arXiv subjects

T. Kato

Publications and source records attributed to T. Kato.

At least 109 records · Page 6Linked to original sources

arXiv2002

Astrometric authentication of RX J2309.8+2135 as a nearby dwarf nova candidate

RX J2309.8+2135 = 1RXS J230949.6+213523 is an X-ray source discovered through the ROSAT all-sky survey. The object was later identified with a star showing a spectrum resembling a cataclysmic variable (CV). Schwope et al. (2002, astro-ph/0210059) recently proposed the spectral type of M3, which would indicate a small distance. We have detected a large proper motion (~69 mas/yr) of this object. This detection authenticates RX J2309.8+2135 as a nearby dwarf nova candidate. Even at a reasonable distance of ~100 pc, which is inferred from the measured proper motion, the outburst maxima of RX J2309.8+2135 would reach V~9. RX J2309.8+2135 may belong to a hitherto unidentified class of long-period CVs with the lowest mass-transfer rates.

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arXiv2002

Photometric study of new southern SU UMa-type dwarf novae and candidates: V877 Ara, KK Tel and PU CMa

We photometrically observed three dwarf novae V877 Ara, KK Tel and PU CMa. We discovered undisputed presence of superhumps in V877 Ara and KK Tel, with mean periods of 0.08411(2) d and 0.08808(3) d, respectively. Both V877 Ara and KK Tel are confirmed to belong to long-period SU UMa-type dwarf novae. In V877 Ara, we found a large decrease of the superhump period (dot(P)/P = -14.5 +/- 2.1 x 10^(-5)). There is evidence that the period of KK Tel decreased at a similar or a more exceptional rate. Coupled with the past studies of superhump period changes, these findings suggest that a previously neglected diversity of phenomena is present in long-period SU UMa-type dwarf novae. The present discovery of a diversity in long-period SU UMa-type systems would become an additional step toward a full understanding the dwarf nova phenomenon. PU CMa is shown to be an excellent candidate for an SU UMa-type dwarf nova. We examined the outburst properties of these dwarf novae, and derived characteristic outburst recurrence times. Combined with the recently published measurement of the orbital period of PU CMa, we propose that PU CMa is the first object filling the gap between the extreme WZ Sge-type and ER UMa-type stars.

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arXiv2002

Is KT Mon a Classical Nova?

KT Mon has been considered as a classical nova in 1942 based on its light curve and spectroscopy. However, we noticed a unusual feature for a nova soon after its maximum: the early presence of high excitation emission lines such as He II and N III. We propose previously overlooked interpretations that KT Mon can be either a WZ Sge-type dwarf nova and an X-ray transient, although the possibility of an unusual or a recurrent nova is not completely excluded. If KT Mon is a WZ Sge-type dwarf nova, the system is expected to have a brown dwarf secondary. If KT Mon is an X-ray transient, the system is a good candidate for the nearest black-hole binary. Within our knowledge, the observed features seem to more strongly support the latter possibility. In all possibilities, we can expect a recurrent outburst in meaningfully near future.

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arXiv2002

V463 Scuti (Nova Sct 2000): Rapidly Evolving Nova with a Prominent Premaximum Halt

We summarize photometric and spectroscopic observations of V463 Sct (Nova Sct 2000), which was originally thought to be a red variable. The spectrum taken on 2000 March 16.81 UT showed prominent emission lines with a FHWM of 990 km/s (H alpha). The light curve shows a conspicuous premaximum halt lasting at least for 24 d, and a late-phase flare-like maximum. The nova then started rapidly fading at a rate corresponding to t2 = 15 +/- 3 d. Long premaximum halts have been considered as a unique character of the "slowest" novae. The present observation, however, suggests that the long premaximum halts are not a unique character of the slowest novae, but a more general phenomenon spreading over a wider range of nova speed classes than has been previously believed. A recent interpretation of premaximum halts requires that the conditions of thermonuclear runaway was only marginally satisfied. Since such conditions are more difficult to meet in rapidly evolving novae, V463 Sct would provide an unique opportunity in testing this interpretation. The early post-outburst spectrum showed co-existence of Fe II lines and some forbidden lines, which suggests that substantial amount of material may have been ejected before the observed optical maximum. The impact of the modern global network (VSNET) on confirmatory processes of transient objects is briefly discussed.

