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

TL;DR

This study analyzes the 2021/2022 outbursts of 4U~1730--22 after 49 years of quiescence, revealing a soft-to-hard state transition likely caused by the propeller effect and examining spectral evolution with NICER and Insight-HXMT.

Contribution

It provides the first detailed spectral analysis of 4U~1730--22's outbursts, highlighting the role of the propeller effect in state transitions and the magnetic field strength of the neutron star.

Findings

The inner disk radius matches the neutron star radius in the high/soft state.

A steep change in disk emission indicates a soft-to-hard transition due to the propeller effect.

The inner disk radius is truncated at the corotation radius, similar to other sources.

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\%$L_{\rm Edd}$. 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$\times10^{8}$ 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.