Hubble Space Telescope Discovers Optical Emission from the Radio Pulsar PSR 1055–52^*

Optical/UV emission from isolated neutron stars, if detected, represents a valuable indicator of the pulsar's emission mechanisms, and more so for those objects of "intermediate" age, where thermal and nonthermal processes are known to coexist. PSR 1055-52 is a middle-aged pulsar and a strong γ-ray emitter. As such, it represents a logical target, but the presence of a nearby star renders optical ground-based searches totally hopeless. The Hubble Space Telescope Faint Object Camera (FOC), however, was successful in detecting the pulsar's emission through its U (F342) filter. It is interesting to compare such emission with an extrapolation of the soft, thermal part of the ROSAT X-ray spectrum. Within the uncertainties, it appears that the FOC flux is compatible with the Rayleigh-Jeans side of such a spectrum, as was the case for Geminga, in many ways a similar object. PSR 1055-52 becomes one of three pulsars (besides the Crab and Vela) visible at all wavelengths.