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

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.

14 pages, 10 figures

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Cited by in corpus (8)

• A Soft X-Ray Spectral Episode for the Clocked Burster, GS 1826-24 as Measured by Swift and NuSTAR
• Catalog of outbursts of neutron star LMXBs
• A Hard-to-Soft State Transition of Aquila X-1 Observed with Suzaku
• Decades' long-term variations in NS-LMXBs observed with MAXI/GSC, RXTE/ASM and Ginga/ASM
• AstroSat view of `Clocked` burster GS 1826-238:broad-band spectral nature of persistent and burst emission during soft state
• Hard-tail emission in the soft state of low-mass X-ray binaries and their relation to the neutron star magnetic field
• Swift/BAT and MAXI/GSC Broadband Transient Monitor
• Return of the Clocked Burster: Exceptionally Short Recurrence Time in GS 1826-238