arXiv subjects

S. Kitamoto

Publications and source records attributed to S. Kitamoto.

22 records · Page 2Linked to original sources

arXiv2002

Energy Spectra and Normalized Power Spectral Densities of X-Ray Nova GS 2000+25

The X-ray energy spectra and Normalized Power Spectral Densities (NPSDs) of an X-ray nova, GS 2000+25, were investigated. The X-ray energy spectra of the source consist of two components: a hard component, which can be represented by a power-law, and an ultra-soft component, represented by radiation from an optically-thick accretion disk (the disk component). In a model in which the power-law component is the Compton-scattered radiation, it is found that the temperature of the incident blackbody radiation to the Compton cloud decrease from 0.8 keV to 0.2 keV according to the decay of the intensity, which coincides with that of the inner accretion disk. When the source changed from the high-state to the low-state, both the photon index of the power-law component (or Compton y-parameter) and the NPSD of the hard component dramatically changed as did GS 1124-683. That is, the photon index changed from 2.2--2.6 to 1.7--1.8 and the absolute values of the NPSDs at 0.3 Hz of the hard component in the low-state became about 10-times larger than those of the hard component in the high-state. These X-ray properties were similar to those of other black-hole candidates, such as Cyg X-1, GX 339-4, and LMC X-3.

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arXiv2001

Long-term X-ray variability and state transition of GX 339-4

With extensive monitoring data spanning over 30 years from Vela 5B, Ariel 5, Ginga, Compton Gamma Ray Observatory, Rossi X-ray Timing Explorer and BeppoSAX, we find evidence for long-term X-ray variability on timescales ~190-240 d from the black hole low-mass X-ray binary system GX 339-4. Such variability resembles the outburst cycle of Z Cam-type dwarf novae, in which the standard disc instability model plays a crucial role. If such a model is applicable to GX 339-4, then the observed variability might be due to the irradiation of an unstable accretion disc. We show that within the framework of the X-ray irradiation model, when the accretion rate exceeds a critical value, GX 339-4 enters a `flat-topped' high/soft state, such as seen in 1998, which we suggest corresponds to the `standstill' state of Z Cam systems.

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arXiv2000

Changes in the long term intensity variations in Cyg X-2 and LMC X-3

We report the detection of changes in the long-term intensity variations in two X-ray binaries, Cyg X-2 and LMC X-3. In this work, we have used the long-term light curves obtained with the All-Sky Monitors (ASMs) of the Rossi X-Ray Timing Explorer (RXTE), Ginga, Ariel 5, and Vela 5B and the scanning modulation collimator of HEAO 1. It is found that in the light curves of both the sources, obtained with these instruments at various times over the last 30 years, more than one periodic or quasi-periodic component is always present. The multiple prominent peaks in the periodograms have frequencies unrelated to each other. In Cyg X-2, RXTE-ASM data show strong peaks at 40.4 and 68.8 days, and Ginga-ASM data show strong peaks at 53.7 and 61.3 days. Multiple peaks are also observed in LMC X-3. The various strong peaks in the periodograms of LMC X-3 appear at 104, 169, and 216 days (observed with RXTE-ASM) and 105, 214, and 328 days (observed with Ginga-ASM). The present results, when compared with the earlier observations of periodicities in these two systems, demonstrate the absence of any stable long period. The 78 day periodicity detected earlier in Cyg X-2 was probably due to the short time base in the RXTE data that were used, and the periodicity of 198 days in LMC X-3 was due to a relatively short duration of observation with HEAO 1.

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arXiv1997

Complex outburst behaviour from the black-hole candidate 4U 1630-47

We present data from different epochs in 1978, 1987-1991 and 1996 from the black-hole candidate 4U1630-47. For the first time we present almost complete coverage of the outbursts which started in 1987, 1988 and 1996. We find that the outburst behaviour of 4U1630-47 is more complex than previously realized. The source shows outbursts with durations on the order of ~100-200 days and sometimes intervals of long-term X-ray activity. The relatively short outbursts which occurred in 1987 and 1996 exhibited different colour behaviour: the outburst in 1987 showed softening of the X-ray spectrum, whereas the outburst in 1996 showed hardening of the X-ray spectrum, as the outbursts proceeded. The outburst which started in 1977 may have lasted for up to ~10 months, whereas the outburst which started in 1988 showed activity for up to ~2.4 years. Such long-term activity is reminiscent of that seen in GRS1716-249 and in the galactic superluminal sources GRS1915+105 and GROJ1655-40. We refine the outburst ephemeris of 4U\1630-47 and find that the outburst recurrence time scale may have changed from ~600 days to ~690 days between the 1984 and 1987 outbursts. If the recurrence interval of ~690 days continues, the next outburst of 4U1630-47 is predicted to occur in 1998 January.

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