Soft X-ray Observation of the Prompt Emission of GRB100418A

We have observed the prompt emission of GRB100418A, from its beginning by the MAXI/SSC (0.7-7 keV) on board the International Space Station followed by the Swift/XRT (0.3-10 keV) observation. The light curve can be fitted by a combination of a power law component and an exponential component (decay constant is $31.6\pm 1.6$ sec). The X-ray spectrum is well expressed by the Band function with $E_{\rm p}\leq$8.3 keV. This is the brightest GRB showing a very low value of $E_{\rm p}$. It satisfies the Yonetoku-relation ($E_{\rm p}$-$L_{\rm p}$). It is also consistent with the Amati relation ($E_{\rm p}$-$E_{\rm iso}$) in 2.5$\sigma$ level.