Search for the elusive optical counterpart of PSR J0537–6910 with the HST Advanced Camera for Surveys

We present the results of deep, high-resolution, multi-band optical observations of the field of the young (${\sim} 5000$ yr) 16 ms X-ray pulsar PSR J0537-6910 performed with the Advanced Camera for Surveys (ACS) aboard the Hubble Space Telescope (HST). Although a few new potential counterparts have been detected within or close to the revised Chandra X-ray error circle (${\simeq} 1 \arcsec$) of the pulsar, only two of them (with magnitudes $m_{\it 814W} \approx 23.9$ and $m_{\it 814W} \approx 24.2$) show indications of a peculiar spectrum which could be related to optical emission from the pulsar. This might be true also for a third, fainter, candidate detected only in one filter (with magnitude $m_{\it 814W} \approx 26.7$). If either of the two brighter candidates is indeed the actual counterpart, the optical output of PSR J0537-6910 would make it similar to young Crab-like pulsars. If not, it would mean that PSR J0537-6910 is significantly underluminous with respect to all pulsars detected in the optical