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arXiv2002

WZ Sge-Type Star V592 Herculis

We observed the entire course of the 1998 outburst of V592 Her, which was originally reported as a nova in 1968. We have been able to construct a full light curve of the outburst, which is characterized by a rapid initial decline (0.98 mag/d), which smoothly developed into a plateau phase with a slower linear decline. We detected superhumps characteristic to SU UMa-type dwarf novae ~7 d after the optical maximum. The overall behavior of the light curve and the development of superhumps were characteristic to a WZ Sge-type dwarf nova. Combined with the past literature, we have been able to uniquely determine the superhump period to be 0.05648(2) d. From this period, together with a modern interpretation of the absolute magnitude of the outburst light curve, we conclude that the overall picture of V592 Her is not inconsistent with a lower main-sequence secondary star in contrast to a previous claim that V592 Her contains a brown dwarf.

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arXiv2002

Two X-Ray Bright Cataclysmic Variables with Unusual Activities: GZ Cnc and NSV 10934

We report on a discovery of unexpected activities in two X-ray bright dwarf novae. GZ Cnc showed an anomalous clustering of outbursts in 2002, in contrast to a low outburst frequency in the past record. The activity resembles an increased activity seen in some intermediate polars or candidates. We identified NSV 10934, X-ray selected high-amplitude variable star, as a dwarf nova with unusually rapid decline. The outburst characteristics make NSV 10934 a twin of recently discovered intermediate polar (HT Cam) with dwarf nova-type outbursts. We propose that these activities in X-ray strong dwarf novae may be a previously overlooked manifestation of outburst activities in magnetic cataclysmic variables.

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arXiv2002

Time-Resolved Photometry of GK Per during the 1996 Outburst

GK Per is a unique cataclysmic variable star which has showed a nova explosion as well as dwarf nova-type outbursts, and has the intermediate-polar nature. We carried out V-band time-resolved photometry and B-band monitoring during the 1996 outburst. This outburst lasted about 60 d and is divided into three parts: the slow rise branch for 35 d, the gradual decay branch with a decay rate of 20.0 d/mag for ~16 d, and the rapid decline branch with a rate of 5.6 d/mag for ~10 d. The $B-V$ color became bluest (B-V~0.18) about 10 d before the outburst maximum, which supports an idea that the outburst in GK Per is of the inside-out type. The spin pulse, 440-s quasi periodic oscillations (QPOs), and ~5,000-s QPOs were detected in our light curve, as previously seen in X-ray and optical observations. In addition, we report the discovery of ~300-s periodicity, which is shorter than the spin period.

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arXiv2002

Photometric Study of KR Aurigae during the High State in 2001

We photometrically observed the VY Scl-type cataclysmic variable KR Aurigae after its final rise from the fading episode in 2000-2001. Time-resolved observation revealed that the light curve is dominated by persistent short-term variation with time-scales of minutes to tens of minutes. On some nights, quasi-periodic variations with periods of 10--15 min were observed. No coherent variation was detected. The power spectral density of the variation has a power law component (f^(-1.63)). The temporal properties of the short-term variations in KR Aur present additional support for the possibility that flickering in CVs may be better understood as a result of self-organized critical state as in black-hole candidates. The light curve lacks "superhump"-type signals, which are relatively frequently seen in VY Scl-type systems and which are suggested to arise from tidal instability of the accretion disk induced by changing mass-transfer rates. The present observation suggests a borderline of superhump excitation in VY Scl-type stars between mass ratios q=0.43 (MV Lyr) and q=0.60 (KR Aur).

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arXiv2002

Superhumps of CC Cancri Revisited

We observed the 2001 November superoutburst of CC Cnc. This observation makes the first detailed coverage of a superoutburst of this object. The best-determined mean superhump period is 0.075518 +/- 0.000018 d, which is 2.7% longer than the reported orbital period. This fractional superhump excess is a quite typical value for a normal SU UMa-type dwarf nova, excluding the previously raised possibility that CC Cnc may have an anomalously large fractional superhump excess. During the superoutburst plateau, the object showed a decrease of the superhump period at dot(P)/P = -10.2(1.3) x 10^(-5), which is one of the largest negative period derivative known in all SU UMa-type dwarf novae.

