Akira Imada - Kagoshima University

Akira Imada
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Akira Imada
Kagoshima University

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Solar and Stellar Astrophysics (17)
Astrophysics (9)
High Energy Astrophysical Phenomena (2)

Publications Authored By Akira Imada

We observed RZ LMi, which is renowned for the extremely (~19d) short supercycle and is a member of a small, unusual class of cataclysmic variables called ER UMa-type dwarf novae, in 2013 and 2016. In 2016, the supercycles of this object substantially lengthened in comparison to the previous measurements to 35, 32, 60d for three consecutive superoutbursts. We consider that the object virtually experienced a transition to the novalike state (permanent superhumper). Read More

How black holes accrete surrounding matter is a fundamental, yet unsolved question in astrophysics. It is generally believed that matter is absorbed into black holes via accretion disks, the state of which depends primarily on the mass-accretion rate. When this rate approaches the critical rate (the Eddington limit), thermal instability is supposed to occur in the inner disc, causing repetitive patterns of large-amplitude X-ray variability (oscillations) on timescales of minutes to hours. Read More

Continuing the project described by Kato et al. (2009, arXiv:0905.1757), we collected times of superhump maxima for 128 SU UMa-type dwarf novae observed mainly during the 2015-2016 season and characterized these objects. Read More

We present optical photometry of a WZ Sge-type dwarf nova (DN), ASASSN-15jd. Its light curve showed a small dip in the middle of the superoutburst in 2015 for the first time among WZ Sge-type DNe. The unusual light curve implies a delay in the growth of the 3:1 resonance tidal instability. Read More

In 2015 March, the notable WZ Sge-type dwarf nova AL Com exhibited an unusual outburst with a recurrence time of ${\sim}$1.5 yr, which is the shortest interval of superoutbursts among WZ Sge-type dwarf novae. Early superhumps in the superoutburst light curve were absent, and a precursor was observed at the onset of the superoutburst for the first time in WZ Sge-type dwarf novae. Read More

We report on monitoring observations of the TeV gamma-ray binary HESS J0632+057, which were carried out to constrain the interaction between the Be circumstellar disk and the compact object of unknown nature, and provide for the first time high-dispersion (R > 50000) optical spectra in the second half of the orbital cycle, from apastron through periastron. The Halpha, Hbeta, and Hgamma line profiles are found to exhibit remarkable short-term variability for ~1 month after the apastron (phase 0.6--0. Read More

We report on simultaneous $g'$, $R_{\rm c}$ and $I_{\rm c}$ photometry of SU Ursae Majoris during 2011 December - 2012 February using OAO/MITSuME. Our photometry revealed that quiescence is divided into three types based on the magnitude and color. Quiescent light curves showed complicated profiles with various amplitudes and time scales. Read More

Affiliations: 1Hiroshima Astrophysical Science Center, Hiroshima University, 2Kwasan Observatory, Kyoto University, 3Faculty of Engineering, Hokkai-Gakuen University, 4Okayama Astrophysical Observatory, National Astronomical Observatory of Japan, 5Okayama Astrophysical Observatory, National Astronomical Observatory of Japan, 6Kwasan Observatory, Kyoto University, 7Gunma Astronomical Observatory, 8Astronomy Institute, Osaka Kyoiku University, 9Astronomy Institute, Osaka Kyoiku University, 10Astronomy Institute, Osaka Kyoiku University, 11Department of Astronomy, Faculty of Science, Kyoto University

We carried out optical high-dispersion spectroscopic monitoring of the Be disk in a Be/X-ray binary A 0535+262/V725 Tau from 2009 to 2012, covering two giant outbursts and several normal outbursts. This monitoring was performed in order to investigate variabilities of the Be disk due to the interaction with the neutron star in recent X-ray active phase from 2008 to 2011. Such variabilities give a clue to uncleared detailed mechanism for very bright X-ray outbursts, which are unique to some Be/X-ray binaries with relatively wide and eccentric orbit. Read More

We report on time-resolved photometry during a 2012 January normaloutburst of SU UMa. The light curve shows hump-like modulations with a period of 0.07903(11) d, which coincides with the known superhump period of SU UMa during superoutbursts. Read More


We report on high-dispersion optical spectroscopic observations of the Be/X-ray binary A0535+262/V725 Tau during the giant outburst in November/December 2009 and after it. The observed emission line profiles, reflecting the structure of the geometrically thin circumstellar envelope of the Be star (Be disk), show drastic variabilities and indicate the existence of a warped component. The enhanced blue shoulder seen after periastron passage implies the gas stream from a dense part of the Be disk to the neutron star. Read More

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. Read More

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. Read More

We carried out an international spectroscopic observation campaign of the dwarf nova GW Librae (GW Lib) during the 2007 superoutburst. Our observation period covered the rising phase of the superoutburst, maximum, slowly decaying phase (plateau), and long fading tail after the rapid decline from the plateau. The spectral features dramatically changed during the observations. Read More

Affiliations: 1Kyoto U., 2Crimean Astrophysical Observatory, 3Kyoto U., 4VSOLJ, 5Institute of Astronomy, Russian Academy of Sciences, 6Sternberg Astronomical Institute, Astronomical Institute of the Slovak Academy of Sciences, 7AAVSO, 8AAVSO, 9Variable Star Section, Norwegian Astronomical Society, 10BAA VSS, 11Kagoshima U.

We observed the 2006 superoutburst of SDSS J080434.20+510349.2 during its plateau phase, rebrightening phase, and post-superoutburst final decline. Read More

We report on our time-resolved CCD photometry during the 2005 June superoutburst of a WZ Sge-type dwarf nova candidate, ASAS 160048-4846.2. The ordinary superhumps underwent a complex evolution during the superoutburst. Read More

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. Read More

We report time-resolved optical CCD photometry on newly discovered SU UMa-type dwarf novae, FL TrA and CTCV J0549-4921. During the 2006 August outburst, we detected superhumps with a period of 0.59897(11) days for FL TrA, clarifying the SU UMa nature of the system. Read More

We report on the time-resolved CCD photometry of a newly discovered variable star, TSS J022216.4+412259.9 during the outburst in 2005 November-December brightening. Read More

We report on time-resolved photometry of the 2005 July superoutburst of the dwarf nova, 2QZ J021927.9-304545. The resultant light curves showed conspicuous superhumps with a period of 0. Read More

We report on time-resolved CCD photometry during the 2005 June outburst of a dwarf nova, ASAS160048-4846.2. The observed light curves unambiguously showed embryonic humps with a period of 0. Read More