X-ray variability with spectral state transitions in NS-LMXBs observed with MAXI/GSC and Swift/BAT

TL;DR

This study analyzes X-ray variability and spectral state transitions in neutron star low-mass X-ray binaries using MAXI/GSC and Swift/BAT data, identifying mini-outbursts that align with disk instability models.

Contribution

It reports the detection of mini-outbursts with smaller amplitudes and shorter durations, interpreting them as a distinct class of variability predicted by disk instability theory.

Findings

Mini-outbursts have smaller amplitudes and shorter durations than normal outbursts.

Mini-outbursts are interpreted as purr-type outbursts predicted by disk instability models.

Similar variability without spectral state transitions may be repeated mini-outbursts.

Abstract

X-ray variabilities with spectral state transitions in bright low mass X-ray binaries containing a neutron star are investigated by using the one-day bin light curves of MAXI/GSC (Gas Slit Camera) and Swift/BAT (Burst Alert Telescope). Four sources (4U 1636$-$536, 4U 1705$-$44, 4U 1608$-$52, and GS 1826$-$238) exhibited small-amplitude X-ray variabilities with spectral state transitions. Such "mini-outbursts" were characterized by smaller amplitudes (several times) and shorter duration (less than several tens of days) than those of "normal outbursts." Theoretical model of disk instability by Mineshige and Osaki (PASJ, 37, 1, 1985) predicts both large-amplitude outbursts and small-amplitude variabilities. We interpret the normal outbursts as the former prediction of this model, and the mini-outbursts as the latter. Here, we can also call the mini-outburst as "purr-type outburst" referring to theoretical work. We suggest that similar variabilities lasting for several tens of days without spectral state transitions, which are often observed in the hard state, may be a repeat of mini-outbursts.