Kaori Kubota

Kyoto University, Kioto, Kyōto, Japan

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Publications (25)32.31 Total impact

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    ABSTRACT: We report on results of the 2006 April multiwavelength campaign of SS 433, focusing on X-ray data observed with Suzaku at two orbital phases (in- and out-of-eclipse phases) and on simultaneous optical spectroscopic observations. Analyzing the FeXXV Kα lines originating from the jets, we detected a rapid variability of the Doppler shifts, dz/dt ≈ 0.019/0.33d-1, which is larger than those expected from the precession and/or nodding motion. This phenomenon probably corresponds to ``jitter'' motions observed for the first time in X-rays, for which significant variabilities in both the jet angle and the intrinsic speed are required. From time lags between optical Doppler curves and X-ray ones, we estimated a distance between the optical jets and the base to be ˜(3--4) × 1014cm. Based on the radiatively cooling jet model, we determined that the innermost temperatures of the jets are T0 = 13±2 keV for the out-of-eclipse phase and 16±3keV for the in-eclipse (the average of the blue and red jets) from the line intensity ratio of FeXXV Kα and FeXXVI Kα. While the broad-band-continuum spectra over the 5-40keV band in eclipse is consistent with a multitemperature-bremsstrahlung emission expected from the jets and its reflection component from cold matter, the out-of-eclipse spectrum is harder than the jet emission with the base temperature determined above, implying the presence of an additional hard component.
    Publications- Astronomical Society of Japan 03/2010; 62:323. · 2.44 Impact Factor
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    ABSTRACT: We report results of the 2006 April multi-wavelengths campaign of SS 433, focusing on X-ray data observed with Suzaku at two orbital phases (in- and out-of- eclipse) and simultaneous optical spectroscopic observations. By analyzing the Fe25 K_alpha lines originating from the jets, we detect rapid variability of the Doppler shifts, dz/dt ~ 0.019/0.33 day^-1, which is larger than those expected from the precession and/or nodding motion. This phenomenon probably corresponding to "jitter" motions observed for the first time in X-rays, for which significant variability both in the jet angle and intrinsic speed is required. From the time lag of optical Doppler curves from those of X-rays, we estimate the distance of the optical jets from the base to be ~(3-4) \times 10^14 cm. Based on the radiatively cooling jet model, we determine the innermost temperature of the jets to be T_0 = 13 +/- 2 keV and 16 +/- 3 keV (the average of the blue and red jets) for the out-of-eclipse and in-eclipse phase, respectively, from the line intensity ratio of Fe25 K_alpha and Fe26 K_alpha. While the broad band continuum spectra over the 5--40 keV band in eclipse is consistent with a multi-temperature bremsstrahlung emission expected from the jets, and its reflection component from cold matter, the out-of-eclipse spectrum is harder than the jet emission with the base temperature determined above, implying the presence of an additional hard component. Comment: 12 pages, 15 figures, accepted for publication in PASJ
    01/2010;
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    ABSTRACT: We report on the observation of a new dwarf nova (DN), OT J055717+683226 (OT: optical transient), during the period of its first-recorded superoutburst in 2006 December. Our observation shows that this object is an SU UMa-type dwarf nova having a very short superhump period of 76.67±0.03 min (0.05324±0.00002 d). The next superoutburst was observed in 2008 March. The recurrence time of superoutbursts (supercycle) is, hence, estimated to be ˜480 d. The supercycle (˜480 d) is much shorter than those (≳ 10 yr) of WZ Sge-type dwarf novae which are in the major of dwarf novae in the shortest orbital period regime (≲ 85 min). Using a hierarchical cluster analysis, we divided the dwarf nova in the shortest orbital period regime in seven groups. We found that objects, which have short supercycles, small outburst amplitudes, and large superhump period excesses compared with those of WZ Sge stars, form a small group. OT J055717+683226 probably belongs to this group.
