Hajime Baba

The Infrared Astronomical Mission AKARI

AKARI, the first Japanese satellite dedicated to infrared astronomy, was launched on 2006 February 21, and started observations in May of the same year. AKARI has a 68.5 cm cooled telescope, together with two focal-plane instruments, which survey the sky in six wavelength bands from mid- to far-infrared. The instruments also have a capability for imaging and spectroscopy in the wavelength range 2-180 mu m in the pointed observation mode, occasionally inserted into a continuous survey operation. The in-orbit cryogen lifetime is expected to be one and a half years. The All-Sky Survey will cover more than 90% of the whole sky with a higher spatial resolution and a wider wavelength coverage than that of the previous IRAS all-sky survey. Point-source catalogues of the All-Sky Survey will be released to the astronomical community. Pointed observations will be used for deep surveys of selected sky areas and systematic observations of important astronomical targets. These will become an additional future heritage of this mission.

The Far-Infrared Surveyor (FIS) for AKARI

The Far-Infrared Surveyor (FIS) is one of two focal-plane instruments on the AKARI satellite. FIS has four photometric bands at 65, 90, 140, and 160 mu m, and uses two kinds of array detectors. The FIS arrays and optics are designed to sweep the sky with high spatial resolution and redundancy. The actual scan width is more than eight arcminutes, and the pixel pitch matches the diffraction limit of the telescope. Derived point-spread functions (PSFs) from observations of asteroids are similar to those given by the optical model. Significant excesses, however, are clearly seen around tails of the PSFs, whose contributions are about 30% of the total power. All FIS functions are operating well in orbit, and the performance meets the laboratory characterizations, except for the two longer wavelength bands, which are not performing as well as characterized. Furthermore, the FIS has a spectroscopic capability using a Fourier transform spectrometer (FTS). Because the FTS takes advantage of the optics and detectors of the photometer, it can simultaneously make a spectral map. This paper summarizes the in-flight technical and operational performance of the FIS.

Variable Star Network: World Center for Transient Object Astronomy and Variable Stars

Variable Star Network (VSNET) is a global professional-amateur network of researchers in variable stars and related objects, particularly in transient objects, such as cataclysmic variables, black-hole binaries, supernovae, and gamma-ray bursts. The VSNET has been playing a pioneering role in establishing the field of transient object astronomy, by effectivelyincorporating modern advancesin observationalastronomy and global electronic networks, as well as collaborative progress in theoretical astronomy and astronomical computing. The VSNET is now one of the best-featured global networks in this field of astronomy. We review the historical progress, design concept, associated technology, and a wealth of scientific achievements powered by VSNET.

The 1995 Superoutburst of the WZ Sge-type Dwarf Nova AL Comae Berenices

We report time-resolved photometric observations of the first superoutburst confirmed since 1975 in a most enigmatic cataclysmic variable, AL Comae Berenices, during 1995 April-June. We detected double-peaked modulations in the early phase of the outburst, the period of which is slightly shorter than the superhump period. Three possibilities for the mechanism causing these oscillations are discussed. We also refer to similar modulations observed in ER UMa stars. As seen during the 1978 outburst of WZ Sge, a dip of over 2 mag in the light curve occurred in the 1995 outburst. After recovery from this dip, AL Com again declined 0.5 mag and rebrightened, showing modulations with the same period as that of the superhump, which indicates that a second superoutburst occurred. These modulations disappeared before the final decline. After decline, AL Com stayed about or over 1 mag brighter than its quiescence level for at least 2 weeks. It is likely that the accretion disk remained in its hot state near the inner edge during the dip and after the end of the outburst.

SUPERHUMPS IN CATACLYSMIC BINARIES. XII. CR BOOTIS, A HELIUM DWARF NOVA

CR Bootis is an enigmatic blue variable star with rapid photmetric variations and a spectrum dominated by helium. It consists of two white dwarfs in close orbit, with a probable underlying binary period of 1471 s. For years we have marveled at the stars large nightly variations - ramping up or down at a rate of ~0.1/hr. An intensive photometry campaign in 1996 showed that this variability is cyclic with a quasiperiod of about 19 hr, and demonstrated association of 14990 s photometric variations (superhumps) with extended bright states (superoutbursts). During superoutburst, the 1490 s signal initially decreased with dot P = -2x 10-5 , but then stabilized at 1487.29±0.02 s after ~300-600 binary orbits. Spectroscopy reveals variably asymmetric absorption lines, with the asymmetry migrating on a probable period of 36 hr; this may be the period of accretion disk precession. Neither the helium composition, nor the degeneracy of the mass-losing component, nor the shortness of the period (all of the periods) seem to present any barrier to the star in being fully certificable as a bona fide dwarft nova. Stabilization of the superhump period at such a low value (1487.29 s) favors a model in which period changes arise from eccentricity changes rather than mean radius changes in the disk. This naturally explains why decreasing period and decreasing amplitude are strongly linked in the superhumps of dwarf novae.

