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 language | English |
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Article number | 81 |
Journal | Astrophysical Journal |
Volume | 140 |
Issue number | 1 |
Number of pages | 10 |
ISSN | 0004-637X |
DOIs | |
Publication status | Published - 2022 |
Keywords
- Low-mass x-ray binary stars
- X-ray bursts