Return of 4U~1730--22 after 49 years silence: the spectral properties of the 2021/2022 outbursts observed by NICER and Insight-HXMT and the soft-to-hard state transition caused by the propeller effect

Abstract

After in quiescence for 49 years, 4U~1730--22 became active and had two outbursts in 2021 \& 2022, the onset and tail of the outbursts were observed by NICER, which give us a peerless opportunity to study the state transition and its underlying mechanism. In this work, we take both the NS surface and accretion disk emission as the seed photons of the Comptonization and derive their spectral evolution in a bolometric luminosity range of 1\%--15\%$L_{\mathrm{Edd}}$. In the high/soft state, the inferred inner disk radius and the NS radius are consistent well, which implies that the accretion disk is close to the NS surface. For the decay stage, we report a steep change of the accretion disk emission within one day, i.e., the soft-to-hard transition, which could be due to the propeller effect and the corresponding neutron star surface magnetic field is 1.8--2.2$\times 10^8$ G. Moreover, the inner disk radius is truncated at the corotation radius, which is similar to the propeller effect detected from 4U~1608--52. The absence of the propeller effect in the hard-to-soft state transition implies that the transition between the magnetospheric accretion and the disk accretion is not the sole cause of the state transitions.

2023,ApJL,942L,12

References in corpus (10)

• Modelling the behaviour of accretion flows in X-ray binaries
• An Empirical Background Model for the NICER X-ray Timing Instrument
• "Propeller" Regime of Disk Accretion to Rapidly Rotating Stars
• Spectral and temporal properties of Compton scattering by mildly relativistic thermal electrons
• Can disk-magnetosphere interaction models and beat frequency models for quasi-periodic oscillation in accreting X-ray pulsars be reconciled?
• Mass transfer during low mass X-ray transient decays
• Long tails on thermonuclear X-ray bursts from neutron stars: a signature of inward heating?
• The connection between the UV/optical and X-ray emission in the neutron star low-mass X-ray binary Aql X-1
• Discovery of a 584.65 Hz Burst Oscillation in the Low Mass X-Ray Binary 4U 1730--22
• Insight-HXMT observation on 4U~1608--52: evidence of interplay between thermonuclear burst and accretion environments