%0 Journal Article
%T Spectral Evolution of a New X-ray Transient MAXI J0556-332 Observed by MAXI, Swift, and RXTE
%A Mutsumi Sugizaki
%A Kazutaka Yamaoka
%A Masaru Matsuoka
%A Jamie A. Kennea
%A Tatehiro Mihara
%A Kazuo Hiroi
%A Masaki Ishikawa
%A Naoki Isobe
%A Nobuyuki Kawai
%A Masashi Kimura
%A Hiroki Kitayama
%A Mitsuhiro Kohama
%A Takanori Matsumura
%A Mikio Morii
%A Yujin E. Nakagawa
%A Satoshi Nakahira
%A Motoki Nakajima
%A Hitoshi Negoro
%A Motoko Serino
%A Megumi Shidatsu
%A Tetsuya Sootome
%A Kousuke Sugimori
%A Fumitoshi Suwa
%A Takahiro Toizumi
%A Hiroshi Tomida
%A Yoko Tsuboi
%A Hiroshi Tsunemi
%A Yoshihiro Ueda
%A Shiro Ueno
%A Ryuichi Usui
%A Takayuki Yamamoto
%A Makoto Yamauchi
%A Kyohei Yamazaki
%A Atsumasa Yoshida
%J Physics
%D 2013
%I arXiv
%R 10.1093/pasj/65.3.58
%X We report on the spectral evolution of a new X-ray transient, MAXI J0556-332, observed by MAXI, Swift, and RXTE. The source was discovered on 2011 January 11 (MJD=55572) by MAXI Gas Slit Camera all-sky survey at (l,b)=(238.9deg, -25.2deg), relatively away from the Galactic plane. Swift/XRT follow-up observations identified it with a previously uncatalogued bright X-ray source and led to optical identification. For more than one year since its appearance, MAXI J0556-332 has been X-ray active, with a 2-10 keV intensity above 30 mCrab. The MAXI/GSC data revealed rapid X-ray brightening in the first five days, and a hard-to-soft transition in the meantime. For the following ~ 70 days, the 0.5-30 keV spectra, obtained by the Swift/XRT and the RXTE/PCA on an almost daily basis, show a gradual hardening, with large flux variability. These spectra are approximated by a cutoff power-law with a photon index of 0.4-1 and a high-energy exponential cutoff at 1.5-5 keV, throughout the initial 10 months where the spectral evolution is mainly represented by a change of the cutoff energy. To be more physical, the spectra are consistently explained by thermal emission from an accretion disk plus a Comptonized emission from a boundary layer around a neutron star. This supports the source identification as a neutron-star X-ray binary. The obtained spectral parameters agree with those of neutron-star X-ray binaries in the soft state, whose luminosity is higher than 1.8x10^37 erg s^-1. This suggests a source distance of >17 kpc.
%U http://arxiv.org/abs/1301.2098v1