1998 Superoutburst of the Large-Amplitude SU UMa-Type Dwarf Nova WX Ceti

We observed the 1998 November superoutburst of WX Cet, a dwarf nova originally proposed as a WZ Sge-like system. The observation established that WX Cet is an SU UMa-type dwarf nova with a mean superhump period of 0.05949(1) d, which is 2.1% longer than the reported orbital period. The lack of early superhumps at the earliest stage of the superoutburst, the rapid development of usual superhumps, and the possible rapid decay of late superhumps seem to support that WX Cet is a fairly normal large-amplitude SU UMa-type dwarf nova, rather than a WZ Sge-type dwarf nova with a number of peculiarities. However, a period increase of superhumps at a rate dot(P)/P = (+8.5+/-1.0) x 10^-5 was observed, which is one of the largest dot(P)/P ever observed in SU UMa-type dwarf novae. A linear decline of light, with a rate of 0.10 mag/d, was observed in the post-superoutburst stage. This may be an exemplification of the decay of the viscosity in the accretion disk after the termination of a superoutburst, mechanism of which is proposed to explain a variety of post-superoutburst phenomena in some SU UMa-type dwarf novae.Comment: 8 pages, 8 figures, accepted for publication in Publ. Astron. Soc. Japa

C*-algebras associated to boolean dynamical systems

The goal of these notes is to present the C*-algebra C*(B,L,θ) of a Boolean dynamical system (B,L,θ), that generalizes the C*-algebra associated to Labelled graphs introduced by Bates and Pask, and to determine its simplicity, its gauge invariant ideals, as well as compute its K-Theory

C*-crossed products and shift spaces

AbstractWe use Exel's C*-crossed products associated to non-invertible dynamical systems to associate a C*-algebra to arbitrary shift space. We show that this C*-algebra is canonically isomorphic to the C*-algebra associated to a shift space given by Carlsen [Cuntz–Pimsner C*-algebras associated with subshifts, Internat. J. Math. (2004) 28, to appear, available at arXiv:math.OA/0505503], has the C*-algebra defined by Carlsen and Matsumoto [Some remarks on the C*-algebras associated with subshifts, Math. Scand. 95 (1) (2004) 145–160] as a quotient, and possesses properties indicating that it can be thought of as the universal C*-algebra associated to a shift space.We also consider its representations and its relationship to other C*-algebras associated to shift spaces. We show that it can be viewed as a generalization of the universal Cuntz–Krieger algebra, discuss uniqueness and present a faithful representation, show that it is nuclear and satisfies the Universal Coefficient Theorem, provide conditions for it being simple and purely infinite, show that the constructed C*-algebras and thus their K-theory, K0 and K1, are conjugacy invariants of one-sided shift spaces, present formulas for those invariants, and present a description of the structure of gauge invariant ideals

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, see also http://www.kusastro.kyoto-u.ac.jp/vsnet/DNe/egcnc.html), during its superoutburst in 1996-1997. EG Cnc, after the main superoutburst accompanied with 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 the superhumps observed during the rebrightening stage and the fading tail are a "remnant" of usual superhumps, and are not newly triggered by rebrightenings. By 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 considerable amount of remnant matter behind the cooling front in the outer accretion disk, even after the main superoutburst. We consider the 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.Comment: 16 pages, 15 figures, to appear in PASJ (special issue on VSNET), for more information, see http://www.kusastro.kyoto-u.ac.jp/vsnet