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Ken'ichi Torii

Publications and source records attributed to Ken'ichi Torii.

17 recordsLinked to original sources
arXiv2009

Survey of Period Variations of Superhumps in SU UMa-Type Dwarf Novae

We systematically surveyed period variations of superhumps in SU UMa-type dwarf novae based on newly obtained data and past publications. In many systems, the evolution of superhump period are found to be composed of three distinct stages: early evolutionary stage with a longer superhump period, middle stage with systematically varying periods, final stage with a shorter, stable superhump period. During the middle stage, many systems with superhump periods less than 0.08 d show positive period derivatives. Contrary to the earlier claim, we found no clear evidence for variation of period derivatives between superoutburst of the same object. We present an interpretation that the lengthening of the superhump period is a result of outward propagation of the eccentricity wave and is limited by the radius near the tidal truncation. We interpret that late stage superhumps are rejuvenized excitation of 3:1 resonance when the superhumps in the outer disk is effectively quenched. Many of WZ Sge-type dwarf novae showed long-enduring superhumps during the post-superoutburst stage having periods longer than those during the main superoutburst. The period derivatives in WZ Sge-type dwarf novae are found to be strongly correlated with the fractional superhump excess, or consequently, mass ratio. WZ Sge-type dwarf novae with a long-lasting rebrightening or with multiple rebrightenings tend to have smaller period derivatives and are excellent candidate for the systems around or after the period minimum of evolution of cataclysmic variables (abridged).

astro-ph.SR↗
arXiv2009

Photometry of Three Superoutbursts of the SU UMa-type Dwarf Nova, SW Ursae Majoris

We investigated the superhump evolution, analysing optical photometric observations of the 2000 February-March, the 2002 October-November, and the 2006 September superoutbursts of SW UMa. The superhumps evolved in the same way after their appearance during the 2000 and the 2002 superoutbursts, and probably during the 2006 one. This indicates that the superhump evolution may be governed by the invariable binary parameters. We detected a periodicity in light curve after the end of the 2000 superoutburst without phase shift, which seems to be the remains of the superhumps. We found QPOs at the end stage of the 2000 and the 2002 superoutbursts, but failed to find extraordinarily large-amplitude QPOs called `super-QPOs' which previously have been observed in SW UMa.

astro-ph.SR↗
arXiv2008

The 2006 November outburst of EG Aquarii: the SU UMa nature revealed

We report time-resolved CCD photometry of the cataclysmic variable EG Aquarii during the 2006 November outburst During the outburst, superhumps were unambiguously detected with a mean period of 0.078828(6) days, firstly classifying the object as an SU UMa-type dwarf nova. It also turned out that the outburst contained a precursor. At the end of the precursor, immature profiles of humps were observed. By a phase analysis of these humps, we interpreted the features as superhumps. This is the second example that the superhumps were shown during a precursor. Near the maximum stage of the outburst, we discovered an abrupt shift of the superhump period by ${\sim}$ 0.002 days. After the supermaximum, the superhump period decreased at the rate of $\dot{P}/P$=$-8.2{\times}10^{-5}$, which is typical for SU UMa-type dwarf novae. Although the outburst light curve was characteristic of SU UMa-type dwarf novae, long-term monitoring of the variable shows no outbursts over the past decade. We note on the basic properties of long period and inactive SU UMa-type dwarf novae.

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arXiv2007

Long-term monitoring of the short period SU UMa-type dwarf nova, V844 Herculis

We report on time-resolved CCD photometry of four outbursts of a short-period SU UMa-type dwarf nova, V844 Herculis. We successfully determined the mean superhump periods to be 0.05584(64) days, and 0.055883(3) for the 2002 May superoutburst, and the 2006 April-May superoutburst, respectively. During the 2002 October observations, we confirmed that the outburst is a normal outburst, which is the first recorded normal outburst in V844 Her. We also examined superhump period changes during 2002 May and 2006 April-May superoutbursts, both of which showed increasing superhump period over the course of the plateau stage. In order to examine the long-term behavior of V844 Her, we analyzed archival data over the past ten years since the discovery of this binary. Although photometry is not satisfactory in some superoutbursts, we found that V844 Her showed no precursors and rebrightenings. Based on the long-term light curve, we further confirmed V844 Her has shown almost no normal outbursts despite the fact that the supercycle of the system is estimated to be about 300 days. In order to explain the long-term light curves of V844 Her, evaporation in the accretion disk may play a role in the avoidance of several normal outbursts, which does not contradict with the relatively large X-ray luminosity of V844 Her.

