arXiv subjects

Makoto Uemura

Publications and source records attributed to Makoto Uemura.

95 records · Page 6Linked to original sources

arXiv2002

CI Aquilae: a recurrent nova with an unusually long plateau phase

We present the results of optical photometry of the recurrent nova CI Aql in later phase of the outburst which occurred in 2000. Our observation revealed that the object reached the quiescent level between 2001 December and 2002 April and therefore that CI Aql is a unique recurrent nova characterized by an extremely long (1.4--1.7 yr) plateau phase. The light curve obtained in the outburst suggests that the object is the first example of an intermediate between classical novae and recurrent novae. In comparison with estimation given in published theoretical calculations, the long duration of the plateau phase supports a higher hydrogen content of the white-dwarf envelope, while such an abundance of hydrogen requires a later cessation of the wind which is in disagreement with the sudden fading observed in late November of 2000. The light curve obtained in later phase of the outburst indicates that the object was fainter and the gradual decline was steeper than predicted. These discrepancies between the observation and theoretical prediction require drastic modification of the present model of CI Aql.

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arXiv2002

Rapid Optical Fluctuations in the Black Hole Binary, V4641 Sgr

We report on unprecedented short-term variations detected in the optical flux from the black hole binary system, V4641 Sgr. Amplitudes of the optical fluctuations were larger at longer time scales, and surprisingly reached ~60% around a period of ~10 min. The power spectra of fluctuations are characterized by a power law. It is the first case in black hole binaries that the optical emission was revealed to show short-term and large-amplitude variations given by such a power spectrum. The optical emission from black hole binaries is generally dominated by the emission from the outer portion of an accretion disc. The rapid optical fluctuations however indicate that the emission from an inner accretion region significantly contributes to the optical flux. In this case, cyclo-synchrotron emission associated with various scales of magnetic flares is the most promising mechanism for the violently variable optical emission.

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arXiv2002

Dwarf Nova-like Outburst of Short Period Intermediate Polar HT Camelopardalis

We report the first time-series observations of the short outburst of the proposed intermediate polar HT Cam (=RX J0757.0+6306). On 2001 December 29, we detected the object was undergoing a bright outburst at the magnitude of $m_{vis}=12.2$. Following this detection, we started international joint observations through VSNET. The light curve showed a gradual decline for the first 0.5 d. Following this short plateau phase, the rate of decline dramatically increased to more than 4 mag d$^{-1}$. Within 1.5 d from the outburst detection, the object almost declined to the quiescent level. During the rapidly declining phase, long-term modulations with a period of 86 min and strong pulses with a period of 8.6 min were observed. We concluded that 86 min and 8.6 min are the orbital period and the spin period of HT Cam, respectively. By the detection of the spin period, we confirmed the IP classification of HT Cam. However, its outburst behavior rather resembles that of dwarf novae. The discrepancy between the declining rates of the total flux and the pulse flux strongly suggests that the disk instabilities were taking place during the outburst.

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arXiv2001

First detection of the growing humps at the rapidly rising stage of dwarf novae AL Com and WZ Sge

We report on time-series photometric observations in the earliest stages of superoutbursts of the extreme dwarf novae, AL Com and WZ Sge, which started on 2001 May after the 6 years quiescence and on 2001 July after the 23 years quiescence, respectively. We detected growth of ``early superhumps'' during the each rising stage. Our observations reject the mass transfer instability for the trigger of the superoutburst of WZ Sge stars, and show the existence of some relations between the ``early superhumps'' and the spiral structure, which give a hint of the origin of the ``early superhumps.''

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arXiv2001

Large-Amplitude 2.65-d Oscillation in the VY Scl-Type star V425 Cas

From long-term photometry of a VY Scl-type star, V425 Cas, between 1998 and 2000, we discovered a short-term, large-amplitude (up to 1.5 mag) variations. The variation was well represented by a single period of 2.65 d. The large amplitude and the profile of the folded light curve suggest that the dwarf nova-type disk instability is responsible for this variation. The shortness of the period is unprecedented in hydrogen-rich cataclysmic variables. Given the recent emerging evidence that the irradiation from white dwarfs in VY Scl-type systems affect their light behavior, we propose a possibility that this unique variation in V425 Cas can be explained by the combination of the dwarf nova-type disk instability and irradiation. Similar short-period "outbursts" have been known in X-ray transients (V518 Per), and helium cataclysmic variables (CR Boo and V803 Cen). We discuss the possibility that these phenomena have a common origin to the unique variation in V425 Cas.

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