Multiwavelength observations of Hercules X-1/HZ Herculis

We present first results from a multiwavelength campaign to observe the binary pulsar system Her X-1/ HZ Her. The campaign spanned 4 days during 1993 August: observations were taken with five satellites (IUE, EUVE, ROSAT, ASCA, and CGRO/BATSE) and several ground-based facilities. A substantial, unexpected drop in flux at X-ray energies, with no change in absorbing column density, was observed. The pulse period has increased from the previous measurement, contrary to the usual spin-up, and no pulsed emission is detected above 0.9 keV in the low state. The optical and UV fluxes continued to show 1.7 day modulation attributed to X-ray heating of the companion star. However, the significant reduction in UV flux observed around eclipse implies an absence of the normally observed excess attributed to X-ray heating of the disk. We conclude that we have observed an anomalous low state, seen only once before (Parmar et al. 1985), in which the X-ray flux is not redistributed but obscured. We suggest explanations for the behavior of the flux at different wavelengths.