    Publications- Astronomical Society of Japan 01/2010; 62. · 2.44 Impact Factor
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    ABSTRACT: We report the observation of a new dwarf nova, OT J055717+683226, during its first-ever recorded superoutburst in December 2006. Our observation shows that this object is an SU UMa-type dwarf nova having a very short superhump period of 76.67+/- 0.03 min (0.05324+/-0.00002 d). The next superoutburst was observed in March 2008. The recurrence time of superoutbursts (supercycle) is, hence, estimated to be ~480 d. The supercycle is much shorter than those of WZ Sge-type dwarf novae having supercycles of >~ 10 yr, which are a major population of dwarf novae in the shortest orbital period regime (<~85 min). Using a hierarchical cluster analysis, we identified seven groups of dwarf novae in the shortest orbital period regime. We identified a small group of objects that have short supercycles, small outburst amplitudes, and large superhump period excesses, compared with those of WZ Sge stars. OT J055717+683226 probably belongs to this group. Comment: 14 pages, 11 figures, accepted for publication in PASJ
    Publications- Astronomical Society of Japan 12/2009; · 2.44 Impact Factor
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    ABSTRACT: We present results of optical spectroscopic observations of the mass donor star in SS 433 with Subaru and Gemini, with an aim to best constrain the mass of the compact object. Subaru/FOCAS observations were performed on October 6-8 and 10, 2007, covering the orbital phase of phi=0.96-0.26. We first calculate cross correlation function of these spectra with that of the reference star HD 9233 in the wavelength range of 4740-4840 Angstrom. This region is selected to avoid 'strong' absorption lines accompanied with contaminating emission components. The same analysis is applied to archive data of Gemini/GMOS taken at phi=0.84-0.30 by Hillwig & Gies (2008). From the Subaru and Gemini CCF results, the amplitude of radial velocity curve of the donor star is determined to be 58.3+/-3.8 km s-1 with a systemic velocity of 59.2+/-2.5 km s-1. Together with the radial velocity curve of the compact object, we derive the mass of the donor star and compact object to be M_O=12.4+/-1.9 M_sun and M_X=4.3+/-0.6 M_sun, respectively. We conclude, however, that these values should be taken as upper limits. From the analysis of the averaged absorption line profiles of strong lines and weak lines observed with Subaru, we find evidence for heating effects from the compact object. Using a simple model, we find that the true radial velocity amplitude of the donor star could be as low as 40+/-5 km s-1 in order to produce the observed absorption-line profiles. Taking into account the heating of the donor star may lower the derived masses to M_O=10.4 +2.3/-1.9 M_sun and M_X=2.5 +0.7/-0.6 M_sun. Our final constraint, 1.9 M_sun< M_X <4.9 M_sun, indicates that the compact object in SS 433 is most likely a low mass black hole, although the possibility of a massive neutron star cannot be firmly excluded. Comment: 36pages, 16 figures, ApJ accepted
    The Astrophysical Journal 12/2009; · 6.73 Impact Factor
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    ABSTRACT: 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 the superhump period is found to be composed of three distinct stages: an early evolutionary stage with a longer superhump period, a middle stage with systematically varying periods, and a final stage with a shorter, stable superhump period. During the middle stage, many systems with superhump periods of less than 0.08 d show positive period derivatives. We present observational characteristics of these stages and give greatly improved statistics. Contrary to an earlier claim, we found no clear evidence for a variation of period derivatives among different superoutbursts of the same object. We present an interpretation that the lengthening of the superhump period is a result of the outward propagation of an eccentricity wave, which is limited by the radius near the tidal truncation. We interpret that late-stage superhumps are rejuvenated excitation of a 3:1 resonance when superhumps in the outer disk are effectively quenched. The general behavior of the period variation, particularly in systems with short orbital periods, appears to follow a scenario proposed in Kato, Maehara, and Monard (2008, PASJ, 60, L23). We also present an observational summary of WZ Sge-type dwarf novae. Many of them have shown long-enduring superhumps during a post-superoutburst stage having longer periods 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 with the 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 candidates for those systems around or after the period minimum of evolution of cataclysmic variables.
