  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.
