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The Crab nebula originated from a core-collapse supernova (SN) explosionobserved in 1054 A.D. When viewed as a supernova remnant (SNR), it has ananomalously low observed ejecta mass and kinetic energy for an Fe-core collapseSN. Intensive searches were made for a massive shell that solves thisdiscrepancy, but none has been detected. An alternative idea is that the SN1054is an electron-capture (EC) explosion with a lower explosion energy by an orderof magnitude than Fe-core collapse SNe. In the X-rays, imaging searches wereperformed for the plasma emission from the shell in the Crab outskirts to set astringent upper limit to the X-ray emitting mass. However, the extremebrightness of the source hampers access to its vicinity. We thus employedspectroscopic technique using the X-ray micro-calorimeter onboard the Hitomisatellite. By exploiting its superb energy resolution, we set an upper limitfor emission or absorption features from yet undetected thermal plasma in the2-12 keV range. We also re-evaluated the existing Chandra and XMM-Newton data.By assembling these results, a new upper limit was obtained for the X-rayplasma mass of <~ 1Mo for a wide range of assumed shell radius, size, andplasma temperature both in and out of the collisional equilibrium. To comparewith the observation, we further performed hydrodynamic simulations of the CrabSNR for two SN models (Fe-core versus EC) under two SN environments (uniformISM versus progenitor wind). We found that the observed mass limit can becompatible with both SN models if the SN environment has a low density of <~0.03 cm-3 (Fe core) or <~ 0.1 cm-3 (EC) for the uniform density, or aprogenitor wind density somewhat less than that provided by a mass loss rate of10-5 Mo yr-1 at 20 km s-1 for the wind environment.

Search for Thermal X-ray Features from the Crab nebula with Hitomi Soft X-ray Spectrometer