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

PSR 0656 + 14 is a nearby, middle-age, X-ray emitting radio pulsar. Here 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 candidate 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 to fit the ROSAT X-ray data, using simple blackbody approximation. On the other hand, given the uncertainties on the distance to PSR 0656 + 14 as well as on the value of its temperature, a nonthermal origin of the optical emission is equally plausible. The inferred optical luminosity 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 counterpart, the nature of the optical emission from isolated neutron stars is discussed, considering its dependence on the pulsar age.