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


Bult P., Mancuso G. C., Strohmayer T. E., Albayati A. C., Altamirano D., Buisson D. J. K., ...More

Astrophysical Journal, vol.940, no.1, 2022 (SCI-Expanded) identifier identifier

  • Publication Type: Article / Article
  • Volume: 940 Issue: 1
  • Publication Date: 2022
  • Doi Number: 10.3847/1538-4357/ac9b26
  • Journal Name: Astrophysical Journal
  • Journal Indexes: Science Citation Index Expanded (SCI-EXPANDED), Scopus, Academic Search Premier, Aerospace Database, INSPEC, zbMATH, Directory of Open Access Journals, DIALNET
  • Istanbul University Affiliated: Yes

Abstract

© 2022. The Author(s). Published by the American Astronomical Society.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.