Superhumps and Repetitive Rebrightenings of the WZ Sge-Type Dwarf Nova, EG Cancri

We report on time-resolved photometric observations of the WZ Sge-type dwarf nova, EG Cnc (Huruhata’s variable), during its superoutburst in 1996–1997. EG Cnc, after the main superoutburst accompanied by the development of superhumps typical of a WZ Sge-type dwarf nova, exhibited a series of six major rebrightenings. During these rebrightenings and the following long fading tail, EG Cnc persistently showed superhumps having a period equal to the superhump period observed during the main superoutburst. The persistent superhumps had a constant superhump flux with respect to the rebrightening phase. These findings suggest that the superhumps observed during the rebrightening stage and the fading tail are a “remnant” of the usual superhumps, and are not newly triggered by rebrightenings. By a comparison with the 1977 outburst of this object and outbursts of other WZ Sge-type dwarf novae, we propose an activity sequence of WZ Sge-type superoutbursts, in which the current outburst of EG Cnc is placed between a single-rebrightening event and distinct outbursts separated by a dip. The post-superoutburst behavior of WZ Sge-type dwarf novae can be understood in the presence of a considerable amount of remnant matter behind the cooling front in the outer accretion disk, even after the main superoutburst. We consider that a premature quenching of the hot state due to the weak tidal effect under the extreme mass ratio of the WZ Sge-type binary is responsible for the origin of the remnant mass.

The SU UMa Nature of the Dwarf Nova, DM Lyrae

We carried out time-resolved V-band photometry of DM Lyr during long outbursts in 1996 July and in 1996 February–March at Ouda Station, Kyoto University, and atOsaka Kyoiku University. Since superhumps were clearly detected in the light curves, DM Lyr was first identified as being an SU UMa-type dwarf nova. The superhump period is 0.0673(2)d, and the superhump excess is 2.8(3)%. The duration of the superoutburst, the outburst amplitude, the decline rate in the plateau phase, and the superhump excess were typical values for a usual SU UMa star. According to visual and CCD observations reported to VSNET, this star has experienced a dramatic change of the outburst pattern from a superoutburst phase to a normal outburst phase. There may exist mechanisms to decrease the number of normal outbursts between two successive superoutburstsand to elongate the recurrence cycle of the superoutburst.

The 1996 Superoutburst of the New SU UMa–Type Dwarf Nova HS Virginis

We report long‐term and time‐resolved photometric observations of the dwarf nova HS Vir (=PG 1341−079). Time‐resolved photometry during the bright outburst in 1996 March revealed superhumps with a period of 0.08059 ± 0.00003 days, which first firmly confirmed HS Vir as a genuine member of SU UMa–type dwarf novae. HS Vir is rather unusual in the smallness (2.7–3.5 mag for brightest superoutbursts) of outburst amplitudes. The marked diversity inferred from the comparison with SX LMi may reflect different mechanisms responsible for the smallness of outburst amplitudes in these dwarf novae.

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 II at 4686 u A, which were absent on July 23, emerged on July 24 together with emission lines of C III/N III Bowen blend. Analyses of the He 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.

IR Com: Deeply Eclipsing Dwarf Nova Below the Period Gap —A Twin of HT Cas?

We observed an X-ray selected, deeply eclipsing cataclysmic variable IR Com (= S 10932). We detected an outburst occurring on 1996 January 1. The light curve of the outburst closely resembled that of a normal outburst of an SU UMa-type dwarf nova, rather than that of an intermediate polar. Time-resolved photometry during the outburst showed that eclipses became systematically deeper and narrower as the outburst faded. Full-orbit light curves in quiescence showed little evidence of orbital humps or asymmetry of the eclipses. In addition to the presence of high–low transitions in quiescence, the overall behavior of the outbursts and the characteristics of the eclipse profiles suggest that IR Com can be best understood as a twin of HT Cas, a famous eclipsing SU UMa-type dwarf nova with a number of peculiarities.