    Publications- Astronomical Society of Japan 12/2009; · 2.44 Impact Factor
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    ABSTRACT: 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). Comment: 239 pages, 225 figures, PASJ accepted
    05/2009;
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    ABSTRACT: We report on time-resolved CCD photometry of the cataclysmic variable EG Aquarii during a 2006 November outburst. During the outburst, superhumps were unambiguously detected with a mean period of 0.078828(6) d, allowing for the first time to classify 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 being 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 ˜0.002 d. After the supermaximum, the superhump period decreased at a rate of dot{P}/P = -8.2 × 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 also comment on the basic properties of long period and inactive SU UMa-type dwarf novae.
    Publications- Astronomical Society of Japan 10/2008; · 2.44 Impact Factor
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    ABSTRACT: 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.
    Publications- Astronomical Society of Japan 06/2008; · 2.44 Impact Factor
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    01/2008;
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    ABSTRACT: We observed an X-ray afterglow of GRB 060904A with the Swift and Suzaku satellites. We found rapid spectral softening during both the prompt tail phase and the decline phase of an X-ray flare in the BAT and XRT data. The observed spectra were fit by power-law photon indices which rapidly changed from $\Gamma = 1.51^{+0.04}_{-0.03}$ to $\Gamma = 5.30^{+0.69}_{-0.59}$ within a few hundred seconds in the prompt tail. This is one of the steepest X-ray spectra ever observed, making it quite difficult to explain by simple electron acceleration and synchrotron radiation. Then, we applied an alternative spectral fitting using a broken power-law with exponential cutoff (BPEC) model. It is valid to consider the situation that the cutoff energy is equivalent to the synchrotron frequency of the maximum energy electrons in their energy distribution. Since the spectral cutoff appears in the soft X-ray band, we conclude the electron acceleration has been inefficient in the internal shocks of GRB 060904A. These cutoff spectra suddenly disappeared at the transition time from the prompt tail phase to the shallow decay one. After that, typical afterglow spectra with the photon indices of 2.0 are continuously and preciously monitored by both XRT and Suzaku/XIS up to 1 day since the burst trigger time. We could successfully trace the temporal history of two characteristic break energies (peak energy and cutoff energy) and they show the time dependence of $\propto t^{-3} \sim t^{-4}$ while the following afterglow spectra are quite stable. This fact indicates that the emitting material of prompt tail is due to completely different dynamics from the shallow decay component. Therefore we conclude the emission sites of two distinct phenomena obviously differ from each other.
    Publications- Astronomical Society of Japan 09/2007; · 2.44 Impact Factor
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    ABSTRACT: 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.
    Publications- Astronomical Society of Japan 03/2007; · 2.44 Impact Factor
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    ABSTRACT: Development of MITSuME is reported. Two 50-cm optical telescopes have been built at Akeno in Yamanashi prefecture and at Okayama Astrophysical Observatory (OAO) in Okayama prefecture. Three CCD cameras for simultaneous g'RcIc photometry are to be mounted on each focal plane, covering a wide FOV of about 30" x 30". The limiting magnitude at V is fainter than 18. In addition to these two optical telescopes, a 91-cm IR telescope with a 1 deg x 1 deg field of view is being built at OAO, which performs photometry in YJHK bands. These robotic telescopes can start the observation of counterparts of a GRB within a minute from an alert. We aim to obtain photometric redshifts exceeding 10 with these telescopes. The performance and the current construction status of the telescopes are presented.
    Il Nuovo Cimento C 03/2007;
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    ABSTRACT: We present the results of optical spectroscopic observations of the mass donor star in SS 433 with Subaru and Gemini. Subaru/FOCAS observations were performed on 4 nights of 2007 October, covering the orbital phase of φ ≈ 0. We calculate cross correlation function of these spectra with that of the reference star HD 9233 in spectral regions that are selected to avoid "strong" absorption lines accompanied with contaminating emission components. The same analysis is applied to the archive data of Gemini/GMOS by Hillwig and Gies (2008). From the Subaru and Gemini CCF results, the amplitude of radial velocity curve of the donor star is determined to be 56.7±3.7 km s −1 with a systemic velocity of 61.3±2.5 km s −1 . Combining with the radial velocity of the compact object, we derive the mass of the donor star and compact object to be M O = 12.2 +2.3 −2.1 M and M X = 4.1 +0.8 −0.7 M , respectively. However, these values should be taken as upper limits if we consider the heating effects (Kubota et al. 2009). We conclude that the compact star in SS 433 is most likely a low mass black hole, although the possibility of a massive neutron star cannot be firmly ruled out.