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arXiv2006

Suzaku observations of the hard X-ray variability of MCG-6-30-15: the effects of strong gravity around a Kerr black hole

Suzaku has, for the first time, enabled the hard X-ray variability of the Seyfert 1 galaxy MCG-6-30-15 to be measured. The variability in the 14-45 keV band, which is dominated by a strong reflection hump, is quenched relative to that at a few keV. This directly demonstrates that the whole reflection spectrum is much less variable than the power-law continuum. The broadband spectral variability can be decomposed into two components - a highly variable power-law and constant reflection - as previously inferred from other observations in the 2-10 keV band. The strong reflection and high iron abundance give rise to a strong broad iron line, which requires the inner disc radius to be at about 2 gravitational radii. Our results are consistent with the predictions of the light bending model which invokes the very strong gravitational effects expected very close to a rapidly spinning black hole.

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arXiv2005

Early BVRcIc imaging and the discovery of the optical afterglow of GRB 041218

We report early BVRcIc imaging of the INTEGRAL GRB 041218. The observation started 129 s after the burst and a sequence of exposures in V, Rc, Ic, and B bands were repeated. The optical afterglow is detected in Rc and Ic bands while it was not detected in B and V. We find that the early afterglow deviates from a single power law decay and identify two characteristic timescales (breaks). Such breaks are theoretically expected from synchrotron cooling in the relativistic fireball (Sari et al. 1998) and we crudely constrain the physical condition of the relativistic shock.

astro-ph↗
arXiv2004

The 2003 Superoutburst of an SU UMa-type Dwarf Nova, GO Comae Berenices

We photometrically observed the 2003 June superoutburst of GO Gom. The mean superhump period was 0.063059(13) d. The resultant data revealed that (1) the obtained light curve contained a precursor, (2) a plateau stage of the object lasted 8 days, which is remarkably shorter than that of ordinary SU UMa-type dwarf novae, and (3) the amplitude of the superoutburst was less than 5 mag, which is unpredictably small when taking into account the fact that the supercycle of GO Com is about 2800 days. In order to explain these anomalies, a mass elimination process from the accretion disk, such as evaporation, may play an important role.

astro-ph↗
arXiv2002

The radiant of the Leonids meteor storm in 2001

We have measured the radiant of the Leonids meteor storm in November 2001 by using new observational and analysis techniques. The radiant was measured as the intersections of lines which were detected and extrapolated from images obtained at a single observing site (Akeno Observatory, Japan). The images were obtained by two sets of telephoto lenses equipped with cooled CCD cameras. The measured radiant, (R.A., Dec.)=(154$^\circ$.35, 21$^\circ$.55) (J2000), is found to be in reasonable agreement with the theoretical prediction by McNaught and Asher (2001), which verifies their dust trail theory.

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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

ASCA Detection of Pulsed X-ray Emission from PSR J0631+1036

ASCA's long look at the 288 millisecond radio pulsar, PSR J0631+1036, reveals coherent X-ray pulsation from this source for the first time. The source was first detected in the serendipitous Einstein observation and later identified as a radio pulsar. Possible pulsation in the gamma-ray band has been detected from the CGRO EGRET data (Zepka, et al. 1996). The X-ray spectrum in the ASCA band is characterized by a hard power-law type emission with a photon index of about 2.3, when fitted with a single power-law function modified with absorption. An additional blackbody component of about 0.14 keV increases the quality of the spectral fit. The observed X-ray flux is 2.1e-13 ergs/s/cm2 in the 1-10 keV band. We find that many characteristics of PSR J0631+1036 are similar to those of middle-aged gamma-ray pulsars such as PSR B1055-52, PSR B0633+17 (Geminga), and PSR B0656+14.

astro-ph↗
arXiv2000

Discovery of 101-s Pulsations from AX J0057.4-7325 in the SMC with ASCA

The results from two ASCA observations of AX J0057.4-7325 = RX J0057.3-7325 are presented. Coherent pulsations with a barycentric period of 101.45 +/- 0.07 s were discovered in the second observation. The X-ray spectrum was found to be hard (photon index ~ 0.9) and unchanged through these observations, except for the flux. The ROSAT archival data show that AX J0057.4-7325 exhibits a flux variation with a factor >~ 10. A discussion on a possible optical counterpart is given.

