arXiv subjects

Taichi Kato

Publications and source records attributed to Taichi Kato.

At least 163 records · Page 9Linked to original sources

arXiv2003

The Infrared Counterpart of the X-Ray Nova XTE J1720-318

We report on the discovery of an infrared counterpart to the X-ray transient XTE J1720-318 on 2003 January 18, nine days after an X-ray outburst, and the infrared light curve during the first 130 days after the outburst. The infrared light curve shows a decline of about 1.2 mag from the peak magnitude of Ks about 15.3 over the observation period, and a secondary maximum, about 40 days after the outburst. Another small increase in the flux was also recorded about 20 days after the outburst. These increases were also detected in the X-ray light curve. The J H Ks colors are consistent with an X-ray irradiated accretion disk suffering an extinction of Av about 8, which is also inferred from its X-ray spectrum and the extinction map constructed from far-infrared dust emission of this line of sight. These J, H, and Ks observations demonstrate that useful data can be obtained even for such an object, which suffers heavy optical extinction, possibly located beyond the Galactic center.

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arXiv2003

Structure in the early afterglow lightcurve of the gamma-ray burst of 29 March 2003

Gamma-ray bursts (GRBs) are energetic explosions that for 0.01--100 s are the brightest gamma-ray sources in the sky. Observations of the early evolution of afterglows we expected to provide clues about the nature of the bursts, but their rapid fading has hampered such studies; some recent rapid localizations of bursts have improved the situation. Here we report on an early detection of the very bright afterglow of the burst of 29 March 2003 (GRB030329). Our data show that, even early in the aferglow phase, the light curve shows unexpectedly complicated structures superimposed on the fading background.

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arXiv2003

Discovery of a short plateau phase in the early evolution of a gamma-ray burst afterglow

We report optical observations during the first hour of the gamma-ray burst (GRB) afterglow of GRB021004. Our observation revealed the existence of a short plateau phase, in which the afterglow remained at almost constant brightness, before an ordinary rapid fading phase. This plateau phase lasted for about 2 hours from 0.024 to 0.10 d after the burst, which corresponds to a missing blank of the early afterglow light curve of GRB990123. We propose that the plateau phase can be interpreted as the natural evolution of synchrotron emission from the forward shock region of a blast wave. The time when the typical frequency of the synchrotron emission passes through the optical range has been predicted to be about 0.1 d after the burst, which is consistent with the observed light curve. Our scenario hence implies that the observed feature in GRB021004 is a common nature of GRB afterglows.

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arXiv2003

The Recurrent Nova U Scorpii in the 1999 Outburst: the First Detection of a Significant Orbital-Period Change

In this paper we present and discuss our time-resolved photometry of an eclipsing recurrent nova, U Sco, during an outburst in 1999, which was conducted from immediately after the optical maximum to the final fading toward the quiescence. In the first-ever complete light-curve, a few primary and secondary eclipses of the binary system were detected, and the timings of the minima were determined. We found that the eclipses showed no totality during the outburst. The depth of the primary eclipses was 0.4-0.8 mag, much shallower than that in quiescence. In the plateau phase, very little irradiation (< 0.1 mag) was observed in the orbital light curve, which implies the existence and a large flaring rim of the accretion disk during the outburst. The minima of the eclipses were detected at earlier orbital phases for the predicted ephemerides. Thus, we obtained an orbital period change of the binary system as \dot{P}/P = -1.7 (+/- 0.7) x 10^{-6} yr^{-1} from the O-C. Assuming that this period change is a result of the conservative mass transfer between the component stars, its mass-transfer rate reaches \dot{M} = 2.4 (+/- 1.0) x 10^{-6} M_solar yr^{-1} for a 1.37 M_solar white dwarf and a 2.0 M_solar mass-donor companion, which is too high to cause shell flashes, even on a massive white dwarf. Therefore, this large rate of the period change strongly indicates a non-conservative mass transfer in the binary system.

