Emitting Supernova Remnant Candidate , M 33 X - 4

Based on ASCA observations of the Local-Group spiral galaxy M 33, a report is made on the X-ray properties of luminous X-ray source called X-4, which is located in a giant H II region in M 33. The 0.8–8.0 keV spectrum of X-4 is described by a two-component model, consisting of a thin thermal plasma emission with a temperature of ∼ 1.0keV, and a power law emission with a photon index of ∼ 2.0. The two spectral components cross over at ∼ 3.0keV. These spectral properties are similar to those of a young supernova remnant (SNR), except that the 0.8–8.0 keV luminosity of M 33 X-4, ∼ 1.0× 1038 erg s−1, is considerably higher than those of the SNRs in the Milky Way. There is also evidence of excess absorption, or an over-abundance of α-elements relative to iron. If this source is an SNR, the hard component is thought to be synchrotron radiation from high-energy particles, produced either by shock acceleration, like in SN 1006, or by a central neutron star, like in the Crab Nebula.