    Astrophysical Journal - ASTROPHYS J. 01/2007; 676(1).
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    ABSTRACT: Suzaku launched in July 2005 is equipped with the wideband instruments covering 0.3–700keV. In addition to these main instruments, active shield counters of the hard X-ray detector are designed to be a wideband all-sky monitor (WAM). The WAM is a powerful gamma-ray burst monitor with a wide energy range of 50–5000keV and with a large effective area of 400cm2 even around 1MeV. The paper describes the strategy for the prompt GRB observation with the WAM, and the follow-up observation of X-ray afterglows with the narrow field instruments. So far, the WAM has detected 53 GRBs and half of them were detected simultaneously with other satellites. In addition to that, from at least 8 GRBs, including a bright and hard GRB 051008, it is succeeded to observe significant gamma-ray emission up to 1MeV. The X-ray afterglow observations were carried out on January 5 and on September 4, 2006. Their results are reported in separate paper.
    Advances in Space Research 01/2007; 40(8):1255–1258. · 1.18 Impact Factor
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    ABSTRACT: We analyzed the X-ray spectra of SS433 in the 5--10 keV band observed with the pn camera of the European Photon Imaging Camera (EPIC) onboard XMM-Newton in four epochs, 2003 April 4, April 8, October 18, and October 25. We detected an iron K-shell absorption edge feature at E~=7.9 keV (in three epochs) or E~=7.6 keV (in one epoch). The continuum spectrum can be well reproduced with a partial covering model, where the absorber covers most of the X-ray jet with a column density of NH~=1024 cm-2. The observed edge energies are consistently explained in all the epochs if the absorbing matter is cold and is moving along the blue-shifted jet with the same velocity. We discuss the geometry of the jet structure based on these results.
    01/2007;
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    ABSTRACT: A radio-IR-optical-X-ray observation campaign of SS 433 has been performed in April 2006, when the jet axis is almost perpendicular to the line of sight. Five flares have been detected during the campaign by radio monitoring observation with RATAN-600. The X-ray astronomical satellite Suzaku observed the source in and out of eclipse. In the X-ray data out of eclipse, the flux shows a significant variation with a time scale of hours. The source seems to be in the active state during the campaign. The observation logs and preliminary results are presented.
    12/2006;
  • Nuovo Cimento B Serie. 12/2006; 121:1531-1533.
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    ABSTRACT: Results are presented of early X-ray afterglow observations of GRB 060105 by Swift and Suzaku. The bright, long gamma-ray burst GRB 060105 triggered the Swift Burst Alert Telescope (BAT) at 06:49:28 on 5 January 2006. The Suzaku team commenced a pre-planned target of opportunity observation at 19 ks (5.3 hr) after the Swift trigger. Following the prompt emission and successive very steep decay, a shallow decay was observed from T_0+187 s to T_0+1287 s. After an observation gap during T_0 +(1.5-3) ks, an extremely early steep decay was observed in T_0+(4-30) ks. The lightcurve flattened again at T_0+30 ks, and another steep decay followed from T_0+50 ks to the end of observations. Both steep decays exhibited decay indices of 2.3 - 2.4. This very early break, if it is a jet break, is the earliest case among X-ray afterglow observations, suggesting a very narrow jet whose opening angle is well below 1 degree. The unique Suzaku/XIS data allow us to set very tight upper limits on line emission or absorption in this GRB. For the reported pseudo-redshift of z=4.0+/-1.3 the upper limit on the iron line equivalent width is 50 eV.
    Publications- Astronomical Society of Japan 10/2006; · 2.44 Impact Factor
  • Nuovo Cimento B Serie. 10/2006; 121:1253-1256.