Discovery of X-ray Emission from G328.4+0.2, a Crab-like Supernova Remnant

G328.4+0.2 is a moderately small (5×5) Galactic radio supernova remnant (SNR) at a distance of at least 17 kpc that has been long suggested to be Crab-like. Here we report on the detection with ASCA of the X-ray emission from the SNR. The X-ray source is faint with an observed flux of (6.0± 0.8)× 10 erg s cm over the 2–10 keV band. The emission is heavily cut-off at low energies and no flux is detected below 2 keV. Spectral analysis confirms that the column density to the source is indeed large, NH ∼ 10 23 atoms cm, and consistent with the total column density of hydrogen through the Galaxy at this position. Good fits to the spectrum can be obtained for either thermal plasma or nonthermal power-law models, although the lack of detected line emission as well as other evidence argues against the former interpretation. The power-law index we find, αP = 2.9 +0.9 −0.8, is consistent with other Crab-like SNRs. In the radio band G328.4+0.2 is nearly as luminous as the Crab Nebula, yet in the X-ray band luminosity it is some 70 times fainter. Nevertheless its inferred soft X-ray band luminosity is greater than all but the brightest pulsar-powered synchrotron nebulae and implies that G328.4+0.2 contains a rapidly spinning, as yet undetected, pulsar that is losing energy at a rate of ∼10 erg s. Subject headings: ISM: individual (G328.4+0.2, MSH 15−57) — pulsars: general — supernova remnants — X-rays: ISM