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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

Superhump-like variation during the anomalous state of SU UMa

We observed an anomalously outbursting state of SU UMa which occurred in 1992. Time-resolved photometry revealed the presence of signals with a period of 0.0832 +/- 0.0019 d, which is 3.6 sigma longer than the orbital period (0.07635 d) of this system. We attributed this signal to superhumps, based on its deviation from the orbital period and its characteristic profile. During this anomalous state of SU UMa, normal outbursts were almost suppressed, in spite of relatively regular occurrences of superoutbursts. We consider that an ensuing tidally unstable state following the preceding superoutburst can be a viable mechanism to effectively suppress normal outbursts, resulting in an anomalously outbursting state.

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arXiv2001

Changing Supercycle of the ER UMa-Type Star V1159 Ori

We examined the VSNET light curve of the ER UMa-type star V1159 Ori. We detected a large variation of the supercycle (the interval between successive superoutbursts) between extremes of 44.6 and 53.3 d. The outburst activity was also found to decrease when the supercycle was long. The observed variation of the supercycle corresponds to a variation of ~40 % of the mass-transfer rate from the secondary star, totally unexpected for this class of objects. We also detected a hint of ~1800 d periodicity in the variation, whose period is close to what has been suggested for solar-type cycles for cataclysmic variables (CVs). If this periodicity is caused by the magnetic activity of the secondary star, this detection constitutes the first clear evidence of continuing magnetic activity in CV evolution, even after crossing the period gap. This activity may partly explain still poorly understood origins of the high mass-transfer rates in ER UMa-type stars.

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arXiv2001

Spiral Structure in WZ Sagittae around the 2001 Outburst Maximum

Intermediate resolution phase-resolved spectra of WZ Sge were obtained on five consecutive nights (July 23 -- 27) covering the initial stage of the 2001 superoutburst. Double-peaked emission lines of He\textsc{II} at 4686 Å, which were absent on July 23, emerged on July 24 together with emission lines of C\textsc{III} / N\textsc{III} Bowen blend. Analyses of the He\textsc{II} emission lines using the Doppler tomography revealed an asymmetric spiral structure on the accretion disk. This finding demonstrates that spiral shocks with a very short orbital period can arise during the initial stage of an outburst and may be present in all SU UMa stars.

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arXiv2001

Detection of an Eclipse during the 1987 Outburst of U Sco

U Sco is a recurrent nova which showed recorded outbursts in 1863, 1906, 1936, 1979, 1987, and most recently in 1999. The eclipsing nature of the object was discovered after the 1987 outburst. We examined our time-series photographic observations obtained during on 1987 May 20, which fortuitously covered the primary minimum (depth 0.8 mag). This postive detection of an eclipse during the outburst makes the first epoch observation of the U Sco eclipse during the 1987-1999 outburst interval.

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arXiv2001

The 1996-1997 Fading of V651 Mon, the Binary Central Star of the Planetary Nebula NGC 2346

V651 Mon is the binary central star of the bipolar planetary nebula NGC 2346. The star showed the second-ever deep fading in 1996-1997, which was presumably caused by obscuration by a dust cloud in the planetary nebula, as was proposed to explain the 1981-1985 event. The entire duration of the 1996-1997 event was \~400 d, remarkably shorter than the 1981-1985 event, suggesting that the obscuring body was smaller or had a larger tangential velocity. The most remarkable feature in this event was the presence of a sharply defined transient clearing (brightening). From the time-scale of the variation, we propose an upper limit of the projected scale of several times ~10^11 cm of the structure responsible for the brightening. This observation provides the first evidence for a sharply defined, small lucent structure within the obscuring body around the central star of NGC 2346

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arXiv2001

Outburst of Possible Eclipsing Symbiotic Variable AS 289

We discovered major brightenings of the symbiotic variable AS 289 in 1984 and 1995. During the outburst phase in 1996, we detected a temporary drop of the brightness. We consider that an eclipse is responsible for the fading episode of AS 289 during outburst, as in FG Ser or V1413 Aql. The profile of light curve suggests the eclipse may be grazing total, while the brightness at minimum is considerably brighter than the pre-outburst level, implying that a certain fraction of the outbursting source remain uneclipsed.

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arXiv2001

Reanalysis of Ichinohe's Observations of RU Cam

The W Vir-type variable star, RU Cam, once claimed to have stopped pulsation, has been known to show irregularly variable amplitude. It has also shown from the analysis of the VSOLJ archive that visual observers of the VSOLJ have recorded the cessation of pulsation and its possible resumption, together with the phase of large-amplitude variation around 1910. We present the result of reanalysis of Ichinohe's original data until 1911, by using the Hipparcos V-magnitudes of the comparison stars. The range of light variation was 8.26-9.33, giving an amplitude of 1.07 mag. Our new analysis, however, rather disproves the existence of the secondary maximum which Ichinohe (1909) described, giving a more consistent, typical W Vir-type, light curve with those of other observers at that epoch.

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arXiv2001

Remarkable yellow supergiant variable TmzV429

We discovered that GSC 6554.559 is a previously unknown variable star, and named as TmzV429. We noticed that TmzV429 is identified with the IRAS-selected proto-planetary nebula (PPN), IRAS 08005-2356, which is undergoing a vigorous mass-loss episode. The analysis of photometric data suggests that TmzV429 a short-period pulsator, resembling a high-latitude yellow supergiant variable. TmzV429 is considered to be one of rare objects caught in the rapid course of PPN evolution, and shows one of the most striking mass-loss features among variable stars in the PPN stage. Since its evolutionary time-scale is estimated to be quite short (~150 yrs), future observations of pulsations of TmzV429 is expected to provide an excellent opportunity to study the stellar evolution in real time.

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arXiv2001

Photometry of the 1991 Superoutburst of EF Pegasi: Super-Quasi-Periodic Oscillations with Rapidly Decaying Periods

We observed the 1991 October outburst of EF Peg. Prominent superhumps with a period of 0.08705(1) d were observed, qualifying EF Peg as a being long-period SU UMa-type dwarf nova. The superhump period showed a monotonous decrease during the superoutburst, which makes a contrast to the virtually zero period change observed during the 1997 superoutburst of the same object. Large-amplitude, and highly coherent quasi-periodic oscillations (super-QPOs) were observed on October 18, when superhumps were still growing in amplitude. Most strikingly, the QPOs showed a rapid decrease of the period from 18 m to 6.8 m within the 3.2-hr observing run. Such a rapid change in the period has not been observed in any class of QPOs in cataclysmic variables. We propose a hypothesis that the rapid decrease of the QPO period reflects the rapid removal of the angular momentum from an orbiting blob in the accretion disk, via the viscosity in a turbulent disk. A brief comparison is given with the QPOs in X-ray binaries, some of which are known to show a similar rapid decrease in the periods.

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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

V344 Lyr: an unusual large-amplitude SU UMa-type dwarf nova with a short supercycle

We studied the large-amplitude SU UMa-type dwarf nova V344 Lyr. A combination of our observations and reports to VSNET has yielded an extensive coverage of outbursts between 1994 July and 2001 June. The analysis of this data showed a mean supercycle length of 109.6 d. This value is one of the smallest among known SU UMa-type dwarf novae (except unusual ER UMa-type dwarf novae). The outburst amplitude of V344 Lyr (~5.5 mag) is found to be much larger than those (3.5 +/- 0.3 mag) of SU UMa-type dwarf novae with similar supercycle lengths. Such a deviation of the amplitude in V344 Lyr is difficult to explain by the inclination effect. The extreme outburst parameters of V344 Lyr would require an additional mechanism to effectively reduce the quiescent luminosity or to increase the outburst frequency.

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