A CANDIDATE IDENTIFICATION FOR PSR 0656-EMISSION FROM ISOLATED NEUTRON-STARS(14 AND THE OPTICAL)

PSR 0656 + 14 is a nearby, middle-age, X-ray emitting radio pulsar. He re we present two deep V images of the pulsar field taken from ESO in 1989 and 1991. A faint (m(v) approximately 25) object is seen in both images to coincide precisely with the VLA sub-arcsecond radio position . Although the possibility of a chance coincidence cannot be excluded, the remarkable positional agreement prompts us to regard the faint ca ndidate as the optical counterpart of PSR 0656+ 14. Thus, this will be the 5th isolated neutron star with an optical counterpart. Its V mag could be compatible with the lowest temperatures (and distance) used t o fit the ROSAT X-ray data, using simple blackbody approximation. On t he other hand, given the uncertainties on the distance to PSR 0656 + 1 4 as well as on the value of its temperature, a nonthermal origin of t he optical emission is equally plausible. The inferred optical luminos ity appears to be well above that expected using the P(-10) Pacini law . Since this happens to be the case also for the Geminga optical count erpart, the nature of the optical emission from isolated neutron stars is discussed, considering its dependence on the pulsar age.