The Nonthermal Radio, X-Ray, and TeV Gamma-Ray Spectra of MSH 15-52

The spectrum and morphology of the radio emission (∼0°.5 extent) from MSH 15 - 52 (or G320.4-1.2) indicates that there is no association with the 0°.2 plerion of the 150 ms pulsar PSR B1509 - 58. The plerionic synchrotron spectrum is visible only in X-rays, and the steepening of the photon spectral index from ∼1.6 below a few keV to ∼2.1 above ∼10 keV is consistent with a crossover from adiabatic losses to synchrotron losses in a plerionic field strength B ∼ 8 × 10⁻⁶ G. A similar field is derived from equipartition between field and particles. Synchrotron loss in such a weak field is small enough to explain why this plerion is ∼20 times larger than the X-ray plerion of the Crab. Relativistic electrons in the 0?2 source scatter 2.7 K MBR photons and possibly IR photons from surrounding dust grains to TeV energies, and the resulting inverse Compton photon spectrum is expected to be harder than ɛ₁⁻² up to ɛ₁ ∼ 20 TeV, with a softening above this energy. The γ-ray flux is expected to exceed the Crab flux above ɛ₁ ∼ 5 TeV. Nooitgedacht atmospheric Cherenkov TeV observations of MSH 15 - 52 reported here were not sensitive enough to constrain the field strength of the nebula, but future large zenith angle Cherenkov observations should be able to probe the spectrum at ɛ₁ ≫ 1 TeV, allowing a measurement of B.