uemuram – ページ 26 – Makoto Uemura （植村誠）

Infrared/optical – X-ray simultaneous observations of X-ray flares in GRB 071112C and GRB 080506

2010-05-25 uemuram

T. Uehara, M. Uemura, K. S. Kawabata, Y. Fukazawa, R. Yamazaki, “Infrared/optical – X-ray simultaneous observations of X-ray flares in GRB 071112C and GRB 080506”, A&A, vol. 519, p. 56, 2010.

We investigate the origin of short X-ray flares which are occasionally observed in early stages of afterglows of gamma-ray bursts (GRBs). We observed two Swift events, GRB 071112C and GRB 080506, before the start of X-ray flares in the optical and near-infrared (NIR) bands with the 1.5-m Kanata telescope. In conjunction with published X-ray and optical data, we analyzed densely sampled light curves of the early afterglows and spectral energy distributions (SEDs) in the NIR-X-ray ranges. We found that the SEDs had a break between the optical and X-ray bands in the normal decay phases of both GRBs regardless of the model for the correction of the interstellar extinction in host galaxies of GRBs. In the X-ray flares, X-ray flux increased by 3 and 15 times in the case of GRB 071112C and 080506, respectively, and the X-ray spectra became harder than those in the normal decay phases. No significant variation in the optical-NIR range was detected together with the X-ray flares. These results suggest that the X-ray flares were associated with either late internal shocks or external shocks from two-component jets.

Dwarf Novae in the Shortest Orbital Period Regime: II. WZ Sge Stars as the Missing Population near the Period Minimum

2010-03-03 uemuram

M. Uemura, T. Kato, D. Nogami, and T. Ohsugi, “Dwarf Novae in the Shortest Orbital Period Regime: II. WZ Sge Stars as the Missing Population near the Period Minimum,” PASJ, vol. 62, p. 613-, 2010.

WZ Sge-type dwarf novae are characterized by long recurrence times of outbursts (˜10 yr) and short orbital periods (lsim 85 min). A significant part of WZ Sge stars may remain undiscovered because of low outburst activity. Recently, the observed orbital period distribution of cataclysmic variables (CVs) has changed partly because outbursts of new WZ Sge stars have been discovered routinely. Hence, estimations of the intrinsic population of WZ Sge stars are important to study the population and the evolution of CVs. In this paper, we present a Bayesian approach to estimate the intrinsic period distribution of dwarf novae from observed samples. In this Bayesian model, we assumed a simple relationship between the recurrence time and the orbital period, which is consistent with observations of WZ Sge stars and other dwarf novae. As a result, the minimum orbital period was estimated to be ˜70 min. The population of WZ Sge stars exhibited a spike-like feature at the shortest period regime in the orbital period distribution. These features are consistent with the orbital period distribution previously predicted by population synthesis studies. We propose that WZ Sge stars and CVs with a low mass-transfer rate are excellent candidates for the missing population predicted by the evolution theory of CVs.

Dwarf Novae in the Shortest Orbital Period Regime. I A New Short Superhump Period Dwarf Nova, OT J055717+683226

2009-12-22 uemuram

M. Uemura, A. Arai, T. Kato, H. Maehara, D. Nogami, et al., “Dwarf Novae in the Shortest Orbital Period Regime. I A New Short Superhump Period Dwarf Nova, OT J055717+683226,” PASJ, vol. 62, p. 187-, 2010.

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.

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