Return of 4U 1730–22 after 49 yr Silence: The Peculiar Burst Properties of the 2021/2022 Outbursts Observed by Insight-HXMT

Abstract Staying in quiescence for 49 yr, 4U 1730–22 became active and had two outbursts 2021 2022; 10 thermonuclear X-ray bursts were detected with Insight-HXMT. Among them, the faintest burst showed a double-peaked profile, placing source as seventh accreting neutron star (NS) exhibiting type I bursts; other photospheric radius expansion (PRE). The properties of non-PRE indicate that it could be related to stalled burning front. For five bright PRE bursts, apart from emission surface, we find residuals both soft (<3 keV) hard (>10 bands. Time-resolved spectroscopy reveals excess can attributed an enhanced preburst/persistent or Comptonization by corona. We find, shows rise until photosphere touches down NS surface rather than theoretical predicted constant Eddington luminosity. shortage early rising phase is beyond occlusion disk. speculate findings above are due obscured part (not only lower part) exposed line sight evaporation material emission, anisotropic ( ξ > 1) phase. In addition, based on fluxes at their touchdown times, derive distance estimation 9.0–12.4 kpc.