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arXiv2000

Discovery of 172-s Pulsations from a Be/X-ray Binary Candidate AX J0051.6-7311 in the SMC with ASCA

The results from three ASCA observations of AX J0051.6-7311 = RX J0051.9-7311 are reported. Coherent pulsations with a barycentric period of 172.40 +/- 0.03 s were discovered in the third observation, with an exceptionally long exposure time of ~177 ks. The X-ray spectrum was found to remain unchanged through these observations, with a photon index of ~0.9. Energy-resolved pulse profiles in the third observation reveal that the pulsations are mostly due to photons with an energy above ~2 keV. Archival data of ROSAT and Einstein indicate that AX J0051.6-7311 exhibits a flux variation having a factor ~> 20.

astro-ph↗
arXiv2000

Discovery of a Slow X-Ray Pulsator, AX J1740.1-2847, in the Galactic Center Region

We report the discovery of an X-ray pulsar AX J1740.1-2847 from the Galactic center region. This source was found as a faint hard X-ray object on 7--8 September 1998 with the ASCA Galactic center survey observation. Then, coherent pulsations of P=729 +/- 14 sec period were detected. The X-ray spectrum is described by a flat power-law of ~= 0.7 photon index. The large absorption column of log NH ~= 22.4 (cm^-2) indicates that AX J1740.1-2847 is a distant source, larger than 2.4 kpc, and possibly near at the Galactic center region. The luminosity in the 2--10 keV band is larger than 2.5 x 10^33 erg/s, or likely to be 3.2 x 10^34 erg/s at the Galactic center distance. Although the slow pulse period does not discriminate whether AX J1740.1-2847 is a white dwarf or neutron star binary, the flat power-law and moderate luminosity strongly favor a neutron star binary.

astro-ph↗
arXiv2000

ASCA Observation of the Crab-Like Supernova Remnant 3C58

We present here the X-ray observation of a Crab-like supernova remnant (SNR) 3C58 with ASCA. We find that the integrated energy spectrum over the nebula is consistent with previous results, showing a power-law spectrum with the photon index $γ= 2.2-2.4$ modified by interstellar absorption of about $(3-4)\times 10^{21}{\rm cm^{-2}}$. Inclusion of a blackbody component which is attributable to the central compact source significantly improves the spectral fit. Stringent upper limits for any line emitting thin hot plasma are established. We find for the first time that the nebular spectrum is harder in the central part of the SNR, becoming softer toward the periphery, while the absorption column is uniform across the nebula. Correspondingly, the nebular size decreases with increasing photon energy which is a steeper function of radius than that of the Crab nebula. The results are compared with synchrotron energy loss models and the nature of the putative pulsar is discussed. Timing analysis was performed to search for pulsed X-ray emission from the central compact source. No significant pulsations are observed, and we present the upper limit for the pulsed fraction.

astro-ph↗
arXiv1997

Discovery of Non Thermal X-Rays from the Northwest Shell of the New SNR RX J1713.7-3946: The Second SN1006 ?

We report ASCA results of a featureless X-ray spectrum from RX J1713.7-3946, a new shell-like SNR discovered with the ROSAT all sky survey. The northwest part of RX J1713.7-3946 was in the field of the ASCA Galactic Plane Survey Project and was found to exhibit a shell-like structure. The spectrum, however shows neither line emission nor any signature of a thermal origin. Instead, a power-law model with a photon index of 2.4-2.5 gives reasonable fit to the spectrum, suggesting a non-thermal origin. Together with the similarity to SN1006, we propose that RX J1713.7-3946 is the second example, after SN1006, of a synchrotron X-ray radiation from a shell of SNRs. Since the synchrotron X-rays suggest existence of extremely high energy charged particles in the SNR shell, our discovery should have strong impact on the origin of the cosmic X-rays.

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