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Mutsumi Sugizaki, Kazutaka Yamaoka, Masaru Matsuoka, Jamie A. Kennea, Tatehiro Mihara, Kazuo Hiroi, Masaki Ishikawa, Naoki Isobe, Nobuyuki Kawai, Masashi Kimura, Hiroki Kitayama, Mitsuhiro Kohama, Takanori Matsumura, Mikio Morii, Yujin E. Nakagawa, Satoshi Nakahira, Motoki Nakajima, Hitoshi Negoro, Motoko Serino, Megumi Shidatsu, Tetsuya Sootome, Kousuke Sugimori, Fumitoshi Suwa, Takahiro Toizumi, Hiroshi Tomida, Yoko Tsuboi, Hiroshi Tsunemi, Yoshihiro Ueda, Shiro Ueno, Ryuichi Usui, Takayuki Yamamoto, Makoto Yamauchi, Kyohei Yamazaki, Atsumasa Yoshida, Spectral Evolution of a New X-Ray Transient MAXI J0556−332 Observed by MAXI, Swift, and RXTE, Publications of the Astronomical Society of Japan, Volume 65, Issue 3, 25 June 2013, 58, https://doi.org/10.1093/pasj/65.3.58
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Abstract
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 |$ =$| 55572) by the MAXI Gas Slit Camera all-sky survey at (|$ l$|, |$ b$|) |$ =$| (238.|$ ^{\!\!\!\circ}$| 9, |$-$| 25.|$ ^{\!\!\!\circ}$| 2), relatively away from the Galactic plane. Swift/XRT follow-up observations identified it with a previously uncatalogued bright X-ray source, which led to optical identification. For more than one year since its appearance, MAXI J0556|$-$| 332 has been X-ray active, with a 2–10 keV intensity above 30 mCrab. The MAXI/GSC data revealed rapid X-ray brightening in the first five days, and a hard-to-soft transition in the meantime. For the following |$ \sim$| 70 days, the 0.5–30 keV spectra, obtained by the Swift/XRT and the RXTE/PCA on an almost daily basis, show a gradual hardening, with large flux variability. These spectra are approximated by a cutoff power-law with a photon index of 0.4–1 and a high-energy exponential cutoff at 1.5–5 keV, throughout the initial 10 months where the spectral evolution is mainly represented by a change of the cutoff energy. To be more physical, the spectra are consistently explained by thermal emission from an accretion disk plus Comptonized emission from a boundary layer around a neutron star. This supports the source identification as being a neutron-star X-ray binary. The obtained spectral parameters agree with those of neutron-star X-ray binaries in the soft state, whose luminosity is higher than 1.8 |$ \times$| 10|$ ^{37}$| erg s|$ ^{-1}$|. This suggests a source distance of |$ >$| 17 kpc.