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arXiv2002

Dramatic Changes in the Outburst Properties in V503 Cygni

We examined the VSNET light curve of the unusual SU UMa-type dwarf nova V503 Cyg which is known to show a short (89 d) supercycle length and exceptionally small (a few) normal outbursts within a supercycle. In 1999-2000, V503 Cyg displayed frequent normal outbursts with typical recurrence times of 7-9 d. The behavior during this period is characteristic to an usual SU UMa-type dwarf nova with a short supercycle length. On the other hand, V503 Cyg showed very infrequent normal outbursts in 2001-2002. Some of the superoutbursts during this period were observed shorter than usual. The remarkable alternations of the outbursting states in V503 Cyg support the presence of mechanisms of suppressing normal outbursts and premature quenching superoutbursts, which have been proposed to explain some unusual SU UMa-type outbursts. The observed temporal variability of the suppressing/quenching mechanisms in the same object suggests that these mechanisms are not primarily governed by a fixed system parameter but more reflect state changes in the accretion disk.

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arXiv2002

The Nature of V359 Centauri Revealed: New Long-Period SU UMa-Type Dwarf Nova

We detected four outbursts of V359 Cen (possible nova discovered in 1939) between 1999 and 2002. Time-resolved CCD photometry during two outbursts (1999 and 2002) revealed that V359 Cen is actually a long-period SU UMa-type dwarf nova with a mean superhump period of 0.08092(1) d. We identified its supercycle length as 307-397 d. This secure identification of the superhump period precludes the previously supposed possibility that V359 Cen could be related to a WZ Sge-type system with a long persistence of late superhumps. The outburst characteristics of V359 Cen are, however, rather unusual in its low occurrence of normal outbursts.

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arXiv2002

Outburst Photometry of the Eclipsing Dwarf Nova GY Cancri

We observed the ROSAT-selected eclipsing dwarf nova GY Cnc (=RX J0909.8+1849) during the 2001 November outburst. We refined the orbital period to be 0.17544251(5) d. The fading portion of the outburst was indistinguishable from those of typical dwarf novae with similar orbital periods. However, the signature of orbital humps (or a hot spot) was far less prominently observed in the orbital light curves and eclipse profiles than in usual dwarf novae with similar orbital periods. The combination of low frequency of outbursts and the apparent lack of slowly rising, long outbursts in GY Cnc is difficult to reconcile within the standard framework of dwarf novae. We suspect that GY Cnc may be the first above-the-gap counterpart of unusual eclipsing dwarf novae HT Cas and IR Com.

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arXiv2002

Deep Transient Optical Fading in the WC9 Star WR 106

We discovered that the WR9-type star WR 106 (HDE 313643) underwent a deep episodic fading in 2000. The depth of the fading (dV ~ 2.9 mag) surpassed those of all known similar "eclipse-like" fadings in WR stars. This fading episode was likely to be produced by a line-of-sight episodic dust formation rather than a periodic enhancement of dust production in the WR-star wind during the passage of the companion star though an elliptical orbit. The overall 2000 episode was composed of at least two distinct fadings. These individual fadings seem to more support that the initial dust formation triggered a second dust formation, or that the two independent dust formations occurred by the same triggering mechanism rather than a stepwise dust formation. We also discuss on phenomenological similarity of the present fading with the double fading of R CrB observed in 1999-2000.

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arXiv2002

NSV 10934: an Unusual ROSAT-Selected Dwarf Nova: An Analogous Object to HT Cam?

NSV 10934 was discovered as a large-amplitude suspected variable star of unknown classification. We noticed that the object is identified with a bright ROSAT X-ray source (1RXS J184050.3-834305). Systematic monitoring of this object by the VSNET team revealed four outbursts between 2001 and 2002, confirming the dwarf nova-type nature of NSV 10934. The outbursts were characterized by a rapid fading (1.5 mag within 0.90 d) toward the end of the outbursts. Combined with the strong and relatively hard X-ray detection, this outburst behavior suggests that the object may be related to the recently discovered intermediate polar, HT Cam (astro-ph/0206184).

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arXiv2002

Unusually Increased Activity of GZ Cnc

GZ Cnc was discovered as a variable star by Takamizawa (Tmz V34), which turned out to a cataclysmic variable (CV). Past studies have shown that this object is a good candidate for a rarely outbursting, long-period dwarf nova. Only three outbursts were recorded between 1999 and 2001. In 2002 March-May, the object suddenly showed a dramatic increase of the outburst frequency. The shortest observed interval was only 11 d. We suggest that the present behavior may be interpreted as a clustering of short outbursts occasionally seen in intermediate polars (magnetic CVs). The present activity can also be comparable to V426 Oph, another dwarf nova which is known to show occasionally increased activities. The resemblances in the X-ray properties and the outburst behavior suggests that GZ Cnc and V426 Oph may comprise a new class of (weakly magnetic) cataclysmic variables.

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arXiv2002

Observations of the Magnetic Cataclysmic Variable VV Puppis with the Far Ultraviolet Spectroscopic Explorer

We present the first far-ultraviolet (FUV) observations of the magnetic cataclysmic variable VV Puppis, obtained with the Far Ultraviolet Spectroscopic Explorer satellite. In addition, we have obtained simultaneous ground-based optical photometric observations of VV Pup during part of the FUV observation. The shapes of the FUV and optical light curves are consistent with each other and with those of past observations at optical, extreme-ultraviolet, and X-ray wavelengths. Time-resolved FUV spectra during the portion of VV Pup's orbit when the accreting magnetic pole of the white dwarf can be seen show an increasing continuum level as the accretion spot becomes more directly visible. The most prominent features in the spectrum are the O VI 1031.9A, 1037.6A emission lines. We interpret the shape and velocity shift of these lines in the context of an origin in the accretion funnel near the white dwarf surface. A blackbody function with T > 90,000 K provides an adequate fit to the FUV spectral energy distribution of VV Pup.

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arXiv2002

Discovery of Extremely Large-Amplitude Quasi-Periodic Photometric Variability in WC9-Type Wolf-Rayet Binary, WR 104

We discovered that the Wolf-Rayet (WR)+OB star binary, WR 104, renowned for its associated "dusty pinwheel nebula" recently spatially resolved with infrared interferometry, exhibits strong quasi-periodic optical variations with a full amplitude of 2.7 mag. Such a large-amplitude, continuous variation has been unprecedented in a WR star. The optical quasi-period (~241 d) is in almost perfect agreement with the interferometric period (243.5+/-3 d). The remarkable agreement of the dominant period in optical variability with the orbital period supports that the strongly varying dust obscuration is physically related to the binary motion, rather than sporadic dust-forming episodes. Considering the low orbital inclination (11+/-7 deg) and the nearly circular orbit inferred from the interferometric observations, the strongly variable line-of-sight extinction suggests that the highly structured extinction can be being formed via an ejection of dust in the direction of the binary rotation axis. Another viable explanation is that the three-dimensional structure of the shock front, itself is the obscuring body. Depending on the geometry, the dusty shock front near the conjunction phase of the binary can completely obscure the inner WR-star wind and the OB star, which can explain the amplitude of optical fading and the past observation of remarkable spectral variation.

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arXiv2002

The Extended Deep Minimum and the Subsequent Brightening of RX And in 1996-1997

We discovered that RX And, one of prototypical Z Cam-type dwarf novae, underwent a deep, extended faint state in 1996-1997. Time-resolved photometry at the bottom of the fading revealed the presence of strong flickering, and the absence of detectable orbital modulation. This finding indicates that the mass-transfer remained even at the deepest minimum of the fading, contrary to what was observed in a deep minimum of a VY Scl-type star, MV Lyr. RX And subsequently underwent a brightening (outburst) during its recovery stage. The photometric and spectroscopic characteristics of the brightening significantly differed from those of ordinary outbursts of RX And, and are considered to resemble an inside-out type outburst of a long-period dwarf nova. An examination of historical visual observations revealed the possible presence of \~10-yr periodicity, which is close to what has been proposed for MV Lyr. The common observed features between RX And and VY Scl-type stars may suggest a common underlying mechanism for producing temporary deep fadings. The departure from the disk instability model, as observed in VY Scl-type stars, was not apparent in the present fading of RX And. In conjunction with the recently published Hubble Space Telescope observation during the same fading, we can conclude that the phenomenological difference from the VY Scl-type fading is understood as a smaller effect of irradiation on the accretion disk in RX And.

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