Propeller-Effect Interpretation of MAXI/GSC Light Curves of 4U 1608-52 and AqlX-1 and application to XTE J1701-462

TL;DR

This study analyzes the luminosity distributions of neutron star low-mass X-ray binaries, interprets the rapid luminosity decrease as the propeller effect, and estimates magnetic fields, applying the model to multiple sources.

Contribution

It introduces a method to interpret luminosity cut-offs as the propeller effect and estimates neutron star magnetic fields from observational data.

Findings

Luminosity cut-off at ~10^36 erg/s indicates propeller effect.

Estimated magnetic fields are (0.5-1.6)×10^8 G for 4U 1608-52 and AqlX-1.

Applied the model to XTE J1701-462, suggesting magnetic fields around 10^9 G.

Abstract

We present the luminosity dwell-time distributions during the hard states of low-mass X-ray binaries containing a neutron star, 4U 1608-52 and AqlX-1, observed with MAXI/GSC. The luminosity distributions show a steep cut-off in the low-luminosity side at $\sim1.0 \times 10^{36}$ erg s$^{-1}$ in both the two sources. The cut-off implies a rapid luminosity decrease in their outburst decay phases, and the feature can be interpreted as due the propeller effect. We estimated the surface magnetic field of the neutron star to be (0.5--1.6) $\times 10^8$ G in 4U 1608-52 and (0.6--1.9) $\times 10^8$ G in AqlX-1 from the cut-off luminosity. We applied the same propeller mechanism to the similar rapid luminosity decrease observed in the transient Z-source, XTE J1701-462, with RXTE/ASM. Assuming that spin period of the neutron star is in the order of milliseconds, the observed cut-off luminosity deduces surface magnetic field in the order of $10^9$ G.