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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-upobservations identified it with a previously uncatalogued bright X-ray sourceand 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, anda hard-to-soft transition in the meantime. For the following ~ 70 days, the0.5-30 keV spectra, obtained by the Swift/XRT and the RXTE/PCA on an almostdaily basis, show a gradual hardening, with large flux variability. Thesespectra are approximated by a cutoff power-law with a photon index of 0.4-1 anda high-energy exponential cutoff at 1.5-5 keV, throughout the initial 10 monthswhere the spectral evolution is mainly represented by a change of the cutoffenergy. To be more physical, the spectra are consistently explained by thermalemission from an accretion disk plus a Comptonized emission from a boundarylayer around a neutron star. This supports the source identification as aneutron-star X-ray binary. The obtained spectral parameters agree with those ofneutron-star X-ray binaries in the soft state, whose luminosity is higher than1.8x10^37 erg s^-1. This suggests a source distance of >17 kpc.

Spectral Evolution of a New X-ray Transient MAXI J0556-332 Observed by MAXI, Swift, and RXTE