The thermonuclear X-ray bursts of 4U 1730-22

Abstract

We present observations of the historic transient 4U 1730-22 as observed with the Neutron Star Interior Composition Explorer (NICER). After remaining in quiescence since its 1972 discovery, this X-ray binary showed renewed outburst activity in 2021 and 2022. We observed 4U 1730-22 extensively with NICER, detecting a total of 17 thermonuclear X-ray bursts. From a spectroscopic analysis, we find that these X-ray bursts can be divided into a group of bright and weak bursts. All bright bursts showed $1\sim 2$ second rise times and a photospheric radius expansion phase, while the weak bursts showed a slower $\sim 5$ second rise with a tendency for concave shapes. From the photospheric radius expansion flux, we estimate the source distance at $6.9\pm 0.2$ kpc. We consider various interpretations for our observations and suggest that they may be explained if accreted material is burning stably at the stellar equator, and unstable ignition occurs at a range of higher latitudes.

13 pages, 6 figures, 2 tables. Accepted for publication in ApJ

References in corpus (19)

• Thermonuclear (type-I) X-ray bursts observed by the Rossi X-ray Timing Explorer
• Discovery of coherent millisecond X-ray pulsations in Aql X-1
• Evidence for enhanced persistent emission during sub-Eddington thermonuclear bursts
• Accretion disks and coronae in the X-ray flashlight
• The influence of accretion geometry on the spectral evolution during thermonuclear (type-I) X-ray bursts
• Millihertz Oscillation Frequency Drift Predicts the Occurrence of Type I X-ray Bursts
• The direct cooling tail method for X-ray burst analysis to constrain neutron star masses and radii
• Discovery of X-ray burst triplets in EXO 0748-676
• X-ray Reflection and An Exceptionally Long Thermonuclear Helium Burst from IGR J17062-6143
• The Latitude of Type I X-Ray Burst Ignition on Rapidly Rotating Neutron Stars
• Ignition latitude and the shape of Type I X-ray bursts
• X-ray burst induced spectral variability in 4U 1728-34
• Neutron star cooling and the rp process in thermonuclear X-ray bursts
• Burst-Disk Interaction in 4U 1636-536 as observed by NICER
• On the dependence of X-ray burst rate on accretion and spin rate
• The efficiency of nuclear burning during thermonuclear (Type I) bursts as a function of accretion rate
• Thermonuclear X-ray bursts from LMXB 4U 1636$-$536 observed with AstroSat
• The study of multi-peaked type-I X-ray bursts in the neutron-star low mass X-ray binary 4U 1636$-$536 with RXTE
• Broadband time-resolved spectroscopy of thermonuclear X-ray bursts from 4U 1636-536 using AstroSat

Cited by in corpus (1)

• Detection of millihertz quasi-periodic oscillations in the low-mass X-ray binary 4U 1730--22 with NICER