Takahiro Nagayama

Three classical Cepheid variable stars in the nuclear bulge of the Milky Way

The nuclear bulge is a region with a radius of about 200 parsecs around the centre of the Milky Way. It contains stars with ages ranging from a few million years to over a billion years, yet its star-formation history and the triggering process for star formation remain to be resolved. Recently, episodic star formation, powered by changes in the gas content, has been suggested. Classical Cepheid variable stars have pulsation periods that decrease with increasing age, so it is possible to probe the star-formation history on the basis of the distribution of their periods. Here we report the presence of three classical Cepheids in the nuclear bulge with pulsation periods of approximately 20u2009days, within 40 parsecs (projected distance) of the central black hole. No Cepheids with longer or shorter periods were found. We infer that there was a period about 25 million years ago, and possibly lasting until recently, in which star formation increased relative to the period of 30–70 million years ago.

Multi-wavelength study of the first γ-ray emitting LMXB XSS J12270-4859

Fermi is discovering many new γ-ray sources, and 1FGL J1227.9-4852 is one of them. Recent studies suggest that this source is associated with the X-ray source XSS J12270-4859 (hereafter J12270), of which nature is poorly understood. In this paper, we show results of our Suzaku and RXTE X-ray observations which are partly simultaneous with near-infrared (NIR) observations using Infra Red Survey Facility (IRSF). We found that J12270 shows (1) no emission lines in the X-ray spectra, (2) unique rapid temporal variations in X-rays, and (3) almost simultaneous X-ray and NIR flares. To investigate its nature, we constructed a spectral energy distribution (SED) from radio to γ-rays. Because of the similarity of its SED with those of other microquasars, we propose that J12270 is a microquasar with a synchrotron jet, and also the first γ-ray emitting low-mass X-ray binary.