Hard-tail emission in the soft state of low-mass X-ray binaries and their relation to the neutron star magnetic field

TL;DR

This study investigates the hard X-ray emission in the soft state of neutron star low-mass X-ray binaries, linking the emission characteristics to the neutron star's magnetic field through Compton cloud analysis.

Contribution

It introduces a method to estimate neutron star magnetic fields by analyzing the size of the Compton cloud inferred from X-ray hardness ratios.

Findings

Sources with higher hardness ratio have larger Comptonized emission.

The size of the Compton cloud correlates with the neutron star's magnetic field.

Estimated magnetic field limits are consistent with previous studies.

Abstract

Average hard-tail X-ray emission in the soft state of nine bright Atoll low-mass X-ray binaries containing a neutron star (NS-LMXBs) are investigated by using the light curves of MAXI/GSC and Swift/BAT. Two sources (4U 1820$-$30 and 4U 1735$-$44) exhibit large hardness ratio (15--50 keV$/$2--10 keV: {\it HR} $>$ 0.1), while the other sources distribute at {\it HR} $\ltsim$ 0.1. In either case, {\it HR} does not depend on the 2--10 keV luminosity. Therefore the difference of {\it HR} is due to the 15--50 keV luminosity, which is Comptonized emission. The Compton cloud is assumed to be around the neutron star. The size of the Compton cloud would affect the value of {\it HR}. Although the magnetic field of NS-LMXB is weak, we could expect a larger Alfv\'{e}n radius than the innermost stable circular orbit or the neutron star radius in some sources. In such cases, the accretion inflow is stopped at the Alfv\'{e}n radius and would create relatively large Compton cloud. It would result in the observed larger Comptonized emission. By attributing the difference of the size of Compton cloud to the Alfv\'{e}n radius, we can estimate the magnetic fields of neutron star. The obtained lower/upper limits are consistent with the previous results.