Upper limits to the prompt X-ray flux from supernova 1987a.

The absence of a detectable perturbation from SN 1987A in VLF radio signals propagating in the earth-ionosphere waveguide is used to set upper limits to the X-ray flux and temperature. The upper limits obtained are consistent with the lack of formation of a hard X-ray radiating viscous shock, and they are several orders of magnitude lower than those predicted using the method of Lasher and Chan (1978). It is suggested that this discrepancy may be due to shortcomings in the modeling of the flare or of the progenitor star.