• Publications of The Korean Astronomical Society
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• v.30 no.2
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• pp.247-249
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• 2015

We review recent results on superhump period variations in SU UMa-type dwarf novae. Our statistical studies have revealed that the evolution of the superhump period is basically composed of three stages: stage-A, during which the superhump period is long and constant, stage-B, during which the superhump period increases as the superoutburst proceeds, and stage-C, during which the superhump period is short and constant. We also introduce a new method of estimating a mass ratio using the stage-A superhump period. This method can extend to, for example, low mass X-ray binaries or AM CVn stars if the stage-A superhump period is well determined.

• Journal of the Korean earth science society
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• v.29 no.7
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• pp.579-585
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• 2008

The time-dependent evolution of disk mass for outburst limit cycle in a black hole microquasar is calculated based on the non-linear hydrodynamic model of thermally unstable accretion disk. The physical parameters such as black hole mass, disk size and mass transfer rate are adopted to reproduce the historical 1975 outburst observed in a prototype black hole X-ray nova A0620-00. The time-dependent effect of irradiation from the central hot region to the disk is considered in two ways: direct irradiation and indirect irradiation reflected from hot accretion flow above the disk. The accretion disk thermal instability model can account for the bolometric luminosity appropriate to typical characteristics of system luminosity observed in X-ray transients during the whole cycle of the outburst evolution. The maximum mass of the accretion disk, ${\sim}4.03{\times}10^{24}g$, is achieved at the ignition of an outburst, and the minimum value, ${\sim}8.54{\times}10^{23}g$, is reached during the cooling decay to quiescence. The disk mass varies ${\sim}5$ times during outburst limit cycle.

• Publications of The Korean Astronomical Society
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• v.30 no.2
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• pp.605-607
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• 2015

The development of time-frequency analysis techniques allow astronomers to successfully deal with the non-stationary time series that originate from unstable physical mechanisms. We applied a recently developed time-frequency analysis method, the Hilbert-Huang transform (HHT), to two non-stationary phenomena: the superorbital modulation in the high-mass X-ray binary SMC X-1 and the quasi-periodic oscillation (QPO) of the AGN RE J1034+396. From the analysis of SMC X-1, we obtained a Hilbert spectrum that shows more detailed information in both the time and frequency domains. Then, a phase-resolved analysis of both the spectra and the orbital profiles was presented. From the spectral analysis, we noticed that the iron line production is dominated by different regions of this binary system in different superorbital phases. Furthermore, a pre-eclipse dip lying at orbital phase ~0:6-0:85 was discovered during the superorbital transition state. We further applied the HHT to analyze the QPO of RE J1034+396. From the Hilbert spectrum and the O-C analysis results, we suggest that it is better to divide the evolution of the QPO into three epochs according to their different periodicities. The correlations between the QPO periods and corresponding fluxes were also different in these three epochs. The change in periodicity and the relationships could be interpreted as the change in oscillation mode based on the diskoseismology model.