HUBBLE-SPACE-TELESCOPE DISCOVERS OPTICAL-EMISSION FROM THE RADIO PULSAR PSR-1055-52

Optical/UV emission from isolated neutron stars, if detected, represen ts a valuable indicator of the pulsar's emission mechanisms, and more so for those objects of ''intermediate'' age, where thermal and nonthe rmal processes are known to coexist. PSR 1055-52 is a middle-aged puls ar and a strong gamma-ray emitter. As such, it represents a logical ta rget, but the presence of a nearby star renders optical ground-based s earches totally hopeless. The Hubble Space Telescope Faint Object Came ra (FOC), however, was successful in detecting the pulsar's emission t hrough its U (F342) filter. It is interesting to compare such emission with an extrapolation of the soft, thermal part of the ROSAT X-ray sp ectrum. Within the uncertainties, it appears that the FOC flux is comp atible with the Rayleigh-Jeans side of such a spectrum, as was the cas e for Geminga, in many ways a similar object. PSR 1055-52 becomes one of three pulsars (besides the Crab and Vela) visible at all wavelength s.