The Thermonuclear X-Ray Bursts of 4U 1730–22

Peter Bult, Giulio C. Mancuso, Tod E. Strohmayer, Arianna C. Albayati, Diego Altamirano, Douglas J. K. Buisson, Jérôme Chenevez, Sebastien Guillot, Tolga Güver, Wataru Iwakiri, Gaurava K. Jaisawal, Mason Ng, Andrea Sanna, Jean H. Swank

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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 173–22 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 1–2 s rise times and a photospheric radius expansion phase, while the weak bursts showed a slower ∼5 s rise with a tendency for concave shapes. From the photospheric radius expansion flux, we estimate the source distance at 6.9 ± 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.
Original languageEnglish
Article number81
JournalAstrophysical Journal
Volume140
Issue number1
Number of pages10
ISSN0004-637X
DOIs
Publication statusPublished - 2022

Keywords

  • Low-mass x-ray binary stars
  • X-ray bursts

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