resolved spectroscopy of thermonuclear X-ray bursts from 4U 1636-536 using AstroSat
• The thermonuclear X-ray bursts of 4U 1730-22
• The imprint of convection on Type I X-ray bursts: Pauses in photospheric radius expansion lightcurves
• Emission and Absorption Lines in Photospheric Radius Expansion Bursts of 4U 1820$-$30
• NICER observations of type-I X-ray bursts from the ultra-compact X-ray binary M15 X-2