Akira Okazaki

Publications (3)4.03 Total impact

• Article: Multiepoch Optical Spectropolarimetry of Three Microquasars, Cyg X-1, LS 5039, and LS I +61° 303

  Osamu Nagae, Koji S. Kawabata, Yasushi Fukazawa, Akira Okazaki, Mizuki Isogai, and Takuya Yamashita
  [show abstract] [hide abstract]
  ABSTRACT: We present optical spectropolarimetric observations of three microquasars, Cyg X-1, LS 5039, and LS I +61° 303. Polarimetry can bring us geometrical information on the innermost part of the circumstellar media. For Cyg X-1, the observed continuum polarization shows sinusoidal time variation (Δ ~ 0.2%) correlated with the orbital phase, as indicated by past studies. The constant component of the polarization has significantly changed (Δ ~ 0.7%) within a few decades, indicating variation of scattering material surrounding the binary. There is no apparent change of polarization across the Hα emission line in Cyg X-1. For LS 5039, the observed polarization shows no significant orbital variation. The wavelength dependence is almost consistent with the typical interstellar polarization (ISP) and we conclude that the ISP is a dominant component of the observed polarization. For LS I +61° 303, the observed polarization is roughly consistent with our previous paper, except for a slight change in the intrinsic polarization and also in the equivalent width of the Hα emission line. These imply a long-term evolution of the Be disk around the mass donor. The position angle of intrinsic polarization at 25° was constant from 2005 through 2008. Our observations suggest that the optical polarization intrinsic to a microquasar with an high mass donor can be explained by the light scattering in the circumstellar matter around the mass donor. A polarization signal from the vicinity of the compact star (including jet and accretion disk) seems absent.
  The Astronomical Journal 02/2009; 137(3):3509. · 4.03 Impact Factor
• Source

  Available from: cern.ch

  Article: Spectropolarimetric Study on Circumstellar Structure of Microquasar LS I +61deg 303

  Osamu Nagae, Koji S. Kawabata, Yasushi Fukazawa, Takuya Yamashita, Takashi Ohsugi, Makoto Uemura, Shingo Chiyonobu, Mizuki Isogai, Toshinari Cho, Masaaki Suzuki, Akira Okazaki, Kiichi Okita, Kenshi Yanagisawa
  [show abstract] [hide abstract]
  ABSTRACT: We present optical linear spectropolarimetry of the microquasar LS I +61$^{\circ}$ 303. The continuum emission is mildly polarized (up to 1.3 %) and shows almost no temporal change. We find a distinct change of polarization across the H$\alpha$ emission line, indicating the existence of polarization component intrinsic to the microquasar. We estimate the interstellar polarization (ISP) component from polarization of the H$\alpha$ line and derive the intrinsic polarization component. The wavelength dependence of the intrinsic component is well explained by Thomson scattering in equatorial disk of the Be-type mass donor. The position angle (PA) of the intrinsic polarization $\sim 25^{\circ}$ represents the rotational axis of the Be disk. This PA is nearly perpendicular to the PA of the radio jet found during quiescent phases. Assuming an orthogonal disk-jet geometry around the compact star, the rotational axis of the accretion disk is almost perpendicular to that of the Be disk. Moreover, according to the orbital parameters of the microquasar, the compact star is likely to get across the Be disk around their periastron passage. We discuss the peculiar circumstellar structure of this microquasar inferred from our observation and possible connection with its high-energy activities.
  11/2006;
• Source

  Available from: ArXiv

  Article: Low- and Medium-Dispersion Spectropolarimetry of Nova V475 Sct (Nova Scuti 2003): Discovery of an Asymmetric High-Velocity Wind in a Moderately Fast Nova

  Koji S. Kawabata, Youichi Ohyama, Noboru Ebizuka, Tadafumi Takata, Michitoshi Yoshida, Mizuki Isogai, Yuji Norimoto, Akira Okazaki, Masashi S. Saitou
  [show abstract] [hide abstract]
  ABSTRACT: We present low-resolution ($R\sim 90$) and medium-resolution ($R\sim 2500$) spectropolarimetry of Nova V475 Sct with the HBS instrument, mounted on the 0.91-m telescope at the Okayama Astrophysical Observatory, and with FOCAS, mounted on the 8.2-m Subaru telescope. We estimated the interstellar polarization toward the nova from the steady continuum polarization components and H$\alpha$ line emission components. After subtracting the interstellar polarization component from the observations, we found that the H$\alpha$ emission seen on 2003 October 7 was clearly polarized. In the polarized flux spectrum, the H$\alpha$ emission had a distinct red wing extending to $\sim +4900$ km s$^{-1}$ and a shoulder around $+3500$ km s$^{-1}$, showing a constant position angle of linear polarization $\theta_{\rm *}\simeq 155\arcdeg\pm 15\arcdeg$. This suggests that the nova had an asymmetric outflow with a velocity of $v_{\rm wind}\simeq 3500$ km s$^{-1}$ or more, which is six times higher than the expansion velocity of the ionized shell at the same epoch. Such a high-velocity component has not previously been reported for a nova in the `moderately fast' speed class. Our observations suggest the occurrence of violent mass-loss activity in the nova binary system even during the common-envelope phase. The position angle of the polarization in the H$\alpha$ wing is in good agreement with that of the continuum polarization found on 2003 September 26 ($p_{\rm *}\simeq 0.4$--0.6 %), which disappeared within the following 2 d. The uniformity of the PA between the continuum polarization and the wing polarization on October 7 suggests that the axis of the circumstellar asymmetry remained nearly constant during the period of our observations.